Insight on passivity of high entropy alloys: thermal stability and ion transport mechanisms in the passive oxide film on CoCrFeMnNi surfaces

Abstract The ion transport mechanisms in passive films pre-formed on CoCrFeMnNi high entropy alloy surfaces in acid solution were studied in-situ by time-of-flight secondary ion mass spectrometry upon oxidation in 18O2 gas. The bilayer structure of the passive film, with outer Fe, Co-rich and inner Cr, Mn-rich layers, is thermally stable, but with reduction of Fe and Co oxides and formation of Mn oxide in the outer layer above 150 °C. At 170 °C, the transport mechanism is mainly outward cation diffusion with slower inward oxygen diffusion. The re-oxidation growth rate follows a parabolic law and the value of parabolic constant was determined.