Materials Performance

DEC 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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12 DECEMBER 2016 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 55, NO. 12 Continued f rom page 11 THE BLOG What other evaluations are available to determine the condition of this liner for continued service? Of course the owners want another 10 years minimum (20 preferred) from the lining with minimal expense. It has been proposed that the topcoat be removed by abrasive grit blasting down to the intermediate hold primer and that another two coats of the same product be applied. The extra thickness is to deal with an increase in operating temperature. A: I do not often recommend that anyone attempt to overcoat a tank lining, as it is much more difficult to achieve satisfactory results after the coating has absorbed moisture and possi- bly other chemicals. If you continue with this evaluation, you will spend a few thousand dollars, but this is extremely cheap insurance if the value of the exist- ing coating is any thing above zero. In other words, you could easily spend a lot of money to overcoat the existing only to find that money wasted plus it destroyed the existing coating for any of several reasons. A: Adhesive failures between coats is cause for concern! But when you say cohesive failures of the topcoat, what values are you using? Glass flake- filled vinyl ester lining material cohesive values are generally not that high (as compared to epoxies, and urethanes). Do you have any documentation from the installation of the lining in question (i.e., installing contractor, or manufacturer of the lining material itself)? Consulting the manufacturer for lining inspection and repair would be the best place to start. e adhesive failures you have discovered definitely need to be further investigated! A: Here is what my experience has been. A couple of years ago I worked for a large paint and coatings manufacturer, which had a substantial vinyl ester product offering decades of performance history. A customer asked the very question you asked and the answer from technical services was very fuzzy: Lightly abrasive blast the entire lining to see if any areas began peeling, if not then it's "probably okay." e justifica- tion was that over time a vinyl ester loses elasticity and the only way to determine if it had in fact lost its ability to flex with temperature change was to blast the topcoat. e problem was there was no way to definitively tell what temperature the original product was designed for. It was determined that it would be better to blast the vessel and line it with an appro- priate product for the new intended service. A lining having lasted 17 years has most definitely done its job. A: One thing to remember is that the pull tests performed showed you only the "weakest link in the chain." at doesn't mean the underlying coating is strong, just stronger than what failed. If you can get a clean pull on the holding primer that you mentioned, or the actual primer coat of vinyl ester, I would try that, too. Secondly, remember that even if you get a reasonable pull on the holding primer, how much life can remain in a 17-year-old coating? At some point you are going to have the coating against the substrate fail before the coating on top of it. I've also seen times where putting a new coating over on old coating stresses the old coating as the new application begins to cure and contract. One last thing to consider is that it's been my experience that blasters cannot eff iciently remove only one coat of paint at a time, or stop where they want to. There will be low spots and sometimes bare metal. Now you're making spot repairs. I've found that starting to blast at Point A and going to Point Z can be more time and cost effective that "hunt and pecking" the bad spots. Fireproofing Q: I had a client inquire if plaster- and cement-encased steel can act as a f ireproof ing. I was in the basement of a large parking garage that went down six stories and noticed that while it was constructed of solid concrete, the only f ireproof ing I saw was spray foam sprayed on the bottom of the corru- gated decks that formed the ceiling, and this made sense to me. If one has old construction scheduled for remodeling and there is concrete that will remain in place, how do you make the determination that the concrete has the ability to act as f ireproof ing and at what hour rating? Another general comment I have regarding intumescent f ireproof ing is about the claims some coating suppliers make regarding their respective products. It has been my experience that one needs to know the size, shape, and function of each respective structural member that

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