Chronoamperometric studies of pitting corrosion of Al and (Al–Si) alloys by halide ions in neutral sulphate solutions

The pitting corrosion of Al, (Al + 6%Si), and (Al + 18%Si) alloys in neutral 0.50 M Na 2SO 4 solution in the absence and presence of NaCl, NaBr and NaI under the influence of various experimental variables has been studied by using potentiodynamic and chronoamperometric techniques. The results showed that the pitting corrosion resistance of the three Al samples decreases in the order: (Al + 18%Si) > (Al + 6%Si) > Al. The current/time measurements showed that the overall process can be described by three stages. The first stage corresponds to the nucleation and growth of a passive oxide layer. The second and the third stages involving pit nucleation and growth, respectively. Nucleation of pit takes place after an incubation time ( t i). The rate of pit nucleation ( t i −1) increases with increasing halide concentration, temperature, and applied potential. The pit growth current density ( j pit) increases linearly with t 1/2, indicating that the pit growth can be described in terms of an instantaneous three dimensional growth under diffusion control. The effect of Cr 2O 7 2−, CrO 4 2−, WO 4 2−, MoO 4 2−, NO 2 − and NO 3 − as inorganic inhibitors on the pitting corrosion inhibition of pure Al and its alloys in (0.5 M Na 2SO 4 + 0.2 M NaCl) solution has also been studied. The presence of these anions (except NO 3 −) results in an increase in the incubation time and a decrease in the pit growth current density of the three samples to an extent depending on the nature and the concentration of the inorganic inhibitors and the composition of the Al samples. The inhibition efficiency of these inhibitors decreases in the order: Cr 2O 7 2− > CrO 4 2− > WO 4 2− > MoO 4 2− > NO 2 −.