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

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 downward direction by adding SO42− and NO3− ions to NaCl solution, respectively. Based upon experimental results, two different pit-growth-promotion mechanisms by anion additives can be proposed: promotion by attack of SO42− and Cl– to the pit wall in SO42−-containing NaCl solution; promotion by the creation of aggressive environment at the pit bottom resulting from the lower mobility of NO3− than Cl– in NO3−-containing NaCl solution.