Al-Zn coatings for the corrosion protection of steel

Aluminum coatings have been reported to be the most suitable for replacing toxic cadmium for the protection of steel and titanium alloys against corrosion. The relatively poor galvanic corrosion protection of aluminium coatings, however, has led to a search for a more effective coating. To this end, pure aluminium and controlled-composition Al-Zn alloy coatings were ion plated onto steel substrates. Over a range of coating conditions the aluminium and the Al-Zn alloy coatings have very similar columnar structures. They were equally successful in protecting the underlying steel. However, a simulation of the coating damage by masking the steel substrate during plating showed the galvanic corrosion protection of Al-2.5%Zn alloy coatings to be superior to that of aluminium. It is probable that this very effective sacrificial corrosion protection means that the structure of the coating is relatively unimportant and that excellent galvanic corrosion protection can be provided by low density columnar structure coatings of Al-Zn alloys.