Surface layer evolutions of pre-oxidised Fe–30Cr alloy induced by gaseous KCl

Abstract

This study reports on the evolutions of surface structure and composition of Fe–30Cr alloys that were first oxidised in oxygen atmosphere then followed by an exposure in gaseous KCl environment. After the pre-oxidation treatment at 800°C for 10 h, a continuous, dense and protective chromia scale was produced. During the following exposure in KCl contaminated atmospheres at 650°C, the surface chromia layer was gradually transformed into worm-like potassium chromate textures, which quickly coalesced on the substrate surface along with simultaneous volatility. The protective oxide layer was then severely deteriorated by enhancing cracking, spallation, volatility and porosity. Voluminous iron migrated outward through these defective sites and iron oxide became the dominating reaction products. Both chromate and residual chromia were finally incorporated into the thick and porous iron oxide scale. This process accounts for the major catastrophic destruction of chromia forming materials involving reactions with alkali chloride salts, even at minor amounts.