Loss of Adhesion and Corrosion Failure on CRS Frames

Q. On some of the CRS frames (used to build industrial equipment) that I have used wherein we specify powder coating, I have noticed the paint peel off with heavy signs of rust underneath. The painter uses zinc phosphating pre-treatment prior to coating. Can you please help explain the reason for this and what it is you think the painter is doing wrong? K. K.

A.Wait a minute while I shake my “eight-ball” to get your answer.

For those of you who are too young to remember what this was I offer the following explanation. An “eight ball” was a water-filled toy from the 60s that would give you one of several innocuous answers to any question you may have. As an example you would ask: “Am I going to be handsome when I grow up?” You would then shake the “eight-ball” and read the answer in the tiny window. It would read something like the following; “You may not always get what you want.” You would then either accept the answer or shake it again for another try. There were many times in my youth that life-altering decisions were easily made using this prognostication device. Well, at least we thought they were life-altering decisions back then anyways.

Why, you may ask, do I joke about getting the answer to this question using a trivial device like an “eight-ball”? Well, it is because there are numerous answers that can be given to explain loss of adhesion and corrosion failure on CRS, no matter how it was pretreated. Each answer can be correct and can be equally incorrect. Choosing the answer that is most appropriate depends upon the conditions that the CRS frame was exposed to before the coating failed.

For instance, if this part was recently powder coated and kept in relatively dry conditions, then the answer would be related to the processing of the part at the painter’s facility. They may not have totally cleaned the part before coating or cured it properly after coating. They may have tried to coat over a rusted part or it could have flash-rusted during processing. The chemistries in the pretreatment system may have been totally “out-of-wack” or the rinse water could have contained a high amount of salts. The powder that was applied could have been a formulation that provides terrible corrosion resistance. The simple fact is I can’t begin to guess what happened without seeing the part or the process.

Now if the coating was performed properly and the part still failed, then you have to ask: “what conditions was the part exposed to?” A CRS part that was correctly cleaned, had a zinc phosphate pretreatment and was properly powder coated with a good powder formula will provide approximately 750-1,000 hours of continuous salt spray protection. This would relate to approximately two to five years of outdoor exposure without a coating failure. However, if it were exposed to a highly corrosive environment, then the coating life would be much less.

So, as I said—let’s shake the “eight-ball” to see the cause of failure. Shake, Shake, Shake…”Improper pretreatment”. Let’s try again. Shake, Shake, Shake…”Improper powder cure”. Hmmn? I want one more try. Shake, Shake, Shake…”Fielded conditions exceeded powder capability.” Now I have no idea which is right. Maybe I should try again. No, maybe I’ll just leave it to your imagination.