CoatingsPro Magazine

SEP 2015

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24 SEPTEMBER 2015 COATINGSPROMAG.COM compressor, it w i l l g radua l ly star t cooling of f, and w ith this, the relative humid it y of the compressed air w i l l star t to r ise ( because colder air has less capacit y to hold moisture). Once the air has cooled of f below its dew point, moisture w i l l condense f rom the air and col lect just where you don't want it (in the blast pot, on the blasted sur face). T his is ex act ly why the dehumid if iers are introduced in a reg u lar blast setup: Cool the air below its dew point and remove water/ moisture f rom it. A: T he only place where inser- tion of the dehumid if ier is impor tant is in setups where you have long d istances bet ween the compressor and the blast pot. In this case, it wou ld ma ke sense to put the dehumid if iers c lose to the blast pot. At this location, the air a lready w i l l have cooled sig nif icant ly and perhaps be c lose to its dew point, ma k ing it relatively easy to extract large amounts of moisture f rom it r ight before it enters the blast pot. T he opposite situation (dehumid if ier c lose to the compressor) might remove a sig nif icant amount of water to beg in w ith, but f rom then on, the air can only cool dow n f ur ther and perhaps reach its new dew point f ur ther dow n the line — creating possible new moisture/condensation. T he above is hy pothetica l, and I have not been able to establish this on site, but it seems log ica l this way. Coating on underground steam lines Q: Does anybody have ex per ience w ith ther ma l insu lation on underg round steam lines? We have ex per ienced excessive exter na l cor ro - sion. T he ma x imum pipe d iameter is 20 in. (508 mm). T he pipe is bare carbon steel sur rounded by a layer of ther ma l insu lation. I do not believe there is electroly te bet ween the pipe and the ther ma l insu lation. A: T here is a long histor y of corrosion problems related to buried insulated pipe. If there is no outer jacket over the insulation, the insulation w ill become saturated, and corrosion w ill occur. T he electrolyte should boil away, but whenever the line is out of ser vice, corrosion gets started. Even an outer jacket can develop a leak, which then creates a worse condition. Replacing coal tar epoxy Q: We used coal tar epoxy in an immersion (tank) application, with saline acidic and some saline basic environment. We also have an external application with saline acid splash. Does anyone have experience in replac- ing coal tar epoxy with other types of coatings? A: Surface preparation is simple and straightfor ward: see NACE Standards Nos. 1 and 2 for immer- sion and NACE No. 3 for splash and spillage. Verif y the application's profile depth and shape, verif y that the application is free from hydrocar- bons ( grease and oil) contamination, and ensure that soluble salts are no greater than 10 ppm. Be certain that a NACE-certified Coating Inspector is involved. Coating selection should be based on the ser vice environment — chemicals, concentration, and temperature. As chemical concen- tration and temperature exposure increase, move from polyamide epox y to polyamine-cured phenolic epox y, and, finally, novalac epox y. A ll have reasonable application pot lives and recoat w indow opportunities. A ll can be applied w ith single-leg airless spray equipment. A lternative coatings, such as aromatic poly urethane, poly urea, and plural-component epox y, are available but require a much higher level of applicator sophistication to achieve success. A: We had the same situation. We replaced coal tar epoxy with immersion-type epoxy mastics and solvent-free coatings based on the chemical resistance of the coating to the environment specifed in the project. Zinc on stainless steel Q: W hy have inorganic zinc (IOZ) silicate primers not been applied on stainless steel (SS)? Zinc will sacri- fce itself to the SS. W hat would be the recommended remedy if IOZ silicate/ epoxy zinc-rich primer has come into contact with SS piping during painting of adjacent carbon steel piping? A: As temperatures approach the melting point of zinc, you will get liquid metal embrittlement of austenitic SS. It is common practice not to apply zinc directly to the SS in the event of a fre or if someone welds on the material in the future without knowing the zinc is there. A: Te paint can be removed by grit blasting. We even use soda blasting to remove paint in order to dye penetrant check for stress corrosion cracking. However, before spending the money or efort to do that, you have to ask the question, "What are the consequences of failure?" In the unlikely event that there is a fre and the spot gets hot enough to melt the zinc, or someone welds on the line without grit-blasting it clean, what will happen if it leaks? Evaluating the combination of "likelihood of failure" and "consequences of failure" is at the heart of risk-based inspection programs. A: If the SS is a pressure vessel, then take action; if not, you can proba- bly just leave it alone, especially if it is a non- or low-pressure pipe (or even a cable junction box). Coating tank bottoms Q: For an on-g rade aboveg round tank on a sand foundation Notes From the Blog

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