environmental degradation of materials in real world

study on corrosion

1. 1 MT 41013 Corrosion and Environmental Degradation of Materials 3-0-0 Text books: ‘ Corrosion Engineering’ by Mars G. Fontana Principle and Prevention of Corrosion by Denny A. Jones

2. 2 Materials Availability Corrosion resistance Cost Strength Appearance Fabricability Factors affecting choice of an engineering material

3. 3 Corrosion is an irreversible interfacial reaction of a material with its environment which results in consumption of the material or in dissolution into the material of a component of the environment Corrosion is the destructive result of electrochemical reaction between a metal or alloy and its environment Corrosion is often called as ‘ Extractive Metallurgy in reverse ’ Corrosion

4. 4 Factors affecting corrosion resistance of a material Corrosion Resistance Electrochemical Metallurgical Physical chemistry Thermodynamic

5. 5 Costs of Corrosion Economic Human life & safety Direct Indirect

6. The annual cost of corrosion worldwide is over 3% of the world’s GDP UTILITIES: Drinking Water & Sewerage Gas distribution Electrical utilities etc. TRANSPORTATION: Ships & Aircrafts Railroad & vehicles INFRASTRUCTURE: Gas & liquid transmission Airports, Waterways GOVERNMENT: Defense Nuclear etc. PRODUCTION & MANUFACTURING: Chemical, Petrochemical, Pharmaceutical, food, paper, metal, oil& gas Exploration etc. Costs of Corrosion: Economy aspect

7. Landmark NACE study on cost of corrosion Indirect cost to Society (what you and I pay) to industry Add in growth in 15 years Today, corrosion is one of the biggest unseen costs to society NACE Cost of Corrosion study: http://www.nace.org/publications/cost-of-corrosion-study Direct Corrosion Costs: $276 billion (3.1% of U.S. GDP) Effect of Corrosion Cost on US Economy

8. According to National Association of Corrosion Engineers (NACE, India chapter) the corrosion cost of India is 5% of the GDP This is about half of our defense budget and perhaps double of our total annual expenses on education. Singh et al. Petrotech-2010 (31 October-3 November 2010), New Delhi, India Sector wide forecasted corrosion cost of India, 2010 (US$ Billion) Estimating the Cost of Corrosion for India

9. Year Case Cause Cost of Corrosion 2013 Cruise ship (Alabama) Severe corrosion prior to ductile rupture $2.9 million 2012 Rupture of natural gas transmission pipeline Columbia Gas Transmission Co.( Virginia ) Ineffective cathodic protection $2.9 million 2011 Light fixture fell in the travel lanes "Big Dig" I-93 Northbound Tunnel (Boston, Massachusetts ) Aluminum components to contact stainless steel ( Galvanic corrosion) $200 million 2009 Rupture of a high-pressure vessel at Nihon Dempa Kogyo (NDK) Co., Ltd (Illinois) Stress Corrosion Cracking (SCC) of the walls 50 million Yen 2009 Rupture of a natural gas transmission pipeline (Florida Gas Transmission Company) Cracking under a de-bonded coating $606,360.00 Recent major accidents due to corrosion

10. World Corrosion Organization (WCO) Australasian Corrosion Association Chinese Society for Corrosion and Protection European Federation of Corrosion National Association of Corrosion Engineers (NACE) Mission of the WCO To promote education and best practices in corrosion control for the socio-economic benefit of society, preservation of resources, and protection of the environment Organizations focusing on corrosion & its control

11. 11 Costs of Corrosion Indirect Economic Plant Downtime Loss of Product Loss of Efficiency Contamination Overdesign

12. 12 Plant Downtime Costs of Corrosion Loss of production while the plant is inoperable during repairs (Example: Cost to replace power from a shutdown nuclear power plant) Loss of Product Leaking tank, containers and pipelines Loss of Efficiency Accumulated corrosion products on heat exchanger tubing and pipelines decreases the efficiency of heat transfer and reduces the pumping efficiency

13. 13 Costs of Corrosion Contamination Soluble corrosion products Radioactive corrosion products Toxic corrosion products Overdesign In absence of adequate corrosion rate information, overdesign is required resulting in wasted resources and greater power requirements for moving parts. Geological storage of high-level nuclear waste in deep repositories requires container that will maintain their integrity for 300 to 700 years

14. 14 Electrochemical Reactions Zn + 2HCl Zncl 2 + H 2 Zn + 2H + Zn 2+ + H 2 Zn Zn 2+ + 2e . . . . . . . Oxidation (anodic reaction) 2H + + 2e H 2 . . . . . . Reduction (cathodic reaction) During metallic corrosion, the rate of oxidation equals the rate of reduction (in terms of electron production and consumption)

15. 15 Electrochemical Reactions Electrochemical reactions occurring during corrosion of Zn in air-free HCl acid

16. 16 Electrochemical Reactions Electrochemical reactions occurring during corrosion of a metal (M) in air-free HCl acid

17. 17 Anodic Reactions M M n+ + ne Fe Fe 2+ + 2e Ni Ni 2+ + 2e Al Al 3+ + 3e

18. 18 2H + + 2e H 2 Cathodic Reactions Fe 3+ + e Fe 2+ Sn 4+ + 2e Sn 2+ Redox reaction

19. 19 Electrochemical Reactions in aerated solution Electrochemical reactions occurring during corrosion of Zn in aerated HCl acid

20. 20 Cathodic Reactions O 2 + 4H + + 4e – 2H 2 O O 2 + 2H 2 O + 4e – 4OH – . . . . . . . Acid solution . . . . . Neutral solution In the absence of all other reduction reaction, water will be reduced by 2H 2 O + 2e – H 2 + 2OH –

21. 21 Formation of rust due to corrosion Fe Fe 2+ + 2e O 2 + 2H 2 O + 4e – 4OH – 2Fe + O 2 + 2H 2 O 2Fe 2+ + 4OH – 2Fe(OH) 2 2Fe(OH) 3

22. 22 An anode : This is where the damage occurs. Oxidation takes place. A cathode : Here’s where the reduction reaction takes place. An electrolyte : (Almost any moisture will do.) Current Path : A current path between the cathode and anode. Summary: What’s need for corrosion to happen

23. 23 Polarization Cathodic Polarization Anodic Polarization

24. 24 2H + + 2e H 2 . . . . . . Reduction (cathodic reaction) Cathodic Polarization If electrons are available, the potential at the surface becomes negative — the excess electrons accumulate at the interface waiting for reaction — Reaction is not fast enough to accommo- date all the available electron The negative potential change is called ‘ cathodic polarization ’

25. 25 Zn Zn 2+ + 2e . . . . . . . Oxidation (anodic reaction) Anodic Polarization As the deficiency (polarization) becomes greater, the tendency for anodic dissolution becomes greater — Anodic polarization basically represents a driving force for corrosion by the anodic reaction A deficiency of electrons produces a positive potential change called ‘ anodic polarization ’

26. 26 Polarization Schematic increase in corrosion rate with increasing potential E and anodic polarization e a e a = E - E corr E corr is steady state potential

27. 27 Polarization Activation Polarization Concentration Polarization

28. 28 Activation Polarization Activation polarization refers to an electrochemical process that is controlled by the reaction sequence at the metal- electrolyte interface Let us consider the hydrogen – evolution reaction on Zn surface during corrosion in strong acid solution Hydrogen – reduction reaction under activation control (schematic representation)

29. 29 Activation Polarization Step 1 : H + diffuse to the metal surface and adsorbed or attached there Step 2 : Electron transfer occur resulting in reduction of H + to H Step 3 : Two H atoms combine together to form H 2 Step 4 : H 2 molecules combine together to form H 2 bubbles The speed of reduction of hydrogen ions will be controlled by the slowest of these steps

30. 30 Concentration Polarization Concentration polarization refers to electrochemical reactions that are controlled by the diffusion in the electrolyte Let us consider the hydrogen – evolution reaction on Zn surface during corrosion in dilute acid solution Concentration polarization during hydrogen reduction (schematic representation)

31. 31 Concentration Polarization Acid solution is dilute The number of H + ions in solution is quite small Reduction rate is controlled by diffusion of H + ions into the metal surface

32. 32 Concentration polarization is predominant when the concentration of reducible species is low (e.g. dilute acids) Activation polarization is predominant during corrosion in media containing a high concentration of active species (e.g. concentrated acids) Activation vs. Concentration Polarization In most instances, concentration polarization during metal dissolution is small and can be neglected; it is important during reduction reactions

33. 33 Influence of environmental parameters on Polarizati on Depending on what kind of polarization is controlling the reduction rate, environmental variables produce different effects For example, any changes in the system that increase the diffusion rate will decrease the effects of concentration polarization and hence increase the corrosion rate Thus, increasing the velocity or agitation of the corrosive medium will increase the rate only if the cathodic process is controlled by concentration polarization If both the cathodic and anodic processes are controlled by activation polarization, agitation will have no influence on corrosion rate

34. 34 Passivity Passivity refers to the loss of chemical reactivity experienced by certain metals and alloys under particular environmental condition That is, certain metal and alloys become essentially inert and act as if they are noble metal such as gold or platinum

35. 35 Behaviour of normal metal without passivity Corrosion rate of a metal as a function of solution oxidising Power (electrode potential)

36. 36 Passivity Passivity at oxidizing potentials above E p

37. 37 Passivity The corrosion resistance above E p , despite a high driving force for corrosion (i.e. high anodic polarization), is defined as ‘passivity’. Passive corrosion rate is very low — a reduction o f 10 3 to 10 6 times below the corrosion rate in the active state. Passivity is caused by formation of thin, protective, oxide film that acts as a barrier to the anodic dissolution. Passive film is thin — a breakdown in film results very high rate of corrosion.

38. 38 Environmental effect – Effect of O 2 and Oxidizer

39. 39 Environmental effect – Effect of Velocity Type A, Part 1 : if the corrosion process is under cathodic diffusion control Type A, Part 1 & 2 : if the corrosion process is under cathodic diffusion control and the metal is passivated subsequently Type B : if the corrosion process is controlled by activation polarization Type C : if the material is protected by massive, less tenacious film which breaks at high velocity – erosion corrosion

40. 40 Environmental effect – Effect of Temperature Type A : A very rapid or exponential rise in corrosion rate with temperature Type B : A negligible temperature effect followed by exponential rise in corrosion rate with temperature – associated with passive – transpassive behaviour

41. 41 Environmental effect – Effect of corrosive concentration Type A, Part 1 : Materials show passivity Type A, Part 1 & 2 : Materials show passivity, but at high concentration, they show high corrosion due to dissolution of protective film (like lead sulphate) Type B : Associated with acids that are soluble in all concentration of water. Initially rate is increased due to increase in H + with concentration. At very high concentration, the rate decreases as acid ionization is reduced.

42. 42 Eight Forms of Corrosion Galvanic or Two-metal corrosion Crevice Corrosion Pitting Intergranular corrosion Uniform or General attack Selective leaching or parting Erosion corrosion Stress corrosion

43. 43 Uniform or General attack Schematic illustration of uniform attack

44. 44 Galvanic or Two-metal corrosion Schematic illustration of Galvanic or Two-metal corrosion

45. 45 Crevice Corrosion Schematic illustration of crevice corrosion

46. 46 Pitting Corrosion Schematic illustration of pitting corrosion

47. 47 Intergranular Corrosion Schematic illustration of intergranular corrosion

48. 48 Selective leaching Schematic illustration of selective leaching

49. 49 Erosion Corrosion Schematic illustration of erosion corrosion

50. 50 Stress corrosion cracking Schematic illustration of stress corrosion cracking

51. 51 Uniform or General attack Schematic illustration of uniform attack Uniform corrosion accounts for the greatest tonnage of metal consumed Yet this form of corrosion is not difficult to predict and control!

52. 52 Uniform or General attack For uniform corrosion to happen, the corrosive environment must have the same access to all parts of the metal surface Metal must be metallurgically and compositionally uniform Uniform corrosion is preferred from technical viewpoint as it is predictable and thus acceptable for design

53. 53 Free Energy Mechanical analogy of free-energy change

54. 54 Free Energy Effect of reaction path on reaction rate It is not possible to accurately predict the velocity of reaction from the change of free energy

55. 55 Free Energy and Electrode Potential Zn + 2HCl Zncl 2 + H 2 Zn + 2H + Zn 2+ + H 2 Zn Zn 2+ + 2e . . . . . . . Oxidation (anodic reaction) 2H + + 2e H 2 . . . . . . Reduction (cathodic reaction) G = -nFE E = E a + E c

56. 56 Electromotive force or emf series Standard half-cell electrode potential Single electrode potential Redox potential

57. 57 Cell potential

58. 58 Cell potential E = E a + E c G = -nFE A positive E corresponding to a negative G indicates that the reaction is spontaneous in the written direction Zn Zn 2+ + 2e 2H + + 2e H 2 E = +0.762 + 0= +0.762

59. 59 Cell potential

60. 60 Cell potential 3Pb + 2Al 3+ 3Pb 2+ + 2Al Pb Pb 2+ + 2e Al 3+ + 3e Al E =  1.662 + 0.126 = 1.532 Reaction will occur in reverse direction

61. 61 Prediction of corrosion behaviour Cu + H 2 SO 4 No reaction 2Cu + 2H 2 SO 4 + O 2 2CuSO 4 + 2H 2 O Cu/Cu 2+ more +ve than H 2 /H + O 2 /H 2 O more +ve than Cu/Cu 2+

62. 62 Take home thoughts!!!!! 62 As the redox potential of metal become more positive Tendency to corrode decreases Tendency decreases even in presence of oxidizers The metals at the uppermost part of the redox series is extremely inert (e.g. Pt and Gold) Redox potential can be used to state a criterion for corrosion Corrosion will not occur until the spontaneous direction of the reaction is metal oxidation Major use of thermodynamic as far as corrosion is concerned It tells unambiguously that corrosion will not occur It does not tell about the rate of corrosion

63. 63 Concentration effects on electrode potential It is known that

64. 64 Concentration effects on electrode potential Substituting and Substituting

65. 65 Concentration effects on electrode potential 2H + + 2e H 2 As the activity of oxidizer increases, the potential becomes more noble The Nernst equation correctly predicts that increased O 2 activity makes the half cell potential more noble O 2 + 2H 2 O + 4e – 4OH – Potential is commonly considered to be a measure of oxidizing power of the solution

66. 66 Potential/pH diagram (Pourbaix Diagram ) Map showing conditions of solution oxidizing power (potential) and acidity or alkalinity (pH) for various possible phases that are stable in an aqueous electrochemical system Boundary lines on the diagram dividing areas of stability for different phases are derived from the Nernst equation

67. 67 Pourbaix Diagram of water Potential/pH diagram showing conditions of stability for water and its decompositions products, oxygen and hydrogen

68. 68 Pourbaix Diagram of water 2H + + 2e H 2 2H 2 O + 2e – H 2 + 2 OH – Both these equations are equivalent and the second equation clearly demonstrate that electrochemical evolution of hydrogen represents the decomposition of water

69. 69 Pourbaix Diagram of water As potential become more noble (positive), O 2 + 4H + + 4e – 2H 2 O O 2 + 2H 2 O + 4e – 4OH – Both these equations are equivalent At potentials noble (positive) to e O2/H2O at any pH, water is unstable and will be oxidized to O 2, Below e O2/H2O, water is Stable and disolved oxygen is reduced to water, if present.

70. 70 Pourbaix Diagram of Al Corrosion is possible in the aresas of pourbaix diagram where soluble ions Of the metals are stable. The metal is possibly resistant to corrosion or passive in areas where oxide is stable. Al is a amphoteric metal

71. 71 Pourbaix Diagram of Al Metal itself is stable & immune to corrosion Aluminate cation is stable Aluminum cation is stable Al 2 O 3 oxide is stable

72. 72 Pourbaix Diagram of Fe Prtotective oxide forms in the neutral solution (similar to that of Al) For Fe, the field of oxide stability is greater at elevated pH and Fe is far more resistant to alkaline solution

73. 73 Corrosion rate m = mass reacted I = current a = atomic weight Zn Zn 2+ + 2e A = surface area i = current density Current density rather than current is proportional to corrosion rate

74. 74 Exchange current density i = exchange current density Exchange current density is a convenient way of representing rates of oxidation & reduction at equilibrium

75. 75 Exchange current density Exchange current density is a fundamental kinetic parameter G is not affected by the electrode surface Exchange current density is affected by the electrode surface

76. 76 Electrode Kinetics Deviation from equilibrium potential is called as polarization

77. 77 Polarization Polarization can be defined as the displacement of electrode potential from the net current Cathodic Polarization Anodic Polarization

78. 78 Activation Polarization Activation polarization curve of a hydrogen electrode Anodic Polarization Cathodic Polarization Tafel equation represents the expression 2.3RT/nF and known as Tafel constant  is overvoltage

79. 79 Concentration Polarization Low reduction or High concentration High reduction or Low concentration

80. 80 Concentration Polarization i L = Limiting current density C B = bulk concentration of ions x = thickness of the diffusion layer

81. 81 Concentration Polarization

82. 82 Combined Polarization Applies to all cathodic reaction Applies to almost all dissolution process except the metal showing active-passive

83. 83 Combined cathodic polarization

84. 84 Mixed potential theory Any electrochemical reaction can be divided into two or more partial reduction & oxidation reductions There can be no net accumulation of electrical charge during an electro-chemical reaction

85. 85 Mixed Electrode

86. 86 Mixed Electrode

87. 87 Mixed Electrode

88. 88 Corrosion rate and exchange current density

89. 89 Mixed Electrode – effect of oxidizer

90. 90 Mixed Electrode – effect of oxidizer

91. 91 Mixed Electrode – effect of oxidizer No effect on corrosion when oxidizer of low i is added to an acid solution

92. 92 Mixed Electrode – effect of external current

93. 93 Combined activation & concentration polarization

94. 94 Effect of stirring during combined polarization Effect of stirring during corrosion on I L and i corr

95. 95 Passivity

96. 96 Active – passive metal

97. 97 Effect of exchange current density on passivity Case 1: Ti in dilute air free sulfuric or HCl acid Case 2: Fe in dilute nitric acid Case 3 : Stainless steel in acid solution containing oxidizer or dissolved O 2

98. 98 Effect of passivity on engineering design Case 3 is the most desirable from engineering viewpoint Case 2 is the least desirable from engineering viewpoint Case 1 is particularly not desirable as corrosion rate is high

99. 99 Effect of oxidizer concentration on corrosion Let us assume that exchange current density of the oxidizer remain constant Effect of oxidizer concentration on the electrochemical behavior of an active- passive metal The only effect of the oxidizer is to shift the reversible potential in the passive direction

100. 100 Effect of oxidizer concentration on corrosion Effect of oxidizer concentration on the corrosion rate of an active-passive metal Amount of oxidizer necessary to cause passivation is greater than that required to maintain it To safely maintain passivity, the oxidizer concentration should be equal to or greater than the minimum amount necessary to cause spontaneous passivation

101. 101 Effect of deaeration, aeration and stirring on corrosion 2H 2 O + 2e – H 2 + 2OH – O 2 + 2H 2 O + 4e – 4OH –

102. 102 Effect of stirring velocity on corrosion Active metal Active-passive metal

103. 103 Galvanic or Two-metal corrosion The less resistant metal becomes anodic The more resistant metal becomes cathodic The driving force for current and corrosion is the potential developed between the two metal

104. 104 Dry-cell Battery Dry cell Battery Zn( s ) → Zn 2+ ( aq ) + 2 e - - oxidation reaction that happens at zinc = anode 2MnO 2 ( s ) + 2 H + ( aq ) + 2 e - → Mn 2 O 3 ( s ) + H 2 O( l ) - reduction reaction at carbon rod = cathode

105. 105 Galvanic or Two-metal corrosion

106. 106 Standard emf series

107. 107 Galvanic series PASSIVE – will not corrode – act as cathode. These elements are least likely to give up electrons! ACTIVE – will corrode – act as anode. These elements most likely to give up electrons!

108. 108 Corroding metal – inert metal couple Rate of hydrogen evolution on Zn is reduced Corrosion rate of Zn in the couple is increased Corrosion potential of Zn is shifted to a more noble value

109. 109 Corroding metal – inert metal couple Effect of exchange current density on galvanic corrosion

110. 110 Corroding metal – corroding metal couple With corroding metals coupled, the total oxidation rate must be considered, as well as the total reduction rate

111. 111 Corroding metal – corroding metal couple If two corroding metals are galvanically coupled, the corrosion rate of the metal with the most active corrosion potential is accelerated and that of the other metal is retarded. These apply to all galvanic couple and are independent of the absolute corrosion rate of the two metals involved. Hence, in determining the effect of galvanic coupling, the anode should be defined on the basis of corrosion potential rather than corrosion rate The corrosion behaviour of galvanic couple is determined by the reversible electrode potential of the actual processes involved, their exchange current densities and Tafel slopes, and the relative areas of the two metals

112. 112 Galvanic couple between active-passive metal and Platinum

113. 113 Galvanic couple between Ti and Platinum Only two metals, Ti and Chromium produces spontaneous passivation in air-free solution if coupled with Pt

114. 114 Environmental effects on Galvanic Corrosion Galvanic corrosion is greater near the sea-shore than in a dry rural atmosphere Condensation near a sea-shore contains salt and therefore more conductive and corrosive Galvanic corrosion does not occur when the metals are completely dry since there is no electrolyte to carry the currents between the electrodes!

115. 115 Distance Effect on Galvanic Corrosion Accelerated corrosion due to galvanic effect is usually greatest near the junction The attack decreases with increasing distance from the junction The distance effect largely depends on the conductivity of the solution

116. 116 Area Effect on Galvanic Corrosion An unfavorable area ratio consists: Large cathode and small anode For a given current flow in the cell, the current density is greater for smaller electrode than the larger one The greater the current density at an anodic area, the greater the corrosion rate

117. 117 Area Effect on Galvanic Corrosion Large cathode area provides more surface for the reduction reaction and the anodic dissolution current must increase to compensate

118. 118 Area Effect on Galvanic Corrosion.......Examples Steel plates with Copper rivets Copper plates with steel rivets Favorable area ratio Unfavorable area ratio Large anode Small cathode Large cathode Small anode

119. 119 Area Effect on Galvanic Corrosion.......Examples Unfavorable area ratio

120. 120 How to Prevent Galvanic Corrosion Select combination of metal as closely as possible in galvanic series Avoid unfavorable area effect of small anode and large cathode Insulate dissimilar metals wherever applicable – It is important to insulate completely, if possible

121. 121 How to Prevent Galvanic Corrosion Bakelite washers under the bolts and nut assume to isolate these parts, yet the shank of the bolt touches both flange – The problem is solved by putting plastic tubes over the bolt shanks, plus the washer so that the bolts are isolated completely from the flanges Proper insulation of a flanged joint

122. 122 Appling coating with caution How to Prevent Galvanic Corrosion Coat the more noble (i.e. more corrosion resistant) metal of the galvanic couple “ to gild refined gold, to paint the lily – to throw perfume on violet, to smooth the ice, or to add another hue unto the rainbow – is wasteful and ridiculous excess” ………Shakespeare

123. 123 How to Prevent Galvanic Corrosion Adding inhibitors to decrease the aggressiveness of the environment Design for the use of readily replaceable anodic part or make them thicker Install a third metal that is anodic to both metals in galvanic contact

124. 124 Beneficial Application of Galvanic Corrosion Cathodic Protection The protection of metal by simply making it the cathode of a galvanic cell – Galvanized (Zn-coated) steel is the classic example The Zn coating is put on the steel, not because it is corrosion resistant, but because it is not The Zn corrodes preferentially and protects the steel – Zn acts as a sacrificial anode

125. 125 Beneficial Application of Galvanic Corrosion Galvanized steel

126. 126 Tin plating The tin, which is more corrosion resistant than Zn is not desirable – Sn is cathodic to steel At perforations in the Sn coating, the corrosion of the steel is accelerated due to galvanic action Tin plating

127. 127 Crevice Corrosion Intensive localized corrosion frequently occurs within the crevices and other shielded areas on metal surface exposed to corrosives This type of attack usually associated with small volumes of stagnant solution caused by holes, gasket surfaces, lap joints, surface deposits, crevices under bolt and river heads – As a result, this form of corrosion is called crevice corrosion

128. 128 Crevice Corrosion Crevice corrosion on stainless steel – gasket interface A sheet of 18-8 stainless steel can be cut by placing a stretched rubber band around it and then immersing it in sea water

129. 129 Crevice Corrosion……mechanism . . . . . . . Oxidation . . . . . . Reduction M M n+ + ne O 2 + 2H 2 O + 4e – 4OH – Initially the reaction occurs uniformly over the entire surface, including the crevice area After some interval, the O 2 within the crevice is depleted Crevice corrosion….. initial stage

130. 130 Crevice Corrosion……mechanism After O 2 is depleted, no further O 2 reduction occur although dissolution of M continues to happen This tends to produce in excess of M + which is balanced by the migration of Cl – M + Cl - + H 2 O = MOH + H + Cl - Both Cl - and H+ accelerates the dissolution rates This increase in dissolution increases the dissolution, and the result is rapidly accelerating or autocatalytic process

131. 131 Crevice Corrosion……mechanism Crevice corrosion ….. Later stage

132. 132 Crevice Corrosion……Susceptibility Metal that depends on oxide films or passive layers for corrosion resistance are particularly susceptible for crevice corrosion (e.g. 18:8 stainless steel, Al) Optimum crevice corrosion resistance would be achieved with an active-passive metal possessing: A narrow active-passive transition A small critical current density An extended passive region Example: Ti, high-nickel alloys (like Hastelloy C) Alloys like Type 430 stainless steel with a large critical current density, a wide active-passive transition, and a limited passive region is extremely susceptible to crevice corrosion

133. 133 Crevice Corrosion……How to Combat Use welded butt joints instead of riveted or bolted joints in new equipment Close crevices in existing lap joints by continuous welding or soldering Design vessels for complete drainage; avoid sharp corners and stagnant areas Butt Joint Riveted Joint

134. 134 Crevice Corrosion……How to Combat Inspect equipment and remove deposits frequently Remove solids in suspension early in the process or plant flow sheet, if possible Use solid, nonabsorbent gaskets such as Teflon, wherever possible Weld instead of rolling in tubes in tube sheets

135. 135 Pitting Corrosion Pitting is a extremely localized form of attack that results in holes in the metal A pit may be described as a cavity or hole with the surface diameter about the same or less than the depth Pitting of 18-8 stainless steel

136. 136 Pit morphology

137. 137 Pitting Corrosion Pitting is one of the most destructive and insidious form of corrosion It is often difficult to detect pits because of there small size and because pits are often covered with corrosion product Pitting is particularly vicious because it is a localized and intense form of corrosion, and failure often occurs with extreme suddenness

138. 138 Pit morphology Deep & closely spaced Shallow Deep

139. 139 Pitting Corrosion Pitting can be considered as intermediate stage between general overall corrosion and complete resistance to corrosion Schematic representation of pitting corrosion as an intermediate stage Increasing temperature and/or the concentration of FeCl 3

140. 140 Standard rating chart for pitting corrosion

141. 141 Evaluation of pitting damage Relationship between pit depth and number of pits Pit depth as a function of exposed area

142. 142 Autocatalytic Nature of Pitting Pits initiate at certain locations where local dissolution of metal is momentarily high due to a surface scratch, emerging dislocations or other defects, or random variations in chemical composition Autocatalytic processes occurring in a corrosion pit The dissolution of metal within the pit tends to produce excess positive charge This leads to migration of chloride ions to maintain the electro-neutrality

143. 143 Autocatalytic Nature of Pitting Autocatalytic processes in a corrosion pit M + Cl - + H 2 O = MOH + H + Cl - Pits also have high concen- tration of H + ions due to hydrolysis Both hydrogen and chloride ions stimulate the dissolution of metals within the pit and it accelerates with time Since the solubility of O 2 in concentrated solution is virtually zero, no O 2 reduction occurs within the pit but it happens in adjacent to pits Pits cathodically protect rest of the surface

144. 144 Influence of Cl - on pitting

145. 145 FeOOH + H 2 O = Fe 3+ + 3OH - Influence of Cl - on pitting FeOOH + Cl - = FeOCl + OH - FeOCl + H 2 O = Fe 3+ + Cl - + 2OH - In absence of Cl - , the passive film dissolve slowly, as Fe 3+ ion (see Fig. a) Cl - catalyze the liberation of Fe 3+ ion by displacement of the outer layer of the passive film (see Fig. b) (a) (b) (c)

146. 146 Pitting potential and protection potential

147. 147 Cyclic polarization and hysteresis

148. 148 Effect of metallurgical variables on pitting Stainless steel alloys are more susceptible to damage by pitting corrosion than are any other group of metals or alloys Effect of alloying on pitting resistance of stainless steel Ordinary steel is more resistant to pitting than stainless steel

149. 149 Prevention of pitting corrosion Addition of 2% Mo to 18:8 SS (type 304) to produce type 316 results in very large increase in pitting resistance

150. 150 Pitting and Crevice corrosion – Do we need to differentiate them? Mechanism of pit growth is virtually identical to that of crevice corrosion All systems that show pitting are susceptible to crevice corrosion However, reverse is not always correct – many systems that show crevice attack do not suffer pitting on freely exposed surface Pitting is self-initiating form of crevice corrosion – it does not require a crevice – it creates it own!

151. 151 Intergranular Corrosion Schematic illustration of intergranular corrosion Localized attack at and adjacent to GBs with relatively littlie corrosion of the grains

152. 152 Carbide precipitation at GB during sensitization Schematic representation of IG corrosion in stainless steel

153. 153 Effect of Cr content on anodic polarization of Fe-Ni alloys Effect of Cr content on anodic polarization of Fe-Ni alloy Below about 12% Cr, the passive potential region is substantially constricted

154. 154 Sensitization diagram for 18:8 SS with varying C content Sensitization diagram of 18:8 SS with varying C content Cr & C are the primary elements those influence the sensitization. Ni and Mo enhances the sensitization as well

155. 155 Weld Decay Weld decay in heat affected zone (HAZ) of a weld in stainless steel Sensitization of austenitic stainless steel during welding is known as weld decay

156. 156 Thermal treatments producing weld decay (a) (b) (a) Temperature – time relationship (b) Location of thermocouple

157. 157 How to control sensitization in stainless steel Employing high temperature annealing followed by fast quenching Adding elements that are strong carbide former (like Ti or Nb). For example, stabilized stainless steel like Type 321 stainless steel (which is Ti stabilized SS) Type 347 stainless steel (which is Nb stabilized SS) Lowering the C content below 0.03 wt% Modifying the grain boundary character (Grain boundary engineering approach)

158. 158 Knife-Line Attack (KLA) Under certain conditions, the stabilized austenitic SS even are attacked intergranularly Ti or Cb fails to combine with C Severe IG corrosion occurred in a narrow band, on both side of weld and immediately adjacent to it Practically no corrosion occurred in the remainder The mechanism of KLA is based on the solubility of Cb in the stainless steel

159. 159 Knife-Line Attack (KLA) In weld metal everything is in solution, no precipitation Away from the weld metal, columbium carbides precipitate chromium carbides precipitates on both side on weld and immediately adjacent to it During solution annealing During cooling following welding

160. 160 Similarities & differences bet n KLA & weld decay Similarities Both results from intergranular corrosion Both are associated with welding Dissimilarities KLA occurs in the narrow band of metal immediately adjacent to the weld, whereas weld decay occurs at an appreciable distance from the weld KLA occurs in stabilized steel The thermal history is different in both the cases

161. 161 Surface defect Based on misorientation angle (  ) Low angle GB (  ≤15°) High angle GB (>15°) Coincidence Site Lattice (CSL) boundary Grain Boundary

162. 162 High Angle Boundary Low Angle Boundary

163. 163 CSL is basically the sites at which the lattice of the two crystals forming a boundary would coincide if they are extended into one another ‘  ’ is reciprocal density of coinciding sites CSL Boundary

164. Red and Green lattices coincide Points to be brought into coincidence Rotation to Coincidence

165. Relating to the 5 relationship

166. Relating to the 5 relationship

167. Relating to the 5 relationship

168. Relating to the 5 relationship

169. Relating to the 5 relationship

170. Relating to the 5 relationship

171. 171 Relating to the 5 relationship

172. Relating to the 5 relationship

173. Relating to the 5 relationship

174. Red and Green lattices coincide after rotation of 2 tan -1 (1/3 ) =36.9 ° 5 relationship

175. 175 To increase the fraction of low  (≤29) coincidence site lattice (CSL) boundaries To disrupt the random high angle grain boundaries ( HABs ) connectivity Approach D. Wolf, J. Phys. (Paris), 46 (1985) 197 Coherent twin (a) (b) 175 [ courtesy: Watanabe] Grain Boundary Engineering Modification of grain boundary character

176. 176 Grain boundary character of (a) BM and (b) GBE specimen. The black and gray lines indicate random and SBs, respectively Grain Boundary Engineering in 316 SS Base Material (BM) specimen shows less fraction of SBs and high random HABs connectivity GBE specimen shows high fraction of SBs and less random HABs connectivity SBs – Special Boundaries (CSL 29 )

177. 177 SEM images of (a & b) surfaces and cross-sections (c & d) of 316 BM and GBEM after the 120-h ferric sulfate–sulfuric acid test IG Corrosion behaviour of BM and GBE specimen The IG corrosion both in surface and cross-section of GBE specimen is much lower than that of BM

178. 178 Arrest of percolation of intergranular corrosion from the surface by distributed SBs The uniform distribution of high fraction of SBs in the GBEM suppressed the deep propagation of intergranular corrosion from the specimen surface due to extensive disruption of the random boundary network IG Corrosion percolation

179. 179 IG Corrosion in other alloys Because of lower solubility of interstitials in ferrite, the ferritic stainless steel sensitize much more rapidly at lower temperature than austenitic SS

180. 180 IG Corrosion in other alloys Duralumin alloy (Al- Cu) alloy due to formation CuAl 2 precipitates The Nickel alloy C, containing Cr, Mo, Fe can be subjected to IG corrosion due to grain boundary precipitation of carbides or Mo rich intermetallic phase

181. 181 Corrosion Rate expression (from weight loss) Mils per year (mpy) Where W = weight loss (mg) D = density of specimen (g/cm 3 ) A = area of specimen, (sq. in) T = Exposure time, (hour) Relative corrosion resistance mpy Outstanding <1 Excellent 1-5 Good 5-20 Fair 20-50 poor 50-200 Unacceptable >200

182. 182 Corrosion Rate expression (from current density) Where, a = atomic weight of the metal i = current density (µA/cm 2 ) n = number of electron lost D = density (g/cm 3 ) K = constant to express the penetration in desired rate; to express in mpy K is 0.129

183. 183 Electrochemical Potentiokinetic Reactivation (EPR) Test Objective: Quantitative study of the susceptibility or tendency of Materials (SS and alloys) to IGC Advantages of EPR over other tests Meet the dual requirement of quantitative as well as nondestructive rapid process

184. 184 Measures the amount of charge associated with the corrosion of the chromium-depleted region surrounding chromium carbide precipitated particles Based on the preferential breakdown of the passive film on the sensitized grain boundaries, where chromium is depleted during a controlled potential sweep from the passive to active regions(reactivation) Principle of EPR Test

185. Schematic EPR Curves for Sensitized and Solutionized AISI Type 304 SS Characteristics of the formed passive layer depends on the chemical composition, surface constituents and distribution of alloying elements, particularly, chromium on the sample surface Highly sensitized materials should show a greater increase in current density than the less sensitized or unsensitized materials

186. 186 SL-EPR In the single loop EPR test the curve is a reverse curve, with the potential scan from positive to negative. DOS=Area under the reactivation peak DL-EPR In the double loop EPR test this is a cyclic curve consisting of a forward scan followed by a reverse scan. The DL-EPR test is independent of surface finish and the presence of random pitting or metallic inclusions (cleaning effect on the specimen surface during the forward anodic scan) DOS = percent ratio of the maximum current density in the reactivation loop to that of anodic loop. Test solution, test temperature and the scan rate are same for both SL-EPR and DL-EPR test Type of EPR Test Test solution: 1 L of 0.5 M H 2 SO 4 +0.01 M KSCN

187. 187 SL-EPR vs. DL-EPR SL-EPR DL-EPR

188. 188 Integrated charge (Q ): The charge measured, in coulombs, during reactivation as given by the time integral of current density below the reactivation peak of the curve. Maximum anodic current density during reactivation(I r ): The current density measured at the peak of the anodic curve during reactivation. Normalized charge (Pa) : The integrated current normalized to the specimen size and grain size. Pa represents the charge (in coulombs/cm 2 ) of the grain-boundary area. Reactivation : In the electrochemical reactivation (EPR) test, the potential sweep from the passivation potential returning to the corrosion potential. Scan rate : T he rate at which the electrical potential applied to a specimen in a polarization test is changed . Terminology

189. 189 Normalized charge (P a ) P a = Q/X (coulombs/cm 2 ) Q = Charge measured on current integration measuring instrument (coulombs) X = A s [5.1 × 10 -3 e 0.35 G ] A s = specimen area (cm 2 ) G = grain size at 100 x Degree of sensitization (DOS) is defined as the percent ratio of the maximum current density in the reactivation loop to that of anodic loop. DOS % = I r /I a x 100 where I r is the reactivation peak current density and I a is the anodic peak current density

190. 190 Ir/Ia < 1% Unsensitized 1% < Ir/Ia < 5% Partially sensitized Ir / Ia > 5% Fully Sensitized SL-EPR DL-EPR

191. 191 Selective leaching Schematic illustration of selective leaching Selective leaching is the removal one element from a solid alloy by corrosion process The most common example is selective removal of Zn in brass alloy

192. 192 Dezincification characteristics Plug type dezincification Common yellow brass consists of 30% Zn and 70% Cu After dezincification, the alloy assumes a red or copper color that contrasts with the original yellow The dezincified portion is weak, permeable and porous. The material is brittle and possesses little strength

193. 193 Dezincification mechanism Two theories have been proposed for dezincification 1 st theory : Zn is dissolved, leaving vacant sites in the brass lattice structure Strong argument against this theory exists: Dezincification of appreciable depths would be impossible or extremely slow because of difficulty of diffusion of solution and ions through a labyrinth of small vacant sites 2 nd theory : (a) Brass dissolves, (b) the Zn stays in solution, and then (c) the copper redeposit

194. 194 Dezincification mechanism Potential regions in chloride solutions for simultaneous and separate dissolution of Cu and Zn and re-deposition of Cu

195. 195 Dezincification mechanism e zn /zn2+ = – 0.763 + 0.0295 log(Zn 2+ ) = – 0.90V e Cu/CuCl2- = 0.208 + 0.0591 log(CuCl 2 - ) – 0.1182 log( Cl - ) = – 0.03 V e CuCl2/Cu+ = 0.465 + 0.0591 log(Cu 2+ )/(CuCl 2 - ) +0.1182 log( Cl - ) = 0.01 V e Cu/Cu2+ = 0.337 + 0.0295 log(Cu 2+ ) = 0.16 V e CuCl /Cu+ = 0.537 + 0.0591 log(Cu 2+ ) + 0.0591 log( Cl - ) = 0.12 V e Cu/ CuCl = 0.137 – 0.0591 log( Cl - ) = 0.20 V

196. 196 Dezincification mechanism Selective dissolution from brass, which requires solid state diffusion of Zn, is too slow to account for the usual penetration rates observed in service and lab dezincification Re-deposition seems necessary to account for accelerated dezincification between 0.0 to + 0.2 V where copper can be re-deposited dezincification predominantly occurs in unstirred solution where dissolved copper could accumulate leading to copper re-deposition

197. 197 Dezincification prevention Use a less susceptible alloy – like red brass (15% Zn). For severely corrosive environments where dezincification occur, or for critical parts, cupronickels (70 - 90% Cu, 30 – 10% Ni) are utilized A small addition of Sn (~1%) to a 70 – 30 brass (Admiralty Metal) significantly reduces dezincification Further improvement was obtained by adding small amounts of arsenic, antimony, or phosphorous as inhibitors

198. 198 Graphitization Gray cast iron sometimes shows the effect of selective leaching Gray cast iron generally contains about 2 to 4% C and 1 to 3% C The graphite is cathodic to iron and an excellent galvanic cell exists. The iron is dissolved, leaving a porous mass of graphite, void and rust Gray cast iron

199. 199 Graphitization Graphitization is a slow process. Its prevalent in underground pipelines If the cast iron in an environment that corrodes this metal rapidly, then more-or-less uniform corrosion occurs Graphitization does not occur in white cast iron as there is no free carbon Ductile (nodular) cast iron instead of gray cast iron is recommended in underground pipeline. Ductile cast iron with a cement mortar lining gives excellent performances

200. 200 Selective leaching in other alloy systems Less commonly occurs in following systems: Selective removal of Si from silicon bronzes (Cu-Si) Selective removal of Al in aluminum bronzes in HF and other acids Removal of Co from Co-W-Cr alloy