Paint Adhesion on Zinc-Coated Steel

Q. We are a large home appliances manufacturer with a facility in Thailand. In this small satellite, we paint washing machine housings from an external supplier. The material used is zinc coated, and powder painting with a polyester +HAA based paint shows really bad bonding. The supplier confirms the correct and complete pretreatment of the part, including cleaning and phosphating, and assures a complete curing of the powder according to the powder manufacturer suggestion. Do you have any suggestions as to where to start? A.R.

A. It seems like an obvious statement, but adhesion issues always take place at the interface of the substrate and coating, so that would be the first place to start. I would generally start such an investigation with a thorough examination of the incoming material. In my previous experience, I have found zinc coated steels to vary widely along with their subsequent paint performance.

To start, you need to understand the type of zinc coated steel you are receiving. Is it from a hot dip galvanizing operation or an electrodeposited zinc coating? Generally, the electrodeposited zinc coating is thinner, much more uniform in surface, does not have the spangle associated with the hot dip coating and generally performs better. The spangle is the dendritic or snowflake appearance on the surface associated with solidification of the zinc following application in a molten bath. If the coating is hot dipped, you can often specify the amount of spangle. Generally for painting, the less spangle on the surface, the better your paint performance.

Next, you need to find out if the galvanized surface is coated with anything. Often, to prevent “white rust” (galvanized corrosion) in storage, the galvanized will be coated. Sometimes it may just be a light oil, but other times it may be a heavy, long-term rust preventative. Other times there will be conversion coating already applied. In this case, you would not want to try to apply another conversion coating over the surface for risk of partially removing the original phosphate coating and not adequately applying a new coating. Additionally, it is important to ensure the chemistry of the phosphate bath is formulated specifically to provide a coating on zinc coated steel. Once you have understood these parameters, you can move on to investigating the actual process parameters at the external facility doing the painting.