Localized corrosion of stainless steels (SSs) has often occurred in newsprint paper machines, despite the low chloride concentration in the process water. This is now known to be caused by thiosulfateions, which can cause pitting of AISI 304 steel without assistance from chloride if the sulfate-to-thiosulfate ratio is within a certain range. The effects of this ratio, of ionic strength and of inhibitive bisulfite ions, have been studied.
For AISI(1) 316L, thiosulfate pitting did not occur unless the chloride concentration was fairly high (more than 10−2 M, i.e., 350 ppm Cl−) and unless the molar concentration of chloride exceeded that of sulfate. These new results explain the previously unexpected pitting of AISI 316L equipment in splash zones, where concentrated chloride/thiosulfate solutions can develop by evaporation.
