The particles used in magnetic particle testing are made of magnetic materials, usually combinations of iron and iron oxides, that have a high permeability and low retentivity. Particles that have high permeability are easily magnetized by and attracted to the low-level leakage fields at discontinuities. Low retentivity is required to prevent the particles from being permanently magnetized. Strongly retentive particles tend to cling together and to any magnetic surface, resulting in reduced particle mobility and increased background accumulation.
Particles are very small and are various sizes.
Each magnetic particle formulation always contains a range of sizes and shapes to produce optimum results for the intended use. Elongated particles are included, particularly in the case of dry powders, because these rod-shaped particles easily align themselves with leakage fields not sharply defined, such as those that occur over subsurface discontinuities. Global-shaped particles are included to aid in the mobility and uniform dispersion of particles on a surface. Magnetic particles may be applied as a dry powder, or wet, by using either water or a high flash point petroleum distillate as a liquid vehicle carrier. Dry powder is available in various colors, so the user can select the color that contrasts best with the color of the surface upon which it is used.