Uniform and Pitting Corrosion Processes of Al, Al-6061, Al-Zn and Al-Cu Alloys Exposed to SCN^- Solutions and the Effect of Some Inorganic Anions

Abstract

The effect of the alloying elements and some inorganic inhibitors (WO₄^2-, MoO₄^2-, SiO₃^2- and CrO₄^2- anions) on the uniform and pitting corrosion characteristics of Al-6061, Al-4.5% Cu, Al-7.5% Cu, Al-6% Zn and Al-12% Zn alloys were studied in 0.04M KSCN solution. Susceptibility of the tested alloys towards pitting corrosion was investigated using transient (potentiostatic and galvanostatic) measurements. An independent method of chemical analysis, namely ICP-AES (inductively coupled plasma atomic emission spectroscopy) was also used to study the effect of the alloying elements and the tested inorganic anions on the uniform corrosion of these materials. Results obtained were compared with pure Al. ICP measurements revealed that the alloyed Cu and alloyed Zn enhance uniform corrosion of the Al samples, while WO₄^2-, MoO₄^2-, SiO₃^2- and CrO₄^2- anions suppressed it. Transient measurements showed that nucleation of pit takes place after an incubation time (t_i). The rate of pit initiation and growth (t_i^-1) increased with increase in SCN^- concentration, applied anodic current, applied anodic potential and solution temperature. This rate of pit initiation and growth decreased in presence of inorganic inhibitors to an extent depending on the concentration and type of the introduced inhibitor. Alloyed Zn was found to accelerate pitting attack, while alloyed Cu suppressed it. Among the tested Al alloys, Al-6061 presented the highest resistance towards uniform and pitting corrosion processes. SiO₃^2- and CrO₄^2- were the most efficient anions in inhibiting uniform and pitting corrosion processes of Al in these solutions.