Chloride solutions austenitic steels

Based on laboratory tests in boiling 42% magnesium chloride solution, austenitic stainless steel and nickel alloys are subject to chloride SCC if their nickel content is less than about 45%. The heat treatment of an alloy was found to have no effect on its resistance to chloride SCC. In practice, however, stainless steel and nickel alloys containing greater than 30% Ni will be immune in most refinery environments. Figure 2.2 shows typical chloride-induced SCC. 

A specific corrodent. One of the unusual and interesting features of SCC is the specificity of the corrodent. A particular alloy system is susceptible to SCC only when exposed to certain corrodents, some or all of which may be unique to that particular alloy system. For example, austenitic stainless steels (300 series) are susceptible to cracking in chloride solutions but are unaffected by ammonia. Brasses, on the other hand, will crack in ammonia but remain unaffected by chlorides. The corrodent need not be present at high concentrations. Cracking has occurred at corrodent levels measured in parts per million (ppm). 

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

Eliminate unfavorable environments. The presence of oxygen and other oxidizers is a critical factor in stress corrosion cracking. For example, the cracking of austenitic stainless steel in chloride solutions can be reduced or completely eliminated if oxygen is removed. 

Suzuki, T., Yambe, M. and Kitomura, Y., Composition of an Anolyte Within Pit Anode of Austenitic Stainless Steel in Chloride Solution , Corrosion, 29, 18 (1973)... 

Austenitic stainless steels will exhibit stress-corrosion cracking in hot aqueous chloride solutions, in acid chloride containing solutions at room temperature, in hot caustic solutions and in high-temperature high-pressure oxygenated water. 

Fig. 8.31 Effect of element shown on resistance of austenitic stainless steels to stress-corrosion cracking in chloride solutions (after Sedriks )...

The oxygen concentration of the solution, as in many instances of corrosion, can also be critical in stress-corrosion cracking tests. Instances are available in the literature that show very markedly different test results according to the oxygen concentration in systems as widely different as austenitic steels immersed in chloride-containing phosphate-treated boiler water and aluminium alloys immersed in 3% NaCl. 

The chloride stress-corrosion cracking of austenitic stainless steels in chloride solutions with samples under tensile stresses has been known since 1940. There are some reports that claim the retained austenitic structure to be responsible for chloride stress-corrosion cracking. 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

In fact, a linear relationship was found to hold between the crevice protection potential and the logarithm of the crevice depth with b = 0.06 V decade 1 for a cylindrical crevice in austenitic stainless steel in chloride solution [62]. 

K. Sugimoto and Y. Sawada, The Role of Molybdenum Additions to Austenitic Stainless Steels in the Inhibition of Pitting in Acid Chloride Solutions, Corros. Sci., Vol 17, 1977, p 425-445... 

T. Suzuki, M. Yamabe, and Y. Kitamura, Composition of Anolyte within Pit Anode of Austenitic Stainless Steels in Chloride Solution, Corrosion, Vol 29, 1973, p 18-23... 

The greatest problems with austenitic stainless steel piping usually arise when the unit is off stream rather than when it is operating. Such problems must be anticipated. The use of stainless steels requires that the necessary steps be taken to avoid them. Chlorides and caustics can cause any austenitic stainless steel pipe to crack trans-granularly under some conditions. Plain chromium stainless steels do not crack in chloride solutions, but they usually pit badly enough to be only moderately satisfactory. 

Austenitic stainless steels Hot, cone, chloride solutions, chloride-contaminated steam... 

Evidence for dealloying has been reported in austenitic stainless steel and iron-nickel alloys in acidified chloride solutions, reduction of titanium dioxide in molten calcium chloride, Cu-Zn-Al alloy in NaOH solutions giving rise to Raney metal particles (46). 

J.I. Dickson, A.J. Russell, D. Tromans, Stress corrosion crack propagation in annealed and cold worked 310 and 316 austenitic stainless steels in boiling (154 °C) aqueous magnesium chloride solution, Can. Metad. Q. 19 (1980) 161-167. 

T.P. Hoar, J.C. Scully, Mechanochemical anodic dissolution of austenitic stainless steel in hot chloride solution at controlled electrode potential, J. Electrochem. Soc. Ill (1964) 348—352. 

O.M. Alyousif, R. Nishimura, Stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling samrated magnesium chloride solutions, Corros. Sci. 50 (2008) 2353-2359. 

For some material-environment combinations it has been shown that accelerated anodic dissolution of yielding metal is the significant mechanism. This is the case for austenitic stainless steels in acidic chloride solutions. In these steels, plastic deformation is characterized by a dislocation pattern giving wide slip steps on the surface. For such systems, Scully [7.50] has proposed a model for initiation and development of stress corrosion cracks, which has been supported by other scientists [7.51]. The model in its simplest form is illustrated in Figure 7.52. A necessary condition is that flie surface from the beginning is covered by a passivating film (A). 

Transgranular stress corrosion cracks are known [7.49] from i) austenitic steels in acidic chloride solutions, ii) low-strength ferritic steels in acidic media, iii) ferritic steels in phosphate solutions, iv) carbon steel in water saturated with CO2 and CO, v) a-brass in ammonia solutions that do not cause surface films, vi) aluminium alloys in NaCl/K2Cr04 solutions and vii) magnesium alloys in diluted fluoride solutions. For further study of fracture surface appearance, see, e.g. Lees [7.49] and Scully [7.53]. 

Normal anodic stress corrosion cracking is caused by a combination of mechanical tensile stress and loeal eleetrolyte dissolution processes when certain conditions are met. First, the corrosive medium must have a specific effect on the respective alloy, and in addition, the alloy in contact with the electrolyte in this material/corrosive medium system must be prone to stress corrosion cracking. The tensile stress must also be suffleiently high. Susceptible systems, for example, are stainless austenitic steels in chloride-eontaining solution or unalloyed and low-alloy steels in nitrate solutions. In contrast, unalloyed and low-alloy steels are not susceptible to stress eorrosion eraeking in ehloride solutions. 

Figure 11.35 Time to failure for different austenitic stainless steels subjected to constant-load stress corrosion cracking testing in boiling magnesium chloride solution [15].

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