Corrosion of High Alloy Stainless Steels and Non-Ferrous Alloys in Bleaching Stage Conditions

Abstract The influence of the major alloying elements iron, chromium, nickel and molybdenum on the corrosion behaviour of stainless alloys in near neutral pH, oxidizing chloride environments, as in ClO2 bleached D-stages, was investigated. Under these conditions it was earlier observed that the pitting resistant high molybdenum containing nickel base alloys suffer general corrosion thinning whereas lower alloyed materials as stainless steels fail by pitting/crevice corrosion or are fully resistant. Comparative electrochemically controlled experiments in chloride and sulphate solutions respectively permit investigation of transpassive corrosion without interference by pitting. It was found that increasing the iron content in nonferrous alloys shifts the transition to transpassivity to higher potentials. This suggests that for a satisfactory performance in these environments stainless materials have to have not just a sufficiently high pitting resistance equivalent (to avoid pitting/crevice corrosion) but also a minimum iron content to avoid transpassive dissolution.