Comparison of stainless steels in simulated paper machine environments

The localized corrosion behavior of austenitic stainless steels (SS) UNS S30403 and UNS S31603, and duplex SS UNS S31803 was compared in simulated paper machine environments containing chloride, sulfate, and thiosulfate at pH 3 and 65°C. Electrochemical testing of the materials was performed by cyclic polarization scans and scratch tests. Thiosulfate caused a remarkable decrease in repassivation potentials (E_pp) and a slight decrease in pitting potentials (E_np) of UNS S30403 and UNS S31603, when compared to the corresponding E_pp and E_np values determined in similar solutions without thiosulfate addition. Pits on UNS S30403 and UNS S31603 steels were enriched in Cr and Cu and depleted in Fe when compared to the base metal. Thiosulfate being present in the solution pits contained also very high amounts of S. Thiosulfate was proposed to be reduced to hydrogen sulfide (H2S) inside the pits enhancing the anodic dissolution process of SS. In the case of UNS S31803, test environments had hardly any influence on the determined high E_pp and E_np values. Chloride concentration had to be a minimum of 500 mg/L until any localized corrosion could be observed on UNS S31803.