Analysis of Film Formation in High Chromium White Iron Hardfacing Alloys in Alkaline Solution using EIS and SIMS

A popular hardfacing alloy consisting of high chromium white iron (HCWI) was deposited on low carbon steel using shielded metal arc welding. The wear-resistant alloys are not only used in applications requiring wear resistance but also used in applications involving fluids of a range of pH values. It is well known that chromium containing alloys have good corrosion resistance due to the presence of passive film. In this study for understanding the film formation on carbides and eutectic austenite matrix, HCWI alloy were exposed to potentials (0.157 V vs. SCE and 0.758 V vs. SCE) derived from potentiodynamic studies in highly alkaline solution of pH 14, simulating Bayer refining process in alumina industry. Electrochemical impedance spectroscopy (EIS) technique was used to analyse the impedance of films. The thickness and the composition of film properties at various potentials were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At potential 0.157 V versus SCE, chromium carbide had a thick layer of oxide film consisting of mainly FeO with minor amounts of CrO. The eutectic austenite matrix consisted of a much denser and slightly thinner film of FeO. It is proposed in this study that for the formation of an oxide film on carbides, the carbides would have to dissociate on the surface. This dissociation of carbides may likely render the hardfacing alloy less effective in wear-resistant applications. The results were explained through superimposing the Pourbaix diagram of chromium carbides and iron (Fe).