Wednesday, July 16, 2014

Weld Inspection After Coating

Hi Paul,
I see your references on The website and am wondering; how I can inspect/verify welds on painted product without being destructive?

Best regards,
Eric D.

Hey Eric,
On fabricated products coatings come in many types. galvanizing, paint, even the oxidizing of a weathering steel will have negative effects on the inspection of welded products.  For visual inspection (a form of nondestructive evaluation) any coating on the finished weld has the potential to mask or cover rejectable discontinuities such as size, cracks, undercut, overlap, porosity, etc…  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 the inspection.
As for other forms of NDE…
Die penetrant (PT) would not be affective.  With die penetrants the discontinuity needs to come to the surface and the penetrent needs to enter the discontinuity through capillary action.  Any coatings would not allow that to happen.
Magnetic particle testing (MT) does have the potential to reveal sub-surface discontinuities but that ability is limited and any coating will certainly affect its dependability.  MT will not work on your aluminum products, the material tested needs to be ferromagnetic.
It’s not uncommon to do Ultrasonic testing (UT) on coated weldments.  I’ve performed UT on thousands of galvanized and painted products successfully.  Calibrations need to be adapted to allow for loss due to the coating, but those types of inspections are performed every day (at least in my world).
Radiograph (RT) is another nondestructive method that would not be affected by most coatings.  In fact, RT would probably be the least affected.
So there ya’ go.  There are several nondestructive methods of NDE that can be performed on coated products, there are simply some adjustments that may be required due to the coating.  But your most affective form of nondestructive testing is visual inspection performed before, during and after welding and prior to any coating.

Ask me the time and I build you a watch.

No comments: