Powder Coating over Galvanizing

At A Plus Powder Coaters, we specialize in applying powder coating over hot-dip galvanized steel, a process we’ve perfected over more than 25 years of experience. Combining galvanizing with powder coating, known as a duplex system, provides exceptional corrosion resistance and significantly extends the lifespan of steel parts. The metallurgically-bonded galvanized layer serves as a primer, while the powder coating adds an additional protective barrier, enhancing the durability of the steel. When done correctly, this method can last up to 1.

5 to 2.3 times longer than either system alone, with some applications lasting over 100 years.

To ensure optimal results, the galvanized material should meet key preparation requirements. The steel should not be liquid quenched, must be kept dry, and should be sent to our facility as soon as possible for coating. Additionally, it’s crucial that loads are tarped or covered during transport to A Plus Powder Coating to minimize moisture exposure on freshly galvanized steel.

This duplex coating system not only offers unparalleled protection against corrosion but also provides cost savings by extending maintenance cycles by 1.5 to 2 times compared to bare steel. For projects requiring specific colors, the duplex system also allows for design flexibility while maintaining superior corrosion resistance. With our large-scale capabilities and experience, A Plus Powder Coating ensures your steel parts are protected for the long term.

With years of experience in the field, our Sales Manager, Terry Watson, has written a published article on this topic. Continue reading below or click the link to download the original article.

Duplex Powder Coating for Galvanized Substrates

By Terry Watson

From design to the field, proper processing is crucial for powder coating over galvanized substrates. Learn why each step is important to achieve a durable, attractive finish suitable for harsh environments.

When it comes to fabrication, it is important that the designer and fabricator know the intended end-use application of the product so that design, fabrication, and finishing can be planned accordingly. In this article, we discuss products that are specifically designed to be galvanized and powder coated (commonly referred to as a duplex coating system). The assumption is that the product will be used in a relatively harsh and corrosive environment since it is receiving a duplex coating system.

It is absolutely critical that the company performing the galvanizing knows that a powder coating will be applied over the galvanized surface. The party responsible for sending the parts out for galvanizing must clearly communicate that the parts must not be quenched. They must be air-cooled. Quenching is commonly used 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 to both accelerate cooling of the metal and slow the reactivity of the zinc so that the product can be shipped without excessive oxidation. However, all types of quenching must be avoided, as they can create disastrous field failures of the coated product due to adhesion loss. Quenched galvanized surfaces are also likely to outgas during the curing process of the powder coating, which causes a “crater effect.” This not only results in an unattractive appearance but also creates voids in the coating that allow moisture to penetrate directly to the galvanized substrate. Even if the appearance is maintained, the chromate used in the quenching process is designed to oxidize and fall off the substrate, leading to poor coating adhesion and premature field failure.

All galvanized products should be shipped to the powder coater as quickly as possible to minimize the amount of oxidation that can form on the zinc. Ideally, galvanized substrates should be coated within 12 to 24 hours of being galvanized. Since these processes are typically performed at different facilities, this timeline can be difficult to meet.

Once the material is received by the powder coater, their staff must ensure that the galvanized surface is free of oil, grease, and dirt.

Metal Preparation

The galvanized surface should be sweep media blasted to SSPC-SP7 standards (also known as brush blasting). Care must be taken when selecting the type of media to use. It is recommended to avoid steel shot, aluminum oxide, or other types of media that can corrode when exposed to the elements or dissimilar metal substrates. Some coaters recommend soft media like corn cob or walnut shell, but harder media can be used effectively as long as it is applied at low pressure by a trained technician who understands not to dwell in any particular area of the galvanized surface. If the galvanized surface has voids, flakes, or blisters, it indicates over-aggressive blasting. The goal is to remove the shiny appearance and surface oxidation while minimizing the removal of zinc from the workpiece.

Enhancing the Primer Coat

Adding a primer coat is recommended to enhance the overall quality of the coating system. In my experience, the benefits of a primer coat offer excellent value for the additional costs incurred. Primer coats should be sprayed within 12 hours of brush blasting the galvanized surface. The powder coater should apply the primer to the manufacturer’s recommended film thickness. In some cases, the manufacturer will recommend a specific level of curing for the primer, ranging from partial to full cure. It is important to understand the manufacturer’s recommended degree of cure. Most recommend “green curing” or partial curing to allow proper adhesion of the topcoat. The primer coat should cool before applying the topcoat, which should be done as soon as possible, ideally within an hour or two, to avoid dust or other contaminants accumulating on the surface.

Top Coating

As soon as possible after the primer coat, the parts should be coated with the desired UV-stable topcoat. We frequently use super-durable polyester chemistries due to their additional UV stability, which comes at a slightly higher cost than standard polyesters. The topcoat powder must be fully cured if it is intended as the final coat.

We sometimes apply a super-durable anti-graffiti clear coat over the color coat for added UV stability, protection of metallic finishes, or graffiti resistance. Some customers are willing to invest in this additional protection to ensure the long-term durability and aesthetic value of their product.

Final Cure

After the topcoat is applied, the product is sent to the oven for a full cure of both coats. The cure time depends on the metal mass and the recommended cycle time and temperature as specified by the powder manufacturer. The oven time must account for the time required to raise the metal to the desired cure temperature and the recommended dwell time at peak temperature.

Most powders offer flexibility in time and temperature, typically ranging from 10-15 minutes at 350°F to 400°F. In most cases, the longer time is preferred to allow the powder to achieve full film properties. Lower temperatures can reduce energy consumption, and in cases of galvanized steel, where trapped porosity might be released during curing, it is often beneficial to use the lower end of the temperature range.

Conclusion

Duplex powder coatings have the potential to add beauty and lifespan to products intended for harsh outdoor environments. When processes are properly planned and executed by all parties involved, the process can be relatively simple and trouble-free. The key is ensuring that everyone is aware of the intended use of the finished product and the steps required upstream and downstream of each process.

Testing every step of the process is critical, especially when coating galvanized products. It’s helpful to request small samples of the same thickness and type of substrate for destructive testing before production. Powder coating galvanized surfaces is common but can be more challenging than coating steel surfaces, so thorough testing is essential.