Sensitization of stainless steels

Sensitization of stainless steel can occur owing to improper heat treatment, welding, or long-term exposure in service to elevated temperatures (28). While there... 

The idea that an etching reagent behaves like a redox system has been verified by Clerbois et al (45) in the case of the Strauss reagent (2N, H2SO4 + 10% CuS04) which has been recommended for testing of the intergranular sensitivity of stainless steels (47). Provided that metallic copper is present,... 

The single loop electrochemical poten-tiokinetic reactivation (EPR) test is an electrochemical test for the sensitization of stainless steels that is described by ASTM G108 [77]. Type 304 or 304 L stainless steel with a 1 pm polished surface is tested in a 0.5 M H2SO4 + 0.01 M KSCN solution at 30 ( 1 °C). The potential is first held... 

The most common nonelectrochemical approach for the study of corrosion is weight loss measurements. Such measurements are limited by the resolution of the gravimetric device, and, for aqueous corrosion, are usually applied in aggressive environments such as the boiling acids used to evaluate sensitization of stainless steels in ASTM A262 [76]. The Quartz Crystal Microbalance (QCM) is a gravimetric instrument capable of submonolayer sensitivity that has been increasingly applied over recent years in the area of corrosion [85, 86]. 

Exposure of certain alloys to elevated temperatures can produce phase changes within the alloy. These phase changes can sometimes lead to corrosive attack of the aUoy. Corrosion can occur if the phase change alters the formation of protective barriers on surface. Examples of this mechanism are sensitization of stainless steels (discussed earlier) and sigma phase (a) formation. Sigma phase formation occurs in nickel-chromium-iron and chromium-iron alloys containing a minimum of 16.5 %... 

Figure 19-9. EPR test to reveal the degree of sensitization of stainless steel...

Electrochemical potentiodynamic reactivation (EPR) is another polarization method that evaluates the degree of sensitization of stainless steels such as S30400 and S30403 steels. This method uses a potentiodynamic sweep over a range of potentials from passive to active (called reactivation). 

Umemura, F. and Kawamoto, T. (1979) Evaluation of degree of sensitization of stainless steels using electrochemical reactivation method. Boshoku Gijutsu (Corros. Eng. Jpn.), 28(1), 24-31. 

Proceedings of Symposium on Radiation Induced Sensitization of Stainless Steels, CEGB, Berkeley Nuclear Laboratories (D. I. R. Norris, ed.), September 1986. 

Figure 5.17 Sensitization of stainless steel in the heat-adjacent zone.

Annealing is the only way to correct a sensitized stainless steel. Because different stainless steels require different temperatures, times, and quenching procedures, the user should contact the material supplier for such information. A number of tests can detect sensitization resulting from carbide precipitation in anstenitic and ferritic stainless steels. The most widely used tests are described in ASTM Standards A 262 and A 763. More detailed information on sensitization of stainless steels can be fonnd in varions books on metallurgically influenced corrosion. 

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