Yosi Bar-Cohen mentioned already in his message that you can find standards addressing certain components. Here in Germany we have a guideline called US1 'Thickness Measurement with Ultrasound' published by the German Society for NDT, however so far only in German available, but I heard that an English version is on the way. The handbook contains information on ultrasonic testing of corroded components. Also a proper probe characteristic is needed accordingly to the specific geometry of corrosion.
Examples are aircraft structures and pipes.