Pitting and Hydrogen Evolution on Aluminum in Concentrated HCl Solutions

According to the Pourbaix diagram, Al is amphoteric, exhibiting passivity in the mid-pH region and dissolving to form aluminate and Al³⁺ ions at high and low pH, respectively. This prediction, which is based on thermodynamics, would indicate that pitting corrosion should occur only in neutral solutions because a passive film is required for the establishment of pitting. Furthermore, the pH of pit environments in Al is considered to be on the order of 3. This study investigates the behavior of Al in concentrated HCl solutions, not because it is a material of construction for such environments, but because of the insight it might shed on Al pit stability. Some retained passivity and pit formation can be observed on surfaces of pure Al samples polarized to high potentials in solutions with as much as 4 M HCl. In very concentrated HCl solutions, the surface is roughened, with more attack at grain boundaries, but no evidence of pitting. The rate of hydrogen evolution (HE), which has previously been observed to occur to about 20% of the total anodic current density in pits formed in neutral solutions, was measured during anodic polarization. Similar to the situation found on anodically polarized Mg, the rate of HE increases with increasing applied anodic current density. However, unlike Mg, a prepolarized Al surface does not exhibit enhanced cathodic reactivity during a subsequent cathodic polarization measurement. The implications of these observations on the Al pitting mechanism will be addressed.