Year

2010

Degree Name

Doctor of Philosophy

Department

School of Chemistry

Abstract

This thesis describes investigations into the initiation and short term atmospheric corrosion behaviour of 55% Al-Zn metal coating. It explores the influence of ions produced by localised oxidation and reduction reactions to affect nearby sites during bare and cut edge corrosion. Band microelectrode(BME) arrays were used to achieve this aim.

Al-40% Zn alloy exhibiting solid solution precipitate microstructures were prepared as model alloys for the α-phase of a 55% Al-Zn metal coating on steel. The effect of electrolyte composition and pH on the polarisation electrochemistry was investigated. Enhanced anodic processes were observed in Cl- compared to SO4 2- electrolytes. The electrolyte pH influenced both anodic and cathodic processes by affecting the solubility of the native oxide film. The presence of zinc rich islands influenced the electrochemical polarisation behaviour by enhancing zinc dissolution.

The interaction between the α- and β-phases of 55% Al-Zn metal coating was quantified by arranging model alloys into BME arrays. Only a small galvanic driving force was observed between the phases. During coupling, OH- and metal ions from cathodic and anodic sites respectively were transported tens to hundreds of microns. Micro-pH sensing near the surface of the alloys allowed the pH at anodic and cathodic sites to be recorded and the transport of OH- ions to be visualised. When the elements of BME arrays were closely spaced, the changes in the near surface electrolyte volume had the effect of reversing the polarity of the galvanic current. Mechanisms for the polarity reversal are discussed as well as the implications for the surface corrosion of unpainted 55% Al-Zn alloy.

The galvanic interaction of 55% Al-Zn and its constituent phases with steel was also quantified using BME arrays. Near surface pH mapping revealed ubiquitous distribution of OH- from the cathodic processes and localised acidic sites from the anodic processes. Simultaneous polarisation experiments on BME arrays enabled the influence of ion transport during polarisation to be measured. Al3+ and Zn2+ ions were found to have a strong pH buffering effect, which accelerated the cathodic hydrogen evolution reaction on steel. The steel generated an alkaline environment that solubilised the native oxide film on the sacrificial metal coatings. This shifted their corrosion potentials more negative. The relevance of these observations to the cut edge corrosion behaviour of 55% Al-Zn alloy is explored.

Interfacial composition and impedance properties of Al-Zn alloys exposed to sulfate and chloride bearing electrolytes was investigated using X-ray photoelectron spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopy. Results show that the surface of the native oxide on a series of Al-Zn alloys (2 – 40 wt% zinc) was composed predominantly of aluminium oxides. Zinc and aluminium metal content increased with depth through the oxide, however results show negligible zinc oxide was present in the oxide film, regardless of composition. Impedance results provide information regarding the effect of Cl- and SO4 2- on the dissolution mechanism of the alloys. It was concluded that the electrochemical behaviour of the Al-Zn alloys was affected by Cl- and SO4 2- anions by changing ion transport mechanisms through the passive film, whereas electrolyte pH affected the electrochemical behaviour via solubility of the native oxides. A model of the Al-Zn oxide – electrolyte interface is presented which accounts for the data in this thesis.

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