Corrosion Properties of Medium Entropy Alloys under Oxygen Evolution Reaction Conditions in Aqueous Electrolytes

Due to their favorable mechanical and anti-corrosion properties, medium entropy alloys (MEA) are of high academic and industrial interest as novel materials for engineering and catalytic applications. Previous studies indicate high transpassive currents without indication of pitting for MEAs, which makes them promising materials for oxygen evolution catalysts [1].<br/>To understand the stability of MEAs CrCoNi and FeCrNi at high anodic potentials, we have investigated the mechanisms of transpassive dissolution and the oxygen evolution reaction (OER). Scanning electrochemical microscopy (SECM) was applied in the interrogative mode for the detection of evolving metal species and oxygen during controlled polarization. SECM operated with oxygen reduction at the probe precisely determined the onset of the OER. Inductively coupled plasma – mass spectroscopy (ICP-MS) analysis was performed to quantify dissolved metal species. Potentiodynamic, potentiostatic and chronoamperometric techniques as well as different electrolytes were used to induce precisely controlled corrosion loads during the SECM experiments and for the ICP-MS analysis. Macroscopic corrosion properties of the alloys, electrical and chemical properties of the passive films were studied by means of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and X-Ray Photoelectron Spectroscopy (XPS), respectively.<br/>Finally, the analysis of the corrosion morphology and the potential of the surface before, during and after passivity breakdown was carried out by means of in-situ atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM). Our results indicate that even though the OER accounts for the majority of the observed transpassive currents, for alloys showing excellent passivation behavior like CrCoNi, the localization of the corrosion at grain boundaries may still present a challenge for material durability. The presentation will summarize the detailed electrochemical characterization of the corrosion processes of the studied MEAs with a specific focus on the transpassive behavior under OER conditions.<br/><br/>[1] A. Wetzel, M. von der Au, P.M. Dietrich, J. Radnik, O. Ozcan, J. Witt, The comparison of the corrosion behavior of the CrCoNi medium entropy alloy and CrMnFeCoNi high entropy alloy, Appl Surf Sci, 601 (2022) 154171.