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Corrosion full ppt

1. Elctrometallurgy& Corrosion
Syllabus:-
Principles: Faradays’ laws of electrolysis, current efficiency, current density,
electrode potentials, EMF series, Galvanic series,
Nernst Equation, Polarization, Mixed potential theory, Pourbaixdiagrams, passivity-
theory &applications., electrochemical methods of protection.
Electro-deposition: Classification and mechanism of electro-deposition processes.
Electroplating of copper, Nickel and Chromium. Alloy plating and electrolessplating.
Corrosion: The relevance of corrosion studies, forms of corrosion,Uniform,
Galvanic, Crevice, Pitting, intergranular, stress corrosion cracking, corrosion fatigue,
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Galvanic, Crevice, Pitting, intergranular, stress corrosion cracking, corrosion fatigue,
hydrogen embrittlement, Dealloying.
Corrosion prevention and control by various methods-change of metal
composition, design improvement, inhibitors, coatings and electrochemical
methods of protection.
Books:
1. Corrosion Engineering by Mars G. Fontana.
2. Electroplating by Lowenheim
3. Fundamentals of Corrosion, CRC press
4. Principles of corrosion Engineering & Corrosion control, ZakiAhmad, Elsevier

2. Elctrometallurgy& Corrosion
Corrosion????
Deterioration of materials in presence of chemical environment leading to a
loss in their function.
One of the mode of material failure.
Corrosion is the deterioration of materials as a result of reaction with its
environment (Fontana).
Corrosion is the destructive attack of a metal by chemical or electrochemical
reaction with the environment (Uhlig).
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reaction with the environment (Uhlig).
Corrosion is an extractive metallurgy in reverse.

3. Elctrometallurgy& Corrosion
Corrosion????
AsperIUPAC,“Corrosionisanirreversibleinterfacialreactionofamaterial
(metal,ceramic,polymer)withitsenvironmentwhichresultsinitsconsumption
ordissolutionintothematerialofacomponentoftheenvironment.Often,but
notnecessarily,corrosionresultsineffectsdetrimentaltotheusageofthe
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notnecessarily,corrosionresultsineffectsdetrimentaltotheusageofthe
materialconsidered.Exclusivelyphysicalormechanicalprocessessuchas
meltingandevaporation,abrasionormechanicalfracturearenotincludedinthe
termcorrosion”

4. Elctrometallurgy& Corrosion
Corrosion can be classified in different ways, such as
Chemical and electrochemical
High temperature and low temperature
Wet corrosion and dry corrosion.
Wet corrosion occurs when a liquid is present. Involves usually aqueous solutions
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Wet corrosion occurs when a liquid is present. Involves usually aqueous solutions
or electrolytes. Ex. Corrosion of steel in water. Whereas Dry corrosion occurs in the
absence of aqueous environment or above the dew point the environment, usually in
the presence of gases and vapours, mainly at high temperatures.
The presence of small amounts of moisture could change the corrosion chemistry
at all.

5. Elctrometallurgy& Corrosion
CorrosionEngineering:theapplicationofscienceandarttopreventorcontrol
corrosiondamageeconomicallyandsafely.
Environments:practicallyallenvironmentarecorrosivetosomedegree.Ex.Airand
moisture,fresh,distilled,saltandminewaters,rural,urbanandindustrialatmosphere..
However,corrosionisbeneficialordesirableinsomecases.ExAnodizing,chemical
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However,corrosionisbeneficialordesirableinsomecases.ExAnodizing,chemical
machiningormilling,

6. Corrosion Principles
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7. Corrosion Principles:-Electrochemical Aspects
----------------------
----------------------
----------------------
----------------------
H
+
Cl
-
Zn
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During metallic corrosion,
rate of anodic reaction r
anodic
= rate of cathodic reaction r
cathodic

8. Corrosion Principles:-Electrochemical Aspects
Like Zn, Iron and Aluminiumare also corroded by hydrochloric acid. The
reactions are:-
Fe + 2HCl = FeCl
2
+ H
2
Al + 3HCl = AlCl
3
+ H
2
All anodic reactions looks similar. This can be written in general form of anodic
or oxidation reaction:
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9. Corrosion Principles:-Electrochemical Aspects
Case 2:-If Zinc is present in aerated HCl solution
Anodic reaction : (Zn Zn
++
+ 2e) ×3
Cathodic reactions are : 2H
+
+ 2e H
2
O
2
+ 4H
+
+ 4e 2H
2
O
3Zn + 6H
+
+ O
2
= 3Zn
++
+ 2H
2
O + H
2
Case 3: If any oxidizer like FeCl
3
or Cucl
2
is present in aerated acidic solution
Anodic reaction : (Zn Zn
++
+ 2e) ×4
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Anodic reaction : (Zn Zn
++
+ 2e) ×4
Cathodic reactions are : 2H
+
+ 2e H
2
O
2
+ 4H
+
+ 4e 2H
2
O
(Fe
+++
+ e Fe
+2
)×2
4Zn +6H
+
+ O
2
+ 2 Fe
+++
= 4Zn
++
+ 2H
2
O + H
2
+ 2 Fe
+2

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12. Corrosion Principles:-Electrochemical Aspects
Formation of Rust on Iron surface: Consider Iron is immersed in water or seawater
which is exposed to environment. Corrosion occurs.
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Ferrous hydroxide in unstable in oxygenated solution and it will convert to ferric
salt which is rust.
2Fe(OH)
2
+ H
2
O + ½ O
2
= 2Fe(OH)
3
or Fe
2
O
3
. X H
2
O
Rust
NOTE:-Iron will not corrode in air free water or sea water because there is no
cathodic reaction.

13. Modern Approach : Thermodynamic Criteria
General Question come in mind when we think of corrosion:---
Can we predict corrosion?
What about the rate of corrosion?
Free Energy Change ΔG is a direct measure of spontaneity of reaction.
Mg + H
2
O + ½ O
2
= Mg(OH)
2
ΔG = -142.6 Kcals
Au + 3/2 H
2
o + 3/2 O
2
= Au(OH)
3
ΔG = + 157 Kcals
NOTE:-But it is difficult to measure ΔG for all reactions practically.
1
Δ
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1
2A
2B
ΔG change in free energy
Both 2A and 2B are at same energy level and spontaneous process but from 1 to 2A, it
takes less time than that of 2B. Thermodynamics does not explain about velocity of
reaction. It only reflects the direction of reaction by its sign, not velocity of reaction.

14. This reaction is electrochemical in nature as there is involvement of electrons here.
Modern Approach : Electrochemical Criteria
So Think other way around about its nature.
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Relation between Free energy and Electromotive force of an electrochemical reaction is:-
ΔG = -nFEwhere, n = no. of electrons, F = Faraday’s constant & E = Electrode potential
Equilibrium Potential
Fe
------------------------
------------------------
------------------------
------------------------
Fe
++
Fe
Fe
++
r
1
r
2
e
e

15. Equilibrium Potential
r
1
Fe Fe
++
(Oxidation or anodic reaction)
r
2
Fe
++
Fe (reduction or cathodicreaction)
At Equilibrium potential, r
1
= r
2
Equilibrium is established at the metal/solution interface and doing so it is
called equilibrium potential.
So, for the reaction Fe
++
Fe
Relation between free energy and equilibrium constant
ΔG = ΔG
0
+ RT ln K where ΔG
0
= Standard free energy (ΔG)
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K = equilibrium constant = a
products
/ a
reactants
ΔG = ΔG
0
+ RT lna
products
/ a
reactants
ΔG = ΔG
0
+ RT lna
Fe
/a
Fe++
a
2
e
E = E
0
–(RT/nF) lna
Fe
/a
Fe++
a
2
e
At standard condition, activities = unity
E = E
0
(Standard Potential or equilibrium potential)

16. Equilibrium Potential
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17. Equilibrium Potential
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18. How to measure Standard Potential of an Electrode
M
V
Metal Electrode
Probe
M M
n+
M
n+
M
------------------------------
---------- -------
-------------------------------
-------------------------------
------------------------------------
-------------------------------------
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New potential developed between
probe and solution
Potential between
electrode and solution
ΔV
ΔV = Relative potential of an metal M electrode w.r.t.probe or reference electrode.

19. Standard Potential of Zinc Electrode
E
0
H
2
= 0.0V
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20. Standard Potential of Zinc Electrode
Anelectrodepotential
isdefinedasthe
potentialofacellin
whichtheelectrodein
questionistheright-
handelectrodeand
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thestandardhydrogen
electrodeistheleft-
handelectrode.

21. Reference Electrode
There are many reference electrode:-
Hydrogen Electrode
Saturated Calomel Electrode Hg/Hg2Cl2
Potential = +0.2444 V w.r.t.SHE
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Saturated Silver Electrode Ag/AgCl(Cl-is saturated)
Potential = +0.197 V w.r.t.SHE
Saturated Copper Electrode Cu/CuSO4

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23. How to measure Standard Potential of an Electrode
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26. Tutorial -01
The half cell reactions(either anodic or cathodic reactions):-
Zn
++
+ 2e = Zn E
0
= -0.763 V
Cu
++
+ 2e = Cu E
0
= + 0.334 V
Fe
++
+ 2e = Fe E
0
= -0.440 V
Ag
+
+ e = Ag E
0
= + 0.799 V
++ 0
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2H
++
+ 2e = H
2
E
0
= 0.000V
O
2
+ 4H
+
+ 4e = 2H
2
O E
0
= +1.229V
O
2
+ 2H
2
O + 4e = 4 OH
-
E
0
= +0.40 V
We may understand it by writing as
Zn
++
+ 2e = Zn
OX + ne = R

27. Tutorial -01
ReversibleElectrodePotential:-
Ifametalisimmersedinasolutionofitsownions,suchaszincinZnS0
4
solution,orcopperinCUSO
4
solution,thepotentialobtainediscalled
thereversiblepotential(E
rev
)
StandardElectrodePotential:-Astandardpotentialreferstothepotentialof
puremetalmeasuredwithreferencetoahydrogenreferenceelectrode.
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Zn
++
+ 2e = Zn E
0
= -0.763 V

28. Tutorial -01
How to Calculate the half cell potential of zinc electrode under non standard
condition? Zn
++
+ 2e = Zn E
0
= -0.763 V
OX + ne = R
ΔG = ΔG
0
+ RT ln K where ΔG
0
= Standard free energy
K = equilibrium constant = a
products
/ a
reactants
ΔG = ΔG
0
+ RT ln a
products
/ a
reactants
ΔG = ΔG
0
+ RT ln a
Zn
/a
Zn
++
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ΔG = ΔG+ RT ln a
Zn
/a
Zn
++
E = E
0
–(RT/nF) ln a
Zn
/a
Zn
++
E = E
0
+ (RT/nF) ln a
Zn
++
/a
Zn
E = E
0
+ (RT/nF) ln [OX]/[R]

29. Ex. Calculate the reversible potential for a zinc electrode in contact with ZnCl2
when the activity of zinc ion aZn
2+
= 10
-3
, Use IUPAC convention.
Tutorial -01
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30. Tutorial -01
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31. Tutorial -01
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32. Tutorial -01
Ex.1.CalculatethepotentialofoxygenelectrodeatpH=14.
Ex.2.CalculatethepressureofH2requiredtostopcorrosionofiron
immersedin0.1MFeCl
2
,ifpH=4.
Ex.3.TheemfofacellmadeofZn(anode)andH
2
electrode(cathode)
immersedin0.7MZnClis+0.690volts.WhatisthepHofthesolution?
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immersedin0.7MZnCl
2
is+0.690volts.WhatisthepHofthesolution?

33. Corrosion Cell
A corrosion cell (a type of Galvanic cell) is essentially comprised of the following
four components
• Anode • Cathode • Electrolyte • Metallic path
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34. Electrochemical Cell
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35. Galvanic Cell
Types of galvanic cells
a.Dissimilar electrode cells
b.Concentration cells
Important concentration cells
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Important concentration cells
•Salt concentration
•Differential aeration (Oxygen Concentration)
•Differential temperature
•Metallographic / mechanical heterogeneities
•Differences in residual stress levels

36. Differential Electrode Cell
Standard Cell Potential or Cell Voltage:-
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39. How to calculate Standard Cell Potential?
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40. How to calculate Standard Cell Potential?
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43. Eh-pH Diagrams –Fundamental Aspects
Eh-pHdiagramsshowingreactionsandproductsatelectrochemicalequilibriumare
oftenreferredtoasPourbaixdiagrams.therearefourregionsinthediagram
correspondingtooxidizing(acidic),oxidizing(alkaline),reducing(acidic)and
reducing(alkaline)environments.
Thebasicdiagramforaqueousenvironmentinvolvesupperandlower,stabilitylimits
forwater,representedbytheoxygen(universaloxidizingagent)andhydrogen
(universalreducingagent)reactions.
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44. Stability of Water
Acid solution with dissolved oxygen.
1.23
-
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E
pH
0 14
0-1.23
-
59 mV
59 mV
Evolution of H
2
H
2
O
stability
Region

45. Eh-pH Diagram
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46. Eh-pH Diagram
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47. Eh-pH Diagram
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48. Eh-pH Diagram
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50. Electrode Kinetics
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51. Electrode Kinetics
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52. Electrode Kinetics
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53. Exchange Current Density(i
0
)
Exchange current density (i
o
) is dependent on
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Exchange current density (i
o
) is dependent on
a)Nature of the redox reaction
b)Electrode composition / surface
c)Concentration ratio of oxidized and reduced species
d)Temperature.

54. Exchange Current Density(i
0
)
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Forexample,ioforH
+
/H
2
reactiononplatinumisabout10
-2
A/cm
2
andformercuryitisabout10
-12
A/cm
2
,whichmeansitiseasierto
reducehydrogenionsfromacidicelectrolyteonaplatinum
electrodeunlikeonmercury,whichpossessesahighHydrogen-
overvoltage.
Exchangecurrentdensitiesaredeterminedexperimentally.

55. Exchange Current Density(i
0
)
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56. Activation Polarization
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57. Activation Polarization
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58. Activation Polarization
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59. Activation Polarization
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60. Activation Polarization
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61. Polarization
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67. Electrode Kinetics
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68. Electrode Kinetics
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69. Electrode Kinetics
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70. Electrode Kinetics
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71. Electrode Kinetics
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72. Mixed Potentials Theory–Concepts and Basics
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73. Electrode Kinetics
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74. Electrode Kinetics
Sincesuchapolarizedpotentialisamixtureofthetwohalf–cell
potentials,itisreferredtoasMIXEDPOTENTIAL.
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76. Electrode Kinetics
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77. Electrode Kinetics
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78. Electrode Kinetics
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79. Electrode Kinetics
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80. Electrode Kinetics
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81. Electrode Kinetics
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82. Electrode Kinetics
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83. Electrode Kinetics
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To summarize the effect of coupling of zinc to platinum, the following
are the main points of interest:
(a)The rate of hydrogen evolution is decreased on zinc and increased
on platinum.
(b)The rate of oxidation of zinc is increased significantly on coupling
and zinc dissolves vigorously. Nothing happens to platinum.
(c)Platinum is an excellent catalyst for reduction of hydrogen and zinc
is a poor catalyst.

84. Electrode Kinetics
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E
0
Zn
= —0.76V, E
0
Au
= 1.50V & E
0
Pt
= 1.2V gold
i
o
(H on Pt)
= 10
-3
A/cm
2
followed by gold i
o
(H on Au)
= 10
-6
A/cm
2
andi
o
(H on Zn)
=
10
-10
A/cm
2
.

85. Electrode Kinetics
Activation controlled
Diffusion controlled
E
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Log iSuppose any metal M present in sea water.

86. Mixed Potential Theory
Galvanic corrosion (Two Metal Corrosion) rates are influenced by two factors,
namely distance and area effects.
Severity of corrosion is the highest near the junction of the bimetal contacts.
Area effect refers to ratio of anodic to cathodic areas and a larger cathode in
contact with a small anode is considered ‘unfavourablearea ratio’. For a given
current flow in a galvanic cell, the current density is higher for a smaller
electrode than for a larger anode. Higher current density results in larger rates of
anodic corrosion.
Examples demonstrating the area effect: Examples demonstrating the area effect:
a.Copper plates (larger cathodes) connected by steel rivets (smaller anodes)
exposed to sea water.
b.Steel plates (larger anode) connected with copper rivets (smaller cathode)
exposed to sea water.

87. Mixed Potential Theory:-EFFEC T OF AREA RATIO
NOTE:-
ThesmallertheanodetocathoderatioasinthecaseofZncoupled
toPt(10cm
2
),thelargeristhemagnitudeofcorrosion.Avaluablerule:
Avoidasmallanodetocathodearearatiotominimizetheriskofserious
galvaniccorrosion.

88. Mixed Potential Theory:-EFFEC T OF AERATION

89. PASSIVITY
Michael Fraday 1940

90. PASSIVITY
Loss of chemical reactivity experienced by certain metals and alloys
under particular environmental condition.
Iron, Nickel, Chromium, Titanium and alloy of these metals
Zn, Cd, Sn, U and Th under limited condition
The condition of high corrosion resistance due to formation of a
surface film under oxidizing condition with high anodic polarization.

91. PASSIVITION OF IRON IN PRESENCE OF Cr
Sincechromiumiscapableofformingaverystableoxideatmuchlower
potentials,alloyingwithchromium(minimum12%)leadstodevelopmentof
corrosionresistantstainlesssteelsandcastirons.Othermetalsthatcanform
passivesurfacefilmsincludealuminium,silicon,titanium,tantalumandniobium.

92. PASSIVITY
Thepassivefilmsmaybeasthinas2-10nm,andtheyofferalimited
electronicconductivity,andbehavelikesemiconductorswithmetallic
propertiesratherthanthepropertiesshownbybulkoxides.Thefilms
alsoallowalimitedamountofconductivityofcationsbecauseoflattice
defectsandaslowanodicdissolution.

93. PASSIVITY

94. PASSIVITY: Effect of acid concentration & temperature on S -Curve
Similarly, chlorides are detrimental to passivity in case of stainless steel and
other ferrous base alloy.

95. 1-Ti-dilute air free H
2
SO
4
or HCl
(continuously active)
2-Cr in dilute air free H
2
SO
4
or
Fe in air free dilute HNO
3
(unstable system)
3-SS or Tiin acid solution contain
oxidizers (continuously passivating)
1/2 O+ 2H
+
+ 2e = HO1/2 O
2
+ 2H
+
+ 2e = H
2
O
Rate of Oxidation = Rate of Reduction

96. Effect of Oxidizer on MPT
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97. Effect of velocity on Active Metal
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98. Effect of velocity on Active-Passive Metal
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Theeasewithwhichan
activemetalgetting
passivateddependsonthe
valueofcriticalanodic
currentdensity.

99. Control of Passivity
Twogeneralrulescanbeappliedtocontrolpassivity.
Ifcorrosioniscontrolledbyanactivationcontrolreductionprocessanalloywhich
exhibitsaveryactiveprimarypotentialmustbeselected.Conversely,ifthereduction
processisunderdiffusioncontrolanalloywithasmallercriticalcurrentdensitymust
beselected.Thus,ifactivationpolarizationisthecontrollingfactor,alloy2mustbe
selectedandifconcentrationpolarizationisratecontrolling,alloy1mustbeselected.
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100. Control of Passivity
ALLOYINGADDITION
Thosealloyingadditionswhichdecreasei
critical
areeffectiveinincreasingthe
passivatingtendency.Elements,likechromiumandnickel,whichhavealower
i
critical
andE
passive
thaniron,reducetheCritical(criticalcurrentdensity)ofiron.
Additionofupto18%chromiumreducesi
critical
iron.Similarly,additionof
morethan70%nickeltocopperreducesi
critical
anE
passive
.
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101. Environmental Effect on Corrosion Rate
1. Effect of oxygen and oxidizers
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102. Environmental Effect on Corrosion Rate
2. Effect of Temperature
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103. Environmental Effect on Corrosion Rate
3. Effect of Corrosive Concentration
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104. Environmental Effect on Corrosion Rate
4. Effect of Velocity
CR
A
B
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Velocity

105. Measurement of corrosion current
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106. Measurement of corrosion current
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Actual Curve and measured polarization curves for active metals

107. Measurement of corrosion current
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108. Measurement of corrosion current
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109. Cathodic Protection –Principles and Classification
SirHumphreyDavy‘spioneeringwork(1824)onprotectingthecopper
sheathingonwoodenhullsintheBritishNavybysacrificialzincandironanodes
isconsideredtobetheearliestexampleofapplicationofcathodicprotection.
Copper-sheathedshiphullsprotectedbysacrificialblocksofiron.
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Copper-sheathedshiphullsprotectedbysacrificialblocksofiron.
Zincalloyassacrificialanode.
Galvanising–Typicalexampleofsacrificialanodetoprotectsteels

110. Variousdefinitions
Reducingoreliminatingaltogethercorrosionbymakingthemetalacathodeby
applicationofeitheranimpressedDCcurrentorattachingthemetaltoa
sacrificialanode.
Corrosionoccursatanodicareas–ifallanodicareascanbeconvertedto
cathodicareas,theentirestructurewillbecomecathodeandcorrosionis
Cathodic Protection –Principles and Classification
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cathodicareas,theentirestructurewillbecomecathodeandcorrosionis
stopped.
Corrosionoccursattheregionswherecurrentdischargesfrommetalto
environment(soil,water)(anodicareas).Thereisnocorrosionatregionswhere
currententersfromtheenvironmenttometal(cathodicareas).
Objectiveshouldthenbetoforcetheentirestructuretocollectcurrentfromthe
environment(makingitcathodicentirely).

111. Corrosion Control
Material Selection
Design
Coatings
Inhibitors
Electrochemical Method of Protection
(i) Cathodic Protection & (ii) Anodic Protection
If supply eeeeee
11/23/2020 Department of Metallurgical & Materials Engg. MNITJ 111
Fe
Fe
+2
+ 2e
OX, H+
R, H
e + OX = R

112. Cathodic Protection
Aspermixedpotentialtheory,thezerocurrentcriterionisshown.An
equilibriumisestablishedonmetal(M)inwhichanodicoxidationrateis
equaltocathodicreductionrate[E
corr
andi
corr
(A)].Bycathodicpolarizationof
themetalwithanappliedDCcurrent(i
app
),initialcorrosionpotentialisseen
shiftedtoalowervalue[i
corr
(B)].Completestoppageofcorrosion,requires
polarizationofthemetaltothereversiblepotentialofthemetal(E
o
M
).
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113. Corrosion Control:-Cathodic Protection
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i
app
= i
a
–i
c
to completely protect

114. Driving force for corrosion is the potential difference.
Current flow depends on factors such as:
a. Resistivity of environment and
b. Degree of polarization of anode and cathodic areas.
Cathodic protection is achieved by supplying electrons to the structure being
protected.
Cathodic Protection –Principles and Classification
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Two methods of cathodic protection
Use of sacrificial anodes or Galvanic Metal Protection
Impressed current method.

115. Impressed current method
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Factors to be considered in the design and execution of cathodic protection
installations. Impressed current system
a) How much current necessary for complete protection?
b) Source of DC Current.
c) Installation, Design, erection and maintenance.
d) Auxiliary anodes –choice, size, number, installation.
e) How to assess elimination of corrosion through entire structure?

116. Corrosion Control:-Impressed current anode system
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117. Use of sacrificial anodes
Requirements of galvanic sacrificial anodes
a.Potential between the anode and the corroding metal structure should be large enough to
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a.Potential between the anode and the corroding metal structure should be large enough to
overcome the anode-cathode cells.
b.Sacrificial anode to have sufficient Electrical Energy Content (EEC) which predicts its life.
c.Good current efficiency relevant to anodic corrosion.
EEC can be estimated and expressed as ampere hours/weight (kg or lb)
Eg: Pure Zinc that possesses high EEC of 372 ampere hour / pound. This means if the zinc
sacrificial anode has to discharge continuously one ampere, on pound of its weight would be
consumed in 372 hours. Lower current discharge will prolong its life further.

118. Corrosion Control:-Cathodic Protection
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Steel Zn
E
0
= -1.1V (SCE)
E
0
= -0.6V (SCE)
E
cell
= -0.6 –(-1.1) = + 0.5 V

119. Cathodic Protection –Influencing Factors and Monitoring
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120. Design Aspects of Cathodic Protection
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121. Design Aspects of Cathodic Protection
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122. Electrode Kinetics
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123. Electrode Kinetics
Current necessary for protection need be just sufficient; neither less nor excess.
Excess current may do harm!
Lower current do not protect!
Design aspects for galvanic anode cathodic protection
Soil resistivity assessment –Site of lowest resistivity to be chosen for location
of anode.
Choice of anode material –Data from commercially available anodes to be
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carefully assessed.

124. Comparison
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125. Stray Current Corrosion
Stray-currenteffectsareencounteredinseveralimpressedcurrentcathodic
protectionsystems.Thisisverycommoninindustrialprotectedsystems,such
asoilproductionindustrieshavinginnumerableburiedpipelines.Current
leakagefromauxiliaryanodesassociatedwithcathodicprotectionsystemscan
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leakagefromauxiliaryanodesassociatedwithcathodicprotectionsystemscan
enterunintentionallytoanear-byunprotectedstructureandleavefromthe
surfacescreatingseverecorrosion.

126. Stray Current Corrosion
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Ifthereisacurrentpathduetoalowresistancemetallicobject(forexample,a
pipelineoranothermetallicstructure),currentleakagefromanimpressed
currentprotectedsystemwillentersuchunprotectedstructurebeforereturningto
theprotectedobject.Regionsfromwherecurrentleavesaresusceptibleto
straycurrentcorrosion.

127. Stray Current Corrosion
Asolutiontosuchaproblemisthroughelectricalbondingofthenear-by
structure.SimultaneouslyadditionalanodesandincreasingDCpowercapacity
canaccordfullprotectiontoallstructuresinthevicinity.Properlyinsulated
couplingscanhelpreducetheproblem
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128. Anodic Protection
Anodic protection refers to prevention of corrosion through impressed anodic
current. This method of protection tested and demonstrated by Edeleanuin 1954.
However can be applied only to metals and alloys that exhibit active-passive
behavior. The interface potential of the structure is increased to passive domain.
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Ifanactive-passivealloysuchasstainlesssteelismaintainedinthepassive
regionthroughanappliedpotential(orcurrent)fromapotentiostat,itsinitial
corrosionrate(i
corr
)canbeshiftedtoalowvalueati
pass
asshowninFigure.

129. Anodic Protection
As per mixed-potential theory,
Applied anodic current density = oxidation current density –reduction current density.
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130. Anodic Protection
Anodicprotectionismoreeffectiveinacidsolutionsthancathodicprotection.Current
requirementsforcathodicprotectioninacidsolutionsareseveralordersofmagnitude
higherthanthatnecessaryforcompleteanodicprotection.Cathodicprotection
currentsinacidsolutioncanalsoleadtohydrogenliberationandembrittlementof
steels.
Anodicprotectionunlikecathodicprotectionisideallysuitedforprotectionofactive-
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Anodicprotectionunlikecathodicprotectionisideallysuitedforprotectionofactive-
passivealloysinaggressiveenvironmentssuchashighacidityandcorrosive
chemicals.Henceanodicprotectionisthemostpreferredchoiceforprotectionof
chemicalprocessequipment.

131. Anodic Protection
Anodic protection parameters include.
a)Protection range –range of potentials in which the metal/alloy exhibits stable
passivity.
b)Critical anodic current density.
c)Fladepotential.
Potentialcorrespondingtomiddleofthepassiveregioncanbetakenasoptimumfor
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anodicprotection.Whilechoosingthedesirableprotectionpotential,anassessment
oftheaggressivenessoftheenvironmentneedbemade.Sincechlorideionsare
detrimentaltopassivity,higherchlorideconcentrationscandecreasetheprotection
range.Metalsandalloyshavingrelativelylargerpittingandprotectionpotentialscan
onlybechosenforveryaggressivechemicalenvironments.Highertemperaturescan
deleteriouslyinfluencetheprotectionpotential.

132. Comparison
AcomparisonbetweenanodicandcathodicprotectionisgiveninTable:-
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133. Corrosion
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134. Electrode Kinetics
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135. It is the uniform thinning of a metal without any localized attack. Corrosion does
not penetrate very deep inside. The most familiar example is
Dissolution of piece of steel or zinc in dilute sulfuric acid at a uniform rate
Tarnishing of silver in dry air in the presence of H
2
S traces
Rusting of steel in air.
At the anodic areas, anodic reaction takes place: Fe -> Fe
++
+ 2e
At the cathodic areas, reduction of oxygen takes place: 0
2
+ 2H
2
0 + 4e -> 40H
-
Uniform Corrosion
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2 2
Fe
2+
+ 20H
-
Fe(OH)
2
With more access to oxygen in the air, Fe(OH)
2
oxidizes to Fe(OH)
3
and later it
loses its water: 4Fe(OH)
2
+ 0
2
+ 2H
2
0 -> 4Fe(OH)
3
Ferrous hydroxide is converted to hydrated ferric oxide or rust by oxygen:
4Fe(OH)
2
+ 0
2
2Fe
2
0
3
•H
2
0 + 2H
2
0

136. Electrode Kinetics
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137. Galvanic Corrosion
Galvaniccorrosionoccurswhentwometalswithdifferentelectrochemical
potentialsorwithdifferenttendenciestocorrodeareinmetal-to-metalcontactina
corrosiveelectrolyte.
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138. Galvanic Corrosion: Dry Cell
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Why it occurs?
Due to different corrosion rate –not the oblivious reason
Due to difference in potential –Basic reason
A B
I

139. Effect of noble metal on corrosion of active metal
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To summarize the effect of coupling of zinc to platinum, the following
are the main points of interest:
(a)The rate of hydrogen evolution is decreased on zinc and increased
on platinum.
(b)The rate of oxidation of zinc is increased significantly on coupling
and zinc dissolves vigorously. Nothing happens to platinum.
(c)Platinum is an excellent catalyst for reduction of hydrogen and zinc
is a poor catalyst.

140. Effect of less active metal on corrosion of more active metal
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141. Supposeplatinumiscoupledtotitaniuminanacidsolution.Thepointofintersection
ofhydrogenreductionreactionandtitaniumoxidation,givei
corr
fortitanium
(uncoupled).ThereisnooxidationofPt.Thetotalrateofreductionisshownbythe
brokenline.Itisobservedthati
corr
fortheTi-Ptcoupleismuchlowerthani
corr
of
titaniumalone.Titaniumspontaneouslypassivatesandtherateofcorrosionis
reduced.
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142. Galvanic Corrosion
FACTOR S AFFECTIN G GALVANIC CORROSION
The following factors significantly affect the magnitude of galvanic corrosion:
A.Position of metals in the galvanic series.
B.The nature of the environment.
C.Area, distance and geometric effects.
A.PositionofMetalsintheGalvanicSeries:
Themagnitudeofgalvaniccorrosionprimarilydependsonhowmuchpotential
differenceexistsbetweentwometals.Foraparticularenvironment,themetals
selectedshouldbeclosetoeachotherinthegalvanicseriestominimizegalvanic
11/23/2020 Department of Metallurgical & Materials Engg. MNITJ 142
selectedshouldbeclosetoeachotherinthegalvanicseriestominimizegalvanic
corrosion.
Ex.AyachtwithaMonelhullandsteelrivets
Aluminumtubingconnectedtobrassreturnsbends

143. B.TheNatureofEnvironmentDueconsiderationmustbegiventotheenvironment
thatsurroundsthemetal.Forinstance,watercontainingcopperions,likeseawater,is
likelytoformgalvaniccellsonasteelsurfaceofthetank.Ifthewaterincontactwith
steeliseitheracidicorcontainssalt,thegalvanicreactionisacceleratedbecauseof
theincreasedionizationoftheelectrolyte.Inmarineenvironments,galvanic
corrosionmaybeacceleratedduetoincreasedconductivityoftheelectrolyte.Incold
climates,galvaniccorrosionofburiedmaterialisreducedbecauseoftheincreased
resistivityofsoil.Inwarmclimates,ontheotherhand,itisthereversebecauseofthe
decreasedresistivityofthesoil.
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144. Galvanic Corrosion
C. Area, Distance and Geometric Effects Effect of Area The anode to cathode area ratio
is extremely important as the magnitude of galvanic corrosion is seriously affected by
it. The area ratio can be unfavorable as well as favorable.
The other ratio, large anode/small cathode, would only slightly accelerate the rate of
galvanic corrosion(favorable).
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145. Behaviour of an active passive metal under corrosive medium
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146. 11/23/2020 Department of Metallurgical & Materials Engg. MNITJ 146

147. Environmental Effects:-
1.Effect of oxygen and oxidizers
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148. Environmental Effects:-
2. Effect of Velocity
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149. Environmental Effects:-
3. Temperature
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150. Environmental Effects:-
4. Effect of corrosive concentration
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151. Environmental Effects:-
4. Effect of Galvanic coupling
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152. Forms of Corrosion
Eight forms of corrosion:-
Uniform or general attack
Two metal or Galvanic corrosion
Crevice corrosion
Pitting corrosion
Intergranularcorrosion
Selective leaching or parting
Stress corrosionand
Erosion corrosion
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153. Uniform corrosion
Ex. 1. A piece of steel or zinc immersed in dilute sulphuric acid
2. Rusting of steel or iron structure in air
3. Tarnishing of silver ornaments due to formation of black colour Ag
2
S
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154. Two metal corrosion or galvanic corrosion
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155. Two metal corrosion or galvanic corrosion
Inthegalvanicseries,therearemanystainless
steelsattheanodicendoftheseries(active
end),andmanystainlesssteelsatthecathodic
end(nobleend)oftheseries.Thedualbehavior
ofstainlesssteelisrelatedtoitsabilitytoform
protectivefilmsonthesurfaceinthepresence
ofoxygenorotheroxidizingagents,suchas
nitricacidorsulfuricacid.Thesefilmsare
destroyedandthesteelscorrodefastinacids,destroyedandthesteelscorrodefastinacids,
suchasHC1orHForothernon-oxidizing
acids.Beforeselectingstainlesssteelsfor
applicationinaparticularenvironment,itmust
bedeterminedwhethertheenvironmentswill
causethemtobeinthepassivestateorinthe
activestate.
Galvaniccorrosionmaynotoccuriftwometals
closetoeachotherinthegalvanicseriesare
joined,suchascopperandbrass.Metalsclose
toeachotherofferaminimumriskof
corrosion.
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156. Two metal corrosion or galvanic corrosion
1.Environmental Effect:-
Change in weight of coupled and uncoupled steel and zinc, gm
Exceptionally,indomesticwaterattemperaturegreaterthan180F,thecouple
reversesandtheSteelbecomesanodic.Apparentlythecorrosionproductson
thezincmakeitactasasurfacenobletosteel.
Inhibitingionssuchasnitrates,bicarbonatesand/orcarbonatesinwater.
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157. Two metal corrosion or galvanic corrosion
Tantalum is very corrosion resistant metal but it should not be used in contact with anodic
metals because it absorbs cathodichydrogen and becomes brittle.
Galvanic corrosion also occurs in the atmosphere. The severity depends largely on the
type and amount of moisture present. Ex. Corrosion is greater near the seashore than in
a dry rural atmosphere.
Galvanic corrosion does not occur when the metals are completely dry since there is no
electrolyte to carry the current between two elctrodeareas.
2. Area Effect:-
Two metal corrosion is readily recognized by the localized attack near the junction.Two metal corrosion is readily recognized by the localized attack near the junction.
3. Area Effect:-
Area ratio = ratio of cathodicto anodic area. Unfavourable area ratio if large cathode and
A small anode. For a given current flow in the cell, the current density is greater for a samll
electrode than for a larger one. The greater the current density at an anodic area the greater
the corrosion rate.
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158. Two metal corrosion or galvanic corrosion
3. Area Effect continued :-
Inshort,thesmallertheanodetocathode
ratioasinthecaseofZncoupledtoPt(10
cm2),thelargeristhemagnitudeof
corrosion.Avaluablerule:Avoidasmall
anodetocathodearearatiotominimizethe
riskofseriousgalvaniccorrosion.
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159. APPLICATION OF PRINCIPLES OF GALVANIC CORROSION
A. Non-metallic Conductors:-Many non-metallic materials are cathodicto metals
and alloys. For example, impervious graphite used in heat exchanger applications is noble
to more active metals. The nature of non-metallic conductors must be known before their
application.
B.MetallicCoatings:-Twotypesofmetalliccoatingsaregenerallyused,nobleand
sacrificialtype.Zinccoatingisanexampleofthesacrificialtype.Zinccorrodeseventually
anditprotectsthesteelsubstratebothbyitsbarriereffectandalsobyprovidingelectrons
tothesteelwhichpreventFe++ionsfromescapingfromthesteel(cathodicprotection
-seebelow).Noblecoatingsactasabarrieronlybetweenthemetalsubstrateandthe
environment.Nickel,silver,copper,leadandchromiumarecallednoblemetalcoatings.environment.Nickel,silver,copper,leadandchromiumarecallednoblemetalcoatings.
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160. APPLICATION OF PRINCIPLES OF GALVANIC CORROSION
C.CathodicProtection:-Inasacrificialsystemofcathodicprotection,
anodesofactivemetals,likeZn,MgandAl,areusedforprotectionofsteel
structures.Thesacrificialgalvanicanodesprovideprotectiontothelessactive
metals,likesteelbecausetheycorrodeandreleaseelectrons.Theelectrons
whicharereleasedbythecorrodingmetalsenterthesteelstructures,which
becomecathodicand,therefore,donotcorrode.Thissystemofcathodic
protectionisbasedongalvaniccorrosion,however,inthiscaseabeneficialuse
istheresultofgalvaniccorrosion.istheresultofgalvaniccorrosion.
D.Cleaningsilverwares:-Masyofthestainsonsilverwareareduetosilver
sulfide.Asimpleelectrocleaningmethodconsistsofplacingthesilverinan
aluminumpancontainingwaterandbakingsoda.Thecurrentgeneratedbythe
contactbetweensilverandaluminumcausesthesilversulfidetobereduced
backtosilver.
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161. Methods of prevention of Galvanic Corrosion
(1) Select metals, close together, as far as possible, in the galvanic series.
(2) Do not have the area of the more active metal smaller than the area of the less active
metal.
(3) If dissimilar metals are to be used, insulate them.
(4) Use inhibitors in aqueous systems whenever applicable and eliminate cathodic
depolarizers.
(5) Apply coatings with judgment.
(6) Avoid joining materials by threaded joints.
(7) Use a third metal active to both the metals in the couple.(7) Use a third metal active to both the metals in the couple.
(8) Sacrificial material, such as zinc or magnesium, may be introduced into this
assembly.
(9) In designing the components, use replaceable parts so that only the corroded parts
could be replaced instead of the whole assembly.
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162. Crevice Corrosion
An important condition is the formation of differential aeration cell.
Causes:-
(a)Presence of narrow spaces between metal-to metal or non-metal to metal
components.
(b) Presence of cracks, cavities and other defects on metals.
(c) Deposition of barnacles, biofoulingorganisms and similar deposits.
(d) Deposition of dirt, mud or other deposits on a metal surface.
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163. Crevice Corrosion
An important condition is the formation of differential aeration cell.
It occurs at opening a few thousandths of an inch or less in width.
Causes:-
(a) Presence of narrow spaces between metal-to metal or non-metal to metal components.
(b) Presence of cracks, cavities corrosion products and other defects on metals.
(c) Deposition of barnacles, biofoulingorganisms and similar deposits.
(d) Deposition of dirt, mud or other deposits on a metal surface.
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164. Crevice Corrosion
Rubber band over sssheet
Gasket (crevice) corrosion on a large 18-8 stainless steel flange
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165. Crevice Corrosion
All chloride containing solutions are highly aggressive and contribute to onset of
crevice corrosion. Seawater and brackish water are high aggressive and promote crevice
corrosion of steels.
The four stages of crevice corrosion suggested are
(1) Depletion of oxygen in crevice due to consumption in the cathodicreaction.
(2) Increase of acidity in the pit by the process of hydrolysis.
(3) Breakdown of the passive film on the surface at a critical value of pH.
(4) Propagation of crevice corrosion with further hydrolysis and production of acidity.
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166. Crevice Corrosion
An optimum crevice corrosion resistance will be achieved with an active-passive metal having
A narrow active-passive transition
A small critical current density and
An extended passive region.
PREVENTION OF CREVICE CORROSION:-(1) Use welded joints in preference to bolted or riveted joints.
(2) Seal crevices by using non-corrosive materials.
(3) Eliminate or minimize crevice corrosion at the design stage.(3) Eliminate or minimize crevice corrosion at the design stage.
(4) Minimize contact between metals and plastic, fabrics and debris.
(5) Avoid contact with hygroscopic materials.
(6) Avoid sharp corners, edges and pockets where dirt or debris could be collected.
(7) In critical areas, use weld overlays with highly corrosion resistant alloy.
(8) Use alloys resistant to crevice corrosion, such as titanium or Inconel. Increased
Mo contents (up to 4.5%) in austenitic stainless steels reduce the susceptibility to
crevice corrosion
(9) Apply cathodicprotection to stainless steels by connecting to adjacent mild steel
structure.
(10) For seawater service, maintain a high velocity to keep the solids in suspension.
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167. Pitting Corrosion
It is a form of localized corrosion of a metal surface where small areas corrode
preferentially leading to the formation of cavities or pits, and the bulk of the
surface remains unattacked.
Metals which form passive films, such as aluminum and steels, are more
susceptible to this form of corrosion. It is the most insidious form of corrosion. It
causes failure by penetration with only a small percent weight-loss of the entire
structure.
Pitting usually grow in the direction of gravity.Pitting usually grow in the direction of gravity.
Very dilute, cold, ferric chloride produces no attack(in a short time) on A, but strong
Hot ferric chloride dissolves specimen C.
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168. Pitting Corrosion
Corrosion of steel after 24 hrs in 5% NaCland 500 lb/inch2 oxygen pressure
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169. Pitting Corrosion
Environment:-
Generally,themostconduciveenvironmentforpittingisthemarine
environment.Ions,suchasCl~,Br"andI-,inappreciableconcentrationstendto
causepittingofsteel.Thiosulfateionsalsoinducepittingofsteels.
Aluminumalsopitsinanenvironmentthatcausethepittingofsteel.Iftracesof
Cu2+are
presentinwater,orFe+3ionsareinwater,copperorironwouldbedepositedonpresentinwater,orFe+3ionsareinwater,copperorironwouldbedepositedon
aluminummetalsurfaceandpittingwouldbeinitiated.Oxidizingmetalionswith
chloride,suchascupric,ferricandmercuric,causeseverepitting.
Presenceofdustordirtparticlesinwatermayalsoleadtopittingcorrosionin
copperpipestransportingseawater.
Withsoftwater,pittingincopperoccursinthehottestpartofthesystem,
whereaswithhardwaters,pittingoccursinthecoldestpartofthesystem.
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170. Pitting Corrosion
Condition:-
Themostimportantconditionisthatthemetalmustbeinapassivestatefor
pittingtooccur.
Theprocessofpittingdestroysthisprotectivefilmatcertainsitesresultinginthe
lossofpassivityandinitiationofpitsonthemetalsurface.
Siteswhicharemostsusceptibletopittingaregrainboundaries.
Thepassivemetalsurroundingtheanodeisnotsubjecttopittingasitformsthe
cathodeanditisthesiteforreductionofoxygen.cathodeanditisthesiteforreductionofoxygen.
Thecorrosionproductswhichareformedattheanodecannotspreadontothe
cathodeareas.Therefore,corrosionpenetratesthemetalratherthanspread,and
pittingisinitiated.
Thereisacertainpotentialcharacteristicofapassivemetal,belowwhichpitting
cannotinitiate.Thisiscalledpittingpotential,£p
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171. Pitting Corrosion
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172. Pitting Corrosion
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