Pitting and crevice corrosion of superaustenitic stainless steels

Superaustenites are mainly used in offshore applications, oil production and chemical industry. Most important types of localised corrosion of these steels are pitting and crevice corrosion. Investigated materials were N08028, S31254 and three modified alloys. Chromium content of investigated alloys varied between 20 and 27%, molybdenum between 3.2 and 6.0%, nitrogen between 0.1 and 0.36% and copper between 0 and 1.1%. For means of comparison stainless steel AISI 316L has been included in the study. Pitting and crevice corrosion of these highly corrosion resistant steels has been investigated by use of standardized tests. Critical pitting temperature and critical crevice temperatures were determined according to ASTM G 48, Methods C and D, respectively. Electrochemical measurements for determination of pitting potentials were done according to ASTM G 61 as well as for determination of critical pitting temperatures according to ASTM G 150. Results are presented as function of MARC (Measure of alloying for resistance to corrosion) defined by Speidel since linear correlation coefficients were higher when compared to conventional PREN. Results obtained by different testing methods must not be compared directly. Every test however is sensitive to microstructural defects like precipitations and segregations that decrease corrosion resistance. The higher alloyed a material is, the higher is its tendency to form microstructural defects, and the more difficult is it to reach its theoretical corrosion resistance at given chemical composition.