(A) study on the effects of various ion-additives on the localized corrosion of pure aluminium and Al-Zn-Mg alloy in aqueous chloride solutions

Abstract

The present work involves the studies on the effects of various ion-additives on the localized corrosion of pure aluminium (Al) and Al-Zn-Mg alloy in aqueous chloride solutions. Part A is concerned with the studies on the effects of hydroxide ion ($OH^-$), electrolytical charged hydrogen, i.e., proton ($H^{+}$), sulphate ($SO_4^{2-}$) ion additives on pitting corrosion of pure Al by using electrochemical quartz microbalance (EQCM) technique. In chapter Ⅲ, effects of OH^- ion addition on the anodic dissolution of pure Al were investigated in chloride ion ($Cl^-$)-containing solution using potentiodynamic polarisation experiment, optical microscopy, ac impedance spectroscopy and EQCM technique combined with open circuit potential (OCP) 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 OCP. From the lower fresh inner oxide film resistance $R_{inn,ox}$ values in $OH^-$ ion-containing chloride solution than those in $OH^-$ ion-free chloride solution below the pitting potential ($E_{pit}$), 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. In chapter Ⅳ, effect of prior cathodic polarization in the pitting corrosion of pure Al has been investigated in aqueous 1 M...