Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2003

Hexavalent Chrome, Wash Primers

Q. I have a customer that paints some galvanized poles with epoxy which is delaminating. He has told me that the poles may have had hexavalent chromium on them before applying the epoxy. His surface prep was SP1 and brush blast. He was not aware of the hexavalent chromium on the surface.

You had mentioned that hexavalent chromium primers are good at retarding galvanic oxidation. Being that it is a primer, I would assume that you are able to paint over it.

You also mentioned that it is a type of passivator. I always recommend to remove a passivator before painting. I had them check for a passivator with copper sulfate.

Questions: 1. Is hexavalent chromium used in wash primers?

A. Some wash primers might well contain hexavalent chromium.

2. If so, I know that wash primers can be coated over, but is there a maximum recoat window time?

A. I'm not aware of a maximum recoat time, but I would suggest that the wash primer be recoated within approximately 24 hours to prevent the substrate from corroding under the film. Bear in mind that a wash primer provides only temporary corrosion protection to the substrate.

3. When hexavalent chromium is used as a passivator at the fab shop, should it be removed before coating?

A. Are you referring to the wash primer? Unless the hexavalent chromium is incorporated in some temporary protective coating (not the wash primer) I don't see a need to remove it before painting.

4. Do you see any problems with the procedure used for painting the poles?

A. The most important aspect regards the surface preparation of the galvanizing. If all of the contaminants on the galvanizing have been removed then a wash primer can be beneficial. On the other hand, you might be able to apply a chromate-containing epoxy primer directly to the metal surface and forego the wash primer.

One of the problems associated with wash primers concerns blistering. I have seen cases in which the presence of a wash primer has caused blistering when the final coating system was exposed to humidity. This might have been due to the presence of unreacted phosphoric acid in the primer, before the final coating system was applied.

If you do apply a wash primer, it is often good practice to wash the dried coating with clean water and then allowing the surface to dry before applying the remaining coatings. You do not want unreacted acid to remain on the surface.