I was recently inspecting a 250′ diameter, floating roof waste water treatment tank. The interior coating on the walls of the tank appeared to be, in many places, intact. In other places, it exhibited blistering, with liquid underneath. But to my surprise, when I chipped away areas of the coating which appeared to be in good condition, I found poor adhesion, but more surprising, there was liquid underneath the coating. And, this liquid appeared to be under some pressure.
That is, as I chipped away at the coating (the first photo, which what appears to look like a musical note) at first the newly exposed steel was dry. But then, moments later, water actually started to percolate up the lower portion of the exposed steel and start to drip down the wall. If you look closely, you can see drip at the bottom of the photo.
There were no outward or visual indication that the coating was flawed. And, in fact, when I removed the coating, the steel underneath, in some cases, appeared to be pristine – where the abrasive blast looked like it had just been completed. But then, as you can see in the remaining two photos, there was considerable corrosion in some areas, but not others.
I was pretty comfortable concluding that the bottom two photos were exhibiting some type of reaction from soluble salts that had not been removed from the steel surface during the original coating application. But when I thought about the first photo, where the steel was pristine and there were no apparent blisters, the only thing we could determine was that it was the cold wall effect.
The situation was further exacerbated and complicated when we found out that the existing coating was not compatible with a certain chemical found in the waste water. So, now, was it possible that the coating softened during immersion allowing liquid behind it?
Anyway, I think the most fascinating issue I ran into was the divide over whether or not the cold wall effect was even real. I spoke with one seasoned coating applicator who had taken several NACE and SSPC courses and who had applied hundreds of different coatings who had never heard of it. When we conducting one of our optimal coating identifications, we mentioned the cold wall effect as a concern in our documentation. A number of manufacturers responded by saying it wasn’t a concern. And one said, and I’m quoting, “The cold wall effect is not a real thing.”
As coating consultants, should this be on our radar or not? Is the cold wall effect a real issue? I’m certain the answer is yes. But, of course, as in almost all issues in our industry, nothing is simply – and the answer may be that it depends. It may depend on the temperature gradient between the tank interior and the “cold” on the tank exterior, the thickness of the coating and its permeability rate.
As coating consultants, I believe it needs to be on our radar and be discussed during the coating identification process with all of the involved parties, including the coating manufacturer. They should have data relative to the cold wall effect or certainly data on the permeability rate of the coating which can provide some guidance in how to properly specify a long-lasting, durable coating solution which will resist the cold wall effect, if, of course, it really exists.