Evolution of Oxide Film Formed on Machined Type 304L SS in High Temperature High Pressure Demineralised Water

摘要

Although oxide films formed over austenitic stainless steel (SS) in high temperature high pressure (HTHP) demineralised (DM) water (with or without chemical addition) have been widely investigated, there are limited studies establishing properties of oxide film formed over surface worked SS. Grain fragmentation (increased grain boundary area) and strain in the surface and sub-surface layers of machined SS influence the oxidation mechanism. In this investigation, oxidation behaviour of machined type 304L SS was investigated in HTHP DM water (300 degrees C, 89 bar, dissolved oxygen < 45 ppb) for 15, 30, 45 and 60 days. Defect density (established by Mott-Schottky analysis) decreased with increasing duration of oxidation and a thin Cr-rich film (characterised by Micro laser-Raman spectroscopy and glow discharge optical emission spectroscopy) having the least defect density was detected after 60 days of oxidation. It is proposed that the initial high grain boundary area and high strain resulted in rapid oxidation of the machined surface, forming an inner oxide with high cationic vacancy concentration and a thick outer oxide by the precipitation of dissolved metal cations from the solution. With increasing oxidation time, the Cr-rich inner layer grew more compact and its ionic defect density decreased. The mechanism is explained using the point defect model.