CoatingsPro Magazine

JAN 2018

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20 JANUARY 2018 COATINGSPROMAG.COM Notes From the Blog Editor's Note: Looking for " in the field " information about the latest coating application techniques? We invite you to join the discussion through NACE's new Communities initiative. e following are excerpts from the NACE Corrosion Network 's (NCN) and NACE Coatings Network 's discussion groups for corro- sion professionals with more than 1,000 participants. e following excerpts have been selected for their potential interest to CoatingsPro readers. ey have been edited for clarity and length. Authors are kept anonymous for publication. Please be advised that the items are not peer-reviewed, and opinions and sug gestions are entirely those of the inquirers and respondents. CoatingsPro and NACE do not guarantee the accuracy of the technical solutions discussed. You' ll enjoy a variety of opinions in this practical information exchange forum. To see related discussions from these free list servers, visit volunteer.nace.org/ communities. Coating DFT in Seawater Environment Q: How can we determine the dry film thickness (DFT) of an insulating coating to prevent galvanic corrosion from occurring? We use a copper alloy close to c-steel in a seawater environment. We are looking for a method to calculate the required coating thickness of a pheno- lic, hot-cured coating. Is a formula available to calculate the required insulation, or is an alternative method recommended? We do know the material (grades) used, the surfaces, and the surrounding fluid. anks for your support. A: I wou ld not count on a coating a lone to stop the cor rosion of carbon steel in a seawater ser v ice, especia l ly if it is bonded to a more noble meta l, such as a copper a l loy. You need a good coating and cathod ic protection. First, use a sur face-toler- ant, f lex ible, abrasion-resistant epox y at about 15 mi ls (381.0 microns) thick on the carbon steel. Second, coat the cathode to eliminate the large cathode to sma l l anode area ratio. T he copper a l loy w i l l only need one coat, about 8 mi ls (203.2 microns) thick. T hird, apply cathod ic protection in the for m of ga lvanic anodes. A: T here are a couple of things to consider. First, coating just the carbon steel is ask ing for trouble if it is in meta l lic contact w ith copper, because the inev itable defects or brea kdow n in the coating w i l l a l low accelerated cor rosion to occur. T he tota l cor rosion loss may wel l be under cathod ic control deter mined by the tota l area of the copper cathode and not by the area of ex posed steel. So you shou ld coat both meta ls, if possi- ble. Second, you can model ga lvanic cor rosion using boundar y element ana lysis, and you cou ld inc lude the resistance of the coating. W hy not just do some practica l laborator y or f ield tests or rely on a litera- ture search? A: You are not tr y ing to stop any cor rosion of the noble a l loy. Just cut dow n on the sur face area ex posed to act as a cathode in the ga lvanic cor rosion cel l. About 8 mi ls (203.2 microns) is what you can get w ith a t y pica l coat of f i l led epox y, and this w i l l minimize pinholes in the coating. Application of 100 Percent Solid Epoxy Q: W hat is the best method (trowel, brush, or roller) to apply a 100 percent solid epoxy paint with a pot life of 9 minutes? A: e fastest way to do it, after the proper surface preparation, is airless. Roller will come in a close second place. A: If your material really has a pot life of 9 minutes, the only usable means of application will be a plural-component airless spray machine. Your painters will not be able to trowel, brush, or roll a material with that short a pot life — it will harden before they can spread it. A: I've done an epoxy with an 8-minute pot life with a non-plu- ral airless when I only had to paint a couple of test panels. But don't take that as a recommendation! e appro- priate amount of "A" and "B" were premeasured, chilled down to appro- priate temperature (i.e., below lab temp but 5 degrees above the dew point and within working range) — and we had a series of people ready to go before mixing started: one mixer, one bucket mover (paint to airless pump, then swap the solvent immediately after spraying), one sprayer, and a timer to track things. At 7 minutes, we started pumping solvent, no matter what. It did work for just a few test panels, but I really don't recommend the practice. Just get a plural rig. We did that for later applications. A: With a 9-minute pot life, as others have stated, you will need a plural-component spray rig. However, not even all of those will work well Coating DFT and Applying Solid Epoxies

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