Effect of an inhibitive pigment zinc-aluminum-phosphate (ZAP) on the corrosion mechanisms of steel in waterborne coatings

The effect of the inhibitive pigment ZAP on waterborne paint systems was studied by means of impedance technique and Raman spectroscopy. The protective mechanism given by the action of ZAP in saline media onto low-carbon steel panels is attributed to the formation of a thin zinc-sodium-phosphate layer under cathodic delamination conditions. The chemical interactions between the waterborne paint systems and the metal substrate at the curing time and during the delamination tests were investigated by Raman spectroscopy. The layer developed on the delaminated area is thought to be a phosphate layer known as α-hopeite. This film is though to arise from a dissolution–precipitation mechanism in alkaline media.