Microstructural Study of the Influence of KCl and HCl on Preformed Corrosion Product Layers on Stainless Steel

Abstract

Metallic construction materials in biomass- and waste-fired boilers are exposed to corrosive environments due to the considerable amounts of alkali chlorides and HCl(g) released in renewable fuels combustion. Alkali chlorides corrosivity toward stainless steels exposed at high temperature has been extensively studied. Nevertheless, the corrosion attack propagation is still not fully understood and it is expected that chlorine diffusivity through oxide layers plays a major role in accelerating the corrosion. In order to investigate the role of chlorine on the propagation step of the corrosion attack, tailor-made oxides were produced. The samples were subsequently exposed to chlorine-containing environments for short period of time. The reaction atmospheres were O2 + H2O + KCl(s) and O2 + H2O + HCl(g) at 600 °C. Since in this study chlorine diffusivity through the corrosion product layer is of great interest, samples were analyzed with XRD and SEM/EDX. High-quality BIB cross sections were performed. Summarizing, for the preformed oxide layers on the stainless steel in the presence of HCl(g), chlorine seemed to penetrate to the oxide/metal interfaces in the material. However, in the presence of KCl(s) there seems to be no effect of the salt on the corrosion rate.