Phosphate Pretreatment of Galvanized Steel

Q. We use a seven-tank process for pretreating cold-rolled steel sheet that has been working fine, but our customer has changed the material to galvanized steel. We are pretreating it using the same seven-tank process, but there are now paint-adhesion failures. What must we do to change the process to pretreat GI sheet? —M.T.

A. The following are two possible causes of your problem:

1. The cleaner you are using in your seven-stage pretreatment system is not removing the oil or wax from the surface of the galvanized steel. Oils and waxes are applied to galvanized steel to protect it from white rusting, a coating of unsightly zinc oxide crystals on the surface.

2. You may be using iron phosphate chemicals in your seven-stage pretreatment system. Although you may get light phosphate coating on galvanized steel surfaces using iron phosphate pretreatments, to provide better paint adhesion, it is better to pretreat galvanized steel using zinc phosphate chemicals.