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 85 water treatment tanks, but that recom- mendation was not contained in the latest version published before the start of this project. e chosen system was approved by NSF Standard 61, which listed in the product data sheet two coats at a dry film thickness of 5 to 8 mils (127.0–203.2 microns) per coat, not the three coats as required by the specification. Abrasive blast cleaning of the steel equipment began in the middle of spring and was completed about three weeks later. According to the inspection data, the actual abrasive blast cleaning was performed on 10 separate days over the three-week period. In general, the abrasive blast cleaning started in the morning and continued until mid or late afternoon. Application of the primer was performed for an hour or two after abrasive blast cleaning opera- tions were completed. According to weather data from the local area, the temperature was between 57 °F and 88 °F (13.9–31.1 °C) during any of the days that abrasive blast cleaning and paint- ing was performed on the clarifier. No precipitation was reported in the area on the first nine days; light precipita- tion was reported on the 10th day, but it occurred several hours after the coating application was completed. e steel equipment was installed in the clarifier in the early summer of the same year. e water in the clari- fier is highly alkaline and, according to personnel at the treatment plant, typically ran between pH 10.5 to 11.5. e clarifier was drained in the late summer so that miscellaneous touch-up painting could be performed, and it was then drained again in the spring of the following year so that repairs could be made. No blistering of the coating was noticed either time the clarifier was drained. e clarifier was again drained in the fall three years after it was originally put in service. At that time, blistering of the coating was observed and reported for the first time. e original coating applicator was notified of the problem and contacted an independent coating failure investi- gation firm to determine the cause of the blistering. Site Investigation A visit was made to the site in the late winter approximately one year after the blisters were first noticed. ere was intermittent mixed rain and snow precipitation at the time of the visit. e condition of the coating on the internal steel mechanisms of the clari- fier was accessed from the clarifier floor and from a step ladder. e troughs were not closely accessed, but the condition could be observed from the floor and from the top of the clarifier. e carbon steel in the immersion areas was covered with a white coating. Most of the blisters had cracked. Rust was eminating from the areas, and the underlying steel was covered in black and red corrosion products. No blistering was observed above the waterline. Blistering Clarifier Coating System

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