Accelerated Lab Testing vs. Field Performance

Question:

The expert's response to J.R.'s question about powder coating over hot dip galvanized steel is misleading at best. The expert implies that "mild steel with electrocoat primer and powder topcoat or two-coat powder systems (zinc-rich primer and TGIC powder topcoat) can provide the same or higher corrosion resistance to hot dipped galvanized steel and powder coat" because they perform better in accelerated lab tests. Lab tests are known to be poor indicators of field performance, especially when a number of field conditions exist. Galvanized steel in most cases will provide a corrosion-free, maintenance-free system for 30-70 yr, depending upon the environment of the application. Any comparisons based upon lab accelerated test results are inaccurate and misleading. The real comparison must be made in field performance results. The book, Duplex Systems by J.F. van Eijnsbergen, documents field performance of powder coating over hot dipped galvanizing in many applications. Field performance is far superior to any other duplex system. P.R. (American Galvanizers Association)

Answer:

"Holy mackerel, Batman, it looks like Nick poked another hornet's nest." What's funny about this is that both the Powder Coating Institute and The Electrocoat Association praise my original answer, calling it "balanced and correct". What we have here is a dueling match between different trade organizations, with me in the middle. I am sorry that the American Galvanizers Association's agenda does not coincide with my attempt to provide unbiased and objective answers to readers' questions. But, I just call them as I see them.

First let's start with the following comment: "Lab tests are known to be poor indicators of field performance." This is true, but not as the statement implies. Accelerated performance laboratory testing can create failures in hours that can take years in the field. Often trying to correlate the laboratory results (rated in hours) to field life (rated in years) is, at best, a guess. However, there is plenty of anecdotal evidence that can be helpful in correlating the results. For instance, 5,000 hr accelerated salt spray testing is used in the outdoor furniture industry to provide a 20-yr guarantee on the product's finish. The automotive industry has similar anecdotal evidence. The organic coatings industry has accepted accelerated corrosion and weatherometer testing as a good laboratory facsimile of actual outdoor exposure product life. If anything can be said about accelerated laboratory testing, it is more arduous on the coating than actual fielded conditions. How else can you produce coating failures in hours vs. years? That's why we call it accelerated testing.

The real issue is determining the type of failure that is exhibited on the coated surface. In the organic coatings industry, corrosion failure is rated in corrosion adjacent to a scribed mark or blisters in the organic coating. Either one of these conditions is considered a failure of the organic coating. I suspect that the Galvanizers rate failure only when the galvanizing is breached. This condition can take much more time to occur on the product's surface. I would expect that after the rated life of "30-70 yr" that much of the powder coating has failed when judged by the organic industry standards, but the galvanizing is still intact. So this means we are comparing apples to watermelons. After all, most people who paint or powder coat over galvanizing are doing so to cover the galvanized surface for aesthetic reasons. Therefore, when this organic coating blisters, cracks or otherwise fails, the intent of the organic coating is no longer met, and the customer considers it a paint failure. In this case who cares if the galvanizing is still providing corrosion protection on the base steel, because it looks like crap.