Which type of particle is generally not used in magnetic particle inspection?

In the context of magnetic particle inspection (MPI), non-magnetic materials are generally not suitable for this method. MPI is a nondestructive testing technique used primarily for detecting surface and near-surface defects in ferromagnetic materials, such as steel and iron. The process relies on the principle that magnetic fields can be induced in ferrous materials, allowing the detection of discontinuities when magnetic particles are attracted to the areas of flux leakage created by these defects.

Non-magnetic materials, like aluminum or certain types of stainless steel, do not respond to magnetic fields and thus do not retain or attract the magnetic particles used in the inspection process. Therefore, any defects on these types of materials would go undetected if subjected to MPI.

The other choices listed in the question pertain to aspects of the magnetic particle inspection process: ferrous materials are essential for conducting effective MPI; fluorescent additives can enhance the visibility of detected particles under ultraviolet light; and dry magnetic powders are one of the forms of magnetic particles used in the process. These materials and methods contribute to the efficacy and reliability of magnetic particle inspection, making non-magnetic materials the clear choice for what is not used in this inspection method.