Transpassive Behavior of Equimolar CrMnFeCoNi and CrCoNi Multi-Principal Element Alloys in an Alkaline NaCl Electrolyte

We investigated the corrosion properties and transpassive behavior of CrMnFeCoNi and CrCoNi multi-principal element alloys (MPEAs) in a 0.1 M NaCl electrolyte at pH 12. By using SECM-based tip substrate voltammetry (TSV) in combination with the chemical analysis of the electrolyte, we were able to differentiate between anodic metal dissolution and oxygen evolution in the transpassive range. Our investigations have shown that CrCoNi has a significantly higher corrosion resistance compared to CrMnFeCoNi. In the studied alkaline environment, a transpassive oxide film is formed on the surface of CrCoNi during secondary passivation. This transpassive oxide film appears to play a significant role in oxygen evolution, as the increase in TSV currents at the microelectrode coincides with the corresponding current density plateau of the voltametric current trace. The formation of the transpassive oxide film was not observed in previous studies conducted in acidic environments. Moreover, the alkaline electrolyte induced a positive hysteresis and mild pitting corrosion, in addition to intergranular corrosion, which was the sole corrosion process observed at acidic pH levels. These findings enhance the understanding of the processes governing the transpassivity of CrMnFeCoNi and CrCoNi MPEAs in alkaline environments and have potential implications for the development of application-tailored corrosion-resistant MPEAs.