[charlie]

I am going to assume the client wants to keep any external protective coatings in place?

If such is the case would you know what the acoustic velocity of the coating and thickness is? As well as the acoustic velocity of the substrate you are needing to examine. There is a potential method but you may need to use a crawler system with an umbilical setup and it would be very dependant on access.

There may also be a radiographic solution to this issue. I have used it to approximate depth of corrosion on internal surfaces of sealed or difficult to examine components in aerospace.

Let us know the answers to the above and hopefully we can assist you in finding a viable solution