Ask Joe Powder – October 2024

PPCJ presents its latest column from our resident powder coatings expert, Kevin Biller AKA Ask Joe Powder. Any readers with powder coatings queries can submit questions to askjoepowder@chemquest.com

• Question 1:

Good day, Joe,

We regularly read your column. We are a manufacturer of industrial hardware. We make hardware that is used in industrial environments and usually outdoors. Currently, we plate our steel parts (stampings and castings) with yellow zinc dichromate and then send them for powder coating.

We have had some adhesion and rust problems and wonder if we are doing the right thing here. The plating process seems to clean the products better than the powder coater can and provides a secondary barrier to seal the part from the elements. Do you think we are doing the right thing to best protect our steel hardware items?

Thank you,

Everett Fuddpatz

Sheboygan, Wisconsin

• Answer 1:

Hello Everett,

Powder coatings generally have sporadic problems adhering to zinc dichromate. I’m not sure if it is related to the varying thickness of the zinc dichromate, but it is a recurring problem. The zinc plating process cleans the metal really well but may not be the best surface for the adhesion of a powder coating. In addition, part handling after plating and before powder coating may introduce soils and oils onto the surface.

The ideal way to achieve excellent powder coating adhesion to steel is to skip the zinc chromate and find a powder coater with a high-quality pretreatment system that is used just prior to powder coating. A multi-stage iron phosphate system is good, while a multi-stage zinc phosphate system is so good that the automotive and appliance industries use it.

Another piece of the puzzle is keeping the parts from corroding between the time they are fabricated and when they reach the pretreatment system. This is a matter of logistics, environment, and possibly temporary protection.

This probably makes you think about investing in the installation of your own pretreatment and powder coating system. I would give it some serious thought in your long-term strategic planning.

I hope this helps.

Best regards,

Joe

• Question 2:

Dear Joe,

I’m having a big problem with my powder coating chipping off my customer’s parts. These are automotive seat assembly brackets.

First, we media blast the parts with aluminium oxide to remove any rust, then wipe them clean with acetone. Next, we apply 2.0 to 3.0 mils (50 to 75 microns) of a zinc-based epoxy powder primer, curing per our supplier’s recommended bake conditions (20 minutes at 175°C), then topcoat with a standard-bake polyester at about 3.0 mils. The polyester is cured per the 20 minutes at 190°C bake recommended on the technical data sheet provided by the powder manufacturer.

The loss of adhesion occurs between the epoxy primer and the polyester topcoat. Could we be applying the powder too thick? Is the powder under-cured? Should we bake the powder longer or at a higher temperature? Your advice will be greatly appreciated.

Jamie Wells

Columbus, Ohio

• Answer 2:

Hi Jamie,

Thank you for the question. By applying a two-coat system, you are giving your customer a “Cadillac” coating that should last beyond the life of the vehicle. When combining two or more coatings on a substrate, certain procedures can be used to ensure excellent adhesion and performance.

Let’s go through your process. Your metal preparation appears to be sound. Media blasting can provide an excellent surface for an epoxy primer. It is important to keep your media clean and to thoroughly blast the entire surface. A subsequent acetone wipe is also a good idea, as long as you use a clean, lint-free cloth.

You mentioned that you are using a zinc-rich epoxy primer. This technology can provide outstanding corrosion protection for ferrous substrates. It is important to note that epoxies are incredibly durable and quite hard when completely cured. Because of this, it is wise to under-bake an epoxy-based powder if it is being used as a primer. Under-curing the epoxy primer provides a softer surface for the polyester topcoat to “bite” into.

Furthermore, the under-curing leaves a few unreacted functional groups that can be chemically bonded by the polyester. These unreacted groups are compatible with the carboxyl functional groups in the polyester and provide a much more intimate bond between the primer and topcoat.

I recommend under-curing the primer by about 15°C lower than the recommended bake temperature or at 50% of the recommended time at temperature. I would then apply the polyester topcoat and ensure that it receives the full recommended bake. It is also important to ensure that the topcoat cures at a higher temperature than the primer. Otherwise, you risk under-curing the entire coating system.

Good luck with your project.

Best regards,

Joe Powder