Posted in: Formulators Forum

Formulators Forum—Powder Coating for Corrosion Protection: Basics of Corrosion

Posted on Thursday, January 4, 2024

An important property of a well formulated powder coating is its ability to extend the life of the metal it is applied to. Metal begins its life in an oxidized state, buried in the ground. The process of converting the oxidized metal into pure metal requires the input of energy. Once separated from the ore, the metal must be protected, or passivated, from the outside environment. If not properly protected, the energy added to the metal during the manufacturing process is slowly released back into the environment. This release of energy drives the process of corrosion and is visible as rust on a defective part. A carefully formulated and properly applied powder coating can passivate the metal underneath it and protect it from oxidation for years, even in harsh, corrosive environments. Not all powder applications require this level of corrosion protection, and understanding the operating environment and customer expectations will help a coater determine the level of corrosion protection required for the job.

How Powder Protects
To prevent oxidation of the metal underneath it, a powder coating must act as a barrier to air and moisture. Powder coatings are supplied in a wide range of chemistry, gloss, and color. The unique combination of binder, pigments, and additives determines not only the appearance of the powder coating but also its barrier properties. The longer the powder coating maintains these barrier properties, the longer the coated part will last.

There are many variables that determine how long a coating will maintain its properties, including the amount of UV exposure, thermal stress, and physical damage the coating may experience over its lifespan. In certain applications, a qualified testing lab may perform weathering testing to simulate environmental stresses in an accelerated timeline. This data can be used to screen powder coat systems before making a final selection.

One Coat DTM
The most basic level of corrosion protection from a powder coating comes from a single coat of powder applied directly to metal (DTM.) If appearance and UV protection are of low concern, a thick layer of an epoxy-based powder coat will provide excellent corrosion protection with a long service life.

While a single coat of epoxy may suit some applications, most powder coatings are applied to both protect the metal and improve its appearance. When a powder coating must perform two functions, a polyester or polyester/epoxy hybrid may fulfill the requirements of the job. While providing less corrosion protection than a thick layer of epoxy powder, a polyester or polyester/epoxy hybrid can provide good corrosion protection and can be formulated in a full range of colors and glosses to meet the requirements of the job.

Two Coat: Epoxy Primer + Topcoat
To get the best of both worlds, some applicators may choose to use a two-coat system. The first layer of powder applied may be an epoxy primer, designed for excellent corrosion protection but with little focus on appearance or weathering. A second coat of powder can then be applied which may have poorer corrosion protection on its own but be formulated to any color and gloss the application requires. By applying two layers of powder, with each layer designed for its specific purpose, a part can be coated in any color and gloss while maintaining long levels of service in harsh environments.

Other Factors to Consider
Selecting the right powder for a job is a critical first step in the painting process, but it is only the first step. To get the full protective properties of a powder coating, it must be applied and cured according to the manufacturer supplied instructions located on the product Technical Data Sheet (TDS). During application, whether by automatic gun or hand spray, the applicator must ensure consistent and complete coverage of the coated part. Special attention must be given to all corners and recessed areas which can be difficult to fully coat in a single pass. Any areas of the part with bare metal or low film build are susceptible to corrosion. Once all surfaces of the part have been coated, the part must be heated and the paint allowed to cure at the proper temperature for the TDS specified length of time. Once the part has been fully cured and has come back down in temperature, the cured film should be inspected for defects, low film, or exposed substrate. A solvent rub test can be performed to confirm the powder has been cured.

Through careful selection and application, a powder coating can turn a carefully machined metal part into a beautiful product with the ability to function in harsh environments for many years.

Sam Koerner is chemist II - Performance Coatings Group - Powder Division for The Sherwin-Williams Company.