Electrochemical characterization of passive film and corrosion in Co-rich high entropy alloys in Ringer's solution

This study investigates the corrosion characteristics of Co₃₀Cr₂₀Ni₂₀Fe₂₀Mo₁₀ (Co30HEA), Co₄₀Cr₂₀Ni₁₅Fe₁₅Mo₁₀ (Co40HEA), and Co₅₀Cr₂₀Ni₁₀Fe₁₀Mo₁₀ (Co50HEA), designed as Co-rich high entropy alloys, compared to 316 L stainless steel and L605. Co30HEA has a single FCC phase, whereas the other two alloys exhibit a dual phase of FCC + (Cr, Mo) σ phase. The presence of high Cr and Mo content in the σ phase results in Cr depletion zones. Co30HEA shows the highest corrosion resistance in Ringer's solution, which diminishes with increasing Co content. Its passive film exhibits ideal capacitive behavior at low frequencies, indicating strong passivation. The films, primarily composed of Cr oxides and hydroxides, vary in thickness, with Co30HEA having a film about twice as thick as the others. All three alloys demonstrate pitting corrosion; however, Co30HEA shows the smallest pits mainly at grain boundaries, while the others show pits near the σ phase due to unstable passive films in the Cr and Mo-depleted zones, acting as anodic sites for galvanic corrosion.