Re: Clad Pipe - Metallurgically Bonded First, I assume that your total pipe thickness is under an inch or so with the cladding representing 25% or less of the total thickness.
With a properly set up precision thickness gage like a Panametrics-NDT Model 35 and an appropriate contact transducer, we can normally measure metal pipe wall thickness to an accuracy of +/- 0.001" or better. However in this sort of case the measurement accuracy is potentially affected by the small difference in sound velocity between the cladding and the base metal. The effective sound velocity of a two-metal sandwich is the weighted average of the velocities of the two metals. As the thickness of one or the other varies, changing the ratio between them, the composite velocity varies too. Because the accuracy of any ultrasonic thickness measurement is only as good as the velocity calibration, this introduces some uncertainty.
If you know the sound velocity of the cladding and the substrate, and the range of thickness for each, then you can easily calculate the worst-case uncertainty. Calculate the weighted average velocity of the sandwich, and then calculate the error that would result from measuring either of the two metals alone over the appropriate thickness range. That's your absolute worst case, which you're unlikely to ever see. As a practical matter, velocities of typical claddings like inconel and substrates like carbon steel alloys are within 5% of each other, and the substrate metal will always be the majority of your sound path, so in practice the potential uncertainly seldom exceeds a couple percent.
I'd have to know the specific thickness range involved to recommend an optimum transducer, but assuming a maximum thickness around an inch, a 5 MHz, 0.5" diameter contact transducer would be my first choice.
Corrosion gages and flaw detectors will be slightly less accurate than precision gages, but only on the order of +/- 0.002" with proper setup, so the potential velocity variation is still the limiting factor for accuracy.