Coating Life Expectancy

What are some tests to evaluate and determine coating life cycles?


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Q. Our company manufactures and paints outdoor tubing products ranging from 1-16 inches in diameter, 5-60 feet in length and 3-11 gage thickness. We paint over aluminum, mild steel and galvanized using TGIC powder or sometimes liquid. The current painting process in place is as follows: Parts are hung and run through a steel shot blaster, blown off with an air wand, painted and cured. We have an adhesion problem with some of our product in the field, but no color problems. We are looking at extending the warranty on these products by possibly installing a wash system, but we really have no baseline information that says how long our paint currently lasts. What would be the best test to determine this? I've heard salt spray would be a good test, but how do salt spray hours translate to years?

A. There are several good tests to help you evaluate and determine approximate paint life cycle. But first, as I have stressed often, your paint performance is only as good as its mechanical bond to the substrate. A poor mechanical bond will definitely have an effect on the life of a paint’s performance. If the mechanical bond is poor, moisture and corrosion can occur between the paint and the surface, resulting in an adhesion failure. With that said, you are correct in your thought to begin including a wash system to properly prepare the surface for the coatings you’ve mentioned. Pretreatment will go a long way to help improve the mechanical paint bond, which ultimately would increase the expected life of the coating. You mentioned that you’re coating with a TGIC Polyester powder coat and sometimes liquid. You did not mention what type of powder you were using, as there are two types of powder coatings—thermoplastic and thermoset—with each having different final physical properties.

The thermoplastic coating melts and flows once heat is applied, but continues to have the same chemical composition once cooled back to a solid coating. Thermosetting powder coatings also melt when exposed to heat. But, after they flow to form a continuous film, they chemically continue to crosslink with the additional heating. The final coating has a different chemical structure than an applied powder.

I am assuming that the liquid coating is some type of enamel, but again with either type of powder or liquid a better surface preparation will add to the over all life of the coating. ASTM B117 Salt Fog Spray Test is the standard used to evaluate coatings on metal substrate. As a general rule, 1,000 hours with a 5-percent salt solution is the typical minimum testing cycle with no blistering or loss of adhesion. Some testing can go 3,000 hours, but you may want to determine which standard would satisfy your proposed extended warranty. This applies to both powder and liquid coating applications.

However, ASTM B117 is only a test to establish a threshold at which a coating could fail. The longer the hours in the chamber without a failure, the better the product performance will be. Unfortunately, there is no exact formula or correlation to equate hours of salt testing to life in the field. ASTM B117 is best served as a tool to determine similar coating performances.