Pitting Corrosion and Crevice Corrosion of an Advanced Chromium-based Stainless Steel

Abstract Alloy 33 is a (wt. %) 33Cr-32Fe-31Ni-1.6Mo-0.6Cu-0.4N austenitic stainless steel combining high yield strength of min. 380 N/mm2 (55 KSI) with high resistance to local corrosion and superior resistance to stress corrosion cracking. Ranking the material according to its PRE (pitting resistance equivalent) value, the new alloy fits in between the advanced 6 % Mo superaustenitics and the nickel-base Alloy 625 but due to the balanced chemical composition the alloy shows a lot less sensitivity to segregation in the base material as well as in welded structures. It is recommended to weld the material with matching filler. The critical pitting temperature of such joints in the 10 % FeCl3 · 6H2O solution is reduced by only 10°C in comparison to the base material. Corrosion tests in artificial seawater (20 g/l Cl-) with additions of chloride up to 37 g/l as well as in a NaCl-CaCl2 solution with 62 g/l Cl- revealed that the critical pitting temperature does not differentiate from the 6 % Mo austenitic steel Alloy 926. With respect to crevice corrosion the depassivation pH value has been determined in 1 M NaCl solution according to Crolet9 and again there was no difference between Alloy 33 and Alloy 926. SCC tests performed on Alloy 33 in the solution annealed condition as well as after heavy cold work up to RP0,2 ≈ 1100 – 1200 N/mm2 (160 – 174 KSI) indicate the high resistance to stress corrosion cracking in hot sodium chloride solutions.