Effects of hydroxide ion addition on anodic dissolution of pure aluminium in chloride ion-containing solution

Effects of hydroxide (OH-) ion addition on the anodic dissolution of pure a luminum (Al) were investigated in chloride (Cl-) ion-containing solution us ing potentiodynamic polarisation experiment, optical microscopy, ac impedan ce spectroscopy and electrochemical quartz crystal microbalance technique c ombined with open circuit potential transient technique. The addition of OH - ions to NaCl solution raised the anodic dissolution rate of pure Al in va lue and at the same time extended the constant anodic current density regio n in width on the polarisation curves. This implies that the pitting corros ion is preceded by uniform thinning of the Al oxide film due to the chemica l dissolution by the attack of OH- ion additives, which is validated by opt ical microscopy. The mass change rate enhanced by the addition of 0.5 M Cl- ions during immersion in OH- ion-free 0.01 M NaCl solution indicates that Cl- ions incorporate into the growing Al oxide film even at steady state op en-circuit potential. From the lower fresh inner oxide film resistance R-in n,R-ox values in OH- ion-containing chloride solution than those in OH- ion -free chloride solution below the pitting potential, it is suggested that t he chemical dissolution of the fresh inner oxide film accelerated by the at tack of OH- ions inhibits the formation of the Cl- ion-incorporated outer f ilm, what is responsible for the suppression of the pitting corrosion of pu re Al. (C) 1999 Elsevier Science Ltd. All rights reserved.