Investigating the Initial Stages of KCl-Induced Corrosion of a Chromia Stainless Steel Using ESEM in-Situ and Well-Controlled Ex-Situ Exposures at 450°c

The oxidation behavior of stainless steels is principally affected by the early stages of the oxidation process. In this study, the initial corrosion behavior of a high-alloy stainless steel (Snicro25, 25Ni-22Cr) was investigated through Environmental Scanning Electron Microscopy (ESEM) exposure. The corrosion coupons were first contaminated by KCl(s) and then exposed in-situ for one hour at 450°C. The ESEM in-situ experiment provided continuous imaging during the early stages of the degradation process. Ex-situ exposures were also carried out using well-controlled reference exposures to substantiate the ESEM in-situ experiment and in order to evaluate the effect of the presence of electron beam and the lower pressure in the ESEM chamber. The post-exposure analyses were conducted on the surface and on the cross section of the exposed samples using high-resolution Transmission Electron Backscatter Diffraction (t-EBSD) technique, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Focused Ion Beam milling (FIB). TEM and SEM observations were coupled with the chemical concentration analysis using the energy dispersive X-ray spectroscopy (EDX) technique. Ultrafine grain sized oxide scale was formed with the thickness of around 50 to 100 nm further away from the salt particles. The t-EBSD analysis revealed the changes in the crystallographic orientation of the substrate and also the detection of high misorientation regions (HMRs) beneath the oxide layer and in the depletion zones.