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Thanks to everyone for reading, thinking about and commenting on my last blog on the Cold Wall Effect. For better or for worse, the column seems to have raised more questions than it answered.
One hallmark of my consulting firm is to look for root causes. In the debate over the Cold Wall Effect, that search raises a more fundamental question: How does permeability play a role?
Let’s start with the basics.
Everything is Permeable
I had heard this before and have always been skeptical. However, after much research and discussion with my physicist buddy Kal at NIST (National Institute of Standards and Technology in Washington, D.C.) I’m told it’s true.
So, since everything is permeable, the question becomes: How permeable is it? (I’m sure there’s a punch line in there somewhere.)
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Photos: Warren Brand |
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These are blisters on one of the New York City's most prestigious swimming pools. The cause: a water soluble solvent in the coating, compounded by chlorine in the water.
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First, let’s understand what we’re talking about.
Permeability 101
Permeability is a way of comparing coating materials. It has two components: diffusivity and solubility.
Permeability = Diffusivity x Solubility
Diffusivity is how fast the water moves inside the material. For a polymer, the more crosslinks, the harder it is for water to penetrate the material, and the lower the diffusivity (think of trying to pass through a large crowd of people at a baseball game).
Solubility is the amount of liquid that a material can absorb at a given thickness. For a polymer coating, the question is: How much does the polymer repel water (like a waterproof coating on your jacket) vs. how much does it absorb (like a towel)?
While each material has its own permeability rate, quantifying that rate becomes complex very quickly.
The temperature matters (things become much more permeable at a higher temp), the quality of application matters (if the coating is not applied properly, all bets are off), the coating composition matters, and so on.
In addition, permeability has to be measured separately for each material moving through the coating. Here, we’re focusing on water.
The ABCs of MVTR
Now, a steel substrate doesn’t much care about permeability. Steel cares only about the rate at which moisture comes into contact with it.
That brings us to the Moisture Vapor Transmission Rate (MVTR), a measure of water vapor passage through a substance.
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Steel cares very much about the rate at which moisture comes into contact with it. Permeability? Not so much.
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Here’s where thickness really matters.
MVTR is inversely proportional to the thickness. Double the thickness, and you cut the MVTR in half. Now this is something the steel will notice!
MVTR is also proportional to permeability (kind of obvious, because we all know that the less permeable the coating, the better). So, the more permeable a material, the greater the MVTR.*
Through the Screen Door
Now, some coatings (like those for concrete floors) and polymers are designed to allow moisture vapor transmission.
Or, let’s take concrete itself. Concrete is porous, as we all know. And no matter how thick it is, water or moisture vapor will (for the most part) travel through it.
Picture a screen door. If you dump a five-gallon bucket of water through a screen door, the water will gush right on through. Even if you stack 10 or 100 screen doors one on top of one another, and pour the bucket of water on top, it will still pass all the way through, snaking its way down and through the spaces almost undeterred.
What Polymers Do
But polymers are different. They are designed to be hydrophobic (water repellent). Their entire purpose is to remove the “electrolyte” portion of the corrosion matrix (anode, cathode, electrolyte and metallic pathway).
But if everything is permeable, how does that apply to these barrier-type coatings?
I think the best analogy is to visualize a sponge. (Yes, I know it’s not a perfect analogy, since barrier coatings are designed to be hydrophobic, and sponges are hydrophilic. But bear with me.)
If a barrier coating is like a sponge, there are two ways to make it less permeable to moisture: Make it thicker (see above) or make it more dense (increase the density of the crosslinking).
Making it more dense would relate to the type of polymer, the density of the crosslinking, the various additives, and a great many other issues too complex for this discussion.
Making it thicker, though—that’s simple.
Thick as a Brick
In my days as a contractor, we applied paint and coatings to thousands of substrates, but our sweet spot was tank lining.
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Which is more likely to make your substrate a loser: thin film or bad surface prep?
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And one thing we found was that most failures we came across were from thin-mil coating systems—say, less than 12 mils.
We wanted to differentiate ourselves, so we decided to offer primarily thick-build coatings (40-125 mils) with a whopping 10-year non-prorated guarantee.
We went on to line tanks at The Chicago Board of Trade, Merchandise Mart, Palmer House, IBM, Caterpillar and dozens of other world-class facilities.
And, decades later, not one has a single blister or coating failure.
Thin Stuff
On the other hand, I just checked the coating thickness on my 1994 Jeep Grand Cherokee: 4.0 mils. And despite never having been in a garage, the car does not have a single blister.
Now, is that a function of low permeability?
Is it because there’s no Cold Wall Effect?
Or is the coating permeable, but the surface prep was so good that there are no chlorides or other contaminants on the substrate for the moisture vapor to interact with during permeation?
I don’t have an answer. My guess is that opinions vary.
What’s yours?
*Props to Kal. Source: Progress in Polymer Science, Volume 26, 2001, pp. 985-1017.
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| ABOUT THE THE BLOGGER |
Warren Brand |
Warren Brand’s coatings career has ranged from entry-level field painting to the presidency of two successful companies. Over nearly three decades, he has project-managed thousands of coating installations and developed specs for thousands of paint and coating applications. NACE Level 3 and SSPC PCS certified, Brand, an MBA and martial-arts instructor, now heads Chicago Corrosion Group, a leading coatings consultancy. Contact Warren. |
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Tagged categories:
Coating Materials;
Consultants;
NACE;
Protective Coating Specialist (PCS);
Protective coatings;
Specification writing;
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Comment from Oscar Duyvestyn, (9/23/2013, 3:55 AM)
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This remains an interesting subject Warren and I will keep folowing it closely. As for my opinion on the coating on your Jeep, I guess a few things play a role: a steel was probably pasivated by a chemical conversion coating prior to wet coating application resulting in a less reactive substrate compared to abrasive blasted steel. Second, considerably less water vapour pressure than an immersed coating. A car is not quite the same than the inside of a water tank. I guess we all know what would happen if you use a 4.0 mil automotive coating as a tank lining. Just my two bob. Keep up the good work.
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Comment from David Cerchie, (9/23/2013, 7:35 AM)
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Indeed, moving from the mechanics of the "cold wall effect" to another critical aspect of the whole process is definitely a good step. Just as a quick review, there were really no wrong answers or positions about the cold wall mechanics, however for me I think Mr. Swan's points about vapor pressure/temperature helped me visualize the mechanics best. And to be sure, the fingers of analysis definitely point to coating permeability as a critical factor. I do have one problem with the analogy of the screens and thicker polymer coatings. I don't understand how adding additional thicknesses of a polymer matrix is different than adding additional layers of screen doors. If we look at the diffusivity piece of the puzzle it would seem that only the time element would be changed by increasing the thickness of the coating. That would seem to hold true whether we're looking at different thicknesses of polymers or different thicknesses of screen doors. The idea of solubility is a bit confusing to me also. When I think of solubility I tend to think of molecular changes brought about by a solvent breaking certain chemical bonds and thereby "dissolving" a substance. However my own ignorance is probably limiting my understanding here. I like the sponge idea however in looking at a material's ability to absorb. The dichotomy between repelling and absorbing is even harder for me to understand. These seem to be two mechanical aspects of a material. Sponges, towels and the like seem to me examples of materials with lots of internal void space which allows for absorption. Wouldn't the parts of these materials that are not voids also have repellant properties? So by adding layers of virtually any material into thicker sections would seem to slow the ingress of moisture to its final destination, its just impractical to use a 100 foot thick liner of sponge to protect a steel tank. Is it possible that by adding additional thickness to any coating the tensile and other properties become sufficiently high as to overcome the forces of vapor pressure buildup behind the coating and prevent blisters from forming? Maybe the larger thickness of the material allows greater absorption (solubility?). Are we winding our way into a black hole here? Thanks Warren for keeping us thinking.
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Comment from Lydia Frenzel, (9/23/2013, 12:31 PM)
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In 1994, Tom Aldinger, (retired Bectel); R.t. Vass, and Bala Viswanate (retired PG&E) gave a SSPC presentation "Water Blasting Versus Abrasive Blasting for In Situ Penstock Relining" in which they set up Atlas Cell Testing, for epoxy, polyurethane, and "Foamed" poly over conventional abrasive blast and WJ cleaned surfaces where they could get a side-by-side comparison of performance of coatings over abrasive blast and waterjet cleaned surfaces. They were looking for an accelerated testing method for interior penstock coatings. They used the cold wall effect for that comparison. There was no indication that "salts" were present. They found that water permeated the epoxy and lifted the coating in one large "blister". You could literally see the water condensed between the coatings and the substrate. On the thicker foamed polyurethane, which was closed cell rather than open cell like a sponge, they found a blister which was interior to the coating thickness. It could have resulted from an expansion of a void because of the temperature differential; it could have been a water-soluble spot of material. Aldinger, Vass, and Viswanath didn't publish the paper- just gave it verbally. I have published their findings, with their permission, Australasian Corrosion Association, Nov 2006, Corrosion & Prevention 2006, Steel & Concrete- Nothing Lasts Forever' from their original slides and transcription of the SSPC tape.
Back around 1996?, CCI (Houston) investigated a water storage tank in Houston area. The coating was covered with blisters that had not popped open. The steel was good under the blister. The culprit was a water soluble component. The supplier had sent to the coating manufacturer a component from a different source. While the physical properties of the component appeared the same on paper, it could react with water.
Lastly, some years ago at the Philadelphia Liberty Bell Conference, in a workshop for basics, I heard an "expert" say that the "cold wall effect was migration of the water through the steel towards the interface between the coating and the steel." I didn't comment during his presentation, as I was sure he just mis-spoke. However, after his talk, I queried him- he was adamant- the water migrated through the steel. Nothing that I could say would dissuade him from his belief. The initial blog on "Cold Wall" just reminded me of the numerous times that I have encountered very experienced persons who don't want anything to do with thermodynamics, chemistry, or physics, but who are firm in their concepts.
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Comment from Warren Brand, (9/23/2013, 3:46 PM)
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Thanks all for your responses. So much data here, don't know where to begin. Oscar: thanks for your kinds words of support. Couldn't agree with you more about the Jeep and immersion.
David - I think we're on the same page in terms of diffusivity. In speaking with my physicist buddy, he didn't like the screen door analogy - but he liked the concrete analogy very much. And I also agree, that the time element is a critical consideration, which goes back to the Moisture Vapor Transmission Rate - which is exactly what you referenced - the rate (time it takes) for moisture to pass through a material. Does that make sense? About solubility, I asked my buddy the same question. He explained that in this context, the water is soluble within the polymer matrix. Nothing is being dissolved (as I initially thought as well), in this context, it refers to the ability of the water to enter into the substrate. Diffusivity becomes an issue once the water has entered. And, about your other question of making coatings thicker, the science would indicate that yes - making the same coating thicker will slow MVTR. This begs a question that I'm sure send shivers up the spines of all coating companies. Why don't we have a universal primer for steel (fodder for another blog)?
Lydia - I've looked over some of the Atlas Cell tests and, IMHO, some of them were, perhaps, not carried out properly. In one of the tests (I cannot remember which one), the exterior temperature remained constant while the internal temperature was raised - which is problematic. I'm not familiar with the specific information you've referenced. Also, sadly, not terribly surprising about the "expert" that said the water was migrating through the steel.
I worked with a client out of Singapore who wanted to line two, 100,000 gallon carbon steel digester tanks with tile from Home Depot (they were currently lined with acid brick, that had begun to fail).
My frustrating remains. Look at how far afield we must go in getting definitive answers pertaining to these issues - an SSPC presentation from 1994, presentation in Australia from 2006 and something in 1996?
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Comment from Keith Holdsworth, (9/26/2013, 1:55 PM)
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Speaking from application and lining installation experience I can add that the fillers of the specific coating materials utilized play a significant role in the longevity of a lining system subjected to cold wall effect. We had done an installation on a precipitator at a chemical plant which manufactures acid that was originally lined with a heavy glass flake filled vinyl ester material. This vessel is uninsulated and located in New York with cold ambient temperatures outside in the winter. Due to improper application from the shop coating facility we were called upon to make lining repairs a short time after it was placed in service. This was a temporary fix due to the amount of failure and requiring relining. Because they couldn't afford down time in production for relining they decided to fabricate a second precipitator and have it lined by us then swap the units out during and outage. We installed the originally specified material (the heavy glass filled trowel applied vinyl ester system). As an added protection against permeability we recommended and installed an optional clear gel coat of vinyl ester resin over the trowel applied material. This was a listed option my the original coating manufacturer. This lining survived for over 10 years in this severe environment. towards the end of the service life of our installation the customer wanted to perform steel repairs and relining of the original precipitator that was swapped out when the new was built and placed in service. The customer was in contact with a different material vendor which sold them on their vinyl ester product instead of what was originally specified. The new viny ester was a 35 to 45 mil spray applied vinyl ester with a fine glass filler versus the original which was approximately 80 to 100 mils with a heavy glass flake that is trowel applied.
The new lining material was utilized at the customers request and the units were swapped out again during and outage. The new lining failed within one year of service. It was determined the failure was due to permeation as a result of cold wall effect. So the spare unit which was sitting idle was relined with the original heavy glass flake filled trowel applied lining with option vinyl ester gel coat applied and is performing flawlessly once again.
This is an example of the size of how the size and orientation of the glass flake in a resin can play a large role with regard to cold wall effect in a severe environment.
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Comment from jeff truman, (9/30/2013, 11:42 AM)
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David Cerchie's comment touches on the key aspect of the cold wall phenomenon: Time. Yes, adding more layers like the screen door example simply increases the time it takes for moisture to penetrate the coating. And if Warren Brand has noticed that thicker coating do not fail, there is a clue.
Blisters will form when two things happen: moisture reaches the cold surface, and then cannot get away. The ideal coating is therefore thick enough to limit the moisture reaching the condensing surface, yet porous enough to not allow that moisture to build up.
Water has different properties in its different states, and I wonder if vapour pressure from water vapour is different than the vapour pressure of liquid water, so that once vapour condenses, there is a different force pushing it back out of the coating.
I'm not a physicist, but there is something in this analysis.
With the Jeep, it is just not subject to the same vapour/moisture pressure as in a tank!
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Comment from Warren Brand, (10/9/2013, 11:28 AM)
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Good Day David and Keith. Thanks for your comments. Very insightful and important. As a point of clarification, I had not meant to compare the jeep with a tank, and sorry if I did. In reference to the jeep, I was thinking that there are a great many exterior coatings (bridges, stand pipes, water towers, stadiums, etc). that are subject to the same, or less aggressive environment than my Jeep experienced in 19 years of Chicago winters. Yet we frequently see failures on these exterior coatings which are profoundly thicker than the 4.0 mils on the Jeep.
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Comment from Randy Gordon, (12/6/2013, 7:40 PM)
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Great writing style, Warren...I enjoy reading your posts. That's funny Lydia; water permeating thru the steel. Time, size, pressure, contaminates, temperature, thicknesses, densities, and solubility. Too many variables at play...Makes my head spin.
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