How to test for a passivation (passivated) layer on galvanized steel prior to painting

Passivation is a quenching process on galvanized steel that prohibits the formation of excessive zinc oxides and zinc hydroxides in the first several weeks after the galvanizing process. These unsightly formations are called wet storage stains. If the galvanized items are likely to be stored or shipped in humid conditions that cause wet storage stains, they are often treated with a clear conversion coating called passivation following the galvanizing process. If the galvanized item will be coated with liquid or powder coatings, all wet storage stains and passivators must be removed prior to coating. Because the passivation layer is clear, is cannot be detected with a simple visual inspection. However, it is critical that the powder or liquid coating applicator know if a passivator exists prior to performing coating operations, or adhesion problems are likely to occur.
To test for the presence of a passivation layer, use the following procedure:

1) Dissolve 20 grams of copper sulfate crystals in one liter of water.
2) Using the procedure specified in SSPC SP-1, perform a solvent wash of a 6” x 6” area of the galvanized part that you want to test.
3) Sand half of the test area using emery cloth. Do not sand the other half.
4) Using an absorbent cotton swab, swipe once through the sanded and unsanded areas of the test area.
5) If the sanded and unsanded areas both turn black at the same rate and in less than 10 seconds, there is no passivation on the surface other than light oil. If the unsanded area turns black slower than the sanded area, or not at all, there is a passivator present on the galvanized steel. If neither surface turns black, the surface is probably an alloy of zinc or some other metal.

If a passivator is present, consult with the coatings manufacturer or supplier for the coating that you intend to apply to determine the best treatment method. Treatment using chemicals or mechanical treatments such as brush blasting are possible solutions. In addition, an adhesion test following treatment is always a wise choice.