CORROSION BEHAVIOR OF 2 STAINLESS-STEELS AND A CHROMIUM CAST-IRON IN PHOSPHORIC-ACID CONTAMINATED WITH S2- IONS

Various impurities such as Cl-, F-, SO42- and S2- contained in natural phosphates or introduced by enrichment processes, lead to many corros ion problems in equipment during the production of phosphoric acid by a wet process. Little is known about the action of S2- ions in acidic media. In an attempt to investigate the mechanism of action of S2- ion s on stainless steels, the corrosion behaviour of three types of steel 904 L (X1NiCrMoCu25-20-5), 316 L (X2CrNiMo17-12-2) and cast iron cont aining 32 wt % Cr in industrial phosphoric acid contaminated with S2- ions was studied, using polarization curve analysis. The characterizat ion of the corrosion products formed on the sample surface has enabled the production of a hypothesis on the corrosion and depassivation mec hanisms of steels in these media. The effect of copper and molybdenum on the corrosion resistance of the studied materials in phosphoric aci d contaminated by S2- ions is noted and discussed in detail.