Investigation of the corrosion behavior of AlCoCrFeNi high-entropy alloy in 0.5 M sulfuric acid solution using hard and soft X-ray photoelectron spectroscopy

Recently, high-entropy alloys (HEAs) have demonstrated excellent properties, such as high yield strength, better ductility, and great corrosion resistance, which breaks the conventional alloy design strategy. We investigate the corrosion behavior of AlCoCrFeNi HEAs in 0.5 M H2SO4 solution under ambient conditions using soft and hard X-ray photoelectron spectroscopy (XPS) for surface/bulk (∼5 nm and > 10 nm) analysis. The composition information of the surface and bulk can be achieved and compared via a switchable target system, where the Al (Kα, 1486.6 eV) and Cr (Kα, 5414.8 eV) targets produced X-rays for regular XPS and hard X-ray photoelectron spectroscopy (HAXPES) as the detection source, respectively. The HAXPES provides a precise composition study in the bulk region without the interference of Auger electron signals caused by neighboring transition metals. The dissolution of metallic ions in H2SO4 solution were collected by inductively coupled plasma–mass spectrometry (ICP–MS) and compared with the XPS-HAXPES analysis. The Al and Cr compounds redeposited to the HEA surface, while Fe, Co, and Ni kept dissolving into the acidic solution. It is speculated that the dissolution of Fe, Ni, and especially Co could slow the rate of dissolution of Cr. The passivation layer formed by Cr hydroxide/oxide could thus protect the HEA from further corrosion. Al easily leached before the formation of the Cr passivation layer, however, Al compounds still became the main species on the surface of the HEA. Moreover, our findings examined by the novel analysis technique may provide a new paradigm for the quantification study of HEAs.