There are two basic classes of magnetic particles available for use, wet and dry. The wet method
particles use a liquid vehicle for suspension; the dry method particles are borne by air. Either water
or oil may be used as a vehicle for the wet method. The particles are colored to give good color contrast
with the surface being inspected. The wet particles are best suited for the detection of fine surface
cracks such as fatigue cracks. They are usually used with stationary equipment where the bath can
be reused until it becomes contaminated. For field applications, aerosol cans of magnetic wet bath are
available. Dry particles are more sensitive for detecting defects beneath the surface and are usually
used with portable equipment.
Particle Properties and Their Effects.
The particles used in the magnetic particle inspection process are finely divided ferromagnetic material,
usually combinations of iron and iron oxides. Properties of these particles include the size, shape, density,
magnetic properties, mobility and color. These properties may vary depending on the application.
It is self-evident that size plays an important part in the behavior of magnetic particles in a magnetic field,
which can be quite weak at a discontinuity. A large heavy particle is not likely to be arrested and held by a
weak field when such particles are moving over a part surface. On the other hand, very fine powders will be
held by very weak fields, since their mass is very small. Consequently, extremely fine particles may adhere
to the very weak leakage fields caused by acceptable surface and/or material variations. Particle size has a
profound effect upon mobility. (Refer to paragraphs 22.214.171.124.1 and 126.96.36.199.2.)
In general, for the dry powders, sensitivity to very fine defects increases as particle size decreases, but with
definite limitations. If the particles are extremely small, on the order of a few microns, they behave like a
dust. They accumulate and adhere even on very smooth surfaces. The particles will adhere at any damp or
slightly oily area, whether or not leakage fields exist. Extremely fine powders, though undoubtedly sensitive
to very weak fields, are not desirable for general use because they do leave a heavy, dusty background. In
some special applications, particles of a specific size range are used. For instance, where it is desired to
detect only rather large, coarse discontinuities, only large-sized particles are used. However, most dry
ferromagnetic powders used for detecting discontinuities are mixtures of particles in a range of sizes. The
smaller particles add sensitivity and mobility, while the large particles not only aid in locating large defects,
but also by a sort of sweeping action, counteract the tendency of the fine ones to leave a dusty background.
Thus, by including a wide size range, a balanced powder with sensitivity over most of the range of sizes of
discontinuities is produced.
Wet Method Visible Materials.
When the ferromagnetic particles are applied as a suspension in some liquid medium, much finer particles
can be used. The upper limit of particle size in most wet method visible materials used for magnetic particle
testing purposes is in the range of 20 to 25 microns (about 0.0008 to 0.0010 inch). Particles larger than this
are difficult to hold in suspension, and even the 20 to 25 micron sizes settle out of suspension rather rapidly
and are left behind as the suspension drains off. Such particles often line up in what are called drainage
lines to form a watermark that could be confused with indications of discontinuities.
In the case of the finer particles, the stranding due to the draining away of the liquid occurs much later,
giving the particles mobility long enough to reach the inf luence of leakage fields and accumulate to form the
indications. The minimum size limit for particles to be used in liquid suspensions is indeterminate.
Ferromagnetic materials commonly used include some exceedingly fine particles. In actual use, however,
particles of this size never act as individuals. Because they are magnetized in use, they become actual tiny
magnets. Under conditions of quiet settling in a suspension, these particles are drawn together as a result of
their retained magnetism to form clumps or aggregates of particles. These aggregations then tend to act as a
unit when they are applied to the surface of parts for magnetic particle testing. The speed and extent to
which this process takes place increases with the retentivity of the particle