Effect of Oxide Scale Formation on the Behaviour of Cu in Steel during High Temperature Oxidation in O-2-N-2 and H2O-N-2 atmospheres

Abstract

Cu enrichment at the steel-scale interface and its migration from there was investigated during the heating of steel cast at 1200 degrees C under various oxidizing conditions. The behaviour of Cu enrichment was found to be largely dependent on the morphology of oxide scale formed during oxidation. At the early stage of oxidation, Cu-rich phase formed and accumulated at the steel-scale interface in both O-2-N-2 and H2O-N-2 atmosphere. However, as the oxidation proceeded, the enrichment was vastly different for each oxidizing atmosphere. In the case of O-2-N-2 oxidation, an oxide layer was formed initially at the steel surface, but a gap was developed soon after at the steel-scale interface and grew in size, which practically separated the scale from the steel substrate. The scale layer formed under this condition was porous. The Cu-rich phase initially formed at the interface seemed to migrate to the scale layer, leaving no Cu-rich phase at the interface. In the case of H2O-N-2 oxidation, however, the scale layer formed was dense and tightly attached to the steel surface, and the Cu rich-phase continued to accumulate at the interface. Regarding the behaviour of the Cu-rich phase formed at the interface, it is proposed with experimental evidences that, when a gap forms at the steel-scale interface, it is the vaporization of Cu in the Cu-rich phase through the gap that brings Cu to the scale.