Abstract
Al content is a significant factor that affects the corrosion characteristics of AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00) in sulfuric acids with or without chlorides. In order to investigate the corrosion mechanism of AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00), by use of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and analysis methods such as XPS, AES, ICP, and SEM to vacuum arc remelted and cast AlxCoCrFeNi (x = 0, 0.25, 0.50, 1.00), and after detailed cross comparison of experimental data, this study obtains the following important conclusions: In comparison of results for alloy x = 0 with those of x ≠ 0, it shows that in sulfuric acid the latter possesses better corrosion characteristics than the former at temperatures lower than the room temperature, and vice versa. Data show the activation energy for corrosion is proportional to the amount of Al, x, in the alloys and the intersection point of lines for different x in the Arrhenius plot is at room temperature (around 23 ~ 27oC) and this obviously conforms the corrosion results mentioned above. The thickness of oxides for alloys in sulfuric acid increases with increasing x in that it is easy to form porous structure for aluminum oxides in sulfuric acid. There are two main effects on corrosion characteristics for increasing aluminum amount in the alloys. One is the lowering anticorrosion of passive oxides because of the relative raise in passive current density in presence of porous structure of aluminum oxides. The other is the microstructural change of the alloys from single FCC to duplex BCC-FCC, where the corrosion process principally prevails in the Al, Ni-riched BCC phase and there is a local cell effect in this BCC-FCC duplex phase microstructure. Thus, the corrosion process accelerates. In chloride bearing sulfuric acid solution the corrosion resistance of the alloys free from Al is better than that of the alloys with Al. Increase in Al amount in the alloys causes lowering in pitting potential of alloys, which is similar to that in conventional alloys.