(A) study on the effects of specimen dimension, defect structural and chemical properties of passivating oxide film on corrosion behaviour of Al-1wt.\%Si-0.5wt.\%Cu alloy

Abstract

The present work is concerned with the effects of specimen dimension, defect structural and chemical properties of passivating oxide film on corrosion behaviour of Al-1wt.\%Si-0.5wt.\%Cu alloy. In chapter III, the electrodissolution of aluminium thin films as microband electrodes (length $= 5 \times 10^{-3} m$) has been discussed in terms of mass transfer determinded by voltammetry and ac-impedance techniques as a function of bandwidth (20 to 2000 nm) in 0.1 M NaOH solution. The anodic polarization curves of the Al microband electrodes show that current density is enhanced with decreasing bandwidth. The ac impedance response suggests that a steady-state diffusion layer appears the more markedly, the smaller the bandwidth. The anodic polarization curves are analyzed on the basis of the combined Butler-Volmer high field approximation and the semi-cylindrical diffusion field approximation. As a result of the analysis, the electrodissolution proceeds by a mixed kinetic-mass transfer controlled reaction. The analysis also makes it possible to distinguish the semi-cylindrical diffusive mass transfer contribution to the electrodissolution from the kinetic contribution, i.e., mass transfer index linearly diminishes with decreasing bandwidth. The increased current density is attributable to the decreased mass transfer contribution, i.e., the more predominant semi-cylindrical diffusive mass transfer as compared to laminar diffusive mass transfer. In chapter IV, statistical variation of pitting potential of anodic oxide films galvanostatically formed on Al-1wt.\%Si-0.5wt.\%Cu alloy has been discussed as a function of formation current density and formation potential in 0.5M $Na_2SO_4 + 0.1M$ NaCl solution, based upon a stochastic theory. It was found that distribution of pitting potentials of the anodic films shifted to higher anodic potentials, as formation current density decreased and formation potential increased. This result indicated that corrosion resistance is imp...