The importance of the solution pH in electrochemical studies of aluminum in aqueous media containing chloride

The presence of chloride ions accelerates the corrosion of aluminum in aqueous media. The solution pH is also an important parameter which strongly influences the rates of corrosion and the pitting potential of pure aluminum. A large number of studies of the corrosion behavior of aluminum in aqueous media have been done using polarization measurements. Little attention has been given in the variation of the bulk solution pH during the cathodic or the anodic polarization measurements. In the present work, during the course of obtaining the polarization curves of aluminum specimens in aqueous media containing sodium chloride, significant changes in the solution pH occurred which consisted of a shift to alkaline values during cathodic polarization and to acidic values during anodic polarization. The results were obtained at conditions of sustained pH, by using a pH controller which kept the solution pH constant by the addition of alkali or acid, as needed. This gave markedly different results, reflecting the sensitivity of the electrochemical measurements to the bulk solution pH. The pitting potential at constant pH was shifted to more cathodic values at pH 7 while it was minimum in the pH range of 5.00–7.0. The rates of corrosion of aluminum were found to depend strongly on both the solution pH and the solution temperature.