Effect of surface state of austenitic type 304L SS on oxide characteristics formed in high temperature high pressure water

Abstract

Surface states of machined, 600 grit SiC paper finished (SiC), diamond polished (DP) and electropolished (EP) type 304L SS specimens were characterised. Cr content, defect density (Mott-Schottky analysis) of the oxide formed (360 h and 720 h) in high temperature high pressure demineralised water was measured and correlated to the oxide protectiveness. Thinnest oxide having maximum Cr content and least defect density (most protective) formed over EP surface (no surface strain), attributed to the growth of a Cr-rich air-formed oxide. Ultrafine grain and surface strain in machined specimen increased the Cr content in the oxide and reduced the defect density, which was attributed to enhanced diffusion of Cr. Oxide on DP specimen (low surface strain) showed the least Cr enrichment and the highest defect density. Results show that the mechanism of Cr enrichment in EP specimen is different from the specimens having surface strain (machined, SiC, DP). Cr enrichment and reduction in defect density in oxide makes the oxide more protective since Cr-rich oxides have lower solubility and lower defect density makes ionic mobility across the oxide difficult. Based on results, the oxides can be categorised in decreasing protectiveness as EP > machined > SiC > DP.