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CL BLOG Continued from page 53 Coating system for carbon steel pipelines We have three parallel crude oil carbon steel pipelines, 50 km in length, with no protective internal or external coating. Approximately 34 km of the pipe's Q length are partially to totally covered with sand because of frequent sandstorms. The lines are heavily corroded. No cathodic protection (CP) is provided because the pipeline was installed as an aboveground structure without taking into consideration the sand accumulation during sandstorms that commonly occur in this Kuwait desert location. We are going to repair parts of the line with <4.9 mm wall thickness remaining, maintaining a minimum wall thickness of 5 mm and applying an external protective coating. We plan to do a very thorough blast cleaning (near white metal blast). The first coat is a solvent-borne inorganic zinc (IOZ) silicate primer, at 75 µm dry flm thickness (DFT). The second coat is a high-build, polyamide-cured, twocomponent grey epoxy at 125 µm DFT. The third coat is a two-component, highsolids, self-priming, aluminum-pigmented epoxy coating with good wetting properties, at 125 µm DFT. We believe the painting system is suitable for an aboveground pipe installation. We are concerned, however, that a few months after the coating has been applied, sand will again accumulate, cover the pipelines, and initiate corrosion. There is a very good possibility of getting contamination between coats while waiting for each coat to cure before applying the next coat from blowing sand and chlorides in the atmosphere. Any contaminant of this nature can cause delamination or disbonding of the organic coating applied over it. Since your lines are heavily corroded, you will need a coating system that has good adhesion to heavily pitted metal that is blasted to SA 2.5 condition. There are several high-solids, abrasion-resistant epoxy coatings that can be applied in one coat to a DFT of 500 µm. does cure fast. I do know of one company that makes the epoxy product in a UVresistant epoxy for large orders, and for smaller orders even makes a topcoat that is 100% solids and UV stabilized. This of course requires two coats. A To protect the pipe when it is covered with sand, you should use CP. The sands in the Middle East, when wet, can be of low resistivity and high chloride content. Depending on resistivity, a galvanic anode system would probably work. Coating by itself does not adequately protect underground structures. I suggest you skip the zinc and go with one 100% solids, one-layer coating. I personally like the immersion-rated epoxy system for preventing corrosion for long periods but it has the disadvantage of suffering ultraviolet (UV) degradation when the sand is not covering the pipeline. The thick flm urethane doesn't have this problem and A High-solids, high-build polyester glass fake at 1 by 1,000 µm or 2 by 750 µm would be a good option in your case. Polyester glass fakes are quick-drying and have excellent abrasion resistance, good thermal expansion coeffcient, minimal water absorption, and excellent immersion properties. A A I can see several problems with your suggested coating system. While the IOZ and epoxy system would have good corrosion prevention properties, there are two problems to consider. Can your applicator apply IOZ properly? In the predominantly hot climate of Kuwait, it will dry very quickly and if applied too thick, it will mud crack. The cracking will not be a good substrate for the epoxy topcoat. NACE International, Vol. 52, No. 5 May 2013 MATERIALS PERFORMANCE 55