Zinc Phosphate Pretreatment Systems for Aluminum-Steel Assemblies

The critical component for maintaining uniform zinc phosphate coatings on aluminum is fluoride. The control of free fluoride is critical to the operation of the process, because aluminum is dissolved in a zinc phosphate bath as a step in the coating reaction. If the dissolved aluminum is not removed from the bath, it will inhibit the formation of coatings on all metals being treated.

This paper presents the characteristics and composition of zinc phosphate coatings on aluminum and steel, and lists some practical considerations of the transition from steel to aluminum components. Some possible problem areas discussed are: galvanic corrosion, underpaint quality, surface finish, electro-painting, and new paint surfaces.