Pretreatment of Galvanized Surfaces Prior to Powder Coating

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Abstract
The paper covers the various methods and processes suitable for the pretreatment of galvanized steel surfaces prior to the application of powder coatings, drawing upon results from industrial coating installations as well as those from laboratory testing. It is important to differentiate between the types of zinc coated surfaces and in particular between strip-line galvanized steel sheet and heavy conventional hot dip galvanized steel. The main precleaning processes in use are alkali cleaning/etching and acid etching, the choice being influenced by the method of application and the thickness of the overlying zinc layer. Conversion coatings are necessary to ensure adequate service life and here the choice lies between chromating and phosphating. Accelerated corrosion tests have indicated that certain yellow Chromate processes are to be preferred for the treatment of thick galvanized surfaces, whereas Chromate, chromate/phosphate and micro-crystalline zinc phosphate processes may all be suitable for use on strip-line galvanized sheet. Laboratory tests and the results of extensive industrial experience have shown that, contrary to accepted belief, degassing of yellow chromated HDG sections, which is essential prior to powder coating to prevent pin-holing, does not adversely affect paint performance.

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