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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COATINGSPRO JANUARY 2019 87 various degrees on virtually every metal surface. e severity of blistering varied from area to area, but there were no significant areas where blistering did not occur. However, finding a signif- icant quantity of carboxylic acid salts in the blister liquid was unexpected. Carboxylic acid salts are the reaction product of esters and moisture in the presence of alkalinity (high pH). e reaction is known as saponifica- tion. Two-component epoxy coatings don't contain the organic compounds necessary for saponification to occur; however, alkyd coatings do. e gray weld-through primer that had been applied to the steel components was based on an alkyd resin; however, the weld-through primer was reportedly removed by abrasive blast cleaning before application of the epoxy. e specification required that the field applicator abrasive blast clean the steel in accordance with NACE No. 2/ SSPC-SP 10. By definition, a near-white blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter except for staining. e staining is limited to no more than 5 percent of each 9 in.² (58.1 cm²) area and may consist of light shadows, light streaks, or minor discolorations caused by stains of rust, mill scale, or previously applied coating. e determination as to whether the surface has been adequately abrasive blast cleaned is visual. Unfortunately, in this case, a gray alkyd primer was applied to the steel in the shop. By definition, near-white blast cleaning does not allow remnants of the coating to remain on the surface. However, these remnants consisting of very small particles of paint appeared as a stain on the surface. Since the gray paint is similar in color to the surface of the abrasive blast cleaned steel, the remnants were not visibly detected. Any alkyd primer that remains on the surface, even if not visible to the unaided eye, will deteriorate over time when in contact with the highly alkaline tank contents and turn into a soluble carboxylic acid salt. e conclusion was that the inspection of the surface cleanliness was insufficient to ensure that all of the gray primer was removed. By definition, magnification of the surface during inspection is not permitted by the standard. Key Takeaways A lkyd paints are not recommended for use in immersion, even when the pH is neutral. e deterioration is much faster when the pH is greater than 10. As such, alkyd paints should never be used on steel that is intended for immersion service, even if the paint is going to be removed by abrasive blast cleaning before the application of an appropriate coating system because abrasive blast cleaning frequently does not remove all residues of previously applied paint. In this case, the chance of failure after the application of the weld-through primer was greatly enhanced since the primer color was similar to the color of the steel, and the tank contents were highly alkaline. Interestingly, the weld-through primer was available in two colors: gray and red. It is possible that had the red color primer been used, the blistering could have been greatly reduced since the red primer contrasts the color of the steel and would be more visible in smaller quantities during inspection. Regardless, the specifier should have used a compatible coating, such as an epoxy, as the holding primer. CP Rick Huntley is a chemica l eng ineer and t he technica l manager of consu lt ing ser v ices and a senior coat ings consu ltant for K TA-Tator, Inc. Hunt ley has more t han 30 years of e x per ience in cor rosion prevent ion, coat ing system recommend a- t ions, coat ing fa i lure invest igat ions, and l it igat ion suppor t. For more infor mat ion, contact: R ic k Hunt ley, rhunt ley@kta.com 800.621.2777 www.twrs.com sales@twrs.com INDIRECT HEATERS RESPIRATORY CLIMATE TUBES Re ad e r In qui r y at co ati n g sp ro m a g.co m /i n q0119

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