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

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 degrees C. At 170 degrees 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.

Automated summary

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 degrees C. At 170 degrees 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.