suspension magnetic particle inspection, more commonly known as wet
magnetic particle inspection, involves applying the particles
while they are suspended in a liquid carrier. Wet magnetic particle
inspection is most commonly performed using a stationary, wet,
horizontal inspection unit but suspensions are also available
in spray cans for use with an electromagnetic yoke. A wet inspection
has several advantages over a dry inspection. First, all of the surfaces
of the component can be quickly and easily covered with a relatively
uniform layer of particles. Second, the liquid carrier provides
mobility to the particles for an extended period of time, which
allows enough particles to float to small leakage fields to form
a visible indication. Therefore, wet inspection is considered
best for detecting very small discontinuities on smooth surfaces.
On rough surfaces, however, the particles (which are much smaller
in wet suspensions) can settle in the surface valleys and lose
mobility, rendering them less effective than dry powders under
Steps in performing an inspection using
Prepare the part surface - Just as is required with dry
particle inspections, the surface should be relatively clean.
The surface must be free of grease, oil and other moisture that
could prevent the suspension from wetting the surface and preventing
the particles from moving freely. A thin layer of paint, rust
or scale will reduce test sensitivity, but can sometimes be left
in place with adequate results. Specifications often allow up
to 0.003 inch (0.076 mm) of a nonconductive coating (such as paint)
and 0.001 inch max (0.025 mm) of a ferromagnetic coating (such
as nickel) to be left on the surface. Any loose dirt, paint, rust
or scale must be removed.
Apply the suspension - The suspension is gently sprayed
or flowed over the surface of the part. Usually, the stream of
suspension is diverted from the part just before the magnetizing
field is applied.
Apply the magnetizing force - The magnetizing force should
be applied immediately after applying the suspension of magnetic
particles. When using a wet horizontal inspection unit, the current
is applied in two or three short busts (1/2 second) which helps
to improve particle mobility.
Inspect for indications - Look for areas where the magnetic
particles are clustered. Surface discontinuities will produce
a sharp indication. The indications from subsurface flaws will
be less defined and lose definition as depth increases.