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September 8 - September 11, 2015

What coating problems are likely in storage tanks that have dissimilar metal sides and floors?

Selected Answers

From Ernie Johnson of Brock Group on September 30, 2015:
You can answer this where all can understand it. When a certain coating is chosen for a tank, one of the main concerns is what product is the tank going to hold and what metal is the tank. If you use, say, an epoxy for a steel tank and if by chance it also has stainless or galvanized, sooner or later the paint will peel off the stainless or galvanized. The only question is when and that is determined on how large the area is that's not intended for the coating you're using and how much extra or special prepping you did to those areas, such as acid wash the galvanize. No matter, sooner or later the coating will fail.

From William Slama of International Paint/Ceilcote Products on September 17, 2015:
Going back to Thin Coatings /Metallurgy 101, this was always a no-no. That was because of the differential galvanic effect making the carbon steel anodic to the more noble alloy. So if there was even one pinhole near the stainless, the corrosion would quickly "drill" through the steel. Determining factors include the area of the alloy, the distance to the pinhole, and the conductivity of the solution contained. Even early on, most specifiers solved this by requiring that the alloy also be coated over to electrically insulate the alloy to eliminate the galvanic potential. Later, when high-build linings were used to line vessels to protect against challenging chemical and thermal environments, this effect was not noticed. That was because the high-build linings were high-voltage tested and could not have pinholes. Moreover, the thicker linings provided high electrical resistance through the lining, so the slight voltage difference was not a problem. In many vessels, it was found to be more reliable to use alloy support brackets and other internals than to try to coat those surfaces for protection. The procedure presently used is to bring the polymer lining a few inches over onto the alloy surface. That process has been used successfully for many decades.

From Warren Brand of Chicago Corrosion Group on September 16, 2015:
Having lined hundreds, perhaps thousands of tanks, the most common issue is having a carbon steel tank with stainless steel appurtenances welded to it in the form of nozzles, supports, manholes, etc. I'm still amazed that this is such a common practice. I have many photos showing a perfectly good tank bottom, but for through-corrosion adjacent to a stainless steel support pad. The problem is simply that by welding stainless steel (a more noble alloy) to a less noble element (carbon steel), you make the carbon steel anodic and (depending on the size of the stainless steel piece) dramatically increase the corrosion rate of the adjacent carbon steel. I have not answered the question precisely, however, having never seen and entire storage tank where the bottom was a different material than the sides.

From B Brown of Self on September 15, 2015:
The better you define your question, the more likely you are to receive a good answer to your problem. "Dissimilar metal" could be rolled plate and forged components all made of carbon steel. I'll suppose the dissimilar is more varied such as carbon steel and austenitic stainless or maybe a copper alloy. The most obvious would be that anchor profile will vary, given that each metal is subjected to identical surface preparation conditions. Although the stainless is likely to be of similar hardness to the carbon steel, stainless work hardens as it is abraded, and, typically, a shallower profile depth is produced with stainless. A harder, sharper abrasive might therefore be chosen for abrasive blast cleaning the stainless. A zinc primer would probably be a poor idea for coating that stainless, but then I did start by saying we have been provided almost no detail regarding what you plan to coat, how you intend to clean and prepare the surfaces or what you plan to coat these surfaces with.

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Tagged categories: Asia Pacific; Coating Application; Coating failure; Coatings; EMEA (Europe, Middle East and Africa); Latin America; North America; Tank exteriors; Tank interiors; Tanks

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