Eddy-Currents can be used to inspect for hidden defects such as breaks, cracks, and some types of corrosion and pitting.
Here’s a simplified explanation: https://www.youtube.com/watch?v=oriFJByl6Hs
Here’s a commercial mfr’r of equipment demonstrating their product: https://www.youtube.com/watch?v=D5BnBzf2F_o