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Ask the Expert Question-and-Answer Archive
by Ron Joseph
Zinc Chromate Primers, Wash Primers and Epoxy Primers
Q. I am an aircraft structural engineer. Usually we use three different kinds of primers--zinc chromate primer, wash primer and epoxy primer. I want to know the difference between these three kind of primers.
A. Wash primers are water-thin coatings of phosphoric acid in solutions of vinyl butyral resin, alcohol and other ingredients. They are generally applied to give a dry film thickness of 0.3-0.5 mils (8-13 microns). The purpose of wash primers is to passivate steel and galvanized surfaces before applying a full bodied primer. Their functions are to passivate the surface and temporarily provide corrosion resistance, and they are used to provide an adhesive base for the next coating. Sometimes, wash primers are also applied to aluminum and other metals to enhance adhesion of the next coats of paint. When treating aluminum and other difficult-to-paint metals, wash primers are often specially formulated so that there is no excess unreacted acid on the surface after is has been applied.
Zinc chromate is simply a corrosion resistant pigment that is added to certain coatings. In the case of a wash primer, the phosphoric acid actually reacts with the metal, whereas in the case of zinc chromate this is not the case. In dry, low humidity weather the zinc chromate does not do anything other than remain in the coating. On the other hand, when the humidity in the air increases, or on a rainy day moisture for the air penetrates the primer coating and slightly dissolves the zinc chromate. The dissolved zinc chromate solution now does react with the underlying metal surface and forms a passive layer (like a blanket|) that prevents corrosion. As soon as the weather becomes dry again the zinc chromate no longer pays a role ... at least until the next time it rains, when the process is repeated. If there are frequent cycles of high and low humidly the zinc chromate will eventually be depleted and soon after you might start to see corrosion of the metal..
Zinc chromate pigments can be added to primers made of several different resin types, such as epoxy, polyurethane, alkyd and others.
On the other hand, there are epoxy, polyurethane, alkyd and other primers that do not contain zinc chromate, but might instead contain other corrosion inhibiting pigments.