Thiosulfate pitting corrosion of stainless steels in paper machine environment: Dissertation

Thiosulfate pitting corrosion of austenitic stainless steels of types UNS S30403 (AISI 304L) and UNS S31603 (AISI 316L), and duplex stainless steel of type UNS S31803 (22Cr DSS) was studied in simulated paper machine environments containing chloride, sulfate and thiosulfate by cyclic polarization scans, scratch tests, Contact Electric Resistance (CER) technique and by corrosion coupon tests performed in a real paper machine. The formed pits and crevice corrosion were examined by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The surface films were also analyzed by Electron Spectroscopy for Chemical Analysis (ESCA) and Secondary Ion Mass Spectrometry (SIMS). Thiosulfate and chloride clearly have a synergistic effect in inducing localized corrosion. Thiosulfate is able to prevent passivation of an active stainless steel surface and able to stabilize metastable pits initiated below the actual pitting corrosion potential. The role of chloride is to initiate corrosion pits. Thiosulfate increases the possible potential range of corrosion pits to grow by lowering the repassivation potential. Hydrogen sulfide originating from thiosulfate is proposed to accelerate the anodic dissolution inside corrosion pits and crevices by forming sparingly soluble metal sulfides and by acidifying the local environment. The compositions of the black deposit enriched inside thiosulfate pits and corrosion crevices were similar to the compositions of the low-resistance surface films. Pitting Resistance Indices (PRE) of the test materials increased in the order of UNS S30403 (PRE 19 - 21), UNS S31603 (PRE 24 - 25) and UNS S31803 (PRE 35) in accordance with the observed localized corrosion resistance in both the laboratory and field tests. All the materials corroded technically significantly in corrosion coupon tests performed in a paper machine. The bulk environment of the paper machine was very mild, but the chloride and thiosulfate ions concentrating up to high levels created favourable conditions for corrosion.