A. Soleimani Dorcheh, R.N. Durham, M.C. Galetz
The role of chloride impurities in nitrate mixture NaNO3-KNO3 (solar salt) on corrosion behavior at 600 °C was studied on a low-chromium ferritic-martensitic X20MoV11-1 steel (X20) and a stainless steel 316 (SS316). Gravimetric and metallographic methods were employed to characterize the kinetics of oxidation and the resulting corrosion products. Steel X20 showed non-protective character in both low- (up to 0.02 wt% Cl−) and high- (up to 0.25 wt% Cl−) chloride salts by forming a thick and non-compact oxide scale. A significant increase in weight gain was observed when X20 steel was immersed in the high-chloride containing salt. Furthermore, the scale underwent severe deformation. SS316 showed superior corrosion resistance in both low- and high-chloride salts. Oxide scales formed on both steels included two zones: an outer Na-rich oxide and an inner mixed oxide based on Fe2O3 and Fe3O4 structures. The morphology and composition of these zones were significantly different on X20 and SS316 steels. A passive Cr-rich oxide layer at the metal/oxide interface was characterized as a protective layer. In the case of stainless steel 316 this layer showed even higher continuity when tested in the high-chloride salt resulting in better protection during the isothermal test.