Stability of passive films formed on aluminium in aqueous halide solutions

Potentiostatic current-time plots (i=f(t) curves) are a useful means of assessing the quality and stability of passive oxide layersformed on aluminium. For chloride and bromide solutions in particular, the current decay profiles are dominated by peaks and transients provided the selected potential in the passive region is within 70–100 mVof the potentiodynamically determined pitting potential. Outside this region smooth decay profiles are always obtained. Films improve with time under most test conditions. For iodide solutions the current-time curves indicate a gradual destabilisation of the passive film due to 12 production. The pH of the halide solution was found to exert a greater influence than previously thought. Addition of In³⁺ or Hg²⁺ ions to the test electrolyte after a prepassivation period leads to activation which in the case of indium is manifested in the form of localised attack. For mercury the attack is more general in nature. The induction time before activation was shown to depend on: (i) passivation potential; (ii) halide concentration and type; and (iii) the quality of the passive film formed before In³⁺ or Hg²⁺ addition.