2. • “Stainless steels are not the solution to all corrosion problems”.
Stainless steels do not rust in normal atmospheric exposures, in
natural waters and in oxidizing acids. But they corrode in reducing
acids. In chloride containing media they undergo localized corrosion
like pitting, crevice corrosion, intergranular corrosion and stress
corrosion cracking.
• Stainless steels derive their corrosion resistance due to passivity.
Passivity is achieved with the addition of a minimum of 12% Cr.
Nickel, molybdenum, copper enhance general corrosion resistance
in sulphuric acid. Mo imparts pitting corrosion resistance. Ti, Nb and
Ta are added to combat intergranular corrosion of austenitic
stainless steels.
3.
4.
5.
6.
7.
8.
9.
10.
11. Exchange Current Density
At equilibrium current does not flow through the circuit, but there is
always a finite exchange of ions and atoms at the interface:
Zn = Zn++ + 2e
Some moles of zinc are leaving the surface and entering the electrolyte as
zinc ions. At the same time an equal number of zinc ions from the
electrolyte are getting reduced on the electrode surface.
Since electron transfer is involved, the rate of exchange can be expressed
in terms of current density using Faraday’s law:
roxid = rred = i0 a/nF
where roxid and rred are the equilibrium oxidation rates. And i0 is the
exchange current density.
12.
13.
14.
15. Polarization
As the electrodes of a cell are short-circuited, current starts
flowing through the circuit
The potentials of the electrodes start deviating from their
equilibrium potential values
The deviation from equilibrium potential is called Polarization
and the extent of deviation is termed Overvoltage, which is
expressed by the Greek letter,η
16. There are two principal types of polarization:
1. Activation Polarization
2. Concentration Polarization
There is a third type, Resistance Polarization, arising out of
electrolyte resistance, and represented by IR drop.
17.
18.
19.
20.
21. • Limiting diffusion current density (IL) represents the
maximum rate of reduction possible for a given system.
• It is expressed by:
IL = DnFCB /x
where, D is diffusion coefficient of the reaction ions, CB
is concentration of reacting ions, and x is the thickness of the
diffusion layer.
22.
23.
24.
25.
26. Passivity
Passivity refers to the phenomenon of loss of chemical reactivity
of a metal or an alloy in an environment where
thermodynamically the reaction ought to have occurred.
It results from the formation of a thin oxidized protective film on
the surface of a metal.
Passivity is defined as a condition of corrosion resistance due to
formation of thin films under oxidizing conditions with high
anodic polarization.
27. Important structural metals like aluminium, iron, nickel, chromium,
titanium and their alloys can be passivated simply by exposure to
strong oxidizing media or by anodic polarization or both.
Other metals that show passivity include silicon, tantalum, niobium,
molybdenum and zirconium.
Usual corrosion conditions are not sufficiently oxidizing to induce
passivity in iron, but they do passivate aluminium and titanium.
Iron can be rendered passive by an initial exposure to fuming nitric
acid.
28.
29. • The phenomenon of passivity was first demonstrated by
Faraday. He attributed passivity to the iron oxide film
formed on the surface of metal on exposure to hot
concentrated nitric acid. Once formed, its slow
dissolution in this environment then determines the
corrosion rate of the metal.
• The passive state is not an inert or static state, but a
dynamic condition in which there is continuous
dissolution and repair of the passive film at discrete
points in the surface.
30. Findings of Monnartz on passivity of Fe-Cr alloys (1911):
• Low-chromium iron alloys with 12.5% or more chromium
are resistant at room temperature to nitric acid at all
concentrations. With 14% Cr or more, Fe-Cr alloys resist
such solutions at temperatures up to boiling.
• In reducing acids, additions of chromium to iron increase
the rate of corrosion.
• Mo additions increase resistance in nitric acid containing
chloride salts.
• Passivity depends on a source of oxygen, either from a
compound in solution or from dissolved oxygen gas.
31. • In solutions in which a given Fe-Cr alloy is not passive e.g.
nitric acid containing a chloride salt, passivity can be induced
by contacting the specimen with a platinum wire, adding
platinum as alloying element, or making the Fe-Cr alloy an
anode by means of a cathode and an external EMF (anodic
protection). All of these procedures change the potential of
the alloy in the noble direction.
32.
33.
34.
35.
36.
37. • Case 1 : Titanium or stainless steels in dilute, air-free sulphuric or
hydrochloric acid. The metal corrodes and does not passivate.
• Case 2: Chromium in air-free sulphuric acid and iron in dilute nitric acid
after the initial passivation in hot fuming nitric acid.
Stainless steels exposed to aerated acid solutions or acid solutions
containing traces of oxidizers.
• Case 3: Stainless steels or titanium in acid solutions containing
oxidizers such as ferric salts or dissolved oxygen and also iron in
concentrated nitric acid.
38.
39. Effects of Environmental Factors on Passivity
1. Temperature and pH
2. Velocity
3. Addition of oxidizers
4. Addition of halides
5. Galvanic coupling
40.
41.
42.
43.
44.
45.
46.
47. Alloy Evaluation
Using mixed-potential theory, it is possible to estimate the
corrosion behaviour of an alloy and to determine the effect of
alloying additions from electrochemical data.
If an active-passive metal is exposed to an aerated corrosive
medium, it spontaneously passivates if its critical current density is
less than the limiting diffusion current density for oxygen
reduction.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57. The Role of Alloying Elements
General Corrosion in Acids
Chromium
• The minimum concentration of chromium required for passivity is a
function of the type of acid, its concentration and temperature. Thus, there
is no fixed ratio of iron to chromium concentration in Fe-Cr alloys that
characterizes the passive state.
• In practice, an alloy with 14-18% Cr provides resistance in a number of acid
environments Table 3, Uhlig). In boiling 65% nitric acid, Type 430 shows a
corrosion rate of 0.5 mm per year where carbon steel 1020 shows a rate of
4500 mm/year. . Higher Cr content, as in Type 446 (23-27%Cr), gives still
better performance.
• In 10% sulphuric acid, however, rate is 6400 mm/year and in 1% HCl, the
rate is 1500 mm/year for Type 430.
58. • The effect of Cr concentration on corrosion of iron in an oxidizing
environment is shown in Fig 1 (Uhlig). In boiling H2SO4 with ferric sulphate,
the passive rate is reduced by 99% as the Cr content is increased from 12
to 25%. Ferric sulphate addition raises corrosion potential from -0.6 V to
+0.6 V.(Table 2, Uhlig)
• Ferric sulphate promotes passivity through a cathodic reaction of
reduction of ferric to ferrous ion (Fe+3 + e Fe+2), with anodic dissolution
of Fe, Cr and Ni (Fe Fe+2 + 2e). Hydrogen evolution reaction follows
when all of ferric sulphate is used up.
• In active state, the alloy shows an increase in rate of corrosion (in 5%
H2SO4) by a factor of 3 with increasing Cr from 10 to 35%.(Fig 1)
59.
60.
61. Nickel
• Both in oxidizing and reducing acids, nickel addition may actually
increase the corrosion rate of Fe-Cr alloys. Fig 1 shows that 10% Ni
alloy with <16% Cr have higher corrosion rates than the alloy with
no nickel. Only when Cr is increased to >16%, there is a rapid
reduction in corrosion.
• Thus in the presence of 10% Ni , increasing Cr >16% reverses the
deleterious effect of Ni, indicating that at these concentrations
there is a synergistic effect between Cr and Ni on the rate of
hydrogen evolution corrosion (active state).
62. • The beneficial effect of Ni in higher Cr alloys is evident from
Fig 3 (Uhlig). Corrosion rate of 18 Cr- 8 Ni (Type 304) is
appreciably lower than those of 16% Cr alloy (Type 403) or
even 25% Cr alloy (Type 446).
• To provide a wider range of resistance to corrosion in
sulphuric acid and sea water, 4% Ni is added to 28 Cr-2 Mo
and 2% Ni to 29 Cr-4 Mo new ferritic stainless steels.
• Corrosion rate of Type 304 increases with increased
concentration of sulphuric acid. (Fig 2, Uhlig)
63.
64.
65. Molybdenum
• Molybdenum enhances passivity of stainless steels in chloride
environments. Corrosion resistance of Type 304 steel enhances with
addition of 2.5% Mo (Type 316). Since Mo is a ferrite former, Ni content is
increased by 3% in the alloy that could contribute to resistance.
• Like nickel, molybdenum converts chromium into a beneficial alloying
element in reducing acids. But there is no significant effect on corrosion in
oxidizing acids or solutions.
• Mo contributes to resistance to pitting of austenitic stainless steels in
chloride containing media. All new ferritic alloys contain molybdenum (1
to 4%), primarily for increasing resistance in sulphuric acid and to pitting.
66. Cr-Ni-Mo-Cu alloy (Alloy 20)
• To provide resistance in broader ranges of concentrations and temperatures of
sulphuric acid solutions, Ni content of Type 316 was increased from 11 to 20% and
2.2% Cu was added. A similar type alloy, 20 Cr-29 Ni- 2.25 Mo- 3.25 Cu, was
developed by Fontana.
• Mo enhances resistance at sulphuric acid concentrations between 20 and 70%,
while Cu provides resistance at concentrations <20% and >70%. Increasing Ni
contributes resistance over the entire range of up to 75% acid.
• The alloy also performs well in oxidizing acids and solutions.
• To combat SCC in sulphuric acid in the range of 20 t0 80%, nickel was increased in
the alloy from 29 to 40% (Carpenter 2 Cb -3 alloy). This alloy is usable in all
aggressive environments, only next to nickel-based Hastelloy alloys in
performance.
67.
68. Performance in 65% Nitric Acid
Corrosion rate in mm per year
• Carbon Steel 4500
• Type 430 (16%Cr) 0.5
• Type 446 (25%Cr) 0.2
• Type 304 (18Cr-8Ni) 0.2
• Type 316 (18Cr-10Ni-2,5Mo) 0.3
• Hastelloy Alloy C 11.4
• (16Cr-54Ni-16Mo-4W)
• Carpenter 20 –Cb3 0.5
• (20Cr-34Ni-2.5Mo-3.5Cu)
• Titanium 9.3
69. Performance in 10% Sulphuric Acid
Corrosion rate in mm per year
• Carbon Steel 1300
• Type 430 (16%Cr) 6400
• Type 446 (25%Cr) 6900
• Type 304 (18Cr-8Ni) 400
• Type 316 (18Cr-10Ni-2,5Mo) 22
• Hastelloy Alloy C 0.4
• (16Cr-54Ni-16Mo-4W)
• Carpenter 20 –Cb3 1.1
• (20Cr-34Ni-2.5Mo-3.5Cu)
• Titanium 160
70. Performance in 1% Hydrochloric Acid
Corrosion rate in mm per year
• Carbon Steel 430
• Type 430 (16%Cr) 1500
• Type 446 (25%Cr) 1900
• Type 304 (18Cr-8Ni) 81
• Type 316 (18Cr-10Ni-2,5Mo) 71
• Hastelloy Alloy C 0.3
• (16Cr-54Ni-16Mo-4W)
• Carpenter 20 –Cb3 0.0
• (20Cr-34Ni-2.5Mo-3.5Cu)
• Titanium 5.6
71.
72.
73.
74.
75.
76.
77.
78.
79. Noble metals
• Addition of noble metals like Cu, Pt, Pd in small amounts
decrease corrosion rate of stainless steels drastically.(Figs 5 &
6, Uhlig text) This is related to the low hydrogen overvoltage
of these metals.
80.
81.
82.
83. Pitting Corrosion
• Chloride salts impair the passive state of iron-chromium
alloys. Depending upon the factors involved in the passivity
breakdown and the morphology of the subsequent attack,
localized corrosion can be classified as pitting or crevice
corrosion if a chemical micro- or macro-heterogeneity is
developed at the metal/solution interface.
84. Pitting Corrosion
Pitting is a localized form of corrosion resulting in the
formation of pits i.e. small holes or cavities with surface
diameter equal to or less than the depth.
Depth of pitting is expressed by pitting factor, a ratio of
deepest metal penetration to average metal penetration as
determined by weight loss of the specimen.
85.
86.
87.
88.
89. Characteristic features of pitting
1. The attack is spread over small discrete areas. Sometimes they are close
together. There can be subsurface enlargement of a pit.
2. Pits usually initiate on the upper horizontal surface.
3. Pitting needs an extended initiation period.
4. Pitting is autocatalytic in nature. Once initiated, pit grows in an over-
increasing rate.
5. Stagnant conditions lead to pitting.
6.Stainless steels and aluminium are particularly susceptible to pitting.
7. Most pitting is associated with halide ions, particularly, chlorides.
90.
91.
92. Initiation of Pits
In the absence of chlorides, passive film dissolves slowly,
according to FeOOH + H2O Fe3+ + 3OH-
Chloride ion catalyzes the liberation of Fe3+ ,according to
FeOOH + Cl- FeOCl + OH-
FeOCl + H2O Fe3+ + Cl- + 2OH-
93.
94. Propagation of Pits
As oxygen is used up inside a stabilized pit, anodic dissolution
concentrates there, with accompanying cathodic reduction of
oxygen just outside the pit. A situation of small anode and large
cathode prevails.
As metal cations accumulate inside the pit, and the chloride ions
rush to the pit promoting the hydrolysis reaction:
Fe2+ + 2H2O + 2Cl- Fe(OH)2 + 2HCl
the HCl produced aggravates the dissolution further.
95.
96. Remedial measures of pitting
1. Eliminating chlorides or decreasing its concentration
2. Avoiding stagnation of the solution
3. Addition of passivating inhibitors
4. Use of cathodic protection
5. Using a metal or alloy of higher resistance to pitting
97. Crevice Corrosion
A crevice is a small gap created by contact of a material with
another material, usually not exceeding 3.2 mm.
The crevice area of a metal tends to get corroded
preferentially compared to the area outside the crevice.
Examples of crevices are lap joints, areas under bolts and
rivets, area under a rubber gasket, areas under dirt or
corrosion debris.
The attachment of barnacles or other biofouling organisms in
marine applications provides crevice areas.
98.
99.
100.
101. • Mo addition (2-3%) to Type 304 imparts resistance to pitting and crevice corrosion
(Type 316). Further increase in pitting resistance is obtained by addition of 1-2% Si
to Type 316 and by addition of 0.2% nitrogen to 18 Cr-8 Ni steel.
• Prior passivation of the surface in a hot nitric acid-dichromate solution reduces the
incidence of pitting.
• Lowering of carbon content (Type 316 vs Type 316L) of passivated steels reduces
the incidence of pit initiation even further.
• SP-2, a Type 316L steel with 2.5% Si and 0.23% N shows significant resistance to
pitting and crevice corrosion.
102. • Tests for resistance to pitting corrosion are carried out in Potassium
permanganate-Sodium chloride solution or in Ferric chloride
solution, the latter being more severe.
• Fig 18 (Uhlig) shows that in Fe-Cr-Mo ferritic alloys, chromium alone
without Mo cannot provide resistance to pitting and crevice
corrosion at 36 or even 40% Cr. Neither 26Cr-1Mo nor 28Cr-2Mo
alloy has optimum pitting corrosion resistance.
• 29 Cr-4 Mo alloy provides resistance in FeCl3 solution at 50 C, which
is equivalent to 90 C in KMnO4-NaCl solution. Addition of Ni to this
alloy beyond specified (2.2%) impair resistance to pitting corrosion.
103.
104. • Measurements of critical pitting potential (CPP) are frequently
used to determine resistance to pitting.
• The potential can be used to compare alloys. However, such
relatively rapid measurements are not reliable guides for
predicting immunity to pitting in long-time service in the
given environment.
• Pitting attack has been observed in long-time exposures at
potentials below the CPP.
105. • Pitting Resistance Equivalent Number (PREN) for a stainless steel
illustrates its resistance to pitting corrosion and is denoted in terms of its
alloy content:
PREN = %Cr + 3.3% Mo + 16% N
• PREN value of 32 is considered to be the minimum requirement for marine
environment where the concentration of chloride ion is above 2%. Duplex
grades mostly satisfy this requirement. (Table 7, Uhlig and Fig 4a Rana)
• Seawater above 70 C demands the duplex grades. (Fig 4b, Rana) Pitting
potential for duplex grade steels are higher compared to Type 304 and
Type 316. (Fig 5, Rana)
106.
107.
108.
109.
110. Intergranular Corrosion
Intergranular corrosion is a preferential attack on the grain
boundary phases or the zone immediately adjacent to them.
The factors contributing to the increased reactivity of the grain
boundary are:
1. Segregation of specific elements or compounds at the grain
boundary, as in aluminium alloys.
2. Enrichment of one of the alloying elements at the grain
boundary, as in brass
3. Depletion of the corrosion-resisting constituents at the grain
boundary, as in stainless steels
111. Weld Decay
Intergranular corrosion encountered in welded stainless steels
near or adjacent to the weld is known as weld decay.
Austenitic stainless steels become susceptible to intergranular
corrosion when heated in the temperature range of 500-800 C.
The material is said to be sensitized.
In the sensitizing range carbon diffuses to the grain boundaries,
combines with chromium to form Cr23C6, causing chromium
depletion.
During welding, chromium carbide formation takes place in the
area remaining in the sensitizing temperature range for a
sufficient length of time, thus making it prone to intergranular
corrosion
112.
113.
114.
115.
116.
117.
118.
119.
120.
121. Remedial measures of weld decay
1. Solution annealing: Sensitized components are heated to a
temperature around 1000 C followed by quenching in water.
2. Reduction of carbon content: Carbon in SS is lowered to
<0.03%, e.g. 304L, 316L, etc.
3. Stabilization: Elements having a higher affinity for carbon than
chromium are added to SS, e.g. Ti (type 321), Nb+Ta (type 347)
122.
123. Knife-line Attack
Even stabilized stainless steels can become prone to
intergranular corrosion after welding.
The zone of attack is narrow and close to the weld, hence
termed
knife-line corrosion.
This happens in weldments that are given post-weld stress
relieving heat treatment.
124.
125. • Carbon combines with chromium during exposure of the steels in
the range of 500 C to 875 C in austenitic stainless steels. Such
exposures are involved during welding and during fabrication. So,
the term ‘weld decay’. A precipritate of chromium carbide(Cr23C6)
forms preferentially at the grain boundaries causing depletion of
chromium. This is called ‘sensitization’. Decrease in chromium leads
to increase in corrosion rate.
• Nitrogen also reacts with chromium and forms β-Cr2N at the grain
boundaries.
126. • In ferritic stainless steels, the sensitization range is above
920 C, where the solubility of carbon and nitrogen
becomes significant in ferrite.(Fig.4, Uhlig). Because of
the difference in sensitizing temperature range, the
zones of intergranular corrosion in ferritic stainless steels
are adjacent to the welds.
• Carbide particles nucleate at grain boundaries and grow
into the metal grains on parallel crystallographic planes,
(Fig 7, Uhlig)
127.
128.
129.
130.
131. • Immunity to intergranular corrosion is restored by heating the sensitized
steel between 650-815 C for a short time. As the diffusion of chromium in
the bcc lattice of ferritic steel is significantly higher than in austenitic steel,
a uniform chromium composition is reestablished.
• Reduction of interstitials is an effective means for reduction of
susceptibility. For 18 Cr-2 Mo alloys to be immune to intergranular
corrosion, the maximum level of C+N is 60-80 ppm. Ferritic stainless steels
stabilized with Ti or Nb have also been developed.(Table 7, Uhlig)
• Duplex grades have low carbon content (0.03%) and are not prone to
intergranular corrosion.
133. Stress Corrosion Cracking
Stress corrosion cracking (SCC) may be defined as
the delayed failure of alloys by cracking when
exposed to certain environments in the presence
of a static tensile stress.
134.
135.
136.
137.
138. General features of SCC
1. Pure metals are almost immune in environments where their alloys crack
readily.
2. The existence of a tensile stress, applied or residual, is necessary.
3. SCC of an alloy occurs more readily in some limited environments.
4. The mode of cracking is either intergranular (IG) or transgranular (TG).
5. The brittle nature of cracking is evident in fractography.
6. SCC occurs above a threshold stress or stress intensity.
7. SCC can be accelerated by application of anodic current and can be stopped
by application of cathodic current, showing that the process is, at least partly,
electrochemical.
149. Metallurgical Aspects of SCC
Susceptibility to SCC is affected by the metallurgical factors such
as:
1. Chemical composition of the alloy
2. Size and preferential orientation of grains
3. Composition and distribution of precipitates
4. Dislocation interactions
5. Phases present and phase distribution
150. Influence of Alloy Chemistry
1. High nickel stainless steels are more resistant to SCC.
2. Ferritic stainless steels that are usually resistant to SCC become susceptible with
the addition of 1.5 to 2% nickel.
3. Duplex stainless steels with high Cr and low Ni are more resistant to SCC.
4. Very low carbon steels are resistant to SCC as carbon segregation at grain
boundary provides sites for adsorption of nitrites to promote SCC.
5. Phosphorus segregation at grain boundaries has been shown to promote
intergranular SCC in low alloy Cr-Mo or Ni-Cr-Mo-V steels and in SS.
6. Addition of 1% Si or 0.2% P or 15% Zn makes copper susceptible to SCC.
7. Addition of a third element like Si, Al to 70 Cu-30 Zn brass changes the mode of
cracking from IG to TG.
8. In Cu-Mn alloy system, Mn> 23% changes the mode of cracking from IG to TG
151.
152.
153. The susceptible potential conditions for stainless steels in
chloride containing solutions correspond to the breakdown
potential range for initiating pitting.
For carbon steels in nitrites, hydroxides or carbonates, the
susceptible range corresponds to the ‘borderline passivity’
condition.
Interestingly, the addition of small amounts of nitrites to NaOH
solution and vice versa shifts the corrosion potential beyond the
cracking potential range and the SCC stops.
154.
155.
156.
157. • Austenitic stainless steels are susceptible to SCC in chloride
(boiling MgCl2 or NaCl) solutions.(Table 6, Uhlig)
• Increasing Ni above or below 8% in Fe-18 Cr alloys increases
resistance to SCC.(Fig 12A, Uhlig)
158.
159. • What is to be stressed here is that
the MgCl2 is more severe than
NaCl test, but the latter is more
realistic.
•Type 304 shows <3 hr in MgCl2,
but 72 hr in NaCl test. Type 446
sensitized shows <17 hr in MgCl2,
but 552 hr in NaCl test.
•Even Carpenter20 shows <40 hr in
MgCl2, but no cracking in NaCl test.
160.
161. • Ferritic stainless steels are, in general, resistant to chloride
SCC, but are susceptible when in sensitized state or when they
contain Ni >1%. In new ferritic alloys with high Cr and Mo,
cracking time in MgCl2 is rapidly reduced as Ni content
exceeds 0.5% (Table 9, Uhlig), but these alloys are resistant to
SCC in NaCl solutions.
• 5. Duplex stainless steels have better resistance to SCC than
austenitic grades. (Fig 8, Uhlig)
162.
163.
164.
165. Effect of Stacking Fault Energy on SCC
• A low stacking fault energy (SFE) indicates presence of a wider gap
between dislocation partials. Cross slip of dislocations becomes more
difficult. This causes movement of dislocations along the slip plane.
• Planar movement of dislocations causes emergence of slip steps on the
surface of the material where the slip plane meets the outer surface of the
material.
• Repeated emergence of slip steps lead to disturbance in the passive film
causing it to become prone to breakage at that spot. The same process
occurs at the crack tip during SC crack growth.
• Therefore, materials with low SFE are more prone to SCC. Austenitic SS
(304/304L) has SFE typically of 20 mJ/m2, whereas this is typically 75
mJ/m2 for nickel and nickel-based alloys.
166.
167. Effect of alloying addition on SCC
• Stacking fault energy is determined by the chemical
composition of the material.
• SFE = - 25.7 + 2% Ni + 0.6% Cr + 7.7% Mn – 44.7% Si
Mo addition also increases SFE considerably, N reduces SFE
slightly.
• Increasing Ni content (> 10 %) increases SFE, hence increases
the resistance of ausutenitic SS to chloride SCC (Copson
curve). However, this does not explain the increase in SCC
susceptibilty with Ni addition up to 8%.
168. • Ni addition in small amount (> 0.5%) ferritic stainless
steels become susceptible to SCC in MgCl2 test, but not
in NaCl test.(Table 9, Uhlig)
• Type 316 has shown higher resistance to SCC than Type
304 in both tests. Higher Ni and Mo makes 316 more
resistant to SCC by increasing its SFE.
• SFE of 316 and 316L is quite similar, so their SCC
tendency does not differ.
169. Microstructural effects
• In general, ferritic SS are more resistant to SCC than austenitic
SS. However, types 434, 430 and Mo-containing Fe-18 Cr have
been reported to undergo chloride SCC.
• Sensitization, cold working, high temperature embrittlement
have been reported to increase susceptibility of ferritic SS to
chloride SCC.
• Ni reduces solubility of carbon in austenite phase.
Sensitization becomes easier, and SCC susceptibility enhances.
170. • Some Duplex SS are resistant to chloride SCC. Some others
undergo SCC, but have a much higher threshold stress for SCC.
With increasing ferrite content, resistance to SCC
increases.(Fig 8, Uhlig)
• The presence of cabonitrides, alpha prime phase, sigma
phase, cold working and high temperature sensitization, as in
ferritic SS, also deteriorate SCC resistance of duplex SS.
171. Reference Books
1. Corrosion Engineering– M. G. Fontana (McGraw Hill)
2. Corrosion and Corrosion Control– H.H. Uhlig (John Wiley)
3. Principle and Prevention of Corrosion – D. A. Jones
(Prentice Hall International)
4. Uhlig’s Corrosion Handbook – R. W. Revie (John Wiley)
