Effects of sulphate, nitrate and phosphate on pit initiation of pure aluminium in HCl-based solution

Effects of sulphate, nitrate and phosphate ion additives on the pit initiation of pure aluminium were investigated in a hydrochloric acid solution as a function of anion concentration by using potentiodynamic polarisation experiment and electrochemical noise measurement. From the quantitative analysis of the electrochemical noise by using the Weibull distribution function based upon a stochastic theory, it was found that the pit embryo formation rate was increased by the addition of phosphate ions. On the other hand, the pit embryo formation rate was increased up to the sulphate or nitrate ion concentration around 0.05 M and then decreased gradually with increasing sulphate or nitrate ion concentration. Moreover, from the three linear regions in one Weibull probability plot in anion-free hydrochloric acid solution, it was concluded that pit initiation was clearly distinguished from uniform corrosion and the stochastic process concerning the series of nucleation, growth and detachment of hydrogen bubbles in the stochastic analysis.