Conversion coating of zinc coated substrates prior to powder coating

Abstract

This paper is concerned with studies of the rates of film formation, their composition and surface chemistry, both for laboratory prepared solutions and a proprietary one. Four types of zinc coatings were included in the programme—electroplated, hot-dipped galvanized (batch and continuous), and sprayed. The experimental techniques employed included measurement of open circuit potentials on immersion of zinc coated samples in various solutions, determination of coupon weight change and zinc dissolution rate, and analysis using SEM with EDXA. Chromate conversion coatings were shown to be beneficial in resisting debondment of powder coatings when subjected to acetic acid salt spray tests. pH of the Chromate solution was important but ‘degassing’ did not appear to be deleterious in terms of debonding of the polymer film, even though it caused widening of cracks in the film and edge curling. The Alocrom 100 solution which was developed for use with aluminium, but which is sometimes used with galvanized steel, resulted in an excessive zinc loss which is not acceptable.