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

Abstract

The mechanism of pit growth of pure aluminum (Al) in sulfate ion (SO42-)- or nitrate ion (NO3-)-containing 0.1 M sodium chloride solutions has been studied in terms of the morphological changes of artificial pits using potentiodynamic polarization experiment, potentiostatic current transient technique and optical microscopy. The increase in SO42- and NO3- ion concentrations 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 artificial pits in aqueous 0.1 M NaCl solution, the average value of the pit current 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 increasing NO3- ion concentration. From observations of the morphologies of the pits, 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 NaCl solution, respectively. Based upon the experimental results, two different 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 environment at the pit bottom up to 0.4 M NO3- ion concentration due to the lower mobility of NO3- than Cl- in NO3--containing solutions.