Effect of annealing temperature on microstructure and corrosion behavior of CoCrFeMnNi high-entropy alloy in alkaline soil simulation solution

The effect of different annealing temperatures on the corrosion behavior of equiatomic CoCrFeMnNi high entropy alloy was investigated by electrochemical tests, immersion test, atomic force microscopy and X-ray photoelectron spectroscopy in alkaline soil simulation solution. The results show that with the increase of annealing temperature, the corrosion resistance of the HEA sample exhibits a trend of first increasing and then decreasing. The micro-galvanic corrosion effect between grain interior and grain boundary decreases as grain size increases. Moreover, MnO inclusions distributed in all annealed samples promote the occurrence of pitting corrosion due to the selective dissolution of Fe, Co, and Ni elements. For the HEA sample annealed at 850 °C, the presence of twins further facilitates the formation of pits. The 750°C-annealed sample possesses the optimal corrosion resistance due to passive film which has a stable structure and the highest contents of Cr oxide and H2O, and the lowest content of hydroxide. Therefore, the corrosion behavior of HEA samples treated at different annealing temperatures is closely related to grain size, inclusion and twins within microstructure, as well as the passive film on the sample surface.