Surface Characterization and Pitting Behavior of High-Cr-Ni-Mo Alloys in Simulated White Water Environment

In the last few years corrosion of equipment in the paper industry has become a universal problem. The currently used 316L SS (UNS S31603) is no longer sufficiently resistant to corrosion. An investigation was undertaken to study the pitting behavior and the nature and composition of passive films formed on high-Cr-Ni-Mo alloys in a simulated white water environment. Cyclic polarization studies and x-ray photoelectron spectroscopy (XPS) were deployed to understand the nature of passive films formed. The formation of a stable protective oxide film on the surface of Alloy 59 enhanced the corrosion resistance of the alloy. The major alloying elements Cr, Ni, and Mo were enriched on the surface, thereby preventing the breakdown of the passive film and provoking passivation of the alloy. XPS analysis of the passive film for Alloy 59 indicated the presence of chromium hydroxide (Cr[OH]₃) and chromate (Cr₂O₃) as major constituents in the outermost and sputtered surface, respectively. The formation of stable MoO^2– species along with Ni, which results from increased anodic segregation, enriched the oxide-forming metallic species of Alloy 59.