Morphological studies of the mechanism of pit growth of pure aluminum in sulfate ion- or nitrate ion-containing 0.1 M NaCl solutions

The mechanism of pit growth of pure aluminum (Al) in sulfate ion (SO42-)- o r nitrate ion (NO3-)-containing 0.1 M sodium chloride solutions has been st udied in terms of the morphological changes of artificial pits using potent iodynamic polarization experiment, potentiostatic current transient techniq ue and optical microscopy. The increase in SO42- and NO3- ion concentration s in NaCl solution raised the pitting potential E-pit of pure Al, which is ascribed to the impediment to pit initiation on pure Al by the addition of SO42- or NO3- ions. From the potentiostatic. current transients of artifici al pits in aqueous 0.1 M NaCl solution, the average value of the pit curren t was observed to increase with increasing SO42- ion concentration, whereas that value of the pit current in the presence of NO3- ions increased up to ca. 0.4 M NO3- ion concentration and then decreased abruptly with increasi ng NO3- ion concentration. From observations of the morphologies of the pit s, it appears that the pit grows preferentially in the lateral direction or in the downward direction by adding SO42- or NO3- ions to aqueous 0.1 M Na Cl solution, respectively. Based upon the experimental results, two differe nt pit growth mechanisms by anion additives can be proposed: (1) pit growth by the preferential attack of both SO42- and Cl- to the pit wall in SO42-- containing solutions; (2) pit growth by the creation of an aggressive envir onment at the pit bottom up to 0.4 M NO3- ion concentration due to the lowe r mobility of NO3- than Cl- in NO3--containing solutions.