CoatingsPro Magazine

MAR 2018

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 MARCH 2018 65 these welded connections through direct impact and sliding abrasion as the wastewater f lows across these areas and hits any protruding surface or corner. Additionally, the wastewater contains various chemicals that can attack the lining system by coming into contact w ith it and subsequently causing a chemical reaction, result- ing in softening of the lining and, ultimately, a lining failure. T hese connection areas can also create a crevice environment where chemicals and debris could build up. Photo 1 shows where extensive delamination has occurred at these welded connec- tions and crevices. A lternative solutions to connect- ing the pipes should have been considered on this project — stripe coating, for instance. T he use of stripe coating is often necessar y for coating performance. Stripe coating is apply ing an additional layer of lining product on areas w ith shar p corners or hard-to-access areas. Crevices and shar p corners are perfect examples of areas where stripe coating may also be used. Sometimes, the stripe coating material is formulated to be a thicker, low-sag ging material that w ill prevent running and pulling away from the shar p angles and corners. In the case of this wastewater pipe, all welded connectors would have been an excellent location for stripe coating. A lso, grinding dow n shar p edges or using a gradual sloping welded metal connector could have reduced foreign object impact to these areas and prevented pulling away of the lining material from the shar p edges of the corners. T he lining materials may have contained solvents or moisture that evaporated and shrunk, which could also have caused exposure of edges. Even a 100 percent solids lining system may have internal coating stress as a result of cross-linking, which could result in a tendency of the lining to pull away from the shar p edges. Lining Delamination roughout the entire wastewater piping system of this project, lining delamination was seen to have occurred in various random locations. Dry film thickness measurements ranged from 33 to 85 mils (838.2‒2,159.0 microns), with an average of 65 mils (1,651 microns). A thicker measurement demonstrated areas where over-coating was used. Several areas of delamination occurred in the areas where over-coat- ing had taken place. Reasons for over-coating could have been the presence of pinpoint corrosion, low dry film thickness of the gradually worn down original lining system, or excessive coating application attributed to the idea that more thickness would provide a longer service life. Regardless of the reason for the over-coating, this was identified to be the cause of the delamination in those areas. Photo 2 demonstrates the delamination of the lining system that was applied over the original, intact lining. ere are numerous considerations when doing over-coating work on previ- ously applied lining systems. For example, the original lining chemistry needs to be tested at a laboratory to ensure compati- bility with the new overcoat system. is specific wastewater pipe had originally been lined with a polyurethane. e overcoat then should have been tested to ensure compatibility with the current polyurethane lining material. Also, the original lining system should have been cleaned to remove all chemicals, salts, and loose particles that would prevent adequate adhesion upon over-coating. Additionally, manufacturer recommendations and a well-prepared specification would promote performance with regard to surface preparation of a previously applied lining system. Some examples would be brush-off or sweep blasting, which includes achieving a moderate surface profile to allow the new lining system to adhere. Many manufacturers require an intact, stable substrate and a 2- to 3-mil (50.8‒76.2 microns) profile The lining materials may have contained solvents or moisture that evaporated and shrunk, which could also have caused exposure of edges. Photo 2. The new lining system delaminated from the original, intact lining. Pipe Lining Failure

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