Weld Coating Inspection and Why Non-Destructive Testing is Needed

16 November 2018

A weld coating inspection can’t just cut out a section of the coating, then dispatch it to a lab for testing. That’s an approach that would defeat the whole purpose of the weld overlay, after all. No, a non-destructive test and inspection system should be recruited here, with its wet comb filters and wheels leading the way. Then, for a truly in-depth inspection, we call in electronic instrumentation.

What Are Weld Coatings? 

A full-girth pipe seam is being used to illustrate this hypothetical case study. The weld, perfectly applied, mates the two pipe sections together, and that seal will persevere, no matter the fluid pressure pushing away from inside the pipeline. Outside, protecting the seam from corrosion and environmental attack, a special coating has been applied. Painted on by a roller brush or chemically adhered to the pipe-encircling weld, this is no ordinary paint-based coating. To properly protect the seam from highly corrosive environments, strong polymers and chemically modified fluids cure or shrink-fit to form impressively durable material barriers.

Non-Destructive Weld Coating Inspections 

Tested to destruction, the protective coating would be left seriously weakened. Clearly, we need a non-destructive approach. The initially wet, polymer-rich emulsion is first checked with wet film thickness tools. The special combs or wheels gauge the depth of the curing overlay. Dry film thickness instruments come next. Employing a flexible wire and probe, the device scans the dry coating for concealed discontinuities and other flaws. From here, the instrument readout measures the DFT, the Dry Film Thickness, as a minimum and maximum value from one side of the pipe to the other. And, since many polymer-based protective coatings use three-layered films, then the device can indicate a defect in any or all of those layers.

Internationally Accredited Thickness Measuring 

Employed as a weld protecting blanket, the coatings referenced here can be as thin as a few millimetres, but they can also be as thick as several centimetres. To ensure dry measurement accuracy, the measurement instruments talked about in the above paragraph possess advanced self-calibrating features. However, since we’re talking about internationally recognized engineering codes and standards, weld coating inspections can’t always rely on such features. For example, in an arctic wasteland, a pipe inspection can be impacted by temperatures that affect automatically calibrated instruments. There are zero test plates and calibration films that’ll confirm instrument accuracy.

And we thought the inspection phase was done after all the welds had been exhaustively inspected for application discontinuities. That’s not the case, not when the welds are exposed to tough environmental conditions. Coated with advanced, polymer-reinforced emulsions, there are wet and dry film thickness tests still to conduct.

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