Effect of microstructural evolution on corrosion behavior of Al_0.7Cr₂FeCoNi high-entropy alloy

In the present study, the influence of the microstructural evolution on the corrosion behavior of the B2 nanoprecipitation-strengthened BCC-based Al_0.7Cr₂FeCoNi high-entropy alloy (HEA) in a 3.5 mass% NaCl solution was studied. The alloys in the as-cast state and heat-treated at 673 K for 2 h consisted of the BCC matrix and the B2 nanoparticles, and the heat treatments at 873 K and 1073 K for 2 h induced the coarsening of the B2 particles and the precipitation of the σ phase. It was found that the alloys exhibited good corrosion resistance, as evidenced by the low corrosion rates of less than 10⁻³ mm/year and the spontaneous passivation due to the formation of the Cr-rich surface films. It is notable that the alloys in the as-cast state and heat-treated at 673 K showed the transpassive dissolution above ∼ 0.7 V vs. SCE, while the alloys heat-treated at 873 K and 1073 K suffered localized corrosion on the B2 phase at ∼ 0.40 V and ∼ 0.37 V during the anodic polarization, respectively. This deterioration in the corrosion resistance of the alloys heat-treated at the higher temperatures was attributed to the higher potential difference between the B2 and σ phases than that between the B2 and BCC phases, as well as the coarsening of the Cr-depleted B2 particles. It is demonstrated that the coarsening of the B2 particles and the precipitation of the σ phase should be suppressed to obtain the corrosion-resistant Al–Cr–Fe–Co–Ni HEAs.