How to inspect a coated weldment

Q: How can I inspect/verify welds on painted product without being destructive?

A: Coatings come in many varieties on fabricated products. Galvanizing, painting, and even oxidizing a weathering steel will have a negative impact on the weld inspection process. For visual inspection, which is a form of nondestructive evaluation (NDE), any coating on the finished weld has the potential to mask or cover rejectable discontinuities such as size, cracks, undercut, overlap, and porosity. Keep in mind, when we list defects by criticality, those that come to the surface generally top the list. So there is risk in completing a visual inspection on any coated product.

That being said, as a CWI I’m often asked to do visual inspection on coated product. I do, but I will always note that the weldment viewed was coated and that coating limited my ability to inspect it.

Types of Weld Testing

• Dye penetrant (PT) would not be effective in this case. With dye penetrants the discontinuity needs to come to the surface and the penetrant needs to enter the discontinuity through capillary action. A coating would not allow that to happen.
• Magnetic particle testing (MT) does have the potential to reveal subsurface discontinuities, but that ability is limited, and any coating will certainly affect its accuracy. MT will not work on aluminum or most stainless steel, as the material tested needs to be ferromagnetic.
• It’s not uncommon to perform ultrasonic testing (UT) on coated weldments. I’ve performed UT on thousands of galvanized and painted products successfully. You have to adapt your calibrations to allow for loss due to the coating, but those types of inspections are performed every day, at least in my world.
• Radiograph (RT) testing is another NDE method that would not be affected by most coatings. In fact, RT would probably be the least affected.

So there you are. Several NDE methods can be performed on coated products, but you need to allow for some adjustments that may be required. Your most effective form of NDE is visual inspection performed before, during, and after welding and prior to coating.

Ask me the time and I build you a watch.