“Not this topic again,” you might say. By “this,” you probably inferred that you are about to read a few hundred words describing the difficulties of meaningfully correlating accelerated weathering to real-time performance. Rest easy; that old topic is kids’ stuff compared to understanding the meaningfulness of corrosion testing.
The performance of coil coatings over the lifetime of a product is of paramount importance. Of the markets served by prepainted metal, the building products market poses the greatest challenges. During a recent NCCA meeting, there were plenty of conversations about accelerated corrosion testing, and this got me thinking about the similarities—and distinct differences—when comparing accelerated corrosion testing and accelerated weathering. Of course, corrosion is a form of weathering, but the term “weathering” commonly refers to what happens to a product’s appearance properties (chalk, fade, gloss retention) when exposed to sunlight, heat and moisture. On the other hand, corrosion refers to the degradation of the metal substrate.
Great progress has been made over the last 20 years to understand how to model an accelerated weathering test to better simulate the environment in which a product will be placed. We now have a better understanding of the need to duplicate the solar power distribution, the unrealistic effects of <295 nm UV wavelengths, and, most recently, the importance and necessity of coating moisture imbibition in the physio-chemical degradation of coatings. This level of understanding is mostly absent when it comes to accelerated corrosion testing.
What makes corrosion testing so difficult? Let’s start with the chemistry of corrosion versus accelerated weathering. Don’t worry; I do not intend to get into the chemistry and physics. We’ll leave that to the researchers, but it is important to know that these researchers are always striving to duplicate in an accelerated test cabinet the same chemistry that is taking place in the real world. When done effectively, new products can be introduced with an assurance that they will perform suitably in the field.
As demanding as it is to understand the degradation reactions of an organic coating during typical weathering, understanding corrosion reactions is way more convoluted! Continue reading →