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NOTES FROM THE BLOG Powder, Buried Pipeline, And Rebar Coatings By MP Forum Editor's Note: Looking for "in the field" information about the latest coating application techniques? We invite you to join the discussion. The 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. The follow- ing excerpts have been selected for their potential interest to CoatingsPro readers. They 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 Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the inquirers and respondents.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 infor- mation on how to subscribe to these free list servers, click on the "Community" link on the NACE Website: www.nace.org. Powder Coating Q: A: Has anyone applied powder coatings onto new galvanized steel members such as light poles or bridge rails? We have specif ied this coating system on one of our projects. My concern or question is, will it adhere to the galvanized steel? If the coating is hot-dip galva- nized (HDG), you should make sure the dipper is qualified. Assuming the work will be in the United States, the dipper should be a member of the American Galvanizers Association (AGA) and be qualified to meet ASTM A123 or A153 standards (depending on what is being dipped). The hot dipper 20 CoatingsPro J July 2012 also needs to be told in advance that what will be dipped will be powder coated. The presence of conversion coatings or very thin oil films on the surface of water in cooling tanks at the dipper can create a bond breaker for your coat ing. Abrasive blast ing may not remove this and you coating will easily peel. ASTM D6386 details preparat ion of the zinc surface for coating. If you use abrasive blasting, I suggest the coater use a relatively soft media at lower pressure and with quick nozzle movement with some trial work required in your specif ication. You want profile but you don't want to remove a lot of zinc in the process. AGA calls these "duplex systems," and they can provide excellent service. However, it typically is not an easy job. AGA has information for download at its Website: www.galvanizeit.org. Search for "painting over HDG." Coating System Quality Q: A: Based on experience and a proven track record, what would be the ranking of the following coating systems for buried pipelines? Epoxy-based Polypropylene (PP) Polyethylene (PE) There is no magic coating for every application. You must always consider cost and effectiveness. Fusion-bonded epoxy is effective for many buried applications. If you want a tougher coating, as for high-humidity soils or submerged conditions, then you could go for a three-layer PE for operat- ing temperatures up to 176°F (80°C). For temperatures >176°F (80°C), three- layer PP would be your best choice. Nevertheless, each situation requires some careful considerations, because 100% solids epoxy resins may behave quite well in most situations, depending on the product. If you choose the "best" available coating, you may end up paying a lot more money than needed. A: There are numerous factors to consider, including the advan- tages and disadvantages of each coating type. Factors to consider include soil stress, service conditions, chemical resistance, durability and resistance of the coatings, ultraviolet resistance, resis- tance to mechanical damage, cathodic shielding, cathodic disbanding, impact resistance, flexibility in various weather conditions, transportation needs, girth weld, field repair, and construction. Rebar Coating Q: A: In the rehabilitation of reinforced concrete structures, what are the risks of coating only part of the reinforcing steel with epoxy? Have there been practi- cal cases of macrocell formation between a coated rebar and an uncoated one? We have typical ly observed problems wi th respect to "anodic ring" effects, in which the parent concrete exhibits delamination immedi- ately adjacent to a patch when extensive chloride concentration is present. If large areas of chloride-contaminated concrete remain and the repair patches are small, prepare for rapid delamina- tion of adjacent or unrepaired areas. Another complicating factor is that if any pinholes or discontinuities exist in the coating, rapid, localized corro- sion can result. As is the case with most corrosion problems, there are many variables to consider, particularly with regard to existing contamination and exposure conditions.