Passivity and passivity breakdown of Al and Al–Zn alloys in SCN− solutions and the effect of some inorganic inhibitors – Galvanostatic, potentiostatic and ICP-AES studies

Abstract

The susceptibility of Al and Al–Zn alloys, containing 6% and 12% (in weight) Zn, towards pitting in KSCN solutions was studied using glavanostatic and potentiostatic techniques. Potentiostatic results showed that the pitting process was categorized into the three stages, i.e., the first passivation stage, the second pit formation and growth stage and the final steady-state stage. Galvanostatic measurements allowed the pitting and repassivation potentials to be determined. The rate of pit initiation and growth was found to increase with increase in SCN− concentration, applied anodic current, applied anodic potential and solution temperature. Alloyed Zn was found to enhance pitting attack. ICP-AES method of chemical analysis confirmed results obtained from electrochemical measurements. The inhibiting effects of MoO42−, WO42−, SiO32− or CrO42− anions were also investigated based on polarization and potentiostatic measurements, complemented with SEM and EDX examinations of the electrode surface. The pitting potential (Epit) and the incubation time (ti) were increased, while the pit growth current density (jpit) was decreased with increase in inhibitor concentration. These inorganic anions therefore inhibited pit nucleation and growth of Al in these solutions. SiO32− and MoO42 anions inhibited pitting corrosion more effectively than MoO42− and WO42− anions.