Made by Biplob Kumar Biswas From Jessore University Of Science & Technology(JUST),Bangladesh.

1. Preamble
Corrosion cycle of steel
 Metals made by smelting (reduction of ore or
mineral).
 Mineral is more stable than metal.
 Metals in air want to return to their oxidized state.
 Corrosion is a natural process!!

2. Definition
 Corrosion is defined as degradation
or destruction of metal or an alloy
because of chemical or
electrochemical reaction with the
surrounding environment or medium.
 Rusting is the corrosion related to
iron and iron-based alloys. Non-
ferrous metals corrode but do not rust.

3. Four requirements of
corrosion
 Anode
 Cathode
 Current flow
 Electronic path
} Cell

4. Reasons for corrosion studies
 Economic (due to material losses).
 Safety (to prevent catastrophic
consequences resulting from
operation failure of equipment).
 Conservation (to conserve metal
resources, which are limited).

5. Responsibility for corrosion
failure
Wrong
specification,
16
Bad
inspection, 10
Human error,
12
Lack of
proving, 36
Poor
planning, 14
Unforseeable,
8
Other causes,
4

6. Bhopal disaster (1984)
(Methyle iso cyanate)
4,000 dead
500,000 affected

7. Accident description
As part of routine procedures, the pipes leading from the MIC
distillation column to the storage tanks were regularly flushed
with pressurized water. MIC and any associated products can
be quite corrosive and could form corrosion deposits in the
pipe. These deposits would contaminate the MIC in the tanks
and could initiate unwanted reactions. During cleaning,
valves in the product lines were to be closed and a blank or
slipblind placed in the product line leading to the storage tank
to prevent contamination.
However the valves, although closed, were not sealing
properly because of corrosion and the maintenance crew
forgot about the blank. It appears that about 1000 kg of water
plus metal debris entered into the tank and initiated an
exothermic reaction.

8. Safety features of MIC tank
 Operative:
◦ Usual practice: Tank should be filled upto 50%.
◦ Prime protection: External jacketed cooling system
(0ºC).
◦ Safety valve.
◦ Located under ground.
 Inoperative:
◦ Refrigeration system was turned off 6 months ago
due to economic crisis of the company.
◦ Valves were defective due to lack of maintenance.
◦ Tank filled with more than 50%.

9. MIC tank

10. Economic factors
 Direct loss
 Replacing corroded
structure and
equipment
 Adding corrosion
inhibitors
 Cost for corrosion-
resistant metals
 Indirect loss
 Shutdown
 Loss of product
 Contamination of
product
 Loss of efficiency
 Overdesign

11. Some examples
 rusted chilled water
piping penetration at
deck, due to water
wicking under
insulation that is flush
to deck
 rusted steam piping
under insulation at a
fuel oil heater

12. Some examples (contd…)
 steel deck support
brackets for topside
vertical ladder (Naval
ship)
 topside rusted steel
electrical conduit
clamps

13. Some examples (contd…)
 pipe hanger (Naval
ship)  floodlight positioning
bracket (Naval ship)

14. Examples (contd…)
14

15. Examples (contd…)
 Bicycle rim
15
Aluminum rim
Chromium plated brass spoke
Al  - 1.66 volt
Cr  - 0.74 volt

16. Some examples (contd…)
Railing of a bridge (Karatsu, Saga, Japan 6 May 2014)

17. Some examples (contd…)
 Cast iron pump
impeller

18. Household examples
(conted…)
 Mail box corrosion  Plumbing fixtures
corrosion

19. Pemex Refinery explosion
Mexico (19 Sept 2012)
19
 State-owned petroleum company
 Process crude oil to produce petrol,
diesel, kerosene etc.
 Explosion occurred
 26 died, 40+ injured
 Financial loss: $300 million – $1000
million

20. Guadalajara Sewer Explosion
Mexico (1992)
20
 Gasoline pipeline (Steel) was
underneath the water pipeline (Zn
coated iron)
 Corrosion occurred in both pipelines
 Gasoline came out and entered in a
nearby sewer line
 252 died; 500 injured; 15000 homeless

21. Consequences of corrosion
 Waste of metals
◦ 25% of annual world production of iron is wasted
due to corrosion
 Decrease in efficiency of machineries
 Failure of machineries
 Leakage in the process
◦ Health & fire hazard
 Causes contamination
21

22. Who will study corrosion???
Distribution of disciplines
Chemical Engineering
Chemistry
Civil engineering
Electrical engineering
None
Materials engineering
Business
Physics

23. Expectation from you …
 Ensuring maximum life of new
equipment through corrosion
protection.
 Preservation of existing equipment.
 Improving the quality of product.
 Prevention of spillage or leakage.
 Reducing hazards to life and property.

24. Theories of corrosion
 Chemical or Dry corrosion
 Electrochemical corrosion or Wet
corrosion

25. Chemical or Dry corrosion
 Simplest case of corrosion.
 Corrosion takes place due to direct
chemical attack.
 Oxygen, halogens, hydrogen sulphide,
nitrogen etc.
 Corrosion product may be insoluble,
soluble or liquid product.

26. Classification
Oxidation
corrosion
• Takes place by
direct action of O2
• Absence of moisture
Corrosion
by other
gases
• CO2, SO2, Cl2,
H2S, F2
• Extent of
corrosion varies
Liquid
metal
corrosion
• Flowing liq at
high temp.
Chemical or
Dry corrosion

27. Electrochemical or Wet corrosion
 There must be an anode & a cathode.
 There must be an electrical potential
difference between the electrode.
 There must be a metallic path electrically
connected with both electrodes.
 There must be an electrically conductive
medium.

28. Difference
Chemical corrosion
1. Takes place in dry
condition.
2. Takes place by direct
chemical attack.
3. Can take place on
heterogeneous or
homogeneous metal
surface.
4. Uniform corrosion.
5. Corrosion product
accumulates at the spot.
Electrochemical corr.
1. Takes place in presence
on wet condition.
2. Takes place through the
formation of cell.
3. Can take place only on
heterogeneous metal
surface.
4. Non-uniform corrosion.
5. Corrosion product
accumulates at the
cathode.

29. Mechanism (general)
 Carbon electrode &
zinc cup
 Reduction occurs
at carbon electrode
while oxidation
occurs at zinc cup
 Zn0→Zn2+ + 2e-
 Amnt. of Zn
corrosion W = kIt
 Corrosion occurs at
zinc cup

30. Mechanism of corrosion (iron)
30
2(Fe → Fe2+ + 2e-)
O2 + 2H2O + 4e- → 4OH -
2Fe + O2 + 2H2O → 2Fe(OH)2
2Fe(OH)2 + ½O2 + 2H2O → 2Fe(OH)3
{ }
Redish-brown

31. Local-action current & local-
action cell
 Observed at metal surface while
exposed in solution (water, salt
solution, acids, or alkalies).
 Accompanied by chemical conversion
of the metal to corrosion products.
 This happens due to impurities of a
metal constitute the electrodes.

32. Types of cells
 Dissimilar electrode cells (e.g. dry cell)
 Salt concentration cells
 Differential aeration cells
 Differential temperature cells
While connected, Cu dissolves at the anode and
deposited at the cathode.
Tending the CuSO4 solution to reach the same
concentration.

33. Types of cells
 Same electrode material
 Same electrolyte
 Only difference is O2 concentration (causes potential
difference)
 Example: crevice corrosion at the lamp post.

34. Differential temperature cell
 Same electrode material.
 Same electrolyte.
 Temperature difference in electrodes.
 Example: corrosion inside heat exchangers,
boilers.

35. Forms of Corrosion
 General
◦ Identified by uniform formation of corrosion product
 Localized
◦ Caused by different chemical or physical
conditions
 Bacterial
◦ Caused by formation of bacteria that has affinity to
metal
 Galvanic / Dissimilar metal
◦ Caused when dissimilar metals come to contact
35

36. Corrosion damages
 Uniform corrosion
 Pitting corrosion
 Crevice corrosion
 Galvanic corrosion
 Intergranular corrosion
 Stress corrosion cracking (SCC)
Based on the appearance of corrosion damage:
36

37. Corrosion damages
 Uniform attack
 Pitting
◦ Impingement attack
◦ Fretting corrosion
◦ Cavitation-erosion
 Dezincification and parting
 Intergranular corrosion
 cracking

38. Corrosion rate expression
 mm/y- millimeter penetration per year
 gmd- grams per square meter per day
 ipy- inches-penetration per year
 mpy- mils-penetration per year (1 mil = 0.001 in)
 Corrosion rate < 0.005 ipy (good corrosion resistance).
 0.005 < Corrosion rate < 0.05 ipy (satisfactory).
 Corrosion rate > 0.05 ipy (unsatisfactory).

39. Free energy change (∆G)
 Chemical reaction mechanism
◦ More (-)ve ∆G, greater tendency of reaction to
occur.
kCalGOHMgOOHMg 6.142)(
2
1 0
222 
kCalGOHCuOOHCu 6.28)(
2
1 0
222 
kCalGOHAuOOHAu 7.15)(
4
3
2
3 0
222 
 Electrochemical reaction mechanism
◦ ∆G = - EnF
◦ Higher the value of E, greater tendency of
reaction to occur.

40. Nernst equation
 Nernst equation provides an exact emf of a cell in
terms of activities of products and reactants.
..............  rRqQmMlL


 m
M
l
L
r
R
q
Q
aa
aa
nF
RT
EE ln0
Homework: Derive this equation.
 General reaction for Galvanic cell:

41. EMF series
 Metals arranged according to standard
potential values.
 More positive → noble metals
 More negative → active metals
 Only useful to predict which metal is anodic
to other.
 Valid when activity of metal ions in
equilibrium are unity i.e. 1.
 Alloys are not included (Only pure metals
are considered).

42. Electromotive force series
Noble metals
Active metals

43. Galvanic series
 Arrangements of both metals and alloys.
 Well representative of particular
environment.
 More appropriate for practical situation.

44. Pourbaix diagram
 Represents thermodynamic state
(thermodynamic data: Potential vs pH)
 Represents chemical & electrochemical
equilibria between metal and aqueous
solution and relates corrosion.
 Does not give any data on rate of
reaction.

45. Pourbaix diagram for iron
Horizontal lines represent reaction, which does not involve H+ or OH-.
Vertical lines involve H+ or OH- but no electrons.Fe → Fe2+ + 2e- ; activity ≈ 10-6
Sloping lines involve H+ or OH- and electron.
Fe2O3 + 6H+ + 2e- → 2Fe2+ + 3H2O

46. POLARIZATION
 What is polarization?
 Linkage between polarization and
corrosion.
 Types of polarization.
 Corrosion control through polarization.

47. What is polarization?
 Electrodes are no longer in equilibrium
when a net current flows.
 In a Galvanic cell:
◦ Anode potential moves towards cathode.
◦ Cathode potential moves towards anode.
◦ Thus the difference in potential becomes smaller.
 So the extent of potential change
caused by net current flow to or from
an electrode is called polarization.

48. Cu Zn
CuSO4 ZnSO4
A
V
R
log current
Potential
φCu
φZn Imax
Imax∙Re
I∙(Re+ Rm)
φcorr
Polarization curves can never intersect.

49. Types of polarization
 Concentration polarization
 Activation polarization
 Polarization due to IR drop

50. Concentration polarization
φCu = 0.342 volt
φ1 = Potential of Cu electrode before current passing
 1
2
1 log
2
0592.0
342.0 
 Cu
When current flows, Cu2+ + 2e- → Cu0
 2
2
2 log
2
0592.0
342.0 
 Cu
 
 2
2
1
2
12 log
2
0592.0



Cu
Cu


51. Significance of Concentration
polarization
Larger current flow causes smaller Cu ion
concentration (Cu2+)2, which results larger polarization
When (Cu2+)2, → 0 then (φ2 – φ1)→∞
The current density at this situation is called limiting current
density.
 
 2
2
1
2
12 log
2
0592.0



Cu
Cu


52. Activation polarization
 Causes by slow electrode reaction
 Requires activation energy
 Example: reduction of hydrogen ion
2H+ + e- → H2

53. Influence of polarization
 Anodically controlled polarization
 Cathodically controlled polarization
 Resistance control

54. Anodically controlled
 Polarization occurs mostly at anode.
log current
Potential
φC
φA Imax
φcorr
Icorr
φcorr
Example: Impure
lead surface
immersed in
sulfuric acid.
Lead sulfate film
will be formed
and Cu (the
impurity) will be
exposed for
corrosion.

55. Cathodically controlled
 Polarization occurs mostly at cathode.
log current
Potential
φC
φA Imax
φcorr
Icorr
φcorr
Examples: Zn corrodes
in sulfuric acid.
Iron corrodes in water.

56. Resistance control
 Electrolyte resistance is very high.
 Resultant current is not sufficient to polarize either anode or
cathode.
log current
Potential
φC
φA
Icorr
Examples: Porous
coating covering a
metal surface.
R∙Icorr

57. Principle of cathodic
protection
 Polarization of cathode is done by
supplying external current
 Electrochemical potential of cathode
moves in negative direction (towards
anode)
 Auxiliary anode is used to spread
current
 The material is protected when it
reaches protection potential
57

58. Types of cathodic protection
 CP with sacrificial anode
 CP with impressed current
58
Fe → - 0.44 v (noble)
Mg → - 2.37 v (active)

59. Cathodic protection
+
+
-
-
+ -
+-
Anode
Sacrificialanode

60. Passivity
 Fe in concentrated
HNO3 → No reaction
(Passive state)
 Fe in dil. HNO3 →
Rapid corrosion
reaction (Active state)
 Passivity is the
phenomenon that
demonstrate how the
corrosion is inhibited
in any given
environment.
70% concentrated HNO3
Fe
Dilute

61. Characteristics of Active-Passive
metal
 The same metal can act as active as
well as passive depending on the
situation.
 Passivity occurs because a film is
produced on the metal surface.
 Thickness of film ≤ 30Å. (1Å = 1×10-7 mm)

62. Potentiostatic polarization curve
of active-passive metal (Fe)
 Active state: metal
corrodes (Fe0 → Fe2+ + 2e-)
 Passive state: insulative
film is formed & no
corrosion occurs
 Transpassive zone:
Formation of Fe3+ as
well as O2 evolution
log i
Potential(φ)
passive
icritical
ipassive
P

63. Flade potential of Fe
 When applied potential is
withdrawn, passivity
decays
 Passivity decays in a very
short time
 At Flade potential, active
state of the metal is re-
established.
Time (sec)
Potential(φ)
φF
Important
 P and φ are roughly equal (but not same). WHY?
◦ change in pH
◦ IR drop due to insulating film

64. Passivators of iron
 Passivators are inorganic oxidizing
agents, which reacts slowly when in
direct contact with iron.
 They are adsorbed on the metal
surface.
 Higher the concentration of passivator,
more readily it adsorbs
 CrO4
2-, NO2
-, MoO4
2-, WO4
2-, FeO4
2-

65. Theory of passivity
 Oxide film theory
◦ Metal oxide or other compound is formed
◦ This oxide separates metal from the
environment
◦ Eventually slows down the rate of reaction
◦ Effectiveness of corrosion reduction
depends on the nature & properties of thin
protective film.

66. Theory of passivity
 Adsorption theory
◦ Passivity is achieved due to chemisorbed
film of O2 or other passivating agents
◦ This film separates metal from water or
other corroding environment
◦ Film may be of monolayer or multilayer
H Hmonolayer Thick layer
(multilayer)
H
Oxygen
Metal
Hydrogen

67. Passivity in iron alloys
 Fe alone is not naturally passive (i.e. corrodes in short time)
 Cr is a naturally passive metal (i.e. remains bright & tarnish-free)
 Fe-alloy have passive property when at least 12% Cr is there
CorrosionRate
2 4 14 180
Chromium (wt%)
6 8 10 12 16 20

68. Factors affecting
corrosion

69. Effect of oxygen on MS corrosion
 Critical concentration may change:
◦ Increases with increasing T
◦ Decreases with increase in velocity
Concentration of dissolved O2 (ml/L)
Corrosionrate(gmd)
Critical concentration

70. Effect of temperature
Temperature (°C)
Corrosionrate(ipy)
Open system
 Corrosion rate increases with increase in
T
 In open system:
 Rate increases first
 Then falls down at 100°C
 In closed system:
 O2 can not escape
 Rate increases with T, until all O2 is
consumed
100°C
Such falling off is related to
decrease of O2 solubility in
water as T is raised.

71. Effect of pH on iron corrosion
pH >10
 Higher surface pH
 Because of alkali & dissolved O2 iron gets
passivated
CorrosionRate(ipy)
12 10 8 6 4 214
pH

72. Effect of pH
pH 4 ~10
 Corrosion rate is independent of pH
 Rate depends on O2 diffusion to the iron surface
 Diffusion barrier (FeO) is regenerated
 Surface pH always remains at 9.5 throughout this range (Why??)
CorrosionRate(ipy)
12 10 8 6 4 214
pH

73. Effect of pH
pH < 4
 FeO film dissolved
 Surface pH decreases
 Corrosion increases (because of H2 evolution & O2
development)
CorrosionRate(ipy)
12 10 8 6 4 214
pH

74. Effect of velocity (Freshwater)
 Corrosion increases with velocity because O2 contact with
the surface
 At sufficient high velocity, enough O2 reach at the surface,
which causes partial passivation
 At further increase in velocity, corrosion-product film is
eroded
CorrosionRate(ipy)
2 4 6 80
V (ft/s)
Rough steel
Polished steel

75. Effect of velocity (seawater)
 Corrosion increases with velocity
 Passivity is never achieved
CorrosionRate
2 4 6 80
V (ft/s)
High concentration of Cl-

76. Corrosion damages
 Uniform corrosion
 Pitting corrosion
◦ Impingement attack
◦ Fretting corrosion
◦ Cavitation-erosion
 Crevice corrosion
 Galvanic corrosion
 Intergranular corrosion
 Stress corrosion cracking (SCC)
Based on the appearance of corrosion damage:

77. Uniform corrosion
 Results from uniform penetration over the surface
 Also results from multiple local-action cell
 Location of anodic & cathodic areas move on the
surface
 Examples: atmospheric exposure of metal (rusting of
steel, green patina formation of copper), exposure in salt
water or soil or chemicals
Rusting of steel
highway
bridge

78. Prevention of uniform
corrosion
 Proper material selection
 Use of coating or inhibitor
 Cathodic or anodic protection
 Individual or combination of all the above

79. Pitting corrosion
 Highly localized form of corrosion
 Causes from local inhomogeneneity on metal
surface, local loss of passivity, rupture of protective
oxide coating.
 Produce sharp holes (small or large in diameter)
 Examples: iron buried in soil (shallow pits), carbon
steel in contact with HCl (deep pits), SS immersed
in seawater.

80. Pitting factor
Pitting factor = 1 (uniform attack)
d
p
Pitting factor =
Deepest metal penetration
Average metal penetration
=
p
d

81. Mechanism of Pitting
Example: Metal in NaCl solution

82. Mechanism of Pitting
Example: Metal in NaCl solution
 M+ is pitted by aerated NaCl solution
 Once a pit is created, local environment & surface film
become unstable
 Rapid dissolution occurs within the pit while O2
reduction takes place on the adjacent surface (self
propagating process)
 Rapid dissolution of M+ causes excess +(ve) charge in
the pit, which causes migration of Cl- in the pit.
 High concentration of metal chlorides (M+Cl-) &
hydrogen ion in the pit.
 H+ and Cl- stimulate dissolution of metals and alloys.

83. Impingement attack
 Moving liquid particles cause the damage.
 Metals subject to high-velocity liquid.
 Corrosion-erosion is another name.
 Example: Copper and brass condenser tubes.

84. Fretting corrosion
 Combination of corrosion and wear
 Oxidation is the most common element
 Relative movement between two surfaces
 Metal oxides become trapped between two surfaces and causes
wear
 Examples: rolling contact bearing
Prevention:
 Lubrication
 Restricting the degree of movement

85. Cavitation-erosion
 Cavitation
◦ Repetitive low & high pressure areas
developed
◦ Consequently bubbles form & collapse at
metal-liquid surface
 Damage caused by cavitation is called
cavitation damage
 Metal surface becomes pitted
 Examples: blade/rotor of pumps, water
turbine blades

86. Prevention of Pitting
 Lessen the aggressiveness of the environment
(e.g. Cl- concentration, temperature, acidity etc.)
 Upgrade materials of construction (e.g. Cr (12%)
containing SS, Mo (4-6%) containing SS etc.)
 Modify the design of system (e.g. ensure proper
drainage, avoid crevices etc.)

87. Galvanic corrosion
 Metal or alloy electrically coupled with another metal
or conducting nonmetal
 The system should have common electrolyte
 Materials possessing different surface potential
 Driving force ->>>> potential difference between two
dissimilar metal
Aluminium rim and chromium plated brass spoke.
Mud on the rim acts as electrolyte.

88. Galvanic and electrolytic cell
 In Galvanic cell
reactions occur
spontaneously when
connected by
electrolyte.
 Chemical energy is
converted to
electrical energy.
 Examples: AA
batteries, car battery
(when it is being
discharged).
 In electrolytic cell
reactions do not
occur without
applying an external
potential.
 Electrical energy is
used to cause the
desired chemical
reaction.
 Examples:
electroplating of Cu,
Au, Ag etc., Car
battery (when it is
being charged).

89. Area concept of corrosion
 Corrosion of the anode may be 100 ~ 1,000 times
greater than if the two areas were the same.
 What to do!!!!!!!
Fe => φ = - 0.403 volt
Cu => φ = + 0.521 volt
Rivet = Fe
Plate = Cu
(i) Rivet = Cu
Plate = Fe
(ii)
Corrosion of (i) >> corrosion of (ii)

90. Aloha aircraft incident
1 fatality and 7 injured.
Why this occurred??
Corrosion occurred in lap joint.
Corrosion product was Al(OH)3.
Al(OH)3 expanded inside the lap joint and lead to
pillowing.
This created undesired increased level of stress.
This stress produced cracking.

91. Prevention of Galvanic
corrosion
 Avoid combinations in which the area of the less
noble material is relatively small.
 Insulate dissimilar metals if possible.
 Apply coating e.g. teflon coating.
 Use chemical inhibitors, which reduces
corrosiveness of the environment.

92. Inter-granular corrosion
 Localized type of attack at the grain boundary of
metal.
 Grain boundary (small in area) acts as anode.
 Rest of the grain (larger area) acts as cathode.
 Attack penetrates deeply into the metal.
 Causing catastrophic failure.

93. Stress corrosion cracking
(SCC)
 Metal subject to constant tensile stress &
exposed simultaneously to a corrosive
environment.
 Thus metal suffers cracking called SCC.
 Compressive stress is not damaging.
 Example: Riveted steam boiler.
High strength alumina alloy
SCC

94. Riveted steam boiler
 Boiler water generally treated with alkali.
 Crevice between rivets & boiler plate allow
alkali to concentrate.
 Concentration of alkali in crevices induce
corrosion.
 Such type of corrosion is often called caustic
embrittlement.

95. Remedy from SCC
 Severe cold working.
 Heat treatment (quenching or slow cooling).
 Cathodic protection.
 Use of special alloy (addition of Al, Ti etc.).
 Use of inhibitors (NaNO3 in boiler water, crude quebracho
extract).
CorrosionRate
Carbon steel (0.076% C)
200 400 600 8000
Temp (°C)
1000
Zone refined steel (pure steel)

96. Atmospheric corrosion
 Atmosphere: 79% N2, 21% O2 (CO, CO2,
NH3, H2S, SO2, NOx, suspended particles)
 Based on the pollutants:
◦ Rural atmosphere (little or no contaminants)
◦ Marine atmos. (high moisture & Cl-)
◦ Urban atmos. (NOx, CO, CO2)
◦ Industrial atmos. (CO, CO2, SO2)
 One metal is resistive to a particular
atmosphere but not effective in the other.
 Example: (i) Galvanized steel (C.I. sheet) in
rural atmos. but less resistive in industrial
atmos. (ii) Lead performs better in
industrial atmos. Because PbSO4 film is
developed.

97. Corrosion film-product
 Metal surfaces retaining moisture
corrode rapidly compared to those
exposed fully.
 Why???? Because H2SO4 absorbed
by rust accelerates corrosion.
 Painting just after rainy season is very
efficient than painting in winter.
  4232
2
1
342
4
1
4
2
1
2
3
2
1
2
1 2422242
SOHOFeSOFeFeSOFe
OHSOHOOSOH
   


98. Atmospheric corrosion of
steel
Lossofweight(kg/m2)
2 4 6 80
Time (years)
10
Pure iron => powdery loose product (i.e. unstable film)
Cu bearing low-alloy steel => compact rust film (i.e. stable)

99. Factor affecting atm.
corrosion
Dust content, gases in the atmos.,
moisture etc.
Dust content:
Suspended particle matters (SPM) e.g.
carbon and carbon compound, metal oxides,
NOx etc.
SPM combines with moisture and
produces Galvanic or differential aeration
cell.
Dust free air is less responsible for
corrosion.
In Dhaka: 3000 μg/m3
(allowed 400 μg/m3)

100. Factor affecting atm.
corrosion
Gases in atmosphere:
H2S causes tarnishing of Ag, Cu, Ni.
SO2 is most harmful
S + O2 → SO2
2SO2 + O2 + 2H2O → H2SO4
Patina: Cu exposed to industrial atmosphere
forms a greenish protective layer
(CuSO4∙3Cu(OH)2).
Fogging: Ni exposed to industrial atmosphere
forms a tarnish of nickel sulfate. (But Ni is
resistant to marine atmosphere).

101. Remedial measures of atmos.
cor.
 Use of organic, inorganic or metallic
coating.
 Reduction of relative humidity.
 Use of alloy.
 Slushing compounds (greases, oil, wax,
organic additives etc.).

102. Underground corrosion
 Important because protection needed for
thousands of kilometers of underground
cross-country pipeline.
 NG, crude oil, water.
 Soil corrosion resembles atmospheric
corrosion.
 Performance of any particular metal
varies from one place to another over the
country.
◦ Differences in pH
◦ Differences in soil composition
◦ Differences in moisture content

103. Factors affecting underground
corr.
 Aeration of soil (depends on porosity).
 Electrical conductivity or resistivity.
 Dissolved salts (Na2SO4, NaCl are harmful).
 Moisture or water content (in desert,
corrosion of buried metal is almost zero).
 pH (acidity or alkalinity).

104. Pitting characteristics of buried
metal
 p = ktn (p: depth of the deepest pit, t: time, k & n:
constant).
◦ n ≈ 0.1 for steel in well-aerated soil
◦ n ≈ 0.9 for steel in poorly-aerated soil
 Pits tend to occur more on the bottom
side of the pipeline.
◦ Pipe settles down & air space produced on the top

105. Remedial measures of soil
cor.
 Use of organic or inorganic coating
(coal tar, pigments, portlant cement, vitreous enamel
etc.).
 Metallic coating (Zn coating).
 Alteration of soil (layer of limestone chip
surrounded the buried pipe).
 Cathodic protection (CP).

106. Corrosion prevention
How to do this????
 Change the metallic material.
 Altering the corrosive environment
(pH, acidity, temp.).
 Separating the metal from
environment (insulation).
 Providing appropriate design.

107. Other aspects of corrosion
prevention
 Welding is preferable from riveting
(crevice corrosion).
 Easy drainage and cleaning (design
aspect).
 Avoid sharp bends (erosion-corrosion).
 Hot spots should be avoided (corrosion
due to temperature gradient).
 Avoid electric contact (galvanic
corrosion).

108. Factors influence the service
life

109. Corrosion control by proper
design
Design for drainage (a) poor, (b) better
(a) (b)
Prevention of excessive turbulence
Fluid trap between two metal jointsAvoid condensation droplets

110. Corrosion control by proper
design
Mixing vessel (a) poor, (b) better
Prevention of localized cooling
(a) poor, (b) better

111. Cathodic protection (CP)
Basics of CP:
 External electric current is applied
 Cathodic potential is lowered to anodic direction
 Surface becomes equipotential
 Corrosion current no longer flows
CP can not be used- Where???
 In nonconducting liquids (oil)
 In extremely corrosive environment
(theoretically possible but incurs huge cost)
 In electrically screened areas
 In vapor

112. CP with sacrificial anode
 Directly connect with a more active metal.
 Anode of this system is called sacrificial
anode.

113. Application of CP
 Underground tanks
 Condenser water boxes
 Structures e.g. bridges
 Evaporators
 Valves, piping and other metal
surfaces submerged in a liquids or
constructed underground
113

114. CP with sacrificial anode
 Sacrificial anode is useful when
electric power is not readily
available.
 Low cost installation.
 Low maintenance cost.
 Combination with coating is better.
Mg anode
8 km coated pipe
30 m bare pipe only

115. Overprotection
 Moderate overprotection is not
harmful.
 Waste of electric current.
 Increased consumption of auxiliary
anode.
 So much H2 may be produced, this
may create H2 overvoltage (H2
embrittlement).

116. Alteration of environment
 Corrosion can be reduced by
◦ (i) changing the corrosive environment
◦ (ii) using inhibitors and passivators
 Moisture can be removed by
dehumidification
 Dissolved O2 (by deaeration, saturation with
N2, using O2 excavengers e.g. Na2SO3, N2H4).
 Cl- ions can be removed.
 Particulate solids can be removed.

117. Use of inhibitors
 Very specific to particular
environment.
 Developed by empirical experiments.
 Sometimes proprietory in nature &
composition is not disclosed.
 Usually used in closed or re-circulating
system.
 Not used in once-through system.

118. Classification of inhibirots
 Passivators (inorganic oxidizing
substances e.g. Na2CrO4, NaNO2, MoO4
2-).
 Organic inhibitors (Slushing compounds:
wax, greases, oil).
 Vapor phase inhibitors (dicyclohexyl
ammonium nitrite: nontoxic & odorless).
1 g of DAN saturates 550 m3 (20,000 ft3) of air.

119. Corrosion control through
coatings
 Metallic coatings
 Inorganic coatings
 Organic coatings

120. Metallic coating
How to do???
 Hot dipping (specimen immersed in molten Zn or Steel
bath).
 Electroplating (Nickel on brass).
 Metal spraying.
 Cementation (specimen put into metal powder at high
temperature).
 Coating by gas phase reaction
CrFeClFeCrCl
2
3
2
3
32 
 Coating by chemical reduction (electroless plating
of Ni: Nickel phosphorus or Nickel-boron alloy coating).
 Ion implantation

121. Classification of metal
coatings
 Noble coating (with Ni, Ag, Cu, Pb, Cr on
steel).
 Sacrificial coating (Zn, Cd on steel).
Noble coating

122. Metal cladding
 Cladding is a physical process in which a
thin layer of one metal is brought in contact
with a heavy layer of a base metal and
binding by a combination of heat and
pressure.
 Metal-to-metal laminar composite.
 Techniques: hot-roll bonding, cold-roll
bonding, explosive bonding, weld cladding
etc.
 Most engineering metals & alloys can be
clad.
 Applied in pressure vessels, reactors, heat

123. Inorganic coating
Vitreous enamel coating
 Powdered glass applied on metal surface and heated in
furnace.
 Hard glassy external layer.
 Susceptible to mechanical damage or cracking by thermal
shock.
Portland cement coating
 Used to protect cast iron or steel on water or soil or both.
 Thickness is 5 to 25 mm.
 Low cost coating.
 Susceptible to mechanical damage and thermal shock.
Chemical conversion coating
 Formed in situ by chemical reaction with metal surface.
 Anodic oxidation (anodizing) of metal (e.g. Al2O3).
 Phosphate coating on steel (Parkerizing /Bonderizing); (e.g.

124. Organic coating
Includes paints, varnishes and lacquers.
 Paint: mixture of insoluble pigments
(metal oxides; e.g. TiO2, Pb3O4, Fe2O3,
ZnCrO4, PbCO3, BaSO4, clay etc.) in organic
vehicle (natural oil).
◦ Paint is not useful to protect buried structures.
◦ Natural oil based paints not recommended for metal
structures totally immersed in water.
 Varnishes: mixture of drying oil, dissolve
resin and volatile thinner.
 Lacquers: resin dissolved in volatile
thinner.

125. Filiform corrosion
 Self propagating crevice corrosion.
 Localized form of corrosion that occur under
the coating or paint.
 Steel, aluminum and other alloys are affected.
 Particular concern in food packaging industry.
 “Wormlike” visual appearance.
 Occurred due to microenvironment effect.

126. Basics of a boiler operation
 Steam boiler consists of low carbon
steel.
 Water inside the tube; hot gases around
the tube.
 Generated steam passes through higher
alloy steel.
 Dissolved O2 is removed (deaeration).
 3Fe+4H2O→Fe3O4+4H2 (Inside the tube)
 At T>570°C: FeO formation.
 Cooling of steam: 4FeO→Fe3O4+Fe
Magnetite: protective
film (@570°C)
Four-pass fire-tube boiler

127. Corrosion in boiler
 Protective magnetite layer may be
damaged either chemically or
mechanically.
 Pitting may occur in localized region.
 Excess OH- concentration (chemical
damage).
 Differential concentration of oxide &
metal (mechanical damage).

128. Boiler water treatment
Why needed???
 To control corrosion.
 To prevent scaling of boiler tubes
(lowers heat transfer rate).
 To reduce SiO2 (damages turbine blades).
Steps???
 Removal of dissolved gases (O2, CO2).
 Addition of alkali.
 Use of inhibitors.

129. Removal of dissolved gases
 Dissolved gases causes pitting corrosion
in tubes.
 Deaerated by steam
 Deaerated by O2 scavengers (Na2SO3,
N2H4).
 Dissoved CO2 should be removed
(carbonic acid is corrosive to steel).
 CO2 accumulation is avoided by CO2
release during boiler blowdown.

130. Alkali addition
 Alkali (NaOH) addition is usual
practice.
 Caustic embrittlement may occur.
 NH3 is sometimes added instead of
NaOH.
◦ NH3 is volatile.
◦ Does not accumulate in crevices.
◦ Crevice corrosion or SCC do not occur.
HCl (ppm) NaOH (ppm)
Relativeattack
pH1 4 137

131. Corrosion testing
What is it??
 Is a powerful tool to control corrosion
(flight).
 Is needed in design stage and in
operational phase.
 Provides data useful for selection of
materials: existing or alternative or new.
Classification???
 Laboratory testing
 Pilot-plant testing
 Field testing

132. Purpose of corrosion testing
 To evaluate and select materials.
 To obtain reference or database
information.
 To determine quality-control and material
acceptance requirement.
 To monitor corrosion-control programs.
 To identify research parameters and
corrosion mechanisms.
<Lack of service history, lack of time and budget.>

133. Testing, inspection,
monitoring
Operational data
(On-line and off-line)
Testing
Inspection
Monitoring
Data Management
Analyses
Forecasting
Decision making
Engg. Review
Assessment
Maintenance