Materials and Corrosion Control and Protection.ppt

Types of Corrosion and methods to combat and mitigate corrosion..

1. Corrosion & Corrosion Control
Prof. Yehia ElShazly
Faculty of Engineering
Alexandria University

2. Prof. Yehia ElShazly

3. •What is Corrosion
•Controlling Corrosion
•Materials and Corrosion
Prof. Yehia ElShazly

4. Prof. Yehia ElShazly

5. What is corrosion
•Corrosion is defined as the destruction or
deterioration of a material because of
reaction with its environment.
•Nowadays, the term is not restricted to
metals, but also include nonmetals:
ceramics, glass, polymers, paints,......
Prof. Yehia ElShazly

6. Corrosion damage
•Personal injuries, fatalities,
•Unscheduled shutdowns
•Environmental contamination.
•Product contamination and loss.
•Overdesign
•Appearance
•Maintenance and operating costs
A total direct plus indirect cost in the US is
estimated to be in the range of 552 billion
dollars or 6% of the GNP.
Prof. Yehia ElShazly

7. Corrosion Accidents
Corrosion damage can sometimes be greatly exaggerated by the
circumstances. While many of the accidents due to a failed corroded
components have gone non-public for reasons of liability or simply
because the evidence disappeared in the catastrophic event, others have
made the headlines.
•
Aloha incident
•Bhopal accident
•Carlsbad pipeline explosion
•Guadalajara sewer explosion
•EL AL Boeing 747 crash
•Explosion due to corrosion by
process chemicals
•F-16 fighter aircraft
•Nuclear reactor with a hole in the
head
•Piping rupture caused by flow
accelerated corrosion
•Pitting corrosion accidents and
incidents of aircraft and
helicopters
•Pollution by oil pipeline releases
•Prudhoe Bay 2006 Oil Spill
•Silver bridge
•Sinking of the Erika
•Stress corrosion cracking of
chemical reactor, the Flixborough
explosion
•Swimming Pool Roof Collapse
Prof. Yehia ElShazly

8. Davis-Besse: The Nuclear Reactor with a Hole in its
Head
The reactor core at the Davis-Besse nuclear plant sits within a metal pot
designed to withstand pressures up to 2,500 pounds per square inch. The
reactor vessel has carbon steel walls nearly six inches thick to provide the
necessary strength. Because the water cooling the reactor contains boric
acid that is highly corrosive to carbon steel, the entire inner surface of the
reactor vessel is covered with 3/16-inch thick stainless steel. But water
routinely leaked onto the reactor vessel's outer surface.
Because the outer surface lacked a protective stainless steel coating, boric
acid ate its way through the carbon steel wall until it reached the backside of
the inner liner. High pressure inside the reactor vessel pushed the stainless
steel outward into the cavity formed by the boric acid. The stainless steel
bent but did not break. Cooling water remained inside the reactor vessel not
because of thick carbon steel but due to a thin layer of stainless steel. The
plant's owner ignored numerous warning signs spanning many years to
create the reactor with a hole in its head.
Prof. Yehia ElShazly

9. Silver Bridge Collapse
•On December 15,1967 at
approximately 5 p.m., the U.S.
Highway 35 bridge connecting
Point Pleasant, West Virginia
and Kanauga, Ohio suddenly
collapsed into the Ohio River. At
the time of failure, thirty-seven
vehicles were crossing the
bridge span, and thirty-one of
those automobiles fell with the
bridge. Forty-six individuals
perished with the buckling of the
bridge and nine were seriously
injured.
Prof. Yehia ElShazly

10. Prof. Yehia ElShazly
•The cause of failure was attributed to a cleavage fracture in the lower limb of
eye-bar 330 at joint C13N of the north eye-bar suspension chain in the Ohio
side span." The fracture was caused from a minute crack formed during the
casting of the steel eye-bar. Over the years, stress corrosion and corrosion
fatigue allowed the crack to grow, causing the failure of the entire structure.
At the time of construction, the steel used was not known for subduing to
corrosion fatigue and stress corrosion. Inspection prior to construction would
not have been able to notice the miniature crack. Over the life span of the
bridge, the only way to detect the fracture would have been to disassemble
the eye-bar. The technology used for inspection at the time was not capable
of detecting such cracks.
•Stress corrosion crackingis the formation of brittle cracks in a normally
sound material through the simultaneous action of a tensile stress and a
corrosive environment. Combined withcorrosion fatigue, which occurs as a
result of the combined action of a cyclic stress and a corrosive environment,
disaster was inevitable for the Silver Bridge. The two contributing factors,
over the years continued to weaken the eye-bar and unfortunately the entire
structure.

11. •Another major factor that helped corrosion fatigue and stress corrosion in
bringing down the bridge was the weight of new cars and trucks. When the
bridge was designed, the design vehicle used was the model-T Ford, which
had an approximate weight of less than 1,500 pounds. In 1967, the average
family car weighed 4,000 pounds or more. In 1928, West Virginia law
prohibited the operation of any vehicle whose gross weight, including its
load, was more than 20,000 pounds. In 1967, the weight limit almost tripled
to 60,800 pounds gross, and up to 70,000 with special permits. Civil
engineers must use a projected life span for nearly all projects, but no one
could see that 40 years after the construction of the Silver Bridge that traffic
loads would more than triple.
Prof. Yehia ElShazly

12. Piping Rupture Caused by Flow Accelerated
Corrosion (FAC)
•A piping rupture likely caused byflow accelerated corrosion and /
orcavitation -erosion occurred at Mihama-3 at 3:28pm onAugust 9,
2004, killing five and injuring seve
•The AP reports that sections of the failed line were examined in 1996,
recommended for additional inspections in 2003, and scheduled for
inspection August 14 (five days after the rupture). Although the carbon steel
pipe carried 300-degree steam at high pressure, it had not been inspected
since the power plant opened in 1976. In April 2003, Nihon Arm, a
maintenance subcontractor, informed Kansai Electric Power Co., the plant
owner, that there could be a problem. Last November, the power company
scheduled an ultrasound inspection for Aug. 14.
Prof. Yehia ElShazly

13. •According toJapan's Nuclear and Industrial Safety Agency (NISA), the
rupture was 560 mm in size. The pipe wall at the rupture location had
thinned from 10mm (394 mils) to 1.5mm. Mihama-3 entered commercial
service 28 years ago and is a 826 MWe (gross) Mitsubishi-built PWR
located 200 miles west of Tokyo. Initial measurements showed that the
steam had corroded the pipe from .4 inches to .06 inches, less than one-
third the minimum safety standard. Kansai Electric said in a statement
Tuesday that the pipe showed "large-scale corrosion."
Prof. Yehia ElShazly

14. Sewer explosion, Mexico
Anexampleofcorrosiondamageswithshared
responsibilitieswasthesewerexplosionthatkilledover
200peopleinGuadalajara,Mexico,inApril1992.6
Besidesthefatalities,theseriesofblastsdamaged1600
buildingsandinjured1500people.Damagecostswere
estimatedat75millionU.S.dollars.Thesewerexplosion
wastracedtotheinstallationofawaterpipebya
contractorseveralyearsbeforetheexplosionthatleaked
wateronagasolinelinelayingunderneath.The
subsequentcorrosionofthegasolinepipeline,inturn,
causedleakageofgasolineintothesewers.

16. Loss of USAF F16 fighter aircraft
Graphite-containing grease is a very common lubricant because
graphite is readily available from steel industries. The alternative, a
formulation containing molybdenum disulphide, is much more
expensive. Unfortunately, graphite grease is well known to cause
galvanicallyinduced corrosion in bimetallic couples. In a fleet of over
3000 F16 USAF single-engine fighter aircraft, graphite grease was
used by a contractor despite a general order from the Air Force
banning its use in aircraft. As the flaps were operated, lubricant was
extruded into a part of the aircraft where control of the fuel line
shutoff valve was by means of electrical connectors made from a
combination of gold-and tin-plated steel pins. In many instances
corrosion occurred between these metals and caused loss of control
of the valve, which shut off fuel to the engine in midflight. At least
seven aircraft are believed to have been lost in this way, besides a
multitude of other near accidents and enormous additional
maintenance.

17. The Aloha aircraft incident
The structural failure on April 28, 1988, of a 19-year-old Boeing 737,
operated by Aloha airlines, was a defining event in creating
awareness of aging aircraft in both the public domain and in the
aviation community. This aircraft lost a major portion of the upper
fuselage near the front of the plane in full flight at 24,000 ft.
Miraculously, the pilot managed to land the plane. Multiple fatigue
cracks were detected in the remaining aircraft structure, in the
holes of the upper row of rivets in several fuselage skin lap joints.
Lap joints join large panels of skin together and run longitudinally
along the fuselage. Fatigue cracking was not anticipated to be a
problem, provided the overlapping panels remained strongly
bonded together. Inspection of other similar aircraft revealed
disbonding, corrosion, and cracking problems in the lap joints.
Corrosion processes and the subsequent buildup of voluminous
corrosion products inside the lap joints, lead to “pillowing,”
whereby the faying surfaces are separated.

18. Stress Corrosion Cracking
of Chemical Reactor
•The Flixborough explosion was the largest-ever peacetime explosion in the UK.
There were28 fatalitiesas well as the near complete destruction of the NYPRO plant
in North Lincolnshire by blast and then fire. The catastrophic explosion at Flixborough
in June 1974 has been traced to the failure of a bypass assembly introduced into a
train of six cyclohexane oxidation reactors after one of the reactors was removed
owing to the development of a leak. The leaking reactor, like the others, was
constructed of12.3 mm mild steelplate with3 mm stainless steelbonded to it, and
it developed a vertical crack in the mild steel outer layer of the reactor from which
cyclohexane leaked leading to the removal of the reactor.
•One of the factors contributing to the crack was stress corrosion, resulting from the
presence of nitrates that had contaminated river water being used to cool a leaking
flange. Mild steel exposed to hot nitrate solution and to stresses which in extreme
cases may be well below the yield strength, and may remain in the structure from the
fabrication procedure or may derive from operating stresses, especially if intensified
by a defect, can develop intergranular cracks. Such stress corrosion cracking results
from the conjoint action of stress, of an appropriate magnitude, and a corrosive
environment, of specific composition, upon steel having a wide range of compositions
or structures. Different types of steel are not equally susceptible to cracking as
measured by the minimum stress to promote cracking or the concentration of the salt
responsible for the corrosion reactions that cause crack propagation.
Prof. Yehia ElShazly

19. Prof. Yehia ElShazly

20. •Corrosion is the disintegration of metal through an
unintentional chemical or electrochemical action, starting at
its surface.
•All metals exhibit a tendency to be oxidized, some more
easily than others.
•A tabulation of the relative strength of this tendency is called
the galvanic series.
•Knowledge of a metal's location in the series is an important
piece of information to have in making decisions about its
potential usefulness for structural and other applications.
Prof. Yehia ElShazly

21. The thermodynamic or chemical energy stored in a
metal or that is freed by its corrosion varies from
metal to metal. It is relatively high for metals such as
magnesium, aluminum, and iron, and relatively low for
metals such as copper, silver and gold.
Prof. Yehia ElShazly

22. •Corrosion is the primary means by which metals
deteriorate.
•Most metals corrode on contact with water (and
moisture in the air), acids, bases, salts, oils, aggressive
metal polishes, and other solid and liquid chemicals.
•Metals will also corrode when exposed to gaseous
materials like acid vapors, formaldehyde gas, ammonia
gas, and sulfur containing gases.
•Corrosion specifically refers to any process involving
the deterioration or degradation of metal components.
Prof. Yehia ElShazly

23. •The corrosion
process (anodic
reaction) of the metal
dissolving as ions
generates some
electrons, that are
consumed by a
secondary process
(cathodic reaction).
•These two processes
have to balance their
charges.
Prof. Yehia ElShazly

24. Daniell cell
The Daniell cell was the
first truly practical and
reliable electric battery.
In the process of the
reaction, electrons can
be transferred from the
corroding zinc to the
copper through an
electrically conducting
path as a useful electric
current.
Prof. Yehia ElShazly

25. •The difference in the
susceptibility of two
metals to corrode can
often cause a
situation that is called
galvanic corrosion.
•The salt bridge, in that
case, provides the
electrolytic path that is
necessary to
complete an
electrochemical cell
circuit.
Prof. Yehia ElShazly

26. FARADAY’s Law
•Faraday's empirical laws of electrolysis relate the current of
an electrochemical reaction to the number of moles of the
element being reacted.
•Supposing that the charge required for such reaction was
one electron per molecule, as is the case for the plating or
the corrosion attack of silver.
•According to Faraday’s law, the reaction with one mole of
silver would require one mole of electrons, or one
Avogadro's number of electrons (6.022 x 10
23
).
•The charge carried by one mole of electrons is known as
one Faraday (F)
Prof. Yehia ElShazly

27. •The Faradayis related to other electrical units through the
electronic charge, i.eelectron charge is 1.6 x 10
-19
coulomb.
Multiplying the electronic charge by the Avogadro number means
that one Faraday equals 96485 C/(mole of electrons).
•where Nis the number of moles and ΔN the change in that
amount
•nis the number of electrons per molecule of the species being
reacted
•I is the total current in amperes (A)
•tis the duration of the electrochemical process in seconds (s)
Prof. Yehia ElShazly

28. Corrosion in acidic medium
•A piece of zinc immersed in hydrochloric acid solution is
undergoing corrosion:
zinc is transformed to zinc ions.
This reaction produces electrons.
These electrons pass through the solid conducting
metal to another sites on the metal surface.
On this site, hydrogen ions are reduced to hydrogen
gas.
Prof. Yehia ElShazly

29. •For corrosion to occur there must be a
formation of ions and release of electrons
at an anodic surface where oxidation or
deterioration of the metal occurs.
•There must be a simultaneous
acceptance at the cathodic surface of the
electrons generated at the anode. This
acceptance of electrons can take the form
of neutralization of positive hydrogen ions,
or the formation of negative ions.
•The anodic and cathodic reactions must
go on at the same time and at equivalent
rates.
•Corrosion occurs only at the areas that
serve as anodes.
Prof. Yehia ElShazly

30. Corrosion in Neutral or Alkaline Environments
•The corrosion of metals can also occur in fresh water,
seawater, salt solutions, and alkaline or basic media.
•In almost all of these environments, corrosion occurs
importantly only if dissolved oxygen is also present.
•The most familiar corrosion of this type is the rusting of iron
when exposed to a moist atmosphere.
Prof. Yehia ElShazly

31. Factors Associated Mainly with
the Metal
•Effective electrode potential of a metal in a solution
•Overvoltage of hydrogen on the metal
•Chemical and physical homogeneity of the metal surface
•Inherent ability to form an insoluble protective film
Prof. Yehia ElShazly

32. Factors Which Vary Mainly with the Environment
•Hydrogen-ion concentration (pH) in the solution
•Influence of oxygen in solution adjacent to the metal
•Specific nature and concentration of other ions in solution
•Rate of flow of the solution in contact with the metal
•Ability of environment to form a protective deposit on the
metal
•Temperature
•Cyclic stress (corrosion fatigue)
•Contact between dissimilar metals or other materials as
affecting localized corrosion.
Prof. Yehia ElShazly

33. The anodic reaction occurring during corrosion can be
written in the general form:
The corrosion of metal M results in the oxidation of metal
M to an ion with a valence charge of n+ and the release
of n electrons.
The value of n depends primarily on the nature of the
metal. Some metals, such as silver, are univalent, while
others such as iron, titanium, and uranium are multivalent
and possess positive charges as high as +6.
Anodic Processes
Prof. Yehia ElShazly

34. Oxidationstatesfoundincompoundsofthemetalicelements.
Asolidcirclerepresentsacommonoxidationstate,anda
ringrepresentsalesscommon(lessenergeticallyfavorable)
oxidationstate
Prof. Yehia ElShazly

35. Cathodic processes
•Hydrogen ion reduction, or hydrogen evolution are the
cathodic reaction that occurs during corrosion in acids.
•Oxygen reduction is a very common cathodic reaction, since
oxygen is present in the atmosphere and in solutions exposed
to the atmosphere.
•Metal ion reduction and metal deposition, can cause severe
corrosion problems in special situations but they are less
common
Prof. Yehia ElShazly

36. •During corrosion, more than one oxidation and one
reduction reaction may occur.
•For example, during the corrosion of an alloy, its
component metal atoms go into solution as ions.
•During the corrosion of a chromium-iron alloy, both
chromium and iron are oxidized.
Prof. Yehia ElShazly

37. Prof. Yehia ElShazly

38. •For the case of corrosion of zinc in a hydrochloric
acid solution containing dissolved oxygen , two
cathodic reactions are possible:
the evolution of hydrogen and
the reduction of oxygen.
•Since there are two cathodic reactions or
processes which consume electrons, the overall
corrosion rate of zinc is increased.
•Thus, acid solutions which either contain dissolved
oxygen or are exposed to air are generally more
corrosive than air-free acids.
•Therefore, removing oxygen from acid solutions will
often make these solutions less corrosive.
Prof. Yehia ElShazly

39. Prof. Yehia ElShazly

40. Corrosion rate expression
•Percent weight loss; milligrams per square centimeter
per day; grams per square inch per hour.
•From an engineering point of view, the rate of
penetration or the thinning of a structural piece can be
used to predict the life of a given component.
•Mils per year: mpy = milli-inch per year534W
mpy
DAT

W=weight loss; mg
D=density of specimen;
g/cm
3
A=area of specimen; sq. In
T=exposure time; hr
Prof. Yehia ElShazly

41. Conversion between Current, Mass Loss, and
Penetration Rates for all Metals
Prof. Yehia ElShazly

42. Conversion between Current, Mass Loss and Penetration
Rates for Steel
Prof. Yehia ElShazly

43. Polarization
•Activation Polarization, Concentration Polarization
•Activation polarization refers to an electrochemical
process that is controlled by the reaction sequence at
the metal electrolyte interface.
•Concentration polarization refers to electrochemical
reactions that is controlled by the diffusion in the
electrolyte.
Prof. Yehia ElShazly

44. Activation Polarization
The rate at which hydrogen ions are
transformed into hydrogen gas is in
reality a function of several factors,
including the rate of electron
transfer from a metal to hydrogen
ions. In fact, there is a wide
variability in this transfer rate of
electrons by various metals and, as
a result, the rate of hydrogen
evolution from different metal
surfaces can vary greatly.
Prof. Yehia ElShazly

45. •The exchange current density (i
0) is surely the single most
important variable that explains the large differences in the
rate of hydrogen evolution on metallic surfaces.
Prof. Yehia ElShazly

46. •Note that the value for the exchange current density of
hydrogen evolution on platinum is approximately 10
-2
A/cm
2
whereas on mercury and lead it is 10
-13
A/cm
2
, eleven
orders of magnitude for the rate of this particular reaction, or
one hundred billion times easier on platinum than on
mercury or lead!
•This is the reason why mercury is often added to power
cells such as the popular alkaline primary cells to stifle the
thermodynamically favored production of gaseous hydrogen
and prevent unpleasant incidents. This is also why lead acid
batteries (car batteries) can provide power in a highly acidic
environment in a relatively safe manner.
Prof. Yehia ElShazly

47. Concentration polarization
•Concentration polarization is the polarization component
that is caused by concentration changes in the
environment adjacent to the surface.
•When a chemical species participating in a corrosion
process is in short supply, the mass transport of that
species to the corroding surface can become rate
controlling.
•A frequent case of concentration polarization occurs
when the cathodic processes depend on the reduction of
dissolved oxygen since it is usually in low concentration,
i.e. in parts per million (ppm).
Prof. Yehia ElShazly

48. •Mass transport to a
surface is governed by
three forces, i.e diffusion,
migrationand
convection.
•In the absence of an
electrical field, the
migration term, that only
affects charged ionic
species, is negligible while
the convection force
disappears in stagnant
conditions.
Prof. Yehia ElShazly

49. Prof. Yehia ElShazly

50. Corrosion cells
•Composition Cell (Galvanic cells)
•Stress Cell
•Concentration Cell
•Temperature Cell
Prof. Yehia ElShazly

51. Galvanic cell
•Galvanic cell arise when two metals with dissimilar
compositions or microstructures come into contact in the
presence of an electrolyte. The two most common examples
follow:
•Dissimilar metals: Formed by two single-phase metals in
contact, such as iron and zinc, or nickel and gold. The metal
that is higher on the Electrochemical Series will be the
cathode. The other metal will suffer anodic reactions and will
corrode.
•Incidentally, dissimilar metal contact (while bathed in a
suitable electrolyte) is the technology behind the construction
of batteries. The voltage of a battery directly follows from the
natural electrode potential of the corrosion reactions present
inside the battery. Hence, controlled corrosion is a good thing!
Prof. Yehia ElShazly

52. •Multi-phase alloy:Formed by a metal alloy composed
of multiple phases, such as a stainless steel, a cast iron,
or an aluminum alloy. The individual phases possess
different electrode potentials, resulting in one phase
acting as an anode and subject to corrosion.
Prof. Yehia ElShazly

53. Stress cells
•Stress cells can exist in a single piece of metal where a
portion of the metal's microstructure possesses more stored
strain energy than the rest of the metal. Metal atoms are at
their lowest strain energy state when situated in a regular
crystal array.
•Grain boundaries: By definition, metal atoms situated
along grain boundaries are not located in a regular crystal
array (i.e. a grain). Their increased strain energy translates
into an electrode potential that is anodic to the metal in the
grains proper. Thus, corrosion can selectively occur along
grain boundaries.
•High localized stress:Regions within a metal subject to a
high local stress will contain metal atoms at a higher strain
energy state. As a result, high-stress regions will be anodic
to low-stress regions and can corrode selectively.
Prof. Yehia ElShazly

54. •For example, bolts under load are subject to more corrosion
than similar bolts that are unloaded. A good rule of thumb is
to select fasteners that are cathodic (i.e. higher on the
Electrochemical Series) to the metal being fastened in order
to prevent fastener corrosion.
•Cold worked:Regions within a metal subjected to cold-
work contain a higher concentration of dislocations, and as
a result will be anodic to non-cold-worked regions. Thus,
cold-worked sections of a metal will corrode faster. For
example, nails that are bent will often corrode at the bend,
or at their head where they were worked by the hammer.
Prof. Yehia ElShazly

55. Concentration cells
•Concentration cells can arise when the concentration of one of
the species participating in a corrosion reaction varies within the
electrolyte.
•Electrolyte concentration: Consider a metal bathed in an
electrolyte containing its own ions. The basic corrosion reaction
where a metal atom losses an electron and enters the electrolyte
as an ion can proceed both forward and backwards, and will
eventually reach equilibrium.
•If a region of the electrolyte (adjacent to the metal) were to exhibit
a decreased concentration of metal ions, this region would
become anodic to the other portions of the metal surface. As a
result, this portion of the metal would corrode faster in order to
increase the local ion concentration.
Prof. Yehia ElShazly

56. •The net affect is that local corrosion rates are modulated in order
to homogenize reduction ion concentrations within the electrolyte.
•Oxidation concentration: Perhaps the most common
concentration cell affecting engineered structures is that of
dissolved oxygen. When oxygen has access to a moist metal
surface, corrosion is promoted. However, it is promoted the most
where the oxygen concentration is the least.
•As a result, sections of a metal that are covered by dirt or scale
will often corrode faster, since the flow of oxygen to these
sections is restricted. An increased corrosion rate will lead to
increased residue, further restricting the oxygen flow to worsen
the situation. Pitting often results from this "runaway" reaction.
Prof. Yehia ElShazly

57. Temperature cells
•Components of these cells are electrodes of the same
metal, each of which is at a different temperature,
immersed in an electrolyte of the same initial
composition.
•These cells are found in heat exchangers, boilers,
immersion heaters, and similar equipment.
•In copper sulfate solution, the copper electrode at the
higher temperature is the cathode, and the copper
electrode at the lower temperature is the anode.
•On short -circuiting the cell, copper deposits on the hot
electrode and dissolves from the cold electrode. Lead
acts similarly, but for silver the polarity is reversed.
Prof. Yehia ElShazly

58. Examples
•Iron will be anodic to copper ground mats or to brass bolts
or other brass parts
•An iron plate having some mill scale present may rust
because the iron is anodic to the mill scale
•An apparently homogeneous iron plate may rust because
tiny areas of the surface contain impurities or grain stresses
which cause them to be anodic to other areas of the surface
•Weld areas of a welded pipe may rust because the weld
metal is of different composition, may contain impurities, or
may cause stress which make it anodic to nearby metal
areas
Prof. Yehia ElShazly

59. •Corrosion may be observed on the bottom of a pipeline
while the top remains nearly undamaged. This may be
attributable to higher oxygen concentration in the soil
moisture (electrolyte) at the top of the pipe, leaving the
bottom anodic. The soil being undisturbed at the bottom of
the pipe provides a lower oxygen content and a lower
resistance to current flow than is present in the backfill
covering the top of the pipe.
•Exposed iron areas in contact with concrete. Encased or
embedded iron may rust because the concrete creates a
different and special electrolytic environment which causes
the exposed iron to become cathodicto the embedded iron.
Prof. Yehia ElShazly

60. •At normal temperatures iron will not corrode appreciably in
the absence of moisture.
•The presence of oxygen is also essential for corrosion to
take place in ordinary water. Oxygen alone will cause
considerable corrosion in acid, neutral, or slightly alkaline
water. In natural waters, the rate of corrosion is almost
directly proportional to oxygen concentration, if other factors
do not change. Oxygen also accelerates the corrosion of
iron in non-oxidizing acid solutions of moderate strength.
•Hydrogen gas is usually evolved from the surface of the
metal during corrosion in acid solutions and in concentrated
solutions of alkalies; in nearly neutral solutions the evolution
is usually very much less and may not be appreciable.
Prof. Yehia ElShazly

61. •The products of corrosion consist, mainly, of black or green
ferrous hydroxide next to the metal, and reddish-brown ferric
hydroxide (rust) which forms the outer layer, with graded
mixtures of the two in between. When iron corrodes in the
atmosphere the amount of ferrous rust produced is small,
but when formed under water the corrosion products often
contain a large proportion of ferrous iron.
•In natural water, the precipitated rust usually carries down
some compounds containing lime,, magnesia, and silica,
together with other insoluble material from the water. These
substances have considerable influence on the structure
and density of the rust coating on the metal surface.' A
loose, nonadherent coating under ordinary conditions may
accelerate locally the rate of corrosion; a uniformly dense
and adherent coating may cut down this rate very
considerably.
Prof. Yehia ElShazly

62. •Surface films, sometimes invisible, often play an important
part in controlling the rate and distribution of corrosion.
These films have been made visible by separation from
some metals and have been shown to raise the potential of
these metals making them more resistant in certain
environments. In fact the superior resistance of metals like
chromium and aluminum, for example, is undoubtedly due
largely to the formation of such films.
•In most cases the initial rate of corrosion is much greater
than the rate after a short period of time. This is particularly
noticeable in film-forming solutions, such as the alkalies or
chromates. It should be noted, however, that the initial rate
of corrosion of a highly polished metal surface is abnormally
low.
Prof. Yehia ElShazly

63. •Dissimilarity in the chemical composition of metals in
contact with each other in an electrically conducting solution
sets up a difference in potential (precisely as in the galvanic
cell) and thus accelerates corrosion locally. In corroding
metals these variations in potential are found between a
metal and other reactive materials, or between different
metals in contact. This action is accompanied by an electric
current which flows through the solution from anode to
cathode, i.e. from the more corrodible to the less corrodible
metal in this particular solution.
•Composition of ordinary iron or steel, within the common
variations found commercially, has little effect on corrosion
underwater or underground, but sometimes it has a marked
effect in atmospheric and acid corrosion. From the
standpoint of corrosion, homogeneity of a metal is not
usually so important as external conditions.
Prof. Yehia ElShazly

64. •The condition of the metal surface in submerged corrosion
may not affect the total corrosion, although it may have a
marked tendency to localize the action. Corrosion of iron is
rarely uniform over its entire surface.
•Corrosion at normal temperature increases with increase of
concentration in dilute solutions of many neutral salts,
particularly chlorides, but decreases again in more
concentrated solutions, other things being equal.
•In natural waters the rate of corrosion generally tends to
increase with increase in the velocity of motion of the water
over the metal surface, with some exceptions where the
film-forming tendency predominates (page 156).
Prof. Yehia ElShazly

65. •Variation in the composition or concentration of a solution in
contact with a metal tends to localize corrosion at certain
areas and retard the action at other areas of the surface. A
portion of the metal surface which is protected from diffusion
of oxygen inward becomes anodic to other areas which are
in contact with a solution richer in oxygen, i.e., corrosion is
more active at such protected areas.
•The smaller the anodic areas in relation to the associated
cathodic areas, the greater is the rate of penetration of
corrosion at the anodic points. The polarity of a certain area
often reverses during the progress of natural corrosion.
Prof. Yehia ElShazly

66. Corrosion
Forms of Corrosion

67. It is convenient to classify corrosion by the forms in which it manifests itself, the
basis for this classification being the appearance of the corroded metal.
Each form can be identified by mere visual observation. In most cases the naked
eye is sufficient, but sometimes magnification is helpful or required.
Valuable information for the solution of a corrosion problem can often be obtained
through careful observation of the corroded test specimens or failed equipment.
Prof. Yehia ElShazly

68. three categories
Group I: Corrosion problems readily identifiable by
visual examination.
1. Uniform corrosion
2. Localized corrosion
3. Galvanic corrosion
Prof. Yehia ElShazly

69. Group II: Corrosion damage that may require
supplementary means of examination for
identification.
4. Velocity induced corrosion
5. Intergranular corrosion
6. Dealloying corrosion
Prof. Yehia ElShazly

70. Group III: Corrosion specimens for these types
should usually be verified by microscopy of one
kind or another.
7. Cracking phenomena
8. High-temperature corrosion
9. Microbial effects
Prof. Yehia ElShazly

71. Prof. Yehia ElShazly

72. •The actual importance
of each corrosion type
will also differ between
systems, environments,
and other operational
variables.
•However, there are surprising similarities in
the corrosion failure distributions within the
same industries
Prof. Yehia ElShazly

73. General or Uniform Attack
•Uniform corrosion corresponds to the corrosion
attack with the greatest metal weight loss and is
a common sight where steel structures are
abandoned to rust.
•Uniform corrosion is the attack of a metal at
essentially the same at all exposed areas of its
surface. At no point is the penetration of the
metal by corrosion twice as great as the average
rate.
Prof. Yehia ElShazly

74. Prof. Yehia ElShazly

75. •Uniform corrosionoccurs when there are local anodic and
cathodic sites on the surface of the metal. Due to
polarization effects, these locations shift from time to time
and a given area on a metal will be act as both an anode
and as a cathode over any extended period of time.
•Weight loss is the most commonly used method of
measuring the corrosion rate of metals when uniform
corrosion occurs.
•Designing in a system a corrosion allowance based on the
possible loss of a material thickness is one of the simplest
methods for dealing with uniform attack.
Prof. Yehia ElShazly

76. •Ultrasonic inspection has been used for decades
to measure the thickness of solid objects.
•A piezoelectric crystal serves as a transducer to oscillate at
high frequencies, coupled directly or indirectly to one
surface of the object whose thickness is to be measured.
•The time the wave of known velocity takes to travel through
the material is
used to determine its
thickness.
Prof. Yehia ElShazly

77. Localized Corrosion
•Pitting Corrosion
small volumes of metal are removed by corrosion from certain
areas on the surface to produce craters or pits that may
culminate in complete perforation of a pipe or vessel wall.
•Pitting is considered to be more dangerous than uniform
corrosion damage because it is more difficult to detect,
predict, and design against.
•A small, narrow pit with minimal overall metal loss can lead
to the failure of an entire engineering system. Only a small
amount of metal is corroded, but perforations can lead to
costly repair of expensive equipment.
Prof. Yehia ElShazly

78. While the shapes of pits vary widely they are usually roughly
saucer-shaped, conical, or hemispherical for steel and many
associated alloys.
Prof. Yehia ElShazly

79. •factors contributing to initiation and propagation of
pitting corrosion:
• Localized chemical or mechanical damage to a protective
oxide film
• Water chemistry factors that can cause breakdown of a
passive film such as acidity, low dissolved oxygen
concentration which tend to render a protective oxide film
less stable and high chloride concentrations
• Localized damage to or poor application of a protective
coating
• The presence of non-uniformities in the metal structure of the
component, for example, nonmetallic inclusions.
Prof. Yehia ElShazly

80. Prof. Yehia ElShazly

81. Crevice Corrosion
•Crevice corrosion occurs in cracks or crevices formed
between mating surfaces of metal assemblies, and usually
takes the form of pitting or etched patches.
•It can also occur under scale and surface deposits and
under loose fitting washers and gaskets that do not prevent
the entry of liquid between them and the metal surface.
Prof. Yehia ElShazly

82. Prof. Yehia ElShazly

83. •Firstly, crevice corrosion is believed to initiate as the result of the
differential aeration mechanism.
•Dissolved oxygen in the liquid which is deep in the crevice is consumed
by reaction with the metal.
•Secondly, as oxygen diffusion into the crevice is restricted, a differential
aeration cell tends to be set up between the crevice microenvironment
and the external surface.
•The corrosion reactions now specialize in the crevice (anodic) and on
the surface more accessible to ambient air (cathodic).
•The cathodic oxygen reduction reaction cannot be sustained in the
crevice area, making it the anode of a differential aeration cell.
•This anodic imbalance may lead to the creation of highly corrosive
microenvironmental conditions in the crevice, conducive to further metal
dissolution.
Prof. Yehia ElShazly

84. Prof. Yehia ElShazly

85. Corrosion under Insulation
•Water entering an insulation material and diffusing inward
will eventually reach a region of dryout at the hot pipe or
equipment wall.
•Next to this dryout region is a zone in which the pores of the
insulation are filled with a saturated salt solution.
•When a shutdown or process change occurs and the metal-
wall temperature falls, the zone of saturated salt solution
moves unto the metal wall.
•Upon reheating, the wall will temporarily be in contact with
the saturated solution, a very corrosive situation.
Prof. Yehia ElShazly

86. Prof. Yehia ElShazly

87. Filiform Corrosion.
•Filiform corrosion is a special form of crevice or oxygen cell
corrosion occurring beneath organic or metallic coatings on
steel, zinc, aluminum, or magnesium.
•Filiform corrosion normally starts at small, sometimes
microscopic, defects in the coating. Lacquers and “quick-
dry” paints are most susceptible to the problem.
•The attack results in a fine network of random “threads” of
corrosion product that develop as a shallow grooving of the
metal surface beneath the coating material
Prof. Yehia ElShazly

88. •A differential aeration cell is set up under the coating, with
the lowest concentration of oxygen at the head of the
filament since oxygen has to diffuse through the porous tail
to the head region.
•A characteristic feature of such a differential aeration cell is
the acidification of the environment with a low level of
dissolved oxygen.
•This leads to the formation of an anodic metal dissolution
site at the front of the head of the corrosion filament.
Prof. Yehia ElShazly

89. Prof. Yehia ElShazly

90. •For steel, pH values at the front of the head of one to four
have been reported with a corrosion potential around
−0.44V (SHE). In contrast, at the back of the head where
the cathodic reaction dominates, the prevailing pH is around
12.
Prof. Yehia ElShazly

91. Galvanic Corrosion
•Galvanic corrosion (dissimilar metal corrosion) refers to
corrosion damage induced when two dissimilar materials
are coupled in a corrosive electrolyte.
•In a bimetallic couple, the less noble material becomes
the anode and tends to corrode at an accelerated rate,
compared with the uncoupled condition and the more
noble material will act as the cathode in the corrosion
cell.
Prof. Yehia ElShazly

92. Galvanic Series
•The potential of a metal in a solution is related to the
energy that is released when the metal corrodes.
•Differences in corrosion potentials of dissimilar metals
can be measured in specific environments by measuring
the direction of the current that is generated by the
galvanic action of these metals when exposed in a given
environment.
Prof. Yehia ElShazly

93. In a galvanic couple
involving any two
metals in a galvanic
series, corrosion of
the metal higher in the
list is likely to be
accelerated, while
corrosion of the metal
lower in the list is
likely to be reduced.
Prof. Yehia ElShazly

94. •Metals with more
positive corrosion
potentials are called
noble or cathodic,
and those with more
negative corrosion
potentials are
referred to as active
or anodic.
Prof. Yehia ElShazly

95. •Values of potential can change from one solution to another
or in any solution when influenced by such factors as
temperature, aeration, and velocity of movement.
Prof. Yehia ElShazly

96. Surface Area Effect
•When a piece of metal is freely corroding, the electrons
generated at anodic areas flow through the metal to
react at cathodic areas similarly exposed to the
environment where they restore the electrical balance of
the system.
Prof. Yehia ElShazly

97. •The larger the cathode compared with the anode, the
more oxygen reduction, or other cathodic reaction, can
occur and, hence, the greater the galvanic current.
•the most unfavorable ratio is a very large cathode
connected to a very small anode.
Prof. Yehia ElShazly

98. Prof. Yehia ElShazly

99. Prof. Yehia ElShazly

100. •.
Prof. Yehia ElShazly

101. •galvanized steel pipes can be used with brass connectors,
but serious corrosion to the pipe end is likely to result if the
contact is made directly to a large area of copper, such as a
tank or cylinder.
•Similarly, stainless steel and copper tubes can usually be
joined without causing problems, but accelerated corrosion
of the copper tube is likely to occur if it is attached to a
stainless-steel tank.
Prof. Yehia ElShazly

102. Prof. Yehia ElShazly

103. Deposition Corrosion
•Pitting occurs in a liquid environment following the
plating out of a more cathodic metallic element in
solution onto a metal surface.
•For example, soft water passing through a copper water
•pipe will accumulate some copper ions. If this water then
goes in a galvanized steel or aluminum vessel, particles
of metallic copper may plate out and their deposit on the
vessel’s internal surface will probably initiate pitting
corrosion by local cell action.
Prof. Yehia ElShazly

104. Intergranular Corrosion
•A narrow path is corroded out preferentially along the grain
boundaries of a metal.
•It often initiates on the surface and proceeds by local cell
action n the immediate vicinity of a grain boundary.
•The driving force is a difference in corrosion potential that
develops between the grain boundary and the bulk material
in adjacent grains.
•The difference in potential may be caused by a difference in
chemical composition between the two zones.
Prof. Yehia ElShazly

105. Prof. Yehia ElShazly

106. SENSITIZATION ISSUES (INTERGRANULAR CORROSION)
Prof. Yehia ElShazly

107. Prof. Yehia ElShazly

108. Prof. Yehia ElShazly

109. Dealloying (selective leaching)
•Localized form of corrosion that involves the selective
removal by corrosion of one of the elements of an alloy by
either preferential attack or by dissolution of the matrix
material.
•The various kinds of selective dissolution have been named
after the alloy family that has been affected, usually on the
basis of the element dissolved (except in the case of
graphitic corrosion).
•Dezincification, Graphitic Corrosion..
Prof. Yehia ElShazly

110. Dezincification
•Dezincification refers to the selective leaching of the zinc
phase in alloys such as brasses that contain more than 15
percent Zn.
•The part will have become weak and embrittled, and
therefore subject to failure without warning.
•To the trained observer, dezincification is readily recognized
under the microscope, and even with the unaided eye,
because the red copper color is easily distinguished from
the yellow of brass.
•The most common is layer dezincification that proceeds
uniformly, The second type is referred to as the plug
dezincification and occurs at localized areas.
Prof. Yehia ElShazly

111. Prof. Yehia ElShazly

112. Graphitic Corrosion
•Selective leaching specific to gray cast iron which leaves
only the graphite phase of the material.
•Gray cast iron pipe may suffer graphitic corrosion as a
result of the selective dissolution of the ferrite in the alloy
leaving a porous matrix made of the 4 to 4.5 percent
graphite.
•Graphitic corrosion occurs in salt waters, acidic mine
waters, dilute acids, and soils, especially those
containing chlorides from seawater.
Prof. Yehia ElShazly

113. Prof. Yehia ElShazly

114. Hydrogen-Induced Cracking
•Atomic hydrogen, and not the molecule (is the smallest
atom of the periodic table) and as such it is small enough to
diffuse readily through a metallic structure.
•When the crystal lattice is in contact or is saturated with
atomic hydrogen, the mechanical properties of many metals
and alloys are diminished.
•Nascent atomic hydrogen can be produced as a cathodic
reaction, either during natural corrosion processes or forced
by cathodic protection, when certain chemical species are
present which act as negative catalysts (i.e., poisons) for the
recombination of atomic to molecular hydrogen.
Prof. Yehia ElShazly

115. •The most commonly encountered species is hydrogen
sulfide (H
2S), which is formed in many natural
decompositions, and in many petrochemical processe.
•Processes or conditions involving wet hydrogen sulfide, that
is, sour services, and the high incidence of sulfide-induced
HIC may result in sulfide stress cracking (SSC), which has
been a continuing source of trouble in the exploration and
exploitation of oil and gas fields.
Prof. Yehia ElShazly

116. Hydrogen Blistering
•Hydrogen blistering occurs predominantly in low-strength
steel alloys when atomic hydrogen diffuses to internal
defects such as laminations or nonmetallic inclusions
where it may recombine as molecular hydrogen (H
2).
•When this happens, tremendous internal pressures can
be generated and cause splits, fissures, and even
blisters on the metal surface.
•The tendency of steel alloys to blister can be prevented
by using sound steels containing few inclusions and
defects.
Prof. Yehia ElShazly

117. Prof. Yehia ElShazly

118. Velocity Induced Corrosion
•Velocity induced corrosion refers to problems
caused or accelerated by the relative motion of
the environment and the metal surface.
Prof. Yehia ElShazly

119. Prof. Yehia ElShazly

120. •Mass transport–control: Mass transport–controlled
corrosion implies that the rate of corrosion is dependent
on the convective mass-transfer processes at the
metal/fluid interface.
•When steel is exposed to oxygenated water, the initial
corrosion rate will be closely related to the convective
flux of dissolved oxygen toward the surface, and later by
the oxygen diffusion through the iron oxide layer.
•Corrosion by mass transport will often be streamlined
and smooth.
Prof. Yehia ElShazly

121. •Phase transport–control: Phase transport–controlled
corrosion refers to conditions when wetting of the metal
surface by a corrosive phase is flow-dependent.
•This may occur because one liquid phase separates from
another or because a second phase forms from a liquid.
•An example of the second mechanism is the formation of
discrete bubbles or a vapor phase from boiler water in
horizontal or inclined tubes in high heat-flux areas under low
flow conditions.
•The corroded sites will frequently display rough, irregular
surfaces and be coated with or contain thick, porous
corrosion deposits.
Prof. Yehia ElShazly

122. •Erosion–corrosion: Erosion–corrosion has been associated
with mechanical removal of the protective surface film resulting in a
subsequent corrosion rate increase via either electrochemical or
chemical processes.
•It is often accepted that a critical fluid velocity must be exceeded for a
given material. The mechanical damage by the impacting fluid imposes
disruptive shear stresses or pressure variations on the material surface
and/or the protective surface film.
•Erosion–corrosion may be enhanced by particles (solids or gas bubbles)
and impacted by multi-phase flows.
•The morphology of surfaces affected by erosion–corrosion and FAC may
be in the form of shallow pits or horseshoes or other local phenomena
related to the flow direction
Prof. Yehia ElShazly

123. The destruction of a protective film on a metallic surface
exposed to high flow rates can have a major impact on the
acceleration of corrosion damage. Carbon steel pipe
carrying water, for example, is usually protected by a film of
rust that slows down the rate of mass transfer of dissolved
oxygen to the pipe wall. The resulting corrosion rates are
typically less than 1 mm/y. The removal of the film by flowing
sand slurry has been shown to raise corrosion rates tenfold
to approximately 10 mm/y
Prof. Yehia ElShazly

124. •Sources of various mechanical forces involved in
the erosion of protective films and underlying
metal :
• Turbulent flow, fluctuating shear stresses, and
pressure impacts
• Impact of suspended solid particles
• Impact of suspended liquid droplets in high-speed gas
flow
• Impact of suspended gas bubbles in aqueous flow
• The violent collapse of vapor bubbles following
cavitation
Prof. Yehia ElShazly

125. Prof. Yehia ElShazly

126. •Protective films fall into two categories:
• Relatively thick and porous diffusion barriers such as formed on
carbon steel as red rust and copper oxide on copper
• Thin invisible passive films such as formed on stainless steels,
nickel alloys, and other passive metals such as titanium.
•However, if the flow of liquid becomes turbulent, the random liquid
motion impinges on the surface to remove this protective film.
•Additional oxidation then occurs by reaction with the liquid. This
alternate oxidation and removal of the film will accelerate the rate
of corrosion.
•The resulting erosive attack may be uniform, but quite often
produces pitted areas over the surface that can result in full
perforation
Prof. Yehia ElShazly

127. •Chromium has proven to be most beneficial toward
improving the properties of the passive film of ferrous and
nickel-based alloys while molybdenum, when added to
these alloys, improves their pitting resistance.
•Small alloy additions to carbon steels can have a marked
influence on the resistance to FAC. Carbon steels are
generally susceptible to FAC, while low alloy chromium
steels such as 1.25 Cr-0.5 Mo and 2.25 Cr-1 Mo are very
resistant to FAC.
Prof. Yehia ElShazly

128. Prof. Yehia ElShazly

129. •Cavitation: Cavitation occurs when a fluid's operational pressure
drops below it's vapor pressure causing gas pockets and bubbles to
form and collapse in a liquid near a metal surface.
•Cavitation removes protective surface scales by the implosion of gas
bubbles in a fluid.
•The subsequent corrosion attack is the result of hydromechanical effects
from liquids in regions of low pressure where flow-velocity changes,
disruptions, or alterations in flow direction have occurred.
•Cavitation damage often appears as a collection of closely spaced,
sharp-edged pits or craters on the surface.
•Cavitation is a problem with ship propellers, hydraulic pumps and
turbines, valves, orifice plates, and all places where the static pressure
varies very abruptly following the Bernoulli principle.
Prof. Yehia ElShazly

130. •The cavitation number (σ) is a dimensionless number that
provides an estimate of cavitation tendency in a flowing
stream as described
•where Pv is the liquid vapor pressure.
•When the cavitation number is null, the pressure is reduced
to the vapor pressure and cavitation will occur.
•The cavitation number and the net positive suction head
(NPSH) are related according to
Prof. Yehia ElShazly

131. Prof. Yehia ElShazly

132. Mechanically Assisted Corrosion
•A combination of tensile stresses and a specific corrosive environment is
one of the most important causes of catastrophic cracking of metal
structures.
•SCC and other types of environmental cracking are also the most
insidious forms of corrosion because environmental cracks are
microscopic in their early stages of development.
•In many cases, they can be detected only by microscopic examination
with an optical or a scanning electron microscope.
•As the cracking penetrates farther into the material, it eventually reduces
the effective supporting cross section to the point where the structure
fails by overload or, in the case of vessels and piping, escape (seepage)
of the contained liquid or gas occurs.
Prof. Yehia ElShazly

133. •Cracking is usually either intergranular (intercrystalline) or
transgranular (transcrystalline). Occasionally, both types of
cracking are observed in a failure.
•Failures are not necessarily the result of ordinarily applied
engineering loads or stresses. However, these loads have to
be added to invisible residual stresses already present in a
structure due to various sources such as fabricating
processes.
•Another source of stress may come from corrosion products
from general corrosion or other forms of corrosion may build
up between mating surfaces and, because they occupy so
much more volume than the metal from which they are
produced, generate sufficient stresses to cause SCC.
Prof. Yehia ElShazly

134. Prof. Yehia ElShazly

135. Stress Corrosion Cracking
•SCC is a mechanical–chemical process leading
to the cracking of certain alloys at stresses
below their tensile strengths. A susceptible alloy,
the proper chemical environment, plus an
enduring tensile stress are required.
Prof. Yehia ElShazly

136. where
σ
mis mean stress
a is depth of defect
C is geometric constant
K
Icis critical stress
intensity at which
catastrophic fracture
occurs
Prof. Yehia ElShazly

137. Prof. Yehia ElShazly

138. Prof. Yehia ElShazly

139. Prof. Yehia ElShazly

140. Prof. Yehia ElShazly

141. Corrosion Fatigue
•Fatigue is the failure of a metal by cracking when it is
subjected to cyclic stress.
•Endurance Limit: is the stress level below which no failure
will occur, even with an infinite number of cycles.
•When a metal is subjected to cyclic stress in a corrosive
environment, the number of cycles required to cause failure
at a given stress may be reduced well below the dotted line
obtained for the same metal in air.
•A marked drop in or elimination of the endurance limit may
occur even in a mildly corrosive environment, especially in
the case of a film-protected alloy.
Prof. Yehia ElShazly

142. •This is because stress reversals cause repeated cracking of
the otherwise protective surface film, and this allows access
of the water to the unprotected metal below with resultant
corrosion.
•Failures that occur on vibrating structures (e.g., taut wires or
stranded cables) exposed to the weather under stresses
below the endurance limit are usually caused by corrosion
fatigue.
•Corrosion fatigue also has been observed in steam boilers,
due to alternating stresses caused by thermal cycling
Prof. Yehia ElShazly

143. Prof. Yehia ElShazly

144. •For uniaxialstress systems,
there will be an array of
parallel cracks which are
perpendicular to the direction
of principal stress.
•Torsion loadings tend to
produce a system of
crisscross cracks at roughly
45°from the torsion axis.
•Corrosion fatigue cracks
found in pipes subjected to
thermal cycling will usually
show a pattern made up of
both circumferential and
longitudinal cracks.
Prof. Yehia ElShazly

145. Fretting Corrosion
•Fretting corrosion refers to corrosion damage at the
asperities of contact surfaces.
•This damage is induced under load and in the presence
of repeated relative surface motion, as induced, for
example, by vibration.
•Pits or grooves and oxide debris characterize this
damage, typically found in machinery, bolted assemblies,
and ball or roller bearings.
•The protective film on the metal surfaces is removed by
the rubbing action and exposes fresh, active metal to the
corrosive action of the atmosphere.
Prof. Yehia ElShazly

146. •Conditions necessary for the occurrence of fretting are
(1) the interface must be under load, and
(2) vibratory or oscillatory motion of small amplitude
must result in the surfaces striking or rubbing together.
•The results of fretting are as follows:
1. Metal loss in the area of contact
2. Production of oxide debris
3. Galling, seizing, fatiguing, or cracking of the metal
Prof. Yehia ElShazly

147. Prof. Yehia ElShazly

148. Corrosion
Corrosion Thermodynamics

149. introduction
•Thermodynamic principles can help explain a
corrosion situation in terms of the stability of
chemical species and reactions associated with
corrosion processes. However, these principles
cannot be used to predict corrosion currents or
corrosion rates.
Prof. Yehia ElShazly

150. Free Energy of a Corrosion
Reaction
If this work is done in an electrochemical cell in which the
potential difference between its two half-cells is E, and
the charge is that of one mole of reaction in which n
moles of electrons are transferred, then the electrical
work (-w) done by the cell must be nE.
F is the Faraday constant which is required to obtain
coulombs from moles of electrons.
Prof. Yehia ElShazly

151. Prof. Yehia ElShazly

152. At equilibrium; ΔG=0
Prof. Yehia ElShazly

153. Prof. Yehia ElShazly

154. Nernst Equation0
00
0
0
10
ln
...
...
ln
0.059
log
mn
MN
ab
AB
G G RT K
aa
K
aa
G zFE
G zFE
RT
E E K
zF
E E K
z
   

  
  


Prof. Yehia ElShazly

155. Cell cathode anode
E E E  
00 0.0592
log
Cell cathode anode
E E E K
z
   Prof. Yehia ElShazly

156. Standard Electrode Potentials
•The potential
difference across an
electrochemical cell
is the potential
difference measured
between two
electronic
conductors
connected to the
electrodes.
Prof. Yehia ElShazly

157. •A voltmeter may be used to measure the potential
differences across electrochemical cells.
•To measure the electrode potential, another electrode is
required.
•The most ancient is the standard hydrogen electrode (SHE).
•It consists of a half-cell in which hydrogen gas is bubbled
over a platinum electrode immersed in a solution having a
known concentration of hydrogen ions. SHE
•Other reference electrodes are much preferred for practical
considerations.
Prof. Yehia ElShazly

158. •The potential difference across a reversible cell made up of
any electrode and a SHEis called the reversible potential of
that electrode.
•if any one electrode, operated under standard conditions, is
designated as the standard electrode or standard reference
electrode with which other electrodes can be compared.
•The standard electrode potentials are customarily
determined at solute concentrations of 1 Molar, gas
pressures of 1 atmosphere, and a standard temperature
which is usually 25°C.
Prof. Yehia ElShazly

159. Standard-state reduction half-cell
potentials in alphabetical order
Prof. Yehia ElShazly

160. Standard-state reduction half-cell
potentials by decreasing order of
potential.
Prof. Yehia ElShazly

161. Prof. Yehia ElShazly

162. Prof. Yehia ElShazly

163. Nernst Equation

 
 
2
0
0 2
0 2
0
2
0.763
ln
0.0592
log
0.0592
log
Zn e Zn
E
ZnRT
EE
zFZn
Zn
EE
z Zn
reduced
EE
z oxidized












Prof. Yehia ElShazly

164. The reaction
Al
3+
+ 3e
-
→Al
has a Nernst equation of
= -1.66 + 0.0197 log[Al
3+
]
Prof. Yehia ElShazly

165. For the half-cell reaction
MnO
4
-
+ 4H
+
+ 3e
-
= MnO
2
+ 2H
2
O , E
0= 0.588
the Nernst equation appears as
E= 0.588 + 0.0197 log [MnO
4
-
] -0.0789 pH
Prof. Yehia ElShazly

166. In the reaction
O
2+4H
+
+4e
-
=2H
2O
water is the reduced species and the oxygen gas is the
oxidised species. By convention, electrochemical half-
equations are written as
Oxidised State+ne Reduced State
Prof. Yehia ElShazly

167. Example
Prof. Yehia ElShazly

168. Prof. Yehia ElShazly

169. THE OXYGEN ELECTRODE AND DIFFERENTIAL
AERATION CELL
•The oxygen electrode can be represented by platinized
platinum immersed in an electrolyte saturated with
oxygen.
•The equilibrium for such an electrode is expressed as
Prof. Yehia ElShazly

170. Example
•Consider two oxygen electrodes in an
aqueous environment: one in contact with
O
2at 1 atm (left) and the other with O
2at
0.2 atm (right).
Prof. Yehia ElShazly

171. In any differential aeration cell, the electrode in contact with
lower -pressure oxygen tends to be the anode, and the
electrode in contact with higher -pressure oxygen tends to be
the cathode.
Prof. Yehia ElShazly

172. Equilibrium potential of the main reference electrodes used in corrosion, at
25
o
C
Prof. Yehia ElShazly

173. •Most reference electrodes are used with a saturated
solution and an excess of the salt crystals.
•The extra salt dissolves into the half-cell solution as some of
the ions diffuse out of the reference cell body through the
liquid junction during normal use.
•When reporting electrochemical potential measurements, it
is always important to indicate which reference half-cell was
used to carry out the work.
•This information is required to compare these
measurements to similar data that could have been
obtained using any other reference half-cells.
Prof. Yehia ElShazly

174. We can take the case
of a measurement of
the potential of a steel
pipe buried in the
ground, using a
saturated copper-
copper sulfate
reference electrode
(CCSRE). This might
show a potential of
-0.700 Vmeasured in
this way. To convert
this potential to a
value on the scale in
which the hydrogen
electrode has a
potential of zero, it is
necessary to add
0.318volt to the
potential that was
measured, making it -
0.382 volt vs. SHE.
Prof. Yehia ElShazly

175. •The silver –silver chloride reference
electrode, can be prepared by
electroplating with silver a platinum wire
sealed into a glass tube.
•The silver coating is then converted partly
into silver chloride by making it anode in
dilute hydrochloric acid.
•When the silver –silve chloride electrode
is immersed in a chloride solution, the
following equilibrim is established:
Silver/Silver Chloride Reference Electrode
Prof. Yehia ElShazly

176. •A laboratory electrode is mainly used
with saturated potassium chloride
(KCl) electrolyte, but can be used
with lower concentrations such as 1
M KCl and even directly in seawater.
•Typical laboratory electrodes use a
silver wire that is coated with a thin
layer of silver chloride.
Prof. Yehia ElShazly

177. Prof. Yehia ElShazly

178. Calomel Reference Electrode
•It consists of mercury in
equilibrium with Hg
2
2 +
, the
activity of which is determined
by the solubility of Hg
2Cl
2
•Pure mercury covers a
platinum wire sealed through
the bottom of a glass tube. The
mercury is covered with
powdered mercurouschloride,
which is only slightly soluble in
potassium chloride solution,
the latter filling the cell
Prof. Yehia ElShazly

179. Prof. Yehia ElShazly

180. Copper/Copper Sulfate Reference
Electrode
•Copper/copper
sulfate half-cells are
typically favored for
potential
measurements of
systems buried in
soils.
Prof. Yehia ElShazly

181. A saturated
CCSRE can be
fabricated with a
solution of copper
sulfate made with
40 g of
CuSO
4.5H
2O in
25 mLof distilled
water. The
saturated solution
should contain
approximately 260
g/L of CuSO
4at
22
o
C.
Prof. Yehia ElShazly

182. Measuring the Corrosion
Potential
•The corrosion potential is measured by determining the
voltage difference between a metal immersed in a given
environment and an appropriate reference electrode.
•the corrosion potential of metal M is -0.405 V. The minus
sign indicates that the metal is negative with respect to
the reference electrode.
•It is customary to connect the reference electrode to the
low point or the instrumental ground to avoid any
confusion in reporting.
Prof. Yehia ElShazly

183. Prof. Yehia ElShazly

184. Acidity and alkalinity
•This refers to whether there is an excess of H
+
(hydrogen) or OH
-
(hydroxyl) ions present.
•The H
+
ion is acid while the hydroxyl ion is
alkaline or basic.
Prof. Yehia ElShazly

185. pH
•pHis a measure of the acidity or basicity of a solution. It is
defined as the cologarithm of the activity of dissolved
hydrogen ions (H
+
).
•There are 10 times as many hydrogen ions available at pH 7
than at pH 8.
•For the reaction, At standard conditions
Prof. Yehia ElShazly

186. Prof. Yehia ElShazly

187. Measuring pH
•A pH meter measures the difference in potential between
a reference electrode insensitive to changes in pH and
an electrode sensitive to such changes:
–Electrodes:A pH electrode consists of two half-cells; an
indicating electrode and a reference electrode.
–Meters: A pH meter is in reality a high precision and high
impedance voltmeter capable of reading small millivolt changes
from the pH electrode system.
–Buffers:These solutions of known pH value allow the user to
adjust the system to read accurate measurements.
Prof. Yehia ElShazly

188. pH Glass Electrode
•A glass electrode is a potentiometric sensor made from
glass of a specific composition.
•A typical pH probe is a combination electrode, which
combines both the glass and reference electrodes into
one body. The pH electrode is essentially a galvanic cell.
The measuring part of the electrode, the glass bulb on
the bottom, is coated both inside and out with a ~10nm
layer of a hydrated gel. These two layers are separated
by a layer of dry glass and the potential is created by the
equilibrium in H
+
ions across the membrane.
Prof. Yehia ElShazly

190. pH Antimony Electrode
•Antimony is a unique metal with the characteristic of a
direct relationship between pH and its measured
potential. The potential difference or voltage developed
between antimony and a copper/copper sulfate
reference electrode varies between approximately 0.1
volts to 0.7 voltsdue to variations in the pH.
Prof. Yehia ElShazly

191. Prof. Yehia ElShazly

192. Galvanic series
•The galvanic series is an arrangement of metals and
alloys in order of their corrosion potentials in the
environment. The potentials of the metals and alloys are
measured in the desired environments, with the most
noble (positive) at the top and the most active at the
bottom.
•In the galvanic series, instead of potentials the relative
positions of the metals and alloys are indicated.
•The series is based on practical measurement of
corrosion potential at equilibrium.
Prof. Yehia ElShazly

193. •The galvanic series indicates that alloys can be coupled
without being corroded. For instance, alloys close to each
other in the series can be safely coupled. As shown in the
table, monel can be coupled to copper, or bronze, without
any risk of galvanic corrosion.
•However, brass cannot be coupled with tin, because the two
are far away in the table and coupling them may cause
serious galvanic corrosion.
•Some alloys exist in two places in the table. For instance,
steel (18/8) exists in the passive state as well as in the
active state. Joining of the two types (active and passive)
may lead to serious corrosion.
Prof. Yehia ElShazly

194. •Each environment requires a different galvanic series, for
example, a galvanic series in static seawater cannot be
used to predict galvanic corrosion in turbulently flowing
seawater.
Prof. Yehia ElShazly

195. Prof. Yehia ElShazly

196. Prof. Yehia ElShazly

197. Potential-pH Diagram
•A Pourbaix diagram is a plot of the equilibrium potential of
electrochemical reactions against pH. It shows how corrosion
mechanisms can be examined as a function of factors such as
pH, temperature and the concentrations of reacting species.
•E-pH diagrams are typically plotted for various equilibria on
normal Cartesian coordinates with potential (E) as the ordinate (Y
axis) and pH as the abscissa (X axis).
•The major uses of such diagrams, which can be constructed for
all metals, are:
–Predicting whether or not corrosion can occur
–Estimating the composition of the corrosion products formed
–Predicting environmental changes which will prevent or reduce
corrosion attack
Prof. Yehia ElShazly

198. •This graphical representation of thermodynamic data for corrosion
studies showed three possible states of a metallic material:
•Immunity region: In the conditions of potential and pH of that region a
metal is considered to be totally immune from corrosion attack and safe
to use. Cathodic protection may be used to bring the potential of a metal
closer to that region by forcing a cathodic shift.
•Passive region: In such region a metal tends to become coated with an
oxide or hydroxide that may form on the metal either as a compact and
adherent film practically preventing all direct contact between the metal
itself and the environment, or as a porous deposit which only partially
prevents contact between the metal and the environment;
•Corrosion region: Thermodynamic calculations indicate that, in such
region of an E-pH diagram, a metal is stable as an ionic (soluble)
product and therefore susceptible to corrosion attack.
Prof. Yehia ElShazly

199. •pH is plotted on the horizontal axis and redox potential E vs
SHE on the vertical axis.
•The horizontal lines represent electron transfer reactions.
They are pH -independent, but potential-dependent. These
lines separate the regions of stability, e.g. Fe and Fe
2+
in a
potential-pH diagram for Fe-H
2O system.
•The vertical lines are potential-independent but pH-
dependent and not accompanied by any electron transfer,
e.g. lines corresponding to the following reactions:
Prof. Yehia ElShazly

200. •The sloping, straight lines give the redox potentials of a
solution in equilibrium with hydrogen and oxygen,
respectively. These equilibria indicate electron transfer as
well as pH, e.g.
Prof. Yehia ElShazly

201. Prof. Yehia ElShazly

202. •Line 1
E=0.77
Prof. Yehia ElShazly

203. •Line 2
E=-0.62 V
Prof. Yehia ElShazly

204. •Line 3
Prof. Yehia ElShazly

205. •Line 4
E=1.0852-0.1776 pH
Prof. Yehia ElShazly

206. •Line 5
E=0.98-0.088 log[Fe
2+
]-0.237 pH
E=1.508-0.237 pH
Prof. Yehia ElShazly

207. Other Lines
Prof. Yehia ElShazly

208. •Below hydrogen line (a) hydrogen is produced by a
reduction of H
+
and H
20, and above oxygen line (b), 0
2
is produced by oxidation of H
20 and OH
-
. Between the
lines aand bwater is stable.
•Variation of concentration of Fe
2+
(10
-6
-1) leads to
several parallel lines.
Prof. Yehia ElShazly

209. Prof. Yehia ElShazly

210. E-pH diagram of iron or steel with four concentrations of soluble
species, three soluble species and two dry corrosion products (25
o
C)
Prof. Yehia ElShazly

211. Prof. Yehia ElShazly

212. Zn POURBAIX DIAGRAM
Prof. Yehia ElShazly

213. Prof. Yehia ElShazly

214. Prof. Yehia ElShazly

215. Prof. Yehia ElShazly

216. Prof. Yehia ElShazly

217. Prof. Yehia ElShazly

218. Prof. Yehia ElShazly

219. Prof. Yehia ElShazly

220. Prof. Yehia ElShazly

221. Prof. Yehia ElShazly

222. Corrosion
Corrosion Kinetics

223. •The mass of primary products formed at an electrode by
electrolysis is directly proportional to the quantity of
electricity passed.
mαIt
•The masses of different primary products formed by equal
amounts of electricity are proportional to the ratio of molar
mass to the number of electrons involved with a particular
reaction
Prof. Yehia ElShazly

224. •F = Faraday's constant. It is the quantity of electricity
required to deposit the ratio of mass to the valency of any
substance.
•It has a value of 96 485 coulombs per gram equivalent.
•In terms of loss of weight of a metal with time, we get
Prof. Yehia ElShazly

225. Prof. Yehia ElShazly

226. Exchange current density
•Each reversible process has a characteristic potential,
called electrode potential.
•When the reaction irreversible, if r
f=r
b, no net current flows
•Then,, ir
f= ir
b= i
0(i
0is the exchange current density)
•Each reversible electrode reaction has its own exchange
current density.
•A high value of i
orepresents a high rate of reaction.
Prof. Yehia ElShazly

227. FACTORS A F F E C T I NG EXCHANGE
CURRENT DENSITY
•Electrode composition:
•Surface Roughness:
•Impurities:
•Atoms are more easily pulled from the kink sites than
terrace sites.
Prof. Yehia ElShazly

228. Overpotential
•The corrosion potential that results from this situation is
a compromise between the various equilibrium potentials
of all the anodic and cathodic reactions involved.
•The difference between the resultant potential (E) and
each individual reaction equilibrium potential (E
eq) is
called polarization and quantified in terms of an
overpotential as in equation:
Prof. Yehia ElShazly

229. The polarization is said to be either anodic, when the
anodic processes on the electrode are accelerated by
changing the specimen potential in the positive (noble)
direction or cathodicwhen the cathodic processes are
accelerated by moving the potential in the negative
(active) direction.
Prof. Yehia ElShazly

230. Prof. Yehia ElShazly

231. •There are three distinct types of polarization and these are
additive, as expressed in equation:
•η
actis the activation overpotential, a function describing the
charge transfer kinetics of an electrochemical reaction.
•η
concis the concentration overpotential, a function
describing the mass transport limitations associated with
electrochemical processes.
•iRis the ohmicdrop. This function takes into account the
electrolytic resistivity of an environment when the anodic
and cathodic elements of a corrosion reaction are separated
by this environment while still electrically coupled.
Prof. Yehia ElShazly

232. •Activation polarization is usually the controlling factor during
corrosion in strong acids.
•Concentration polarization usually predominates when the
concentration of the active species is low; for example, in
dilute acids or in aerated waters where the active
component, dissolved oxygen, is only present at very low
levels.
•The ohmic drop will become an extremely important factor
when studying corrosion phenomena for which there is a
clear separation of the anodic and cathodic corrosion sites.
Prof. Yehia ElShazly

233. Activation Polarization
•Activation polarization is caused by a slow electrode
reaction: The reaction at the electrode requires an activation
energy in order to proceed.
•The rate of transformation is controlled by the magnitude of
the energy barrier that an atom or ion must surmount to
transform from metal to ion or from ion to metal.
•Activation polarization, η , increases with current density, i ,
in accord with the Tafel equation :
Prof. Yehia ElShazly

234. •where β and i
0are constants for a given metal and
environment and are both dependent on temperature.
•The larger the value of i 0 and the smaller the value of β ,
the smaller the corresponding overpotential.
Prof. Yehia ElShazly

235. Factors affecting activation
polarization for hydrogen electrode
•Current Density:
•Materials:
•Surface Roughness:
•Temperature:
•pH:
•Adsorption of Ions:
•Pressure:
Prof. Yehia ElShazly

236. For purely
activation
controlled
processes, each
reaction can be
described by a
straight line on an
E vs. Log i plot,
with positive Tafel
slopes for anodic
processes and
negative Tafel
slopes for
cathodic
processes.
Prof. Yehia ElShazly

237. Concentration polarization
•If copper is made cathode in a solution of dilute CuSO
4
E
1=E
0-0.0592/2 log(1/[Cu
++
])
•When current flows, copper is deposited on the electrode,
thereby decreasing surface concentration of copper ions to
an activity [Cu
++
]
s.
E
2=E
0-0.0592/2 log(1/[Cu
++
]
s)
•Since [Cu
++
]
sis less than [Cu
++
]the potential of the polarized
cathode is less noble, or more active, than in the absence of
external current.
Prof. Yehia ElShazly

238. •The difference of potential, E
2-E
1, is the concentration
polarization , equal to
η=E2-E1= -0.0592/2 log([Cu
++
] /[Cu
++
]
s)
•The larger the current, the smaller the surface concentration
of copper ion,, thus the larger the corresponding
polarization.
Prof. Yehia ElShazly

239. •Infinite concentration polarization is approached when
[Cu
++
]
s approaches zero at the electrode surface.
•The corresponding current density that results in this limiting
lower value of [Cu
++
]
s is called the limiting current density .
•If i
Lis the limiting current density for a cathodic process and
iis the applied current density,0.0592
E2 E1 log( )
2
L
L
i
ii
   

Prof. Yehia ElShazly

240. Prof. Yehia ElShazly

241. •The limiting current density (A/cm
2
) can be evaluated from
the expression
•where D is the diffusion coefficient for the ion being
reduced, δ is the thickness of the stagnant layer of
electrolyte next to the electrode surface (about 0.05 cm in
an unstirred solution), t is the transference number of all
ions in solution except the ion being reduced (→1) and c is
the concentration of diffusing ion in moles/liter.
Prof. Yehia ElShazly

242. Effect of oxygen solubility
Prof. Yehia ElShazly

243. Solubility of oxygen in air saturated
water
Prof. Yehia ElShazly

244. Oxygen dissolved in seawater in equilibrium
with a normal atmosphere
Prof. Yehia ElShazly

245. Mass transport to a
surface is governed
by three forces, i.e
diffusion, migration
and convection. In
the absence of an
electrical field, the
migration term, that
only affects charged
ionic species, is
negligible while the
convection force
disappears in
stagnant conditions.
Prof. Yehia ElShazly

246. J
Ois the flux of species O
(mol s
-1
cm
-2
)
D
Ois the diffusion coefficient
of species O (cm
2
s
-1
)
dC
Ois the concentration
gradient of species O across
the interface between the
metallic surface and the bulk
environment (mol cm
-3
)
dx is the thickness of the
interface (cm).
Prof. Yehia ElShazly

247. Diffusion coefficients of selected ions at
infinite dilution in water at 25
o
C
Prof. Yehia ElShazly

248. For more practical situations the diffusion coefficient
can be approximated with the help of equation,
that relates D
Oto the viscosity of the solution (μ) and
absolute temperature (T):
where Ais a constant for the system.
Prof. Yehia ElShazly

249. J
Ois the flux of species O
(mol s
-1
cm
-2
)
D
Ois the diffusion coefficient
of species O (cm
2
s
-1
)
dC
Ois the concentration
gradient of species O across
the interface between the
metallic surface and the bulk
environment (mol cm
-3
)
dx is the thickness of the
interface (cm).
Prof. Yehia ElShazly

250. •For well mixed solutions, the concentration is constant in the
bulk or convective region. This is represented by the
horizontal line where C = C
O.
•There is also a region where the concentration drops, falling
to zero at the electrode surface.
•The diffusion layer, associated with this drop has a specific
thickness (δ) that depends upon the nature of the solution into
which it extends.
•For stirred aqueous solutions the thickness of the diffuse
layer varies between 0.01and 0.001 mm.
Prof. Yehia ElShazly

251. For a chemical species O that is consumed by the cathodic
reaction at the corroding surface, the concentration gradient
(dC
O/dx) is greatest when the concentration of that species is
completely depleted at the surface, i.e. C
O= 0. It follows that
the cathodic current is limited in that condition
For intermediate cases, i.e. when the cathodic current is
smaller than i
L, η
conccan be evaluated using an
expression derived from Nernst equation:
Prof. Yehia ElShazly

252. Concentration Controlled Corrosion Processes
•When one of the reactions is limited by the rate of transport of
the reactant to the metallic surface being corroded, with the
exception that the environment is aerated and stagnant. In
this situation the reduction of oxygen shown in equation now
becomes possible as a second cathodic reaction.
Prof. Yehia ElShazly

253. Prof. Yehia ElShazly

254. EFFECT OF VARIOUS FACTORS ON
CONCENTRATION POLARIZATION
•Agitation
•Temperature
•Velocity
•Concentration of ionic species
•Geometry of electrode and fluid flow.
Prof. Yehia ElShazly

255. MIXED POTENTIAL THEORY
•Electrochemical reactions are composed of two or more
partial anodic and cathodic reactions.
•There cannot be any accumulation of charges.
•When zinc corrodes in a dilute acid,
Prof. Yehia ElShazly

256. Prof. Yehia ElShazly

257. •If Zinc in contact with its ions at unity is polarized, it would
be oxidized at more noble potentials; and reduced at more
active potentials .
Prof. Yehia ElShazly

258. Evans diagram
•The Evans diagram shows the electrode potential in volts in
the ordinate and the reaction rate (ampere) in the abscissa.
•In case of zinc corroding in an acid,
Prof. Yehia ElShazly

259. •The ratio of anodic to cathodic areas is taken as unity and current
(I) is replaced by current density (i).
•On polarizing anodically, the potential is displaced in the positive
direction,
•On cathodic polarization, it is displaced in the negative direction.
•The intersection of anodic and cathodic curve gives E
Corrthe
corrosion potential, which is the mixed potential of the system.
•The rate of corrosion is given by i
corr which represents the total
current associated with the dissolution of zinc and evolution of
hydrogen.
Prof. Yehia ElShazly

260. PREDICTION OFCORROSION
TENDENCY
•According to the emf series, zinc has a more active
potential than iron, E°(Zn) = -0.76 V, E°(Fe) = —0.44
Prof. Yehia ElShazly

261. Prof. Yehia ElShazly

262. EFFECT OF AN OXIDIZER
Prof. Yehia ElShazly

263. consequences of adding an oxidizer:
•The corrosion rate of the metal is increased.
•The corrosion potential is shifted to a more noble direction.
•The rate of hydrogen evolution is decreased.
Prof. Yehia ElShazly

264. COUPLING OF AN ACTIVE METAL TO A NOBLE METAL
The total
cathodic
process is
the sum of
H
2
on Zn and
H
2on Pt,
resulting in
the dotted
line.
Prof. Yehia ElShazly

265. •The rate of hydrogen evolution on zinc (H on Zn) decreases.
•The rate of oxidation of zinc increases.
•The rate of hydrogen evolution on platinum surface (H on
Pt) increases vigorously.
•Platinum is an excellent catalyst for reduction of hydrogen
and zinc is a poor catalyst.
Prof. Yehia ElShazly

266. EFFECT OF GALVANIC COUPLING
•Consider coupling of zinc to gold and zinc to platinum.
According to the thermodynamic approach, the
difference between the potential of zinc (E^Zn = —
0.76V) and gold (E^Au = 1.50V) is higher than the
difference between the potential of the zinc and platinum
(E^Zn = -0.76V,E^Pt = + 1.2V).
•According to the thermodynamic approach, the Zn-Au
couple should corrode faster than Zn-Pt couple.
Prof. Yehia ElShazly

267. Prof. Yehia ElShazly

268. EFFECT OF OXYGEN
Prof. Yehia ElShazly

269. Prof. Yehia ElShazly

270. •It is observed that upon aeration of the acid solution, rate of
reaction (i
Corr) increases and the corrosion potential (E
Corr)
shifts to more noble value.
•The rate of corrosion increases on aeration.
Prof. Yehia ElShazly

271. RESISTANCE POLARIZATION (OHMIC
POLARIZATION)
•The ohmic overpotential appears as the simple product of a
resistance and a current between the anodic and cathodic
sites of a corrosion process.
•For many corrosion situations these sites are adjacent to each
other and the ohmic drop is negligible, particularly so when
the environment itself is a good electrolytic conductor, e.g.
seawater.
•However, there are special conditions where the separation of
the anodic and cathodic sites can be an important factor in the
corrosion progress, e.g. galvanic corrosion, or even an
integral part of protection scheme, e.g. anodic and cathodic
protection.
Prof. Yehia ElShazly

272. With an increasing resistance offered by the electrolytes,
the magnitude of the corrosion current would decrease as
shown by R1, R2 and R3 .
Prof. Yehia ElShazly

273. Water Resistivity Measurement
For the simple cell shown in the following Figure
where:
Ris the measured resistanceacross the cell
Ais the cross-sectional area of each electrode, provided
that both electrodes have the same dimensions
lis the gap separating the electrodes
Prof. Yehia ElShazly

274. Prof. Yehia ElShazly

275. Prof. Yehia ElShazly

276. Soil Resistivity Measurement
•Soil resistivity is a function of soil moisture and the concentrations
of ionic soluble salts and is considered to be most comprehensive
indicator of a soil’s corrosivity.
•Typically, the lower the resistivity, the higher will be the corrosivity.
•Soil resistivity generally decreases with increasing water content
and the concentration of ionic species.
•Sandy soils are high up on the resistivity scale and therefore
considered the least corrosive.
•Clay soils, especially those contaminated with saline water are on
the opposite end of the spectrum
Prof. Yehia ElShazly

277. Prof. Yehia ElShazly

278. •Field soil resistivity measurements are most often conducted
using the Wenner four-pin methodand a soil resistance
meter.
•The Wennermethod requires the use of four metal probes or
electrodes, driven into the ground along a straight line,
equidistant from each other.
•Soil resistivity is a simple function derived from the voltage
drop between the center pair of pins, with current flowing
between the two outside pins.
Prof. Yehia ElShazly

279. Prof. Yehia ElShazly

280. Prof. Yehia ElShazly

281. Prof. Yehia ElShazly

282. Prof. Yehia ElShazly

283. •An alternating current from the soil resistance meter
causes current to flow through the soil, between pins
C1and C2.
•The voltage or potential is then measured between pins
P1and P2. The meter then registers a resistance
reading.
•Resistivity of the soil is then computed from the
instrument reading, according to the following formula:
where:
ρis the soil resistivity (ohm-centimeters)
Ais the distance between probes (centimeters)
Ris the soil resistance (ohms), instrument reading
Prof. Yehia ElShazly

284. •The resistivity values obtained represent the average resistivity of
the soil to a depth equal to the pin spacing. Resistance
measurements are typically performed to a depth equal to that of the
buried system (pipeline) being evaluated.
•Moisture content, temperature and salts also affect soil resistivity.
Soil that contains 10% moisture by weight will as much as five times
lower soil resistivity than that which contains 2.5%.
•Soil at room temperature will be as much as four times lower in
resistivity than that at 32 degrees. So the time of year that you
conduct the test can play a major role in the results.
•Finally salt content factors in the results in a big way. Just changing
the composition by 1% can reduce soil resistivity by as much as a
factor of 20.
Prof. Yehia ElShazly

285. Measurement OF E
corr
Prof. Yehia ElShazly

286. Prof. Yehia ElShazly

287. KINETICS OF PASSIVITY
•If a metal is converted to an oxide and the oxide which is
formed is stable,the metal is considered passive as this oxide
forms a barrier between the metal and the environment.
•For example, iron is not attacked in concentrated HNO
3, as a
very thin film of an oxide is formed on its surface and causes
loss of reactivity.
•A film of solid hydroxide or oxide may be precipitated from an
aqueous solution if there are metal ions in the solution
•Because of film formation, there is a reduction in the current
density for cathodic reduction and an increase in the current
density for an anodic polarization.
•The above factors lead to retardation of corrosion.
Prof. Yehia ElShazly

288. Prof. Yehia ElShazly

289. Corrosion of a passive metal
Effect of oxidizer
Prof. Yehia ElShazly

290. •1-3 nonpssivating metal. Rate of corrosion increases
from A to C
•4 rapid transition in corrosion potential from point D in
active state to point G in passive state.
•5 remains in the passive state
•6-7 Transpassive region. Corrosion rate increases.
Prof. Yehia ElShazly

291. Reducing Oxidizer concentration
Prof. Yehia ElShazly

292. Velocity Effect
Prof. Yehia ElShazly

293. •Solution velocity affects the corrosion rate of a diffusion
controlled system.
•Velocity has no effect on activation controlled systems.
•The corrosion rate of a metal in a diffusion controlled
process becomes independent of solution velocity at
very high velocities.
Prof. Yehia ElShazly

294. Effect of velocity on active passive
metal
Prof. Yehia ElShazly

295. Critical Anodic Density: The smaller the critical anodic
current density, the easier a metal will be passivated
by an increase in velocity.
Prof. Yehia ElShazly

296. Galvanic Coupling
Prof. Yehia ElShazly

297. Prof. Yehia ElShazly

298. •2H
2+
+2e = H
2Au 1N HCl i0=10
-6
•2H
2+
+2e = H
2Pt 1N HCl i0=10
-3
•Au
3+
+3e=Au E
0=1.498
•Pt
2+
+2e=Pt E
0=1.2
•2H
+
+2e=H2 E
0=0
•Zn
2+
+2e=Zn E
0=-0.763
•The reason why gold produces a less severe galvanic
effect is not related to its reversible potential but rather to
the fact that it has a lower hydrogen exchange current
density than platinum.
Prof. Yehia ElShazly

299. Coupling of Metals
Prof. Yehia ElShazly

300. •Metal M is more noble than metal N and has a lower
corrosion rate (i
corr(M)).
•When they are coupled, the resultant mixed potential of
this system occurs at a point where the total oxidation
rate equals total reduction rate.
•In this figure, coupling equal areas of these two metals
decreases the corrosion rate of metal M to i
corr(M-N) and
increases the corrosion rate of metal N to i
corr(M-N)
Prof. Yehia ElShazly

301. Effect of Area in Galvanic
Coupling
Prof. Yehia ElShazly

302. •If two corroding metals are galvanically coupled, the
corrosion rate of the metal with the most active potential
is accelerated and that of the other metal is retarded.
The anode is determined on the basis of the corrosion
potential rather than the corrosion rate.
•The corrosion behavior of a galvanic couple is
determined by the reversible electrode potential of the
actual process involved, their exchange current densities
and Tafel slopes, and the relative areas of the two
metals. Galvanic corrosion behavior cannot be predicted
accurately on the basis of emf potentials.
Prof. Yehia ElShazly

303. Coupling an active passive
metal
•Coupling
titanium to
platinum
has been
found to
reduce
corrosion.
•Only
Titanium
and
Chromium.
Prof. Yehia ElShazly

304. In general
coupling an
active passive
metal will
increase the
rate of
corrosion.
Prof. Yehia ElShazly

305. Corrosion Prevention

306. Corrosion Prevention
•Corrosion Monitoring
•Materials Selection
•Alteration of Environment.
•Design
•Cathodic and Anodic Protection
•Coatings
Prof. Yehia ElShazly

307. Corrosion Monitoring
and Inspection

308. Corrosion Monitoring Basics
•Corrosion inspectionandmonitoringare key activities in ensuring
asset integrity and control of corrosion. Field data and the results of
laboratory evaluations should be trended to obtain up-to-date
corrosion information. Management decisions on equipment
condition, prediction of remnant life and requirements for chemical
treating are only as good as the information input provided from field
experience.
•Modern corrosion monitoring technologies can emphasize the highly
time-dependent nature of corrosion damage. The integration of
corrosion monitoring technology in existing systems can also
provide early warning of costly corrosion damage and
provideinformationon where the damage is taking place.
•In a modern business environment, successful enterprises cannot
tolerate major corrosion failures, especially when these involve
personal injuries, fatalities, unscheduled shutdowns and
environmental contamination.
Prof. Yehia ElShazly

309. Corrosion Monitoring Basics 2
•For this reason considerable effort must be expended incorrosion
controlat all stages of a system's life, from thedesigntable to the
last day of operation.
•Typically, once a system, a plant or any piece of equipment is put
into service, maintenance is required to keep it operating safely and
efficiently. This is particularly true for aging systems and structures
that will be required to operate beyond their original design life.
•Correct and effective corrosion monitoring strategiesshould be used
as a proactive tool to assist with operating a plant more effectively,
thereby prolonging its life and gaining optimum throughput.
•It also enables continuous monitoring of actual corrosion rates,
allowing for timely preventative action if a variance is observed.
Prof. Yehia ElShazly

310. Corrosion Monitoring Basics 3
•Currentcorrosion inspectionand monitoring typically requires
planned periodic shutdowns to inspect equipment. Scheduled
shutdowns are costly in terms of productivity losses, restart energy
and material costs.
•Unscheduled shutdowns are disruptive and often quite expensive.
•Internal corrosion failures result in costly cross contaminations of
product and process streams.
•External corrosion leaks put process fluids into the plant
environment and can create significant safety hazards.
•Corrosion monitoringsystemsvary significantly in complexity, from
simple coupon exposures or hand held data loggers to fully
integrated plant process surveillance units with remote data access
and data management capabilities.
•Other sensors or devices will produce a signal that needs to be
recorded and analyzed further for an estimation of the severity of a
situation. By combining such signal with the knowledge of a process
or environment it allows an operator to be proactive.
Prof. Yehia ElShazly

311. An effective corrosion monitoring program includes a
wide range of activities:
•Identification of all critical components
•Identification of component alloy composition
•Measurement of the location and extent of corrosion
•Prediction of remaining life
•Identification of failure mechanisms
•Determination of fitness for service condition
•Inspection scheduling
•Development of recommendations for remedy and correction of
problems
•Development of corrosion prevention strategies
Prof. Yehia ElShazly

312. Inspection Methods
•Inspection normally refers to the evaluation of the quality of some
characteristic in relation to a standard or a specification. As products
and their manufacturing processes have grown complex and divided
among many departments, the job of inspection has also become
complex and distributed. A flow diagram is useful for showing the
various materials, components, and processes that collectively or
sequentially make up the system.
•Corrosion inspection and monitoring includes assessment of:
–In-line systemscover installation of devices directly into the process,
but which need to be extracted for analysis, e.g. corrosion coupons, bio-
studs, etc.
–On-line monitoring techniquesinclude deployment of corrosion
monitoring devices either directly into the process or fixed permanently
to the equipment.
–Off-line monitoringis mainly achieved through the use of inspection
andNDT techniques.
Prof. Yehia ElShazly

313. Prof. Yehia ElShazly
CLIMAT devices have been used for more than
three decades to monitor atmospheric
corrosivity. These units have been utilized
successfully around the world in marine and
industrial type atmospheres.

314. Prof. Yehia ElShazly
Ultrasonic testing Industrial radiography
Guided wave testing Electromagnetic testing

315. Non destructive Evaluation(NDE) Techniques
•NDEtechnology refers to an array of non destructive techniques
(NDT) and processes to monitor, probe and measure material
response. The measured response is related to a desired material
property or test object attribute by interpretation. The main NDT
methods are:
–Visual inspection
–Liquid penetrant inspection
–Magnetic particle inspection
–Radiographic inspection (X-ray and gamma ray)
–Eddy current inspection
–Ultrasonic inspection
–Thermographic inspection
•No NDE process or procedure produces absolute discrimination of
anomalies but the end output of a procedure may be quantified and
the anomaly or flaw detection capability may be measured,
analyzed, quantified and documented.
Prof. Yehia ElShazly

316. •The selection of inspection points is of paramount importance as
corrosion factors to be considered in a corrosion maintenance
program are often related to the geometry of systems and
components. Selection of inspection points should be based on a
thorough knowledge of process conditions, materials of construction,
geometry of the system, external factors and historical records.
Prof. Yehia ElShazly

317. Risk Based Inspection (RBI)
•Risk is defined as the combination of probability and
consequence.
•Risk-based inspection refers to the application of risk analysis
principles to manage inspection programs for plant equipment.
•RBI has been used in the nuclear power generation industry for some
time and is also employed in refineries and petrochemical plant.
•The ultimate goal of RBI is to develop a cost-effective inspection and
maintenance program that provides assurance of acceptable
mechanical integrity and reliability.
•The highest risk is mostly associated with a small percentage of plant
items.
•Risk based inspection procedures can be based on either qualitative or
quantitative methodologies.
Prof. Yehia ElShazly

318. •Qualitative procedures provide a ranking of equipment, based
largely on experience and engineering judgment.
•Quantitativerisk-based methods use several engineering
disciplines to set priorities and develop programs for equipment
inspection.
•Risk Based Inspection (RBI) schemes are a planning tool used to
develop the optimum plan for the execution of inspection activities.
•RBI uses the findings from a formal risk analysis, such as a
Corrosion Risk Assessment, to guide the direction and emphasis of
the inspection planning and the physical inspection procedures.
•A risk based approach to inspection planning is used to
–Ensure risk is reduced to as low as reasonably practicable
–Optimize the inspection schedule
–Focus inspection effort onto the most critical areas
–Identify and use the most appropriate methods of inspection
Prof. Yehia ElShazly

319. Prof. Yehia ElShazly

320. Corrosion Risk Analysis
•The complexity of engineering systems is growing steadily with the
introduction of advanced materials and modern protective methods.
Even at the most basic level, the multiple interactions between
defects, faults and the failure of a system can be extremely complex.
Prof. Yehia ElShazly
•Defect:departure of a characteristic of
a system from requirements
•Fault:inability of s system to perform a
required function
•Failed State:state of a system unable
to perform a required function
•Failure Mode:effect by which a failure
is observed
•Failure:termination of the ability of a
system to perform a required function

321. •To determine the probability of a failure, two fundamental issues
must be considered:
What are the specific forms of corrosion and their rates?
What is the possible effectiveness of inspection?
•The input of corrosion experts is required to identify the relevant
forms of corrosion in a given situation and to determine the key
variables affecting the propagation rate.
•It is also important to realize that full consensus and
supportingdataon the variables involved is highly unlikely in real-life
complex systems and that simplification will often be necessary.
Prof. Yehia ElShazly

322. •One semi-quantitative approach for ranking process equipment is
based on internal probability of failure (POF).
•The procedure is based on an analysis of equipment process and
inspection parameters, and ranks equipment on a scale of one to
three, with "one" being the highest priority.
•The procedure requires a fair degree of engineering judgment
and experience and, as such, is dependent on the background
and expertise of the analyst. This procedure can facilitate an
efficient use of finite inspection resources when 100% inspection is
not practical.
Prof. Yehia ElShazly

323. Maintenance strategies
•Corrosion maintenance costs represent a significant portion of operating
budgets in most industrial sectors, particularly where aging structures/plant is
involved.
•Modern approaches to maintenance management are designed to minimize
these costs and to improve reliability and availability of plant and equipment.
•In this context, maintenance activities are treated as an investment and not
as an organizational liability.
•Four general types of maintenance philosophies or strategies can be
identified, namely:
–corrective,
–preventive,
–predictive, and
–reliability centered maintenance.
•The challenge is to optimize the balance between the them types for
maximum profitability.
Prof. Yehia ElShazly

324. Corrective Maintenance
•Corrective maintenance refers to action only taken when a system or
component failure has occurred.
•The task of the maintenance team in this scenario is usually to effect
repairs as soon as possible.
•Costs associated with corrective maintenance include repair costs
(replacement components, labor, consumables), lost production and lost
sales.
•To minimize the effects of lost production and speed up repairs, actions
such as increasing the size of maintenance teams, the use of back-up
systems and implementation of emergency procedures can be considered.
•Unfortunately, such measures are relatively costly and/or only effective in
the short-term.
•For example, if heat exchanger tubes have leaked due topitting
corrosionand production must proceed as a matter of urgency, it may be
possible to plug the leaking tubes on a short-term basis.
Prof. Yehia ElShazly

325. Preventive Maintenance
•Inpreventive maintenance, equipment is repaired and serviced before
failures occur.
•The frequency of maintenance activities is pre-determined by schedules.
•Preventive maintenance aims to eliminate unnecessary inspection and
maintenance tasks, to implement additional maintenance tasks when and
where needed and to focus efforts on the most critical items.
•The higher the failure consequences, the greater the level of preventive
maintenance that is justified.
•This ultimately implies a trade-off between the cost of performing preventive
maintenance and the cost to run the equipment to failure.
•Components are essentially inspected for corrosion and other damage at
planned intervals, in order to identify corrective action before failures
actually occur.
•Preventive maintenance performed at regular intervals will usually results in
reduced failure rates.
•As significant costs are involved in performing preventive maintenance,
especially in terms of scheduled downtime, good planning is vital.
Prof. Yehia ElShazly

326. Predictive Maintenance
•Predictive maintenance refers to maintenance based on the actual condition
of a component.
•Maintenance is not performed according to fixed preventive schedules but
rather when a certain change in characteristics are noted.
•Corrosion sensors supplying diagnosticinformationon the condition of a
system or component play an important role in this maintenance strategy.
•A useful analogy can be made with automobile oil changes:
–Changing the oil every 5000 km to prolong engine life, irrespective of whether
the oil change is really needed or not, is a preventive maintenance strategy.
–Predictive maintenance would entail changing the oil based on changes in its
properties, such as the build-up of wear debris. When a car is used exclusively
for long distance highway travel and driven in a very responsible manner, oil
analysis may indicate a longer critical service interval.
–Some of the resources required to perform predictive maintenance will be
available from the reduction in breakdown maintenance and the increased
utilization that results from pro-active planning and scheduling.
–Good record keeping is very important to identify repetitive problems, and the
problem areas with the highest potential impact.
Prof. Yehia ElShazly

327. Reliability Centered Maintenance
•Reliability centered maintenance (RCM) involves the establishment or
improvement of a maintenance program in the most cost-effective and
technically feasible manner.
•It utilizes a systematic, structured approach that is based on the
consequences of failure.
•As such it represents a shift away from time-based maintenance tasks and
emphasizes the functional importance of system components and their
failure/maintenance history.
•The concept of RCM finds its roots in the early 1960's, with RCM
strategies for commercial aircraft developed in the late 1960s, when wide-
body jets were introduced to commercial airline service. A major concern
of airlines was that existing time-based preventive maintenance programs
would threaten the economic viability of larger, more complex aircraft. The
experience of airlines with the RCM approach was that maintenance costs
remained roughly constant but that the availability and reliability of their
planes improved. RCM is now standard practice for most of the world's
airlines.
Prof. Yehia ElShazly

328. •This starts with the seven questions below, worked through in the order
that they are listed:
1. What is the item supposed to do and its associated performance
standards?
2. In what ways can it fail to provide the required functions?
3. What are the events that cause each failure?
4. What happens when each failure occurs?
5. In what way does each failure matter?
6. What systematic task can be performed proactively to prevent,
or to diminish to a satisfactory degree, the consequences of
the failure?
7. What must be done if a suitable preventive task cannot be
found?
Prof. Yehia ElShazly

329. Material Selection

330. •From a purely technical standpoint, an obvious answer to corrosion
problems would be to use morecorrosion resistantmaterials.
•In many cases, this approach is an economical alternative to other
corrosion control methods.
•The choice of a material is the result of several compromises:the
technical appraisal of analloywill generally be a compromise
between corrosion resistance and some other properties such as
strength and weldability. And the final selection will be a compromise
between technical competence and economic factors.
•In specifying a material, the task usually requires three stages:
Listing the requirements
Selecting and evaluating the candidate materials
Choosing the most economical material.
Prof. Yehia ElShazly

331. Prof. Yehia ElShazly

332. MATERIALSSELECTION
1-Selection
of the
proper
material.
Combinations of
metals and
corrosive
environment
for the least
amount of
money
Stainless Steel Nitric Acid
Nickel and nickel alloysCaustic
Monel HF
Hastelloys Hot HCl
Lead Dilute Sulphuric Acid
Aluminuim Non-stainingatmosphere
Tin Distilled water
Titanium Hot strong oxidizing
solutions
Tantalum Ultimate resistance
Steel Conc. Sulphuric Acid
Prof. Yehia ElShazly

333. 2-Metal Purification
•Corrosion resistance of a pure metal is usually better
than that of one containing impurities or small amounts
of other elements.
3-Nonmetallics
•Rubbers, Plastics: much weaker and softer, more
resistant to Chloride ion and HCl, less resistant to
sulphuric acid and oxidizing acids and solvents, low
temperature limitations
•Ceramics: Excellent corrosion and high temperature
resistance, brittle.
•Carbon and Graphite: good corrosion resistance, good
electric and heat conductivity, Fragile.
•Wood: attacked by aggressive environment.
Prof. Yehia ElShazly

334. Making Choices Based on Economic
Calculations
•Initial cost
•Best estimate of expected life
•Length of typical shutdown for
emergency repair
•Cost of planned maintenance during
scheduled shutdowns
•Effect of failure on total plant operation
Prof. Yehia ElShazly

335. Summary
of
estimated
direct cost
of
corrosion
for
industry
sectors
analyzed
in the 2001
study (US)
Prof. Yehia ElShazly

336. •Even the best design cannot be expected to anticipate
all conditions that may arise during the life of a system.
•Corrosion inspection and monitoring : traditional
corrosion inspection practices typically require planned
periodic shutdowns or service interruptions to allow the
inspection process.
•Accidental interruptions or shutdowns are potentially
much more disruptive and expensive.
Prof. Yehia ElShazly

337. Choosing the Right Material
•Of the 103 elements, 80 are metals, each possessing
different mechanical, chemical, and physical properties.
•All these metals can corrode, they do it in many different
manners.
•Furthermore, many of these 80 metals have been alloyed to
make tens of thousands of different alloys.
•From a purely technical standpoint, an obvious answer to
corrosion problems would be to use more resistant materials.
In many cases, this approach is an economical alternative to
other corrosion control methods.
Prof. Yehia ElShazly

338. Prof. Yehia ElShazly

339. Mixed acid isocorrosion
diagram
•the corrosion behavior
of metals is expressed
in units of penetration
rates, i.e. mm y
-1
or
milli-inch per year
(mpy),
•The corrosion behavior
of non-metals is
expressed in qualitative
terms such as
‘recommended’,
‘questionable’ and ‘not
recommended’.
Prof. Yehia ElShazly

340. Prof. Yehia ElShazly

341. Stainless Steel
•Stainless steels are engineering materials with good corrosion-
resistance, strength and fabrication characteristics. They can readily
meet a wide range of design criteria, including load, service life and low
maintenance.
•Selecting the proper stainless steel grades involves weighing four
qualities in the following order of importance:
–Corrosion or Heat Resistance,the primary reason for specifying stainless.
The specifier needs to know the nature of the environment and the degree of
corrosion or heat resistance required.
–Mechanical Properties,particularly strength at room, elevated or low
temperature. The combination of corrosion resistance and strength is the
basis for selection.
–Fabrication Operationsand how the product will be made (e.g., forging,
machining, forming, welding, stamping, roll forming, four-slide operations).
–Total Cost,includingmaterial and production costs and considering the
cumulative savings of a maintenance-free product with longevity.
Prof. Yehia ElShazly

342. Types of Stainless Steel
•Normally Stainless Steels are classified by their crystalline
structure.
•Austenitic, or 200 and 300 series, stainless steels
–Have an austenitic crystalline structure, which is aface-centered
cubic crystal structure.
–Austenite steels make up over 70% of total stainless steel
production.
–They contain a maximum of 0.15% carbon, a minimum of 16%
chromium, and sufficient nickel and/or manganese to retain an
austenitic structure at all temperatures.
–200 Series—austenitic chromium-nickel-manganese alloys. Type
201 is hardenable through cold working; Type 202 is a general
purpose stainless steel.
–Decreasing nickel content and increasing manganese results in
weak corrosion resistance.
Prof. Yehia ElShazly

343. –300 Series. The most widely used austenite steel is the304, also
known as18/8for its composition of 18% chromium and 8%
nickel.
–The second most common austenite steel is the316grade, used
primarily for its increased resistance to corrosion. A typical
composition of 18% chromium and 10% nickel, commonly known
as18/10 stainless, is often used incutleryand high-
qualitycookware.
–Superausteniticstainless steels, exhibit great resistance to
chloride pitting and crevice corrosion because of
highmolybdenumcontent (>6%) and nitrogen additions, and the
higher nickel content ensures better resistance to stress-
corrosion cracking versus the 300 series.
Prof. Yehia ElShazly

344. •Ferriticstainless steels
–have better engineering properties than austenitic grades, but
have reduced corrosion resistance, because of the lower
chromium and nickel content.
–They are also usually less expensive.
–Ferritic stainless steels have abody-centered cubiccrystal
system and contain between 10.5% and 27% chromium with
very little nickel, if any, but some types can contain lead.
–Most compositions include molybdenum; some, aluminium or
titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo,
29Cr-4Mo, and 29Cr-4Mo-2Ni.
Prof. Yehia ElShazly

345. •Martensiticstainless steels
–are not as corrosion-resistant as the other two classes but are
extremely strong and tough, as well as highlymachinable, and
can be hardened by heat treatment.
–Martensitic stainless steel contains chromium (12–14%),
molybdenum (0.2–1%), nickel (less than 2%), and carbon (about
0.1–1%) .
–It is quenched and magnetic.
Prof. Yehia ElShazly

346. •Duplex steelstainless steels
–Theyhave a mixed microstructure of austenite and ferrite.
–Duplex stainless steels have roughly twice the strength
compared to austenitic stainless steels and also improved
resistance to localized corrosion, particularlypitting, crevice
corrosion and stress corrosion cracking.
–They are characterized by high chromium (19–32%) and
molybdenum (up to 5%) and lower nickel contents than
austenitic stainless steels.
•Lean duplexrefers to grades such asUNSS32101 (LDX 2101), S32202 (UR2202),
S32304, and S32003.
•Standard duplexrefers to grades with 22% chromium, such as UNS S31803/S32205,
with 2205 being the most widely used.
•Super duplexis by definition a duplex stainless steel with aPitting Resistance
Equivalent Number(PREN) > 40, Usually super duplex grades have 25% or more
chromium. Some common examples are S32760, S32750 (2507), and S32550.
•Hyper duplexrefers to duplex grades with a PREN > 48. UNS S32707 and S33207 are
the only grades currently available on the market.
Prof. Yehia ElShazly
PREN =%Cr + 3.3x(%Mo + 0.5x%W) + 16x%N.

347. •The corrosion, heat resistance and mechanical properties are all
affected by the chemical composition of the stainless steel. As the
composition of the steel is varied, so are the properties. The major
alloying elements of stainless steel include:
•Chromium
–Forms a passive surface film to make stainless steel resistant to corrosion.
–Increases the scaling resistance, tensile strength and wear resistance.
•Manganese
–Improves hot-working properties.
–Up to 2% has no effect on strength, ductility and toughness.
–Above 2% increases yield strength and tensile strength (as in the 201 grade).
–Important as a partial replacement of nickel in the 201 grade.
–Stabilizes the austenitic structure.
Prof. Yehia ElShazly

348. •Molybdenum
–Increases creep resistance; strength at high temperatures; and corrosion
resistance, particularly in sulfite, sulfate, acetic acid and acetate solutions and in
a salt-water atmosphere.
–Expands range of passivity and counteracts tendency to pit.
•Nickel
–Stabilizes the austenitic structure.
–Increases high-temperature strength; ductility, which makes stainless steel easier
to form; and corrosion resistance, particularly in industrial and marine
atmospheres and the chemical-, food-and textile-processing industries.
•Silicon
–Increases scaling resistance by forming a tight initial scale that will withstand
cyclic temperature changes; also slightly increases tensile strength and
hardness.
–Resists carburizing at high temperatures.
–When 1% or more, improves resistance to strong sulfuric acid but offers little
improvement for dilution and is unfavorable in nitric-acid service.
–*Note:Ductility decreases as silicon content increases.
Prof. Yehia ElShazly

349. •Sulfur –Phosphorous –Selenium
–Increases machinability.
–Decreases ductility and transverse tensile strength.
•Titanium –Columbium –Tantalum
–Prevents intergranular corrosion by stabilizing the carbon as titanium or
columbium carbides.
–Produces finer grain size.
–Reduces stretcher strains from drawing and forming by their addition to type 430.
Prof. Yehia ElShazly

350. Prof. Yehia ElShazly

351. the Environment
Some environments are more corrosive than others.
While there are exceptions, the following statements
are generally accepted as facts.
•Moist air is more corrosive than dry air
•Hot air is more corrosive than cold air
•Hot water is more corrosive than cold water
•Polluted air is more corrosive than clean air
•Acids are more corrosive than bases (alkalies) to
steels
•Salt water is more corrosive than fresh water
•Stainless steel will outlast ordinary steel
•No corrosion will occur in a vacuum, even at very high
temperatures
Prof. Yehia ElShazly

352. Prof. Yehia ElShazly

353. Prof. Yehia ElShazly

354. Prof. Yehia ElShazly

355. Prof. Yehia ElShazly

356. Prof. Yehia ElShazly

357. Prof. Yehia ElShazly

358. ALTERATIONOFENVIRONMENT
1-Changing Mediums:
•Lowering temperature. (in some cases: high temperature
–less oxygen solubility)
•Decreasing velocity (concentration (diffusion) controlled
corrosion –Concentration cells –erosion corrosion)
•Removing oxygen or oxidizers (vacuum treatment –inert
gas sparging –oxygen scavengers –dearation is not
recommended for active passive metals and alloys)
•Changing concentration
Prof. Yehia ElShazly

359. Prof. Yehia ElShazly

360. 2-Inhibitors:
•An inhibitor is a substance that when added in small
concentrations to an environment, decreases the
corrosion rate.
•Can be considered as a retarding catalyst
•Most inhibitors have been developed by empirical
experimentation.
•They are classified according to their mechanism and
composition
Prof. Yehia ElShazly

361. Adsorption-type Inhibitors
•Organic compounds that are adsorbed on the
metal surface and suppress metal dissolution.
•Affect anodic and cathodic processes.
•Organic Amines
Prof. Yehia ElShazly

362. Hydrogen-evolution poisons
•Arsenic and Antimony ions.
•Specifically retard the hydrogen evolution
reaction.
•Thus effective in acid solutions but ineffective in
other cathodic reactions.
Prof. Yehia ElShazly

363. Scavengers
•Removing corrosive reagents from solution.
•Sodium sulfite and Hydrazine which remove
dissolved oxygen from aqueous solutions
2Na
2SO
3+ O
2→ 2Na
2SO
4
N
2H
4+ O
2→ N
2+ H
2O
•Effective in solutions where the oxygen
reduction is the cathodic reaction.
Prof. Yehia ElShazly

364. Oxidizers
•Cromate, Nitrate, Ferric salts.
•Used where the metal is an active passive metal
(such as iron and its alloys and Stainless steel)
Prof. Yehia ElShazly

365. Vapor-Phase Inhibitors
•Possess a very high vapor pressure.
•They are placed in the vicinity of the metal and
they are transferred to the metal surface through
sublimation and condensation to the metal
surface.
•Used in closed spaces.
Prof. Yehia ElShazly

366. Limitations
•It may not be possible to add inhibitors to all corrosive
systems because they may contaminate the
environment.
•Some are toxic.
•Used in closed systems as they are not practical in
“once-through” systems.
•Rapidly lose their effectiveness as the concentration and
temperature of the environment increase.
Prof. Yehia ElShazly

367. DESIGN
•Design should consider mechanical and strength
requirements together with an allowance for corrosion.
•It is necessary to make allowances for the reduction of
thickness in designing pipes, tanks,..etc.
•Design should take in consideration the variation in the
depth of penetration.
•The wall thickness must meet mechanical requirements
throughout its operating lifetime.
•Thickness monitoring.
Prof. Yehia ElShazly

368. •Weld rather than rivet tanks: riveted joints provide sites
for crevice corrosion.
•Design tanks and other containers for easy draining and
easy cleaning.
•Design systems for easy replacement of components
that are expected to fail rapidly in service.
•Avoid excessive mechanical stresses and stress
concentrations in components exposed to corrosive
mediums.
•Avoid residual stresses in components exposed to
corrosive mediums.
Prof. Yehia ElShazly

369. •Avoid electrical contact between dissimilar metals to
prevent galvanic corrosion.
•Avoid sharp bends in piping systems when high
velocities and/or solids in suspensions are involved.
•Provide thicker structure where impingement takes
place.
•Make sure materials are properly selected.
•Specify procedure for cleaning and storage of parts.
•Avoid hot spots during heat transfer operations and
uneven temperature distribution.
•Design to exclude air.
Prof. Yehia ElShazly

370. CATHODICPROTECTION
•Supplying electrons to the metal structure to be
protected to suppress metal dissolution and
increase the rate of hydrogen evolution.
•Impressed Current: by an external power supply.
•Sacrificial Cathodic protection: through galvanic
coupling to a more active metal (magnesium)
(anode) which corrodes preferentially
Prof. Yehia ElShazly

371. •Steel structures exposed to soils, fresh and
brackish water and seawater are protected if
they are polarized to a potential of -0.85 volts
versus a copper/copper sulfate reference
electrode.
Prof. Yehia ElShazly

372. Prof. Yehia ElShazly

373. Prof. Yehia ElShazly

374. Prof. Yehia ElShazly

375. Prof. Yehia ElShazly

376. Prof. Yehia ElShazly

377. Prof. Yehia ElShazly

378. Prof. Yehia ElShazly

379. Prof. Yehia ElShazly

380. Prof. Yehia ElShazly

381. Prof. Yehia ElShazly

382. Sacrificial Cathodic Protection
Advantages
•It requires no external source, which might fail.
•It is economical.
•It can be easily installed.
•It can be easily maintained.
•It can be used in areas where the soil resistivity is low.
•Lesser interference with the other metallic structures is
caused because of a relatively low current output.
•The current is evenly distributed.
Prof. Yehia ElShazly

383. Disadvantages
•It has limited applications compared to impressed
current.
•Driving voltage is fixed and cannot be manipulated,
except by selecting Mg instead of Zn for example.
•The cost of protection is high for bare systems (uncoated
structures).
•As no above-ground equipment is used, it is difficult to
trace the protected system, unless contact posts are
provided.
Prof. Yehia ElShazly

384. Impressed Current Protection
Advantages
•One installation can protect a large area of metal.
•The system can be used for a wide variety of voltage
and current requirement.
•Schemes can be designed for life in excess of 20 years,
if required.
•Current requirements and potentials can be easily
adjusted to the varying needs of protection.
•Can be applied to a wide range of structures.
•Requires generally a small total number of anodes.
•Requires simple controls which can be automated.
•A large area can be protected by one installation.
Prof. Yehia ElShazly

385. Disadvantages
•External power is essential.
•More complicated system for installation.
•Less economical for smaller jobs.
•Limited to use below a soil resistivity of 3000 ohms-cm.
•Possibility of interference effects on other buried
structures.
•Regular maintenance is essential.
•Power failures can cause serious problems, and faults
may go unnoticed for long times.
Prof. Yehia ElShazly

386. Stray Current Corrosion
•If an underground metallic structure is present, the point
at which the current is discharged from the metallic
structure to the ground becomes the anode and,
therefore, corrodes .
•The undesired current which enters the metallic structure
on its way to the cathode is called stray current and the
corrosion caused by stray current is called stray current
corrosion.
Prof. Yehia ElShazly

387. Prof. Yehia ElShazly

388. ANODICPROTECTION
•Formation of protective film on metals by externally
applied anodic current.
•Carefully applied anodic currents to metals showing
active passive transition, they are passivated and the
rate of metal dissolution is decreased.
•A potentiostat is required to maintain the metal at a
constant potential with respect to a reference electrode.
•In particular on mild or stainless steel equipment use for
concentrated sulphuric acid storage.
•This method, however, requires careful monitoring and
control otherwise it may instead hasten the corrosion
process.
Prof. Yehia ElShazly

389. Prof. Yehia ElShazly

390. Prof. Yehia ElShazly

391. Prof. Yehia ElShazly

392. COATINGS
•Provide a barrier between the metal and its environment.
•Metal coating: electrodeposition, flame spraying,
cladding, hot dipping and vapor deposition.
•In/Organic coatings: sparying, diffusion, chemical
conversion.
•A complete barrier must be provided, porosity or other
defects can result in accelerated localized attack.
Prof. Yehia ElShazly

393. •Electroplating: metal on the base metal –done in an
electrolytic bath –Zinc, nickel, tin and cadmium are the
most used –Gold, silver and platinum are common.
•Flame spraying: the metal is finely divided as liquid
particles and is blown onto the surface to be protected.
Oxygen, acetylene or propane are commonly used as
the flame.
•Cladding: Rolling two sheets of metal together to
produce a composite sheet.
•Hot dipping: Galvanized steel. (dipping the substrate in a
molten bath of the coat metal.-Zinc, tin, lead and
Aluminium.
Prof. Yehia ElShazly

394. •Vapor deposition: expensive where coat metal vapor is
deposited on the substrate.
•Diffusion: Surface alloying-Heat treatment to cause alloy
formation by diffusion of one metal into the other. Parts
to be coated are packed in solid materials or exposed to
gaseous environments containing the metal that forms
the coating.
•Chemical conversion: Corroding the metal surface to
form an adherent and protective corrosion product.
Anodizing Aluminium by formation of a protective film of
Al
2O
3.
Prof. Yehia ElShazly

395. Organic coatings:
•Paints, Varnishes, Lacquers, and similar coatings.
•Widely used.
•Three main factors:
–Surface preparation: roughening to obtain mechanical
teething and good bonding, as well as removal of dirt,
corrosion products, scale, oils,grease. The best
method is to grit-blast or sand blast or pickling in
chromic acid, scraping, wire brushing, solvent
cleaning, degreasing.
–Selection of primer: Acts as a barrier , may be a rust
inhibitive pigments such as zinc chromate. Good
sealing is important.
–Selection of top coat: Multiple coat to complete
sealing and to close any pinhole in the coat.
Prof. Yehia ElShazly

396. Failure Analysis

397. Failure Analysis in Corrosion
Engineering
•The depth of the analysis into the roots of the failure is the key to
accurately unearthing all of the failure sources.
•Looking at machinery failures one finds that there are:
–Physical Roots:The physical reasons why the parts failed.
–Human Roots:The human errors of omission or commission that resulted
in the physical roots.
–Latent Roots (Management System Weaknesses):The deficiencies in
the management systems or the management approaches that allow the
human errors to continue unchecked.
•The more detailed the analysis, the better we understand all the events
and mechanisms that contribute as the roots of the problem.
Prof. Yehia ElShazly

398. •The analysis is divided into three categories in order of complexity
and depth of investigation and they are:
–Component Failure Analysis (CFA), which looks at the piece of the
machine that failed, for example, a bearing or a gear, and determines
that it resulted from a specific cause such as fatigue or overload or
corrosion and that there were these x, y, and z influences.
–Root Cause Investigation (RCI)is conducted in much greater depth
than the CFA and goes substantially beyond the physical root of a
problem to find the human errors involved. It stops at the major human
causes and doesn't involve management system deficiencies. RCI's are
generally confined to a single operating unit.
–Root Cause Analyses (RCA)which includes everything the RCI covers
plus the minor human error causes and, more importantly, the
management system problems that allow the human errors and other
system weaknesses to exist. An RCA can sometimes extend to sites
other than the one involved in the original problem.
Prof. Yehia ElShazly

399. •Although the cost increases as the analyses become more complex
the benefit is that there is a much more complete recognition of the
true origins.
•Using a CFA to solve the causes of a component failure answers
why that specific part or machine failed and can be used to prevent
similar future failures.
•Progressing to an RCI; the cost is five to ten times that of a CFA but
the RCI adds a detailed understanding of the human errors
contributing to the breakdown and can be used to eliminate groups
of similar problems in the future.
•However conducting an RCA and correcting the major roots will
eliminate huge classes of problems.
Prof. Yehia ElShazly

400. How to Conduct a Failure Analysis
•A failure analysis is much like the work of a detective.
•Important clues are discovered throughout the investigation
that provides insight into what may have caused the failure
and what contributing factors may have been involved.
•The failure analyst is aided by a broad knowledge of materials
in general.
•Success is more likely if the analyst is aware of the failed
material’s mechanical and physical properties and its
fabrication and historical performance characteristics.
•The analyst must also possess a working knowledge of
structural design and stress behavior.
Prof. Yehia ElShazly

401. •A component is considered to have failed when it has deteriorated to the
point at which it is unsafe or only marginally capable of performing its
intended function.
•For an item to be classified as a failure it need not be completely broken.
•Fractures occur in materials when cracks are initiated and propagate to a
greater or lesser degree. They may not go to completion. Cracks may be
initiated by mechanical stresses or environmental-or chemical-influences,
by the effects of heat, by impurities in the material or by a combination of
these and many other factors. Understanding the relative importance of
those factors in the specific case at hand is the job of the failure analyst.
Prof. Yehia ElShazly

402. How to Conduct a Failure Analysis ?
•Step one:Determine when, where and how the failure occurred
•Step two:Collect samples for laboratory examination
•Step three:Take on-site photographs
•Step Four:Visually examine the sample
•Step five:Identify defects Non-Destructively
•Step six:Conduct appropriate chemical analysis
•Step seven:Confirm material composition and identify
contaminants through EDS analysis
•Step eight:Analyze via Fractography
•Step nine:Analyze via Metallography
•Step ten:Conduct Appropriate Mechanical and Materials
Testing and Analysis
Prof. Yehia ElShazly

403. Determine when, where and how the failure
occurred
•It is important to visit the failure site in the field if possible.
•All operators involved in the failure should be interviewed personally.
•Determine what the conditions were at the time of failure.
•Were there prior indications suggesting failure was about to occur?
•Was the failure gradual or catastrophic?
•Was the part protected after failure? How was the fracture handled?
•Did the failure involve any fire or other condition which could have altered
the microstructure of the base metal or of some part of the sample such as
a weld?
•It may be important to obtain documentation on maintenance procedures
during the lifetime of the equipment that failed including, if applicable,
maintenance personnel, records of scheduled maintenance, and suppliers
and products used.
•As a part of this preliminary information gathering, it is also important to
obtain the physical and chemical specifications for the product which failed,
against which performance may be measured.
Prof. Yehia ElShazly

404. Collect samples for laboratory examination
•Samples selected should be characteristic of the material and contain a
representation of the failure or corrosive attack.
•For comparative purposes, a sample should also be taken from a sound
and normal section.
•Sampling handling is a paramount issue on which the whole remaining
analysis depends.
•Fracture surfaces must be protected from damage during shipment by
rigorously careful packaging.
•Surfaces should not be touched, cleaned or put back together.
•Surface chemistry must not be contaminated by careless handling.
•Materials specifications and service history reveal much about the nature of
failure. If submitting a sample for analysis background information will need
to be provided.
•Samples can be removed by acetylene torch, air-arc, saw, trepan, or drill. All
cuts with an acetylene torch should be made at least six inches and cuts by
air-arc at least four inches away from the area to be examined to avoid
altering the microstructure or obscuring corrosive attack.
Prof. Yehia ElShazly

405. Collect samples for laboratory examination 2
•Samples can be removed by acetylene torch, air-arc, saw, trepan, or drill. All
cuts with an acetylene torch should be made at least six inches and cuts by
air-arc at least four inches away from the area to be examined to avoid
altering the microstructure or obscuring corrosive attack.
•If pipe failures are involved, careful observation of the pipe conditions is
important both prior to sample removal and as the cut separates the two
ends of the pipe, as those may indicate stress conditions in the pipe at the
time of failure.
•All of these characteristics should be noted and documented
photographically.
•Be careful to include in the samples any failure-related materials such as
coatings, soils in which a pipe may have been buried, corrosion deposits,
waters, etc.
Prof. Yehia ElShazly

406. Collect samples for laboratory examination 3
•It is vital to prevent liquid samples from going septic:
–If bacterial content is a potentially important issue the samples must be
taken in clean containers, refrigerated and delivered to microbiological
labs for culturing within 24 hours.
–If bacterial content is irrelevant to the study, then two drops of
household bleach per quart of sample will sterilize the contents. Note
that the bleach addition will change the sodium and chlorine contents of
the samples. A detailed knowledge of the final purpose for the samples
has to control how they are to be handled.
Prof. Yehia ElShazly

407. On-site failure photographs
•Photographs should be taken of the failed piece of equipment including the
samples to be removed and their surroundings.
•These should show the relationship of the questioned area to the remainder
of the piece of equipment.
•Additional photos should be taken of the samples after removal to fully
identify them.
•If more than one sample is to be taken, proper designation of the sample
and its location relative to the piece of equipment should be noted. The
dimensions of the sample, the date the failure occurred, and the date of the
photographs should be noted.
•Consider the use of video recording if complex disassembly is required.
Prof. Yehia ElShazly

408. Visual examination
Prof. Yehia ElShazly
•Visually examine the sample. Examine the sample with unaided eye,
hand lens and/or low magnification field microscopes. Note the
condition of the accessible surface documenting all sorts of
anomalies, searching for cracks, corrosion damage, the presence of
foreign material, erosion or wear damage, or evidence of impact or
other distress. Also consider the condition of protective coatings.
Manufacturing defects are important.
•If pipe failure is involved, it is important to carefully measure wall
thicknesses both at the failure site and some distance away from it
at four locations 90 degrees apart around the pipe circumference,
starting a the failure site. At the same time note the presence of any
corrosion and map its general distribution.

409. Non-destructive examination
Prof. Yehia ElShazly
•Search for material imperfections with anon-destructive
techniquesuch as radiography, magnetic particle, ultrasonic,
liquid/dye penetrant, eddy current, leak, and/or acoustic
emissions non-destructive testing procedures.

410. Chemical analysis
Prof. Yehia ElShazly
•Chemical analysisshould be conducted on the original material to
determine if the material was of proper type and grade, whether it
met appropriate standards, and whether deviation from the
specifications contributed to the fracture, wear, breaks corrosion and
failure.
•Wet chemical analysis, Atomic Absorption, X-ray Photoelectron,
Auger Electron and Secondary Ion Mass Spectroscopy are all
potentially suitable methods of chemical analysis, depending on the
particular need of the situation. The techniques differ in important
ways. Other parts of the failure “system” may also require analysis,
including corrosion products, coatings and liquids.

411. Material composition and contaminants
Prof. Yehia ElShazly
•Confirm material composition and identify contaminants
through EDS analysis.
•EDS (Energy-Dispersive Spectroscopy) is an analytical
method based on the differences in energy of the
characteristic x-rays emitted by the various elements.
•It is used in conjunction with scanning electron microscopy
(SEM) to identify the elements present at a particular spot on
a sample.
•Advantages of EDS are that it is easily performed and is
reliable as a qualitative method. Limitations are that it is only
marginally useful as a quantitative method.

412. Prof. Yehia ElShazly

413. Fractography
•Fractography is used to determine
–the mode of fracture (intergranular, cleavage, or shear),
–the origin of fracture,
–location and nature of flaws that may have initiated failure.
•With this information, the answer as to why a part failed can
usually be determined.
•The major use of fractography is to reveal the relationship
between physical and mechanical processes involved in the
fracture mechanism.
•The size of fracture characteristics range from gross features,
easily seen with the unaided eye, down to minute features just
a few micrometers across.
Prof. Yehia ElShazly

414. Fractography 2
•Light and electron microscopy are the two more common techniques
used in fractography.
•An important advantage of electron microscopy over conventional
light microscopy is that the depth of field in the SEM is much higher.
•The texture of a fracture surface, that is, the roughness and the
color, gives a good indication of the interactions between the
fracture path and the microstructure of the alloy. For instance, at low
stress a fatigue fracture is typically silky and smooth in appearance.
Stress corrosion fractures show extensive corrosion features and
corrosion “beach marks.” A discontinuous ductile fracture shows
some stages of crack tip blunting, crack arrest and "pop-in".
Prof. Yehia ElShazly

415. Prof. Yehia ElShazly
Crankshaft fatigue fracture
Fatigue striation in austenitic
stainless steel

416. Prof. Yehia ElShazly
Reversed bending fatigue initiating at the root of a stainless steel bolt.

417. Metallography
•Prepare a laboratory specimen with care not to remove inclusions,
erode grain boundaries or compromise the sample in some other
way.
•Study structural characteristics in relation to its physical and
mechanical properties at low and high magnification.
•Take careful note of grain size, shape, and distribution of secondary
phases and nonmetallic inclusions.
•Segregation and other heterogeneous conditions also influence the
mechanical properties and behavior characteristics of metal.
•Metallography for the analyst may be concerned with pit depth,
intergranular corrosion, hydrogen attack and embrittlement, caustic
embrittlement, stress corrosion cracking (intergranular or
transgranular), and corrosion, mechanical or thermal fatigue.
•Also, within limits, an almost complete history of the mechanical and
thermal treatment received by a metal is reflected in its
microstructure.
Prof. Yehia ElShazly

418. Physical Testing
•It may be necessary to conduct physical tests to determine if the
mechanical properties of the materials involved conform to
specifications.
•Hardness, tensile strength, impact, fatigue resistance, wear,
flexibility and many other physical tests are relatively common.
These tests often compare the material in the failed component with
standards.
•Test specimens for determination of mechanical properties should
not be taken from areas of the component that have been plastically
deformed during the failure.
•In general, structural members and machine parts can fail to perform
their intended functions by:
–excessive elastic deformation (deflection under applied loads),
–yielding (permanent material deformation as a result of stress), or
–fracture.
Prof. Yehia ElShazly