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July 24 - July 28, 2017

After applying a thermal spray coating on a petrochemical industry live line that operates at 48 degrees Celsius (118 degrees Fahrenheit), the coating sustained a brown discoloration on the bottom of the pipe. What would cause this discoloration of a TSA coating?

Selected Answers

From wenbo chen of Morimatsu on November 28, 2017:
I also want to know the degree of acceptance of such stains or rust bloom that may occur on TSA (DFT 250-500 microns), and any industry code or practice that can support this acceptance, so I can convince our client....

From David Grove of The Grove Real Estate Group on August 15, 2017:
Both of the above answers are correct; however, both display visual differences. As Om Prakash Jat indicated, environmental conditions will carry discoloration and stains to a previously non-weathered surfaces (or coatings), just like soot from a fire can stain many surrounding surfaces. In this case, moisture will display distinct color patterns from dripping on the sides and collecting on the bottom of the pipe. Clamps and supports will also show visible patterns of this. As Ron Pluenneke indicated, it is highly likely that the cause was due to access or applicator error since applying TSA to the bottoms or backside of pipes is so difficult. This is a main cause of failure with TSA since the coating can be expected to have insufficient thickness to prevent corrosion from redeveloping. If the coating was or was not thick enough, minor destructive testing or spot cleaning with certain chemicals may easily identify this. The discoloration may also display application patterns or variances to the location for weathering. In both cases, further inspection is required.

From RON PLUENNEKE of Sentinel integrity solutions on August 8, 2017:
Thermal spray aluminum (electric arc & flame) is applied on a daily basis in the refinery I am employed at. Specification for low chrome & carbon steel: SSPC SP-10 with a 3-4.5 anchor profile. Dry film thickness is set at 10-20 Mils. We have a 4 hour max on covering blasted substrate with TSA. The pipe that showed discoloration on the bottom side probably didn't have enough TSA applied. When it is applied at 10+ mils with good equipment and workmanship, it should not show any discoloration. Another thought would be that corrosion (rust) from an adjacent pipe might have leeched onto and accumulated on the bottom side of pipe in question.

Moisture in the air emanates discoloration and rust bloom over newly sprayed coating. Moisture flew towards bottom side of the pipeline, which can be a cause of discoloration.

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Tagged categories: Galvanized/thermal spray coatings; Quality Control; Thermal spray

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