The Influence of Zinc Phosphate Crystalline Morphology on the Corrosion Resistance of Electropainted Steel

Summary

Systematically varied forms of zinc phosphated steel have been overpainted with a cathodic primer, a surfacer and a thermosetting acrylic top coat. The painted systems were then subjected to accelerated salt spray and exterior weathering tests. Their corrosion performance has been correlated with the P:P+H ratio, where P and H are the phosphophyllite and hopeite zinc phosphate levels respectively, determined by X-ray and electron diffraction techniques. Under the specific conditions chosen, two trends were discerned. Firstly, with the spray process, corrosion resistance increased with increase in P:P+H ratio. Secondly, with the spray/dip process, corrosion resistance decreased with increase in the P:P+H ratio. Thus unequivocal acceptance of this ratio as a unique indicator of corrosion performance is challenged and attention drawn to other experimental variables that may have significant influence, such as physical crystalline structure and porosity, etc.