Re: PE Coating defeats If the coating is external, a simple straight beam test with an ultrasonic flaw detector and a contact transducer can usually find bubbles larger than 0.5 mm or so. Transducer selection will depend on polyethylene thickness -- try 10 MHz up to about 5 mm thickness, and 5 MHz up to about 25 mm.
If the coating is internal and there is access only from the outside, the test is significantly more difficult because of the poor acoustic impedance match between steel and polyeythlene and the numerous multiple echoes that will occur within the steel. If the PE is less than half the thickness of the steel, then any bubble echoes will occur between the first and second backwall echoes from the steel, but if the ratio is larger there will be difficulties. Testing from the ID with special fixturing is a possibility.