Aspects of Chemical Conversion Coating Processes

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Abstract
The mechanisms for the formation of conversion coatings on metals in acidic solutions have been studied and individual reactions for the production of crystalline and amorphous phosphate coatings, oxalate coatings, fluoride coatings and Chromate coatings on a variety of metals have been deduced from the compositions of the coatings and other evidence. It is postulated that, apart from direct oxidation, the initial reaction in all cases is attack by acid on the metal which can lead to local rise in pH, to hydrogen evolution and reducing conditions or to a local increase in metal ion concentration, any or all of which can bring about the precipitation of insoluble compounds.