(A) study on the effects of sulphate and nitrate ion additives on pit growth of pure aluminium in 0.1 M NaCl solution

Abstract

The effects of sulphate $(SO_{4}^{2-})$ and nitrate $(NO_{3}^{-})$ ion additives on pit growth of pure aluminium (Al) have been investigated in 0.1 M NaCl solution as a function of anion concentration using potentiodynamic polarization experiment, potentiostatic current transient technique and optical microscopy. The increase in $SO_{4}^{2-}$ and $NO_{3}^{-}$ ion concentrations in NaCl solution raised the pitting potential E_{pit} of pure Al in value and at the same time the steady-state current density at potentials much higher than the $E_{pit}$ on the potentiodynamic polarization curves. This means that $SO_{4}^{2-}$ and $NO_{3}^{-}$ ion additives impede the pit initiation on pure Al surface, but enhance the growth of pre-existing pits. In order to experimentally establish the condition for pure pit growth process, we artificially made a pit on pure Al surface. Potentiostatic current transients obtained from the moment just after adding $SO_{4}^{2-}$ and $NO_{3}^{-}$ ions to NaCl solution revealed that the artificial pit grows more rapidly in the presence of $SO_{4}^{2-}$ and $NO_{3}^{-}$ ions than the absence over time. From observation of the cross-sectional views of the artificial pit, it appeared that the pit grows preferentially in the lateral direction and in the downward direction by adding $SO_{4}^{2-}$ and $NO_{3}^{-}$ ions to NaCl solution, respectively. Based upon the experimental results, two different pit growth promotion mechanisms by anion additives can be proposed: promotion by preferential attack of $SO_{4}^{2-}$ ions more mobile than $Cl^{-}$ ions in $SO_{4}^{2-}$ ion-containing solution; promotion by the production of nitrate salt from $NO_{3}^{-}$ ions more oxidizing than chloride salt from $Cl^{-}$ ions in $NO_{3}^{-}$ ion-containing solution. Both cases may enhance the dissolution of bare Al metal, thus accelerating the pit growth of pure Al in NaCl solution.``