Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Corrosion Monitoring
Ultrasonic inspection can be used for the in situ monitoring of corrosion by
measuring the thickness of vessel walls with ultrasonic thickness gages. The
advantage of this method is that internal corrosion of a vessel can be
monitored without penetration.
There are, however, some disadvantages. Serious problems may exist in
equipment that has a metallurgically bonded internal lining, because it is not
obvious from which surface the returning signal will originate. A poor surface
finish, paint, or a vessel at high or low temperature may also complicate the
use of contact piezoelectric transducers (although this difficulty might be
addressed by noncontact in situ inspection with an EMA transducer).