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

Wednesday, 8 October 2014: 16:40
Expo Center, 1st Floor, Universal 11 (Moon Palace Resort)
N. Mortazavi (Chalmers University of Technology), L. Intiso (CENTRO SVILUPPO MATERIALI S.p.A.), M. Halvarsson, and L. G. Johansson (Chalmers University of Technology)
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.