"Small Volcanoes"

Q. I work for a company called Fuel Systems. We are a Tier one manufacturer of fuel tanks for the heavy truck industry. We are being forced to move to an aluminized steel product to help in the reduction of rust on the inside of the tanks. We have a powder coat line that we put all our steel tanks through, and we’re finding that many, not all, of the aluminized tanks are showing blistering after final curing.

A metallurgist looked at the surface and found that the blisters looked like small volcanoes. There were fisheyes as well. Most of the blistering occurs on the radius, and there is absolutely no blistering on the side heads which are blanked and formed. Our paint supplier, Valspar, has some specimens as well, but they have not turned up any answers.

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I am not a chemist and I do not know the interactions of the aluminized steel with the bath or the powder coat. We have other aluminized tanks that show no blistering. We can put a steel tank in front and in the back of an aluminized tank and the only blistering will be found on the aluminized tank. We have tried preheating the tank to 400° and then painting, but there is no positive outcome. I am purchasing some MEK tomorrow and I am going to clean one tank and put the other through the wash to see if the problem may be impurities on the surface. I realize that the aluminum and steel create a unique environment for the paint process. If you can be of any help I would greatly appreciate your time. M.D.

A. Your description of the failure looking like “volcanoes” leads me to believe that these are pinholes and not blisters. Pinholes are breaches in the coating caused when a gas pierces the coating during the hardening process. Pinholes are caused by one, or more, of the following.

Substrate failure (outgassing) occurs when the substrate is subjected to high temperatures in the powder coating cure oven. This can be corrected by preheating the product to release the gas before coating.

Surface contamination (oils and the like) can volatize in the cure oven. This can be corrected by removing these contaminants in the pretreatment system prior to powder coating.

Coating cure by-products can build up in thicker films. Reducing coating thickness and adjusting the cure oven temperature should correct this problem. Otherwise, evaluate other powder coating formulas to replace the one you are using.

You will have to run several well-thought-out design of experiments (DOE) processes to determine which of these may be the root cause of your problems. As a consolation, you should be aware that aluminized steel has been successfully powder coated for many years. Snowmobile and Jet Ski mufflers and exhaust systems are just a couple of examples of powder coated aluminized steel products. Therefore, I expect that you will solve your difficulties, as well.