Corrosion-Basics for beginners and professionals

Basics on corrosion

1. Basics of Corrosion
M. G. Pujar
CSTG, IGCAR

2. Performance and Failures of Industrial Structures
 Corrosion
Uniform, Localized
 Wear
Adhesive Wear, Abrasive
Wear, Corrosive Wear,
Fretting Wear
 Fracture
Ductile, Brittle
 Fatigue
Low Cycle, High Cycle,
Fretting

3. Component Failure FrequencyComponent Failure Frequency
Corrosion occurs in various forms in different materials
depending on the conditions of environment

4. What is corrosion?
Corrosion is the deterioration of materials/
components due to interaction with the environment.
The interaction can be of the following types :
• PhysicalPhysical : Metals in liquid metals (Stainless
Steels in liquid sodium)
• ChemicalChemical : Iron in high temperature air
• ElectrochemicalElectrochemical : Iron in water

5. Common examples
•Rusting of iron and steel
(Rusted iron bolt)
•Pitting and cracking in
stainless steel components
•Wall thinning of metallic pipes
and tubes due to flowing liquids
– erosion corrosion
•Green patina formation over
copper & bronze objects

6. •Loss of aesthetic value
•Wall thinning in components, even up
to unsafe levels
•Leaking of process fluids
•Loss of function, property or failure of
components
•Economic loss
•Injury or loss of life
•Environmental damage
Consequences of Corrosion

7. Why Metals and Alloys Corrode ?Why Metals and Alloys Corrode ?

8. Why does corrosion take place ?
•Metals exist in nature as ores –stable
compounds such as oxides, carbides, sulphides
etc.
•Metals are in a higher energy state compared to
their compounds (ores) – Since energy has to be
spent for extracting metals from ores
•Metals thus react spontaneously with the
environment to revert back to the stable
compounds

9. Material
Composition
Heat treatment
Microstructure
Surface & Stress
Interfaces
Composition
Structure
Stability of film
Diffusion of ions
Environment
Composition
Temperature
pH, Impurities
Flow rate
Corrosion
Corrosion is interdisciplinary in nature; synergistic influence of various
parameters of material, environment and their interfaces play a key role
in the failure of components due to corrosion

10. Chemical DifferencesChemical Differences
Segregation of elements
Elemental partitioning
Physical DifferencesPhysical Differences
Variable thickness of oxide film
Metallurgical factors
Grain boundaries
Point and line defects
Grain orientation
Localised sresses
Fabrication ProcessesFabrication Processes
Bending, rolling, threading, stamping, welding, drilling
Heterogeneities

11. How does corrosion take place?
Four essential requirements Four essential requirements
for corrosionfor corrosion
•Anode - Metal oxidation takes
place releasing electrons
•Cathode – electrons are
consumed to form reduced
species
•Electrical contact between
anode and cathode for electron
transport
•Electrolyte for ionic transport
between the electrodes
Chemical or Electrochemical Mechanism

12. Many corrosion processes are electrochemical in nature and the basic
conditions for corrosion by electrolytes is the occurrence of reduction
reaction
An oxidation reaction, which occurs at a location called as
‘anode’, is
M M  M M
n+n+
+ ne + ne
--

Fe Fe  Fe Fe
2+2+
+ 2e + 2e
--

The electrons released during this reaction are consumed at a different
location called as ‘cathode’. Examples of reactions at the cathode are
2H 2H
+ +
+ 2e+ 2e
- -
 H H
22 (acidic, without oxygen) (acidic, without oxygen)
O O
22 + 4H + 4H
++
+ 4e + 4e
- -
 2H 2H
22O (acidic, with oxygen) O (acidic, with oxygen)
2H 2H
22O + OO + O
2 2 + 4e+ 4e
- -
 4(OH) 4(OH)
--
(basic, with oxygen) (basic, with oxygen)
2H2H
22O + 2eO + 2e
- -
 2(OH) 2(OH)
--
+ H + H
22 (basic, without oxygen) (basic, without oxygen)
M M
+ +
+ e+ e
- -
 M (impurity ions) M (impurity ions)
M M
n+n+
+ e + e
- -
 M M
(n-1)+(n-1)+
(impurity ions) (impurity ions)
The Electrochemical Nature of Corrosion

13. Electrochemistry of Aqueous Corrosion
Corrosion occurs by metal dissolution (oxidation) at anode;
electron released thereby is consumed (reduction) by ions at
cathode;
During corrosion anodic current  cathodic current

14. El
ectrode potential
When a Metal is immersed in solution, it acquires either +ve
or –ve charge w.r. t. the solution.
A definite potential difference is generated between metal
and the solution
The potential difference is called as Electrode Potential.
M(s) M
n+
(aq) + ne


Results in –ve Electrode

Potential, whereas,
M
n+
(aq) + ne

M(s)
Results in +ve Electrode
Potential

15. Electrode Potential
Standard Electrode Potentials (Electromotive or emf Seriesemf Series)
Mg
2+
(aq) + 2e  Mg (s) -2.34V
Zn
2+
(aq) + 2e  Zn (s) -0.76 V
Fe
2+
(aq) + 2e  Fe (s) -0.44V
Cu
2+
(aq) + 2e  Cu (s) 0.34 V
These potentials were measured against Standard
Hydrogen Electrode (SHE)
For an electrochemical reaction, Ox + ne-  Red
Nernst equation,
Free Energy Change ΔG = -nFE
Corrosion proceeds when ΔG is negative
Ox
d
nF
RT
EE
o Re
ln

16. Environmental factors affecting
corrosion rate
•pH
•Oxygen content
•Flow rate
•Temperature
•Conductivity
•Nature and concentration of various
constituents in the medium, both ionic and
non ionic

17. Manifestations of Corrosion
Uniform CorrosionUniform Corrosion
Galvanic CorrosionGalvanic Corrosion
Selective Leaching / DealloyingSelective Leaching / Dealloying
Localised corrosionLocalised corrosion
Pitting CorrosionPitting Corrosion
Crevice CorrosionCrevice Corrosion
Intergranular CorrosionIntergranular Corrosion
Environment sensitive crackingEnvironment sensitive cracking
Stress Corrosion CrackingStress Corrosion Cracking
Hydrogen EmbrittlementHydrogen Embrittlement
Corrosion FatigueCorrosion Fatigue
Fretting CorrosionFretting Corrosion
Erosion CorrosionErosion Corrosion
Microbiologically Influenced CorrosionMicrobiologically Influenced Corrosion
High Temperature CorrosionHigh Temperature Corrosion

18. Uniform corrosion can be described as corrosion
reaction that takes place uniformly all over the
surface of the material, thereby causing a general
thinning of the component and an eventual failure of
the material.
Factors affecting
general corrosion
- Moisture
- Temperature
- Contaminants
- Corrosion Control
Uniform Corrosion of
Carbon Steel Manhole
Uniform Corrosion

19. The most commonly observed corrosion of
metals and alloys is uniform or general
corrosion, which is characterized by uniform
thinning and corrosion attack of the surface.
Uniform Corrosion

20. Corrosion Rate
Rate of uniform corrosion is expressed as loss of Rate of uniform corrosion is expressed as loss of
thickness per unit timethickness per unit time
Corrosion rate = where k is constant, W is the
weight loss, D is density, A is area of the specimen
and T is the time of exposure to the corrosive
medium
•mpy (mils per year)  1 mil = inch
•µm/year
•mdd ( milligram/dm
2
/day) – weight loss per unit area
per unit time
1000
1
DAT
kW

21. Pitting Corrosion Crevice Corrosion
Intergranular CorrosionStress Corrosion Cracking
Manifestation of Localised Corrosion

22. 1.Pitting is a localized form of corrosion attack which
produces pits. It may cause perforation of equipment.
2.It is a highly localized corrosion occurring on a metal
surface. Pitting is commonly observed on surfaces with
little or no general corrosion.
3.Pitting typically occurs as a process of local anodic
dissolution where metal loss is accelerated by the
presence of a small anode and a large cathode.
4.Pits are initiated at inclusions, second phases and regions
of compositional heterogeneities.
Pitting corrosion

23. Pitting Corrosion

24. Mechanism of Mechanism of
Pitting Pitting
CorrosionCorrosion
Pits Grow by Pits Grow by
“Autocatalytic “Autocatalytic
Mechanism “ Shown Mechanism “ Shown
AsideAside
METAL
Cathodic
Anodic
Oxygen poor at
base of crack

25. (a)
(a) Crevice corrosion of type 316 stainless steel in acid
condensate under a PTFE spacer; (b) typical schematic
morphology with attack greatest at the mouth of the crevice.
Crevice corrosion is localized corrosion which
might occur in small areas (lesser than mm
2

area) of stagnant solution in crevices, joints
and under corrosion deposits (metal/metal or
metal/non-metal).
Crevice CorrosionCrevice Corrosion
(b)

26. Crevice Corrosion - Prevention or Control
Redesign of equipment to eliminate crevices.
Close crevices with non-absorbent materials or
incorporate a barrier to prevent moisture
penetration into crevice.
Prevent or remove builds-up of scale or other solids
on surface of material .
Use of one-piece or welded construction instead of
bolting or riveting .
Select more corrosion-resistant or inert alloy

27. Active – Passive Metals
Iron, Chromium, Nickel, Cobalt and their alloys Iron, Chromium, Nickel, Cobalt and their alloys
show active-passive behaviourshow active-passive behaviour

28. Galvanic Corrosion
•Galvanic corrosion takes place when two dissimilar metals are
electrically connected in an electrolyte
•Results from a difference in redox potentials of metallic ions
between two or more metals. The greater the difference in
redox potential, the greater the galvanic corrosion.
•The less noble metal will corrode (i.e. will act as the anode)
and the more noble metal will not corrode (acts as cathode).
•Perhaps the best known of all corrosion types is galvanic
corrosion, which occurs at the contact point of two metals or
alloys with different electrode potentials.
•The extent of galvanic corrosion depends on the area ratio of The extent of galvanic corrosion depends on the area ratio of
two metals, cathodic efficiency of noble metal and difference two metals, cathodic efficiency of noble metal and difference
in corrosion potentials of two uncoupled metals.in corrosion potentials of two uncoupled metals.

29. Engineering aspect -
Galvanic corrosion
•Galvanic series obtained
for various materials in
flowing seawater (2.5-4
m/s) at temperatures in
the range 10-26C.
•Dark boxes indicate
active behavior for active-
passive alloys.
NOBLE END
Titanium
Monel
Passive Stainless Steel
Inconel Alloy
Nickel
Copper-Nickel (70/30)
Copper-Nickel (90/10)
Aluminium Bronze
Copper
Alpha Brass (70Cu-30Zn)
Aluminium Brass
Muntz Metal (60Cu-40Zn)
Tin
Lead
Active Stainless Steel
Cast Iron
Mild Steel
Aluminium
Zinc
Magnesium
ACTIVE END

30. Big Cathode, Small Anode = Big TroubleBig Cathode, Small Anode = Big Trouble
AnodeAnode
CathodeCathode

31. Steps to be taken to prevent Galvanic Corrosion:Steps to be taken to prevent Galvanic Corrosion:
- Selection of alloys which are similar in
electrochemical behavior and/or alloy content.
- Area ratio of more actively corroding material
(anode) should be large relative to the more inert
material (cathode).
- Use coatings to limit cathode area.
- Insulate dissimilar metals.
- Use of effective corrosion inhibitor.
Galvanic Corrosion - Prevention or Control

32. Definition: Stress corrosion cracking requires presence of
tensile stress, either residual, applied or a combination
of both, and the presence of a Specific Corrodent.
Cracks propagate at right angle to the direction of the
tensile stress at stress levels much lower than those
required to fracture the material in the absence of the
corrodent.
Stress Corrosion Cracking (SCC)

33. Stress corrosion Cracking
Stress corrosion cracking (SCC) is
caused by the simultaneous effects
of tensile stress and a specific
corrosive environment. Stresses may
be due to applied loads, residual
stresses from the manufacturing
process, or a combination of both.
Cross sections of SCC frequently
show branched cracks. This river
branching pattern is unique to SCC
and is used in failure analysis to
identify when this form of corrosion
has occurred.

34. Stress Corrosion Cracking
Unfortunate Facts of SCC
Cracking takes place in
otherwise best alloys
Passive alloys resistant
to general corrosion
High strength steels
prone to cracking
Cracking at RT
in pure water
Material-Environment for SCC
Material Environment
Austenitic SS
(annealed)
Cl
-
, OH
-
, HTHP Water
Austenitic SS
(sensitised)
HTHP Water, dilute Cl
-
,
OH
-
, F
-
, polythionic acid
Carbon & Low
Alloy Steel
OH
-
, NO
3
-
, CO
3
-
& HCO
3
-

solutions, anhydrous NH
3
High Strength Low
Alloy Steel
Water, moist air, Cl
-
& Cl
-
-
H
2
S
Ti alloys Red fuming HNO
3, hot
molten salts, methanol, salt
water

35. Corrosion Fatigue
1.“Fatigue” is described as the cracking of a metal
under repeated cyclic stress.
2.Fatigue cracks initiate and propagate at stresses
below the yield strength after numerous cyclic
applications of stress.
3.Presence of a corrosive environment can accelerate
the initiation and propagation of fatigue cracks.
This process is known as “Corrosion Fatigue”.
4.The corrosion fatigue crack path is usually
transgranular, but it can become intergranular in
sensitized Type 304 Stainless Steel.

36. •Synergistic interaction of cyclic plastic
deformation and environment
• Both crack initiation and propagation
are influenced
• Unlike SCC no specific corrosive
environment / materials combination is
necessary
Fatigue life data, S-N curves, for a high-
strength steel under different environmental
conditions
Corrosion Fatigue