the unfavorable aggregate shapes, formed by simple agglomeration in suspension, will line up into magnetically held
elongated aggregates under the influence of local, low-level leakage fields. This effect contributes to the high
sensitivity of the fine particles comprising wet method materials.
Most ferromagnetic materials have fairly high densities. The densities of the materials in common use vary from
around 5 to nearly 8 times the density of water. Large, heavy particles will settle out of a suspension faster than either
smaller and/or lighter particles. This constitutes one more reason for requiring magnetic particles to be small. The
density of many ferromagnetic particles is lowered somewhat by compounding or coating them with pigment with
densities lower than that of the particles; with the obvious advantage of the particles remaining suspended longer than
uncoated particles. This is true of both the dry, pigmented powders, and the fluorescent particles in liquid suspension.
Magnetic particles used for magnetic particle testing should have as high a permeability and as low a retentivity as
possible. This is so they can be readily magnetized by the low-level leakage fields that occur in the vicinity of a
discontinuity and can be drawn by these fields to the discontinuity itself to form a visible indication. However, there is
little connection between permeability and sensitivity for magnetic powders. For instance, the iron-based dry-method
powders have permeabilities that are higher than the oxides used in the wet method. Yet a typical dry powder has less
ability in detecting the extremely fine surface cracks than the wet-method particles. This is because the higher
permeability is insufficient to overcome the handicaps of the other less desirable characteristics of the dry powders.
Unless all other factors are in the proper range for the application at hand, high permeability alone is of little value.
As a general principle, low coercive force and low retentivity are desirable properties for magnetic particles. If these
values were high in a dry powder, the particles would become magnetized during manufacture or in first use, and thus
become small, strong, permanent magnets. Once magnetized, their tendency to be controlled by the weak fields at
discontinuities would be overshadowed by their tendency to stick magnetically to each other and to the test surface.
This acts to reduce mobility of the powder, and also to form a high level of background that obscures defect indications.
Wet method particles that could become strongly magnetized because of high coercive force would also form this same
objectionable background. In addition, such particles would stick to any iron or steel in the tank or plumbing of an
inspection unit, and cause heavy settling-out losses that would have to be made up by frequent additions of new
particles to the bath. Another undesirable feature displayed by highly retentive wet method particles is their tendency
to clump together quickly in large aggregates on the test surface. Excessively large clumps of material have low
mobility and indications are distorted or obscured by the heavy, coarse-grained backgrounds. Therefore, particles
having high coercive force and retentivity are not desirable for wet method use either.
Both theory and experience have shown that low coercive force and retentivity are advantageous. But low does not
necessarily mean minimum or none. Dry powders with some residual magnetism appear more sensitive, especially in
the diffuse leakage fields formed by defects lying wholly below the surface. The reason may be that the small amount
of polarity established in weakly magnetized, elongated particles aids in lining them up into strings when the leakage
fields from discontinuities act upon them. The action is similar to that of the compass needle swinging in the very
weak field of the earth. Similarly, wet-method particles benefit from higher-than-minimum values of retentivity and
coercive force. These ultra-fine particles begin to collect at discontinuities as soon as they are applied to the test
surface, when the agitation, which had been present in the bath, ceases. With insufficient retained magnetism; the
particles remain fine and migrate very slowly through the liquid, due to the weak leakage fields, and the viscosity of the
liquid suspending medium. The indications of discontinuities will build up, but very slowly, taking as long as five to
ten seconds. On the other hand, if excessively magnetized particles are used; the test surface is covered with large
immobile clumps as soon as the bath is applied. Particles having intermediate magnetic properties collect into clumps
more slowly while the indications are forming. The leakage field, strongest at the actual discontinuity, draws particles
toward it, while the particles themselves are constantly enlarging due to agglomeration. At the same time they sweep
up the ultrafine particles as they move toward the defect. In this way all the magnetic fields present work together.