Powder Over Galvanized Steel

Q. We are bidding a project for a department of transportation that specifies application of powder on hot dip galvanized steel. The specification also require that the galvanize company and powder coat company provide a minimum 2-year guarantee against failure. Our powder coating company will not provide such a guarantee. Is this “duplex” system a common practice? In your opinion, why would a powder coat company not guarantee this system? Do you know of powder coating company that will provide a guarantee for the “duplex” system?—C.W.

A. The “duplex” system is fairly common. Powder can provide ultra-violet protection and additional performance in addition to adding a color to a galvanized product. Most powder coating companies will not provide guarantees, however. There are so many variables that it is usually not wise. Be aware that there is a difference between a warranty and a guarantee. A warranty can be limited to the aspects of the process that the coater has control over and list the specific remedies if the coating does fail. A guarantee makes the coater liable for any type of failure and is not practical given the many possible types of failure and the things that can cause a failure.

The powder coater cannot control the galvanizing process. The coater does not know what type of abuse the product will be subjected to. However, a limited warranty against failure based on the things the coater can control is common. The powder coater could warranty it against a certain level of fade or chipping and other performance criteria. There are certain powder coating companies that have better knowledge of this process and will do a more reliable job of it and some that are not as good. Check for experience with galvanized as a substrate, and see what the powder coater can offer for a warranty. Make sure the coater has had previous experience with powder over galvanized steel.

If you do go ahead with the project, you should understand how it has to be done so you can properly evaluate a coater. The description below is paraphrased from a paper I co-wrote with Terry Watson of A-Plus Coating. It has a great description of the process:

It is critical that the company performing the galvanizing know that it will have a powder coating applied over top of the galvanized surface. The party responsible for sending the parts out for galvanizing will need to clearly communicate that the parts cannot be quenched. They must be air-cooled. Quenching is a commonly used process in the galvanizing industry where either water or, more commonly, a chromate solution is sprayed on the parts after they exit the molten zinc. This is done both to accelerate cooling of the metal and slow the reactivity of the zinc so that the product can be shipped from the galvanizer without the presence of excessive oxidation. Quenching can create disastrous field failure of coated product due to adhesion loss. Quenched galvanized surfaces are also likely to outgas during the cure process of the powder coating. The relief of trapped air during the heating of the coating can cause crates in the molten film and leave an ugly appearance. The craters can also create voids in the coated surface and allow moisture to penetrate directly to the galvanized substrate. 

All galvanized products should be shipped to the powder coater as quickly as possible so that the amount of oxidation that is allowed to form on the zinc can be minimized. Ideally, the galvanized substrate should be coated within 12–24 hours of being galvanized. Since these processes are typically performed at different facilities, this is highly unlikely to occur. Once material is received by the powder coater, the coater will need to make sure that the galvanized surface is free of oil, grease and dirt.  

The galvanized surface should be sweep-media-blasted to SSPC-SP7 standard (also known as brush blasting.) Care must be taken when choosing what type of media to use. Steel shot or other spherical media is not recommended, because it can peen contamination into the steel and generate corrosion after the part is coated and cured. Some coaters recommend soft media such as corn cob and walnut shell, but harder media can be used effectively as long as it is used at low pressure by a trained technician who understands that he should not dwell in any particular area of the galvanized surface. If the galvanized surface has voids or the appearance of flaking of the zinc from the metal surface, chipping from the edges, or blisters, then the blast is too aggressive. The goal is to take away the shiny appearance and surface oxidation while removing as little zinc as possible from the workpiece. 

A primer coat is recommended to enhance the overall quality of the coating system. The primer coat adds significantly to the performance of the coating. Primer coats should be sprayed within 12 hours of brush blasting the galvanized surface. The powder coater should apply the primer to recommended film thickness and make certain that he understands the recommended level of cure for the primer coat. Most powder manufacturers recommend “green curing” or partial curing of the primer so that proper intercoat adhesion can be accomplished once the topcoat is applied. The primer coat should be allowed to cool, and then a top coat should be applied as quickly as possible. 

The parts should be coated with the desired topcoat as soon as possible after the primer coat is complete. The topcoat is often a “super-durable” polyester chemistry, a material that is specifically engineered to add significantly to the ultraviolet resistance and durability of the finish.

As you can see, there is a lot to think about with this type of coating process. A knowledgeable powder coater will be able to give you a good finish and should be able to provide a reasonable warranty against failure.