Effects of Sulfate and Nitrate Ion Additives on Pit Growth of Pure Aluminum in 0.1 M Sodium Chloride Solution

Abstract

Abstract Effects of sulfate (SO42−) and nitrate (NO3−) ion additives on pit growth of pure aluminum were investigated in 0.1 M sodium chloride (NaCl) solution as a function of anion concentration using a potentiodynamic polarization experiment, a potentiostatic current transient technique, and optical microscopy. The increase in SO42− and NO3− ion concentrations in NaCl solution raised the pitting potential (Epit) of pure aluminum and increased the steady-state current density at potentials above Epit. This means that SO42− and NO3− ion additives impede pit initiation on pure aluminum, but enhance the growth of preexisting pits. Potentiostatic current transients of artificial pits obtained from the moment just after adding SO42− and NO3− ions to NaCl solution revealed that the artificial pit grows more rapidly in the presence of SO42− and NO3− ions. 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 downwar...