Mechanisms of simultaneous oxidation-chloridation of austenitic stainless steels by NaCl in air

Abstract

Simultaneous oxidation-chloridation behavior of austenitic stainless steels by NaCl in air was investigated at 650°C. Surface oxide scales of the test alloys had double-layered structures that consisted of inner spinel layer and outer Fe2O3 layer. Condensed and gaseous chlorides were formed below the scale surface by NaCl vapor diffusing inward via pores or cracks of the scales. The rate of internal and uniform attack mainly depended on the porosity of the inner spinel layer. In order to predict the complex mixed oxidation behavior of alloys, new types of thermodynamic stability diagrams were utilized, which was constructed by computer-assisted calculations of phase equilibria of the multi-component reacting systems using the Thermo-CaIc program. The overall mixed oxidation process including the initial stage of reaction, the rapid formation and growth of the scale and the transport of gaseous species through the oxide scale was elucidated.