Effect of the grain size on the corrosion behavior of CoCrFeMnNi HEAs in a 0.5 M H2SO4 solution

This work investigated the corrosion behavior of the equiatomic CoCrFeMnNi high entropy alloy (HEA) in a 0.5 M H2SO4 solution. The grain size of the HEA was adjusted by annealing. The electrochemical analysis showed that the corrosion resistance of the CoCrFeMnNi HEA increases first and then decreases with the increase of grain size. When the grain size is lower than a certain value, increasing the grain size can improve the corrosion resistance. Fine-grained (≤1.24 μm) HEA forms abundant grain boundaries, which promotes the occurrence of galvanic corrosion at the grain boundaries with inter-grains, resulting in reduced corrosion resistance. However, coarse-grained (≥145.9 μm) HEA provides too few ionic diffusion channels to form a stable and continuous passive film, which also reduces the corrosion resistance. Nanoindentation and XPS tests showed that a high content of Fe (Ⅲ) oxides and Cr (Ⅲ) oxides are formed on the medium-grained HEA, which improves the chemical stability and mechanical strength of the passive films. Thus, the corrosion resistance of medium-grained HEA is much better than those of fine-grained and coarse-grained HEA. [Display omitted] •Corrosion behaviors of CoCrFeMnNi high entropy alloys with different grain sizes were investigated.•Galvanic corrosion at grain boundaries will reduce the global corrosion resistance.•Continuous passive films cannot be formed on coarse-grained HEA.•Combination of reduced grain boundary density and continuous passive film can improve corrosion resistance.