28 Jul The Cold War over the Cold Wall Effect
Reality is not always as firm a thing as we would wish, alas, even in the realm of coatings science.
Take the Cold Wall Effect: to some experts, a key cause of coating failure; to others, a myth.
I was recently reminded of this controversy while inspecting a 250-foot-diameter, floating roof wastewater treatment tank. The interior coating on the walls of the tank appeared to be intact many places. In others, the coating was blistered, with liquid underneath.
Why the Weeping?
To my surprise, though, even when I chipped away areas of coating that appeared to be in good condition, I found poor adhesion—and, more surprising, liquid underneath the coating. And that liquid appeared to be under some pressure.
As I chipped away (see the photo below, with the scrape in the shape of a musical note), the newly exposed steel was initially dry.
But then, moments later, water actually started to percolate up the lower portion of the exposed steel and drip down the wall. (If you look closely, you can see drip at the bottom of the photo.)
There was no outward or visual indication that the coating was flawed. In fact, when I removed the coating, the steel underneath appeared pristine in some areas—as if the abrasive blast had just been completed.
Meanwhile, as you can see in the remaining two photos below, there was considerable corrosion in some areas, but not in others.
I was pretty comfortable concluding that the bottom two photos were revealing 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, another possibility arose: 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 wastewater.
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.
The Cold Wall Effect is sort of like a cold can of beer on a hot, humid day, except inside out. That is, the cold can draws moisture from the atmosphere in the form of condensation.
To take the analogy a bit further, if you covered that cold can with a tissue or piece of paper, the water vapor would still find its way to the cold metallic surface, after the molecules drove through the tissue or paper.
So, the theory goes, if you have a water tank (or a tank that contains water) in, say, Chicago, where the outside winter temperatures can make a polar bear cry, and the inside temperature is above 90°F, then the water vapor molecules in the tank will be drawn to that cold wall.
Depending on the permeation rate of the coating (or on whom you talk to), those molecules will then physically drive through the coating, hit the steel wall, turn into liquid condensation, and begin the process of forming a blister.
Or so say inspectors and applicators who have seen the problem over and over again.
Still, the concept has its detractors.
I spoke with one seasoned coating applicator who had never heard of the Cold Wall Effect, despite having completed several NACE and SSPC courses and applied hundreds of different coatings.
When our firm was 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.”
A Definite Maybe
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 with almost all issues in our industry, this one isn’t simple—and the answer may be that old fallback, “It depends.”
In this case, it may depend on the temperature gradient between the tank interior and the “cold” on the tank exterior, or on the thickness of the coating and its permeability rate.
In any case, as a coating consultant, I believe it needs to be not only on our radar but actively discussed during the coating identification process with all involved parties, including the coating manufacturer.
The coating maker should have data relative to the Cold Wall Effect (or, certainly, data on the permeability rate of the coating) that can provide some guidance in how to properly specify a long-lasting, durable coating solution that will resist the effect.
If, of course, it really exists.