“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! Think about the cut edge of any prepainted product, and let’s keep the discussion to 0.015″ G90 hot-dipped galvanized steel (HDG) just to keep things simple (as if!). If you could look head-on at the drip edge, you’d see about 13.5 mils of cold-rolled steel (CRS) coated on both sides with less than one mil of zinc (the galvanized layer), another mil of primer and topcoat on one side and about a half-mil of primer and backer on the other. The cut edge, therefore, is basically unprotected steel. This drawing may help to illustrate the various layers.
And yet with a 13.5 mil layer of unprotected CRS we do not have, in reality, much of an edge corrosion issue in our industry. How can this be?
Corrosion is a complex set of electrochemical reactions, and it is simply not all that easy to study these reactions. By comparison, studying the degradation reactions during weathering of organic coatings is not trivial, but over the last 30 years excellent techniques have been developed and are within the reach of any interested laboratory. And consider this: When studying coating degradation, there are plenty of surfaces to study. When studying the early stages of corrosion at the cut edge, you need to study a surface (see above) that is only 13.5 mils thick. That may sound thick compared to the zinc and paint thicknesses involved, but that’s a mighty thin surface for chemical analysis of the corrosion mechanism.
Some corrosion chemistry is described as anodic corrosion, and other reactions fall under the term cathodic corrosion. As if it is not complex enough, some of the electrochemical reactions produce extremely alkaline microenvironments. Organic coatings are particularly susceptible to alkaline conditions, so, in addition to all that inorganic chemistry taking place within the metal, now you have the interaction of the corrosion byproducts (those alkaline moieties) attacking the primer and actually breaking chemical bonds. These reactions take place at the interface of the zinc layer (or zinc-pretreatment layer…it’s hard to tell exactly where all the action is!) and the primer. Once the corrosion reaction starts at the cut edge, it begins to move away from this edge and creep up the panel. This is the “creepage” that we often describe in corrosion test methods. Is it any wonder that we satisfy ourselves not with what is happening chemically, but rather with the simple measurement of creepage at the scribe and cut-edge?
What does it all mean? How do we consider entering a chromate-free world when our understanding of corrosion and test methods seem so inadequate? That’s a topic for another blog. Stay tuned for “Accelerated Weathering: Part Two.” Coming soon…
-David A. Cocuzzi, NCCA Technical Director