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 aluminum (Al) were investigated in chloride (Cl-) ion-containing solution using potentiodynamic polarisation experiment, optical microscopy, ac impedance spectroscopy and electrochemical quartz crystal microbalance technique combined with open circuit potential transient technique. The addition of OH- ions to NaCl solution raised the anodic dissolution rate of pure Al in value and at the same time extended the constant anodic current density region in width on the polarisation curves. This implies that the pitting corrosion is preceded by uniform thinning of the Al oxide film due to the chemical dissolution by the attack of OH- ion additives, which is validated by optical 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 open-circuit potential. From the lower fresh inner oxide film resistance R-inn,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 the chemical dissolution of the fresh inner oxide film accelerated by the attack of OH- ions inhibits the formation of the Cl- ion-incorporated outer film, what is responsible for the suppression of the pitting corrosion of pure Al. (C) 1999 Elsevier Science Ltd. All rights reserved.