Microstructural Investigation of the HCl-Induced Corrosion of the Austenitic Alloy 310S (52Fe26Cr19Ni) at 500 °C

This paper investigates the influence of 500 ppm HCl in a 5 %O-2-95 %N-2 atmosphere on the oxidation of the austenitic stainless steel AISI 310S at 500 A degrees C. Laboratory exposures were made for one, 24, 72 and 168 h and the samples were analysed with XRD, SEM/EDX, FIB and TEM/EDX. When exposed in oxygen a thin and protective chromium-rich oxide scale forms. Addition of HCl causes significantly accelerated corrosion. Within the first hour of exposure, accumulations of FeCl2, CrCl2 and NiCl2 forms below the chromium-rich oxide, especially at steel grain boundaries. The chlorine-induced corrosion is suggested to occur through an electrochemical reaction, in which the dissociation of HCl to form chloride ions at the scale surface is coupled to the oxidation of the metal surface beneath the scale by an outwards electronic current and inwards diffusion of chloride ions along oxide grain boundaries.