Corrosion of Austenitic Stainless Steels

Austenitic stainless steels are the most significant class of corrosion-resistant alloys for which intergranular corrosion can be a major problem in their satisfactory use. The problem is most often encountered as a result of welding but also may result from stress-relief annealing or incorrect heat treatments. Intergranular corrosion also can occur in ferritic stainless steels and in nickel- and aluminum-base alloys. 

The Fe-Cr-C Equilibrium Relationships in Stainless Steels. The metallurgical processes occurring in austenitic stainless steels causing susceptibility to intergranular corrosion (sensitization) and methods to either prevent or remove susceptibility, are illustrated by the physical metallurgy of the selected alloys in Table 7.5. These are all austenitic stainless steels, and after quenching from elevated temperatures are es- 

Effect of Thermal History of Austenitic Stainless Steels on Susceptibility to Intergranular Corrosion. The time dependence for the local depletion of chromium sufficient to cause susceptibility to intergranular corrosion as functions of temperature and carbon content is of the form represented in Fig. 7.54 (Ref 83). The curves are typical of type 3xx alloys with nominal chromium concentrations of 17 to 25 wt% and, since they represent times for initiation of intergranular corrosion, 

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M. O. Speidel, in R. W. Staehle and M. O. Speidel, eds.. Stress Corrosion of Austenitic Stainless Steels, ARPA report, 1979. 

Other Considerations Autoignition can occur if combustible fluids are absorbed by wicking-type insulations. Chloride stress corrosion of austenitic stainless steel can occur when chlorides are concentrated on metal surfaces at or above approximately 60°C (140°F). The chlorides can come from sources other than the insulation. Some calcium sihcates are formulated to exceed the requirements of the MIL-I-24244A specification. Fire resistance of insulations varies widely. Calcium sihcate, cellular glass, glass fiber, and mineral wool are fire-resistant but do not perform equally under actual fire conditions. A steel jacket provides protection, but aluminum does not. 

Intergranular Corrosion of Austenitic Stainless Steels (Section 3.3)... 

In practice, three methods are available for preventing sensitisation and intergranular corrosion of austenitic stainless steels ... 

A more detailed treatment of sensitisation of austenitic stainless steels, of intergranular corrosion of austenitic stainless steels without sensitisation, and of sensitisation and intergranular corrosion of ferritic stainless steels and high-nickel alloys, is given by Cowan and Tedmon . 

Ceilings, P. J. and de Jongh, M. A., Grain Boundary Oxidation and the Chromium-depletion Theory of Intercrystalline Corrosion of Austenitic Stainless Steels , Corros. Sc/., 7,413 (1967) Armijo, J. S., Impurity Adsorption and Intergranular Corrosion of Austenitic Stainless Steel in Boiling HNOj-KjCrjO, Solutions , Corros. Sci., 7, 143 (1967)... 

France, W. D. and Greene, N. D., Some Effects of Ex(>erimental Procedures on Controlled Potential Corrosion Tests of Sensitised Austenitic Stainless Steel , Corros. Sci., 10,379(1970) Tedmon, C. S. (Jr.), Intergranular Corrosion of Austenitic Stainless Steel , J. Electrochem. Soc., 118, 192(1971)... 

Wilson, F. G., Mechanism of Intergranular Corrosion of Austenitic Stainless Steel , Br. Corros. J., 6, 100 (1971)... 

Gizhermo, R. and Khristo, E., Effect of the Deoxidation Method on the Intercrystalline-corrosion Tendency of Cr-Ni Austenitic Steels , Melalurgiye, 5, 17 (1972) C.A., 80, 98898y Joshi, A. and Stein, D. F., Chemistry of Grain Boundaries and its Relation to Intergranular Corrosion of Austenitic Stainless Steel , Corrosion, 28, 321 (1972)... 

Grassiani, M., Pit Corrosion of Austenitic Stainless Steels in Artificial Sea-water , Trib. CEBEDEAU, 25, 515 (1972) C.A., 79, 8531a... 

Test methods for determining electrolytic corrosion with electrical insulating materials Method for determination of resistance to intergranular corrosion of austenitic stainless steels copper sulphate-sulphuric acid method (Moneypenny Strauss test) Specification for electroplated coatings of tin/lead alloys... 

Auscor Prediction of corrosion of austenitic stainless steels UK Savior... 

Localized biological corrosion of stainless steels. There are three general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Figure 6.29). These conditions should be examined for metals that show active-passive corrosion behavior. Microbiological corrosion in austenitic steel weldments has been documented. (Wahid)61, (Krysiak)14... 

Secondly, we focused on the role of oxygen impurities in element-selective corrosion of austenitic stainless steels in liquid sodium. Calculation was carried out for an oxygen impurity associated with an Na atom located at the site on top of a Cr atom at the Fe(OOl) surface in sodium. The result shows that a positively charged Cr atom will be released selectively into sodium by an 0 anion. 

Secondly, we examined the mechanism of element-selective corrosion of austenitic stainless steels in liquid sodium. We performed calculations for a system where an 0 impurity atom accompanying Na atom approaches Or atom at an Fe(OOl) surface exposed to liquid sodium. The energy levels attributed to the Cr 3d orbitals are located above the Fermi level, while the energy levels originated from the 0 2p orbitals are below the Fermi level. Furthermore, the level for the Na 3s orbital is above the Fermi level. By the energy differences of these orbitals, the Cr atom is charged positively, while an 0 atom accompa-... 

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