EFFECT OF HALIDE ION AND APPLIED POTENTIAL ON REPASSIVATION BEHAVIOR OF AL-1WT.PERCENT-SI-0.5WT.PERCENT-CU ALLOY

Abstract

Repassivation behaviour of Al-1 wt.%Si-0.5 wt.%Cu alloy has been investigated as a function of applied potential in 0.5 M Na2SO4 solutions containing various chloride ion concentrations ranging from 0 to 0.5 M in terms of potentiostatic current transients by using an abrading electrode technique. The current transients obtained were found to be classified into three groups. The anodic processes included in each group are as follows: (i) only repassivation related to the formation of passivating oxide in low concentrations (0-0.01 M); (ii) competition between oxide repassivation and film breakdown, the growth of a stable pit after an induction time of t(m) followed by a uniform metal dissolution through aluminium salt film in intermediate concentrations (0.05-0.3 M); (iii) only a uniform metal dissolution through aluminium salt film in high concentrations (>0.3 M). Transition from (i) to (ii) occurs at 0.05 M Cl-, regardless of applied potential and (ii) to (iii) transition does at -0.1 V/sce and 0.3 M Cl-. In connection with the results of the current-time curves, potential-time curves measured on oxide-covered Al alloy in various halide (F-, Cl-, Br- and I-) solutions demonstrated that the attack by halide ions occurs on the bare metal surface simultaneously with the formation of anodic oxide film.