CoatingsPro Magazine

JAN 2019

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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86 JANUARY 2019 COATINGSPROMAG.COM ere was a layer of an off-white precipitate on much of the surface of the coating. e coatings in virtually all immer- sion areas were experiencing blistering to some degree. e severity of blister- ing varied considerably from relatively sparse blistering inside the cone area to relatively dense blistering on some of the troughs and in sections of the exterior of the center cone. e blisters ranged in size from about ¼" (0.6 cm) in diameter up to ¾" (1.9 cm) in diameter. ere was no obvious pattern to the blistering with the exception that all the blisters were in immersion areas. No blistering was observed above the waterline. Most of the blisters had cracked, and there was rust emanating from the cracks. W hen the caps were removed from these blisters, the underlying steel was covered with a relatively light layer of black and red corrosion products. Usually, the anchor pattern of the steel would still be visible in the corroded areas. W hen the surrounding coating was forcibly removed with a utility knife, the steel substrate was free of corro- sion. In no case was pitting or any other sign of advanced corrosion/section loss observed under the removed blisters. Additionally, some of the blisters had not broken. Many of those blisters were probed to determine whether there was liquid inside. W hen liquid was found in a blister, it was extracted using a syringe and placed in a septum vial for subsequent analysis. e firm used an electronic gage in several areas to determine whether the severity of the blistering could be correlated to the thickness of the coating. e coating thickness was measured as thin as approximately 10 mils (254.0 microns) to as thick as 30 mils (762.0 microns) in areas of relatively dense blistering. e thick- ness of the coating was not significantly different in areas of sparse blistering. ere seemed to be no correlation between thickness of the coating and severity of blistering. e adhesion of the coating was subjectively assessed by picking and probing at the coating with the blade of a utility knife. e adhesion of the non-blistered coating was typical for a coating system that had been in immersion for several years. e coating could only be removed in small chips, and it was not possible to insert the knife under the coating and remove the coating in sections larger than a fraction of a square centimeter (0.2 in.²). Samples of the coating were removed for subsequent labora- tory analysis. Laboratory Analysis e key to determining what causes blistering of a coating often lies within the blister liquid. e laboratory performed a detailed microscopic examination of the samples removed from the clarifier equipment and analyzed blister liquid for the presence of solvents and organic residue. e microscopic examination of the paint chips removed from the clari- fier did not reveal anything unusual. e back surface of the chips exhibited a roughness that replicated an abrasive blast cleaned surface. ere was a slight black residue on the back that is similar in appearance to abrasive residue and is commonly found on the back of chips removed from abrasive blast cleaned surfaces. Cross-sectional examination revealed two to four layers of coating; the total coating thickness was 14 to 27 mils (355.6–685.8 microns). e blister liquid was analyzed using a gas chromatograph/mass spectrometer to determine whether organic solvent was present in the coating. e analysis revealed no signif- icant amount of organic solvent in the blister liquid. e blister liquid was dried, and the residue was examined by Fourier transform infrared spectroscopy. e infrared spectrum of the residue was very similar to that of carboxylic acid salts, which was an unusual finding when analyzing residue from an epoxy coating. Failure Mechanism It was clear that the blistering of the coatings on the metal components was a systematic problem and was not caused by random surface preparation or application inconsistencies. It was observed during the field investiga- tion that the blistering occurred to Blistering Clarifier Coating System The surrounding coatings were removed with a putty knife, and no pitting or other signs of advanced corrosion/section loss were observed under the removed blisters.

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