INVESTIGATION OF THE FORMATION AND PROPERTIES OF PROTECTIVE OXIDE LAYERS ON HIGH-PURITY CHROMIUM WITH SIMS IMAGING TECHNIQUES

Spallation of the protective oxide layer formed during hot gas oxidati on is the main reason for the corrosion of high purity powder metallur gically produced chromium[1]. To explain the formation and occasional spallation of the oxide layer a chromium sample subjected to two conse cutive oxidation processes in O-16 and O-18 atmosphere at high tempera ture was characterised by 2D and 3D SIMS. The formation of the protect ive oxide can be described by the diffusion of chromium from the bulk to the surface and the reaction of the chromium atoms with the oxygen from the gaseous phase. The very low solubility of nitrogen in chromiu m oxide indicates its inability to diffuse through the growing oxide l ayer acid explains the enrichment of nitrogen (same mechanism applies for chlorine) in the interface metal/oxide. The accumulation of trace elements within the interface during the oxidation process explains th e reduced adhesion power of the passivation layer and its spallation.