Zinc-rich coatings for improved corrosion resistance of extruded aluminium heat-exchange tubing in acidified chloride solution
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Thin walled heat exchanger Al alloy tubes, with solid-solution Zn-rich surface, are widely used by the automotive industry and are considered by the heat, ventilation, air-conditioning and refrigeration market. This work investigates protection of Al surface with Zn-rich layers for improved corrosion resistance of Al tubes against pitting. Zinc was applied on AlMn alloy extruded tubes by thermal-arc spraying. The tubes were subsequently subjected to heat treatment at various temperatures and durations to obtain solid-state AlZn alloy diffusion layer at the surface with varying thickness and Zn concentration profiles. Concentration depth profiles were obtained by glow discharge optical emission spectroscopy (GD-OES). Corrosion rate was determined by weight loss resulting from immersion for predetermined times in acidified artificial sea water solution of pH 3. The as-sprayed Zn coating was found to be highly non-uniform which resulted in non-uniformity of the Zn-rich layers laterally and in depth. Thickness of Zn-rich layer increased with heat treatment time and temperature, while Zn concentration at the surface decreased. Zn-rich layers were found capable of protecting the AlMn alloy substrate against pitting at the expense of increased self-corrosion rate. SEM and EDS area analysis of corroded samples after immersion test showed that corrosion of Zn-rich layers was non-uniform due to non-uniformity of the layers themselves. Regions with higher Zn concentration, ρZn, corroded first and protected galvanically regions with lower ρZn. The well-known relationship between the corrosion potential of AlZn alloys and Zn concentration was extended over a wide concentration range using ρZn of corroded areas and related further to the corrosion rate in acidified synthetic seawater. The database thus developed is useful for optimizing the Zn-rich layer thickness and Zn concentration profile in the layer to minimize uniform corrosion, and therefore maximize service lifetime, without reducing the protection against pitting.