Chemical Resistant Coating

Q. We're having a problem with rusting of a large sprocket on one of the machines in our food processing plant. This sprocket is 71 inches diameter and 0.689-inch thick, and was originally treated by electrogalvanizing. It's washed daily with an aqueous cleaner consisting of a 20 percent phosphoric acid and 20 percent nitric acid. Following the acid solution, it is treated with a foaming agent that consists of 3 percent octyl decyl dimethyl ammonium chloride, 1.5 percent didecyl dimethyl ammonium chloride, 1.5 percent dioctyl dimethyl ammonium chloride and 4 percent alkyl dimethybenzyl ammonium chloride. Can you help us to remedy the rusting problem? —C.S.
A. Since the sprocket was originally electrogalvanized, I recommend you disassemble the machine and send the sprocket to a shop for re-electrogalvanizing. Pretreatment and painting can solve the rust problem temporarily, but the paint will eventually wear off in the areas where the chain links contact the sprocket teeth. That is why I recommend electrogalvanizing and not painting.

If you insist on painting, the most acid-resistant and wear-resistant organic coating that can be applied in situ is a two-component epoxy. Please remember that it will eventually wear off where the chain contacts the sprocket teeth, and rusting can reoccur in those areas. Painting must be done immediately after de-rusting following this procedure: first, remove the rust from the sprocket, then immediately after de-rusting apply a two-component epoxy primer. After the primer is dry, apply a two-component epoxy enamel.