What can be a consequence of having oil or rust on the surface during MPI?

Having oil or rust on the surface during Magnetic Particle Inspection (MPI) can indeed lead to false negatives. When the inspection surface is contaminated with oil, rust, or other debris, it can act as a barrier that prevents the magnetic particles from making proper contact with any existing surface defects. This contamination can obscure or hide defects from detection by the magnetic particles, which rely on the ability to interact with the magnetic flux leakage caused by those defects.

False negatives occur when a defect is present but not detected due to the interference of these contaminants. Therefore, surface cleanliness is crucial in ensuring accurate and reliable results in MPI, as it allows for optimal particle spread and adherence to any cracks or discontinuities present in the material being inspected.