previously, the particles that are used for magnetic particle
inspection are a key ingredient as they form the indications that
alert the inspector to defects. Particles start out as tiny milled
(a machining process) pieces of iron or iron oxide. A pigment
(somewhat like paint) is bonded to their surfaces to give the
particles color. The metal used for the particles has high magnetic
permeability and low retentivity. High magnetic permeability is
important because it makes the particles attract easily to small
magnetic leakage fields from discontinuities, such as flaws. Low
retentivity is important because the particles themselves never
become strongly magnetized so they do not stick to each other
or the surface of the part. Particles are available in a dry mix
or a wet solution.
Dry Magnetic Particles
magnetic particles can typically be purchased in red, black, gray,
yellow and several other colors so that a high level of contrast
between the particles and the part being inspected can be achieved.
The size of the magnetic particles is also very important. Dry
magnetic particle products are produced to include a range of
particle sizes. The fine particles are around 50 mm
(0.002 inch) in size, and are about three times smaller in diameter
and more than 20 times lighter than the coarse particles (150
mm or 0.006 inch). This make them
more sensitive to the leakage fields from very small discontinuities.
However, dry testing particles cannot be made exclusively of the
fine particles. Coarser particles are needed to bridge large discontinuities
and to reduce the powder's dusty nature. Additionally, small particles
easily adhere to surface contamination, such as remnant dirt
or moisture, and get trapped in surface roughness features. It should also be recognized that
finer particles will be more easily blown away by the wind;
therefore, windy conditions can reduce the sensitivity of an inspection.
Also, reclaiming the dry particles is not recommended because
the small particles are less likely to be recaptured and the "once
used" mix will result in less sensitive inspections.
particle shape is also important. Long, slender particles tend
align themselves along the lines of magnetic force. However, research
has shown that if dry powder consists only of long, slender particles,
the application process would be less than desirable. Elongated
particles come from the dispenser in clumps and lack the ability
to flow freely and form the desired "cloud" of particles
floating on the component. Therefore, globular particles are added
that are shorter. The mix of globular and elongated particles
result in a dry powder that flows well and maintains good sensitivity.
Most dry particle mixes have particles with L/D ratios between
one and two.
Wet Magnetic Particles
Magnetic particles are also supplied in a wet suspension
such as water or oil. The wet magnetic particle testing method
is generally more sensitive than the dry because the suspension
provides the particles with more mobility and makes it possible
for smaller particles to be used since dust and adherence to surface
contamination is reduced or eliminated. The wet method also makes
it easy to apply the particles uniformly to a relatively large
Wet method magnetic particles products differ from
dry powder products in a number of ways. One way is that both
visible and fluorescent particles are available. Most nonfluorescent
particles are ferromagnetic iron oxides, which are either black
or brown in color. Fluorescent particles are coated with pigments
that fluoresce when exposed to ultraviolet light. Particles that
fluoresce green-yellow are most common to take advantage of the
peak color sensitivity of the eye but other fluorescent colors
are also available. (For more information on the color sensitivity
of the eye...see the material on penetrant inspection.)
The particles used with the wet method are smaller
in size than those used in the dry method for the reasons mentioned
above. The particles are typically 10 mm
(0.0004 inch) and smaller and the synthetic iron oxides have particle
diameters around 0.1 mm (0.000004 inch).
This very small size is a result of the process used to form the
particles and is not particularly desirable, as the particles
are almost too fine to settle out of suspension. However, due
to their slight residual magnetism, the oxide particles are present
mostly in clusters that settle out of suspension much faster than
the individual particles. This makes it possible to see and measure
the concentration of the particles for process control purposes.
Wet particles are also a mix of long slender and globular particles.
The carrier solutions can be water or oil-based.
Water-based carriers form quicker indications, are generally less
expensive, present little or no fire hazard, give off no petrochemical
fumes, and are easier to clean from the part. Water-based solutions
are usually formulated with a corrosion inhibitor to offer some
corrosion protection. However, oil-based carrier solutions offer
superior corrosion and hydrogen embrittlement protection to those
materials that are prone to attack by these mechanisms.