A comprehensive approach to the atmospheric corrosion of zinc and zinc (alloy) coated steel based on the surface chemistry in nonequilibrium gradient conditions

摘要

Zinc coatings compete with paints as the most widely used protection against atmosphericcorrosion of low alloyed steels. Accordingly, numerous studies have been published on thecorrosion behaviour and corrosion mechanisms of zinc and its alloys. Still, there is a need to unitethe various experimental findings from field and testing into a fully consistent and comprehensibletheory. The objective of this presentation is to modestly contribute to this quest for zinc and zinccoated steel in salt laden atmospheric environments.By considering the surface chemistry implications of the localized anode/ distributed cathode cellformation and its associated lateral mass transport gradients in thin electrolyte films, anunderstanding of the macroscopic manifestations of the atmospheric corrosion, consistent withexperimental observations, can be approached.The formation of simonkolleite, Zn_5(OH)_8Cl_2.H_2O, and its seemingly anomalous role in promotinglocal high surface alkalinity and surface wetting resulting in high cathodic activity, is a key for thisunderstanding. The presence of e.g. magnesium ions can suppress this mechanism. For thecorrosion development of zinc (alloy) coated steels a decisive factor is how electrochemicallyactive areas of exposed steel become for oxygen reduction after dressed-off of zinc. The abilityand conditions for formation of protective films of simonkolleite and zinc hydroxy carbonatesdirectly on the steel is discussed, especially in the context of rinsed vs. non-rinsed conditions. Thedistinction between the corrosion behaviour of zinc as such and in zinc(alloy) coated steel isextended beyond zinc as mere cathodic protection of the steel. In corrosion exposures thepresence of AlFe intermetallics in hot-dip zinc products can be shown to reduce the onset andpropagation rate of red rust compared to electro-zinc coatings. It is suggested that slightprecipitates of AlZn-LDH act as cathodic suppressor in the same, but more subtle way as foraluminium alloyed zinc coatings.A number of experiments will exemplify, including results from time resolved corrosion monitoring,covering exposures at different temperatures, humidity, salt types and salt loads.