What Causes Outgassing?

Question:

I read with interest your comment regarding powder coating over galvanized surfaces in the April issue of Products Finishing. Although we use conventional air-dry coatings rather than powder, we experience adhesion problems. I have been assigned to find out why.

I have talked to a number of galvanizers, paint suppliers and coating application people. The term “outgassing” was mentioned on several occasions. I am having trouble understanding where the gas comes from or what the gas is.

This adhesion problem occurs with 100% solids (no VOCs) liquid paints and powder paints. Therefore, I have ruled out solvents as being the source of the gas. I am told that a galvanized surface will be composed of pure zinc, zinc oxide, zinc hydroxide and zinc carbonates depending upon the length of time it is exposed to the atmosphere. I find it very difficult to believe that the zinc would turn into a gas. Would you please help me understand what is going on? Thanks for the help. R.B.

Answer:

You’re absolutely right to say that the zinc itself is not the origin of the gas. Air is entrapped between the layers of zinc during the dipping process. This air will be released when the product is heated during cure. This entrapped air will “outgas” through the coating causing pinholes in the coating. Having said that, this defect will allow moisture to penetrate the coating and react with the substrate causing corrosion. This eventually may cause a loss of adhesion of the coating. But, I suspect that this is not your main problem.

Normally, adhesion failures on hot dipped galvanized parts are caused by the failure of the bond between the zinc and the underlying steel substrate or the failure of the bond between numerous coats of zinc. This can easily be verified by examining the paint chip. If there is a coating of zinc on the backside of the chip, then the bond between the zinc and the steel was broken and outgassing had nothing to do with the failure. Furthermore, zinc oxide will form on the zinc surface when exposed to the atmosphere for even a relatively short period of time. This oxide layer must be removed or the topcoat will lose adhesion because the bond between the zinc oxide and the zinc will be broken. In this case, the paint chip may not have any zinc on the backside but will have a powdery residue of zinc oxide. This will be visible under magnification and will look like a dull, silver-looking, speckled material.

The first principle in painting is to ensure that the surface is cleaned and prepared properly. This is especially true if you are looking for good adhesion. Ensuring that the galvanized material is adequately bonded to the steel substrate and that any zinc oxide is removed prior to coating will make adhesion problems disappear. Having a good bond between the zinc and the steel substrate may be difficult when using hot dipped galvanized steel. Paintable grades of zinc steels are galvalume,electroplated zinc and galvannealed steel. Selecting one of these paintable grades from your steel supplier will make your life much more bearable by ensuring an excellent bond between the zinc and the steel substrate. Just don’t forget that even these products will have zinc oxides that must be removed prior to painting. Good luck.