Passive film composition and stability of CoCrFeNi and CoCrFeNiAl high entropy alloys in chloride solution

In this work, CoCrFeNi and CoCrFeNiAl high entropy alloys (HEAs) were produced by arc melting. The aim was to investigate the correlation of the electrochemical behavior with microstructural features and chemical compositions of the surfaces during the first hours of immersion in 3.5 wt% NaCl solution at room temperature. Scanning electron microscopy (SEM) analysis and X-ray diffraction (XRD) patterns indicated that the CoCrFeNi alloy was comprised of one single face-centered cubic phase with distinct dendritic and interdendritic regions. The CoCrFeNiAl, in turn, was comprised of B2 and body-centered cubic (BCC) crystalline structures. The BCC phase was associated with dendritic regions while the B2+BCC structure was formed in interdendritic regions. The evolution of the electrochemical behavior was monitored by electrochemical impedance spectroscopy (EIS) up to 24 h of immersion. Al addition decreased the corrosion resistance of the CoCrFeNi alloy, as indicated by the lower charge transfer resistance values. X-ray photoelectron analysis of the passive films revealed that the oxide layer on the CoCrFeNi alloy was enriched with Cr2O3, improving its corrosion resistance. Conversely, the Al2O3-rich passive film formed on the CoCrFeNiAl alloy decreased its corrosion resistance. [Display omitted] •Evolution of the corrosion behavior of the HEAs is explained based on passive film composition.•Cr and Fe oxidized species plays a central role for the corrosion resistance of CoCrFeNi alloy.•Al2O3 governs the EIS response of the CoCrFeNiAl alloy.