Adhesion on Galvannealed Material
What causes paint adhesion problems on galvanneal material?
Q. We are having paint adhesion problem on galvanneal material. We have a degreasing facility with a seven-tank process. —A.K.
A. Zinc coated steels can be notoriously difficult to achieve good paint adhesion. Some of the problems are related to the mill coatings and oil that are applied to the galvanneal to ensure they do not oxidize. These can be very tenacious, particularly if shipping from overseas and needing the associated extended corrosion resistance.
Additionally, the zinc coating of a simple galvanized can be one of the most difficult of the zinc coated steels to get good paint adhesion. The galvannealed variety essentially takes the hot dip galvanized steel and bakes it, which will form a zinc-iron alloy that significantly improves the paint adhesion while maintaining the material corrosion resistance.
It is unclear what is involved in your seven-tank cleaning system, but time, temperature, cleaner concentration and agitation are the most important factors to control to maximize the potential of your cleaning system. As mentioned, the lubricants applied at some of the mills can be particularly difficult to remove. I would recommend increasing the time, temperature and cleaner concentration in an effort to better remove the contaminants.
Specific questions about zinc phosphate and pretreatment are answered in one article...
What you should know about cleaning and electrocleaning.
Better adhesion, enhanced corrosion and blister resistance, and reduced coating-part interactions make pretreatment a must.