When magnetic particles are applied over the surface of a magnetized part, they must move and gather at a
discontinuity under the influence of the leakage field to form a visible indication. Any factor that interferes with this
required movement of the particles will have a direct effect on the sensitivity of the powder and the test. Conditions
promoting or interfering with mobility are different for dry and wet method materials.
Dry powder should be applied in such a way that the particles reach the magnetized surface in a uniform cloud with a
minimum of motion. When this can be done, the particles come under the influence of the leakage fields while
suspended in air, and have three-dimensional mobility. This condition can be approximated when the magnetized
surfaces are vertical or overhead. When the particles are applied on a horizontal or sloping surface they settle directly
to the surface and do not have the same degree of mobility. Mobility can be achieved in this case by tapping or
vibrating the part, which jars the powder loose from the surface and permits it to move toward the leakage fields.
When AC, or half-wave rectified AC (pulsating DC) are used for magnetization, the rapid variation in field strength
while the current is on imparts a vibratory motion to the magnetic particles on the surface of the part. This gives the
particles excellent mobility for the formation of indications. The coatings applied to some of the dry-method powders
to give color to the indications also reduce friction between particles and the surface of the part, aiding mobility.
Wet Method Materials.
The suspension of particles in a liquid, which may be water or a petroleum distillate, allows mobility for the particles in
two dimensions when the suspension is flowed over the surface of the part, and in three dimensions when the
magnetized part is immersed in the suspension. Wet method particles readily settle out of suspension. To be effective,
the magnetic particles must move with the liquid and reach every surface that the liquid covers without settling out
somewhere along the way. Particles settle out of suspension at a rate that is directly proportional to their size and their
density and inversely proportional to the liquid's viscosity. While it must be balanced off against many other
properties, mobility is one of the factors which is important to wet method results. The viscosity of the suspension
medium is also important to mobility. In thicker liquids the magnetic particles migrate to the leakage field more
slowly. If the suspension liquid is too viscous and the magnetizing cycle too short, the indication may not form
adequately. As a practical rule for sensitive inspection, the viscosity of the suspension medium should not exceed 3
Visibility and Contrast.
These are important properties that have a great deal to do with making a magnetic powder suitable for its intended
purpose. Size, shape, and magnetic properties of a particle may be adequate, but if the indication is not visible to the
inspector the inspection fails.
Visibility and contrast are promoted by choosing colors of particles that are easy to see against the color of the surface
of the test part. The natural color of the metallic powders is silver-gray. The colors in the iron oxides commonly used
as the base for the wet method materials is limited to black and red. Visibility against certain colors can be increased
by coloring the powder particles in some way. By use of pigments the silvery iron particles are colored white, black,
red or yellow, all with comparable magnetic properties. One or another of these colors gives good contrast against the
surfaces of most of the parts that are tested. Among the dry powders, the gray-white powder gives good contrast
against the surfaces of many test parts. It fails to give good visibility, however, against the silver-gray of a sand- or
grit-blasted surface, or against bright machined or ground surfaces. Choice of colors must be made by the inspector to
provide the best possible visibility against the surfaces of the test part under the conditions of shop lighting that prevail.
Similarly, the choice of either the black or the red wet method material is made to suit particular lighting conditions.
In some cases it has been found advantageous to coat the part being tested with a color to improve contrast. Chalk or
whiting, in alcohol, has been used in the past for the inspection of large castings and weldments, when lighting
conditions were poor in the areas where the inspection was being conducted. Aluminum paint has been similarly used.