Effects of sulfate and nitrate ion additives on pit growth of pure aluminum in 0.1 M sodium chloride solution

Abstract

Effects of sulfate (SO42-) and nitrate (NO(3) over bar) 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 NO(3) over bar ion concentrations in NaCl solution raised the pitting potential (E-pit) of pure aluminum and increased the steady-state current density at potentials above E-pit. This means that SO42- and NO(3) over bar 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 NO(3) over bar ions to NaCl solution revealed that the artificial pit grows more rapidly in the presence of SO42- 4 and NO 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 NO(3) over bar 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 NO than Cl- in NO(3) over bar-containing NaCl solution.