CoatingsPro Magazine

JAN 2009

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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Page 15 of 107

NOTES FROM THE BLOG Severely Corroded Tanks, IOZ, and Galvanizing 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 follow- ing are excerpts from the NACE Corrosion Network (NCN) and NACE Coatings Network e-mail-based discussion groups for corrosion professionals, with more than 1,000 participants. The following excerpts have been selected for their potential inter- est 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 Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the inquir- ers and respondents.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 Web site: Coatings for Severely Corroded Tank Plates We are facing severe external corrosion on our tank plates and structural items. The atmosphere is very corrosive, with humidity up to 100% during the rainy season. On average, we experience humidity up to 60 and 70% throughout the year. The atmosphere is further corrosive because of emission from various nearby sources. Any remedial recommendations? Q A You need a good maintenance painting program. I suggest a good inorganic zinc (IOZ) primer with an acrylic aliphatic polyurethane topcoat, assuming this system is compatible with the existing 16 CoatingsPro J January 2009 coating system on your tanks. (I assume your tanks are coated now.) This system should perform well in your environment. A I am hesitant to recommend using an IOZ unless there is good surface preparation. IOZ generally requires NACE No. 1/SSPC-SP 5 white metal blast cleaning and sometimes NACE No. 2/SSPC-SP 10 near-white metal blast cleaning. With your "severe external corrosion," this may be hard to obtain. Also, if the surface is pitted, IOZ is not the best choice. If you do use an IOZ primer, you may be looking at the likeliness of a complete and full reblast. There are other types of primers on the market that are more suitable for the maintenance coating market that would not require a full blast. Perhaps ultra-high-pressure water jetting also could be an interesting option for you. The epoxy intermediate coat(s) is standard and an acrylic polyurethane topcoat is advisable for UV resistance. Recoating IOZ with IOZ Q A My client wants to recoat an inorganic zinc (IOZ) coating that was applied some time ago. In other words, apply an IOZ coat on top of an existing IOZ coat. Any problems? IOZ coatings develop a zinc oxide and carbonate surface, compounds that make it difficult — if not impos- sible — to recoat IOZ with itself. This is a problem with both solvent-based and waterborne IOZ. I once had a manufacturer's salesman tell me that IOZ can be recoated with itself if it is done almost immediately and while in the shop or before it is exposed to the environment. I strongly question whether he had any real shop or field experience. Recoating an IOZ coat with another zinc-rich coating is virtually always done to repair damaged areas. Little damage usually occurs in the shop or immediately after applying the first coat, so recoating or repairing before or very soon (a day or two) after the original coat is seldom done. Repair generally needs to be done in the field after several weeks, if not months, of exposure to the elements. The general rule used in industry is that because the zinc oxide and zinc carbonate begin to form so quickly on the zinc after normal exposure (the oxygen and carbonates are in the water and in the atmosphere as the IOZ cures), they function as a bond breaker. Therefore, one cannot assure good adhesion between coats of IOZ. The most common repair system that I am familiar with is to coat damaged areas with a zinc-rich epoxy or other zinc-rich organics. The bond of zinc-rich epoxies is not as adversely affected, and they will bond well to IOZ after a wash or mist coat to reduce bubbles from air entrapment. I would have a problem with recoat- ing (repairing) IOZ with IOZ. I would not take the chance. A As a general rule in the industry, it is not a good practice to recoat IOZ with itself. This is because of the rapid formation of zinc oxide/zinc carbonate. Even if you recoat quickly, this second layer either will not show proper bonding or will remain very powdery. Standard IOZs, both water- and solvent-based, are formulated in such a way that the normal critical pigment

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