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Notes From The Blog Painting and Corrosion Allowance and Blister Formation Editor's Note: Looking for "in the field" information about the latest coating application techniques? We invite you to join the discussion. Te following are excerpts from the NACE Corrosion Network's (NCN) and NACE Coatings Network's email-based discussion groups for corrosion professionals, with more than 1,000 participants. Te following excerpts have been selected for their potential interest to CoatingsPro readers. Tey 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 Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the inquirers and respondents.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. For information on how to subscribe to these free list servers, click on the "Community" link on the NACE Website: www.nace.org. Painting and corrosion allowance Q: Does primer need to be applied to all equipment and pipelines? Which piping and vessels are designed using zero corrosion allowance? A: Many coatings are self-priming, so they do not need a special primer. Most pipes do not have a "corrosion allowance" per se. Primer does not need to be applied to all equipment and pipelines. For example, on insulated steam pipelines and vessels we don't apply a protective coating because the surface temperature is hot and corrosion is negligible. Tere may be some piping and vessels designed using zero corrosion allowance, especially if they A: 24 are not pressurized and just exposed to atmosphere or in an environment where the corrosion rate is negligible. Corrosion allowance is an additional protective measure for carbon and low-alloy steel. Its value can be calculated from a combination of De Waard's equation(s), design life, etc. It also ensures that there is enough thickness for pressure containment. It is unusual to associate corrosion allowance with corrosion-resistant alloys. A: Blister formation in bimetallic immersion Q: A coated, insulated carbon steel (CS) tank (~12-foot [4 m] diameter by 55-foot [17 m] long) contains beer ready for bottling (pH 4.2) in full immersion maintained at a constant temperature of 33° F (0.6° C). Failure has occurred on the lining system (single coat 100 percent solids/vol epoxy) in generalized areas in the form of erasersized blisters that are very uniform in JANUARY 2014 COATINGSPROMAG.COM size. Te areas have very high blister density (the blisters are almost touching). Te remaining 60–75 percent of the areas are without evident failure. Corrosion is evident on the steel underneath the blisters. Unfortunately, the pH of the liquid inside the blisters was undetermined because the tank was out of service for some time. Te manufacturer's thickness recommendation was 20 mils (508 µm). Te blistered areas, however, were strongly correlated to flm build. Blister formation occurred only in areas below a fairly consistent flm build threshold of 22 mils (559 µm). Film builds >25 mils (635 µm) exhibited no blisters. Te bimetallic aspect results from the construction of cooling tubes inside the tank. Six stainless steel (SS) tubes (3-inch [76 mm] diameter by 45-foot [14 m] long) run nearly the full length of the tank and provide coolant to maintain the liquid at a constant 33° F (0.6° C). Te tubes are welded to the wall of the