T.O. 33B-1-1
3-20
3.2.4.1.2
Paint or plating on the surface of a part has the effect of making a surface defect behave like a subsurface defect. The
relative thickness of the plating or paint film and the size of the defects sought, determine whether or not the coatings
should be stripped. The dry method is more effective than the wet method in producing indications through such non-
magnetic coatings. If fine cracks are suspected, the surface should be stripped of the coating if its thickness exceeds
0.003 inch. Most coatings of cadmium, nickel or chromium are usually thinner than this, and the plating makes an
excellent background for viewing indications. Hot galvanized coatings are thicker than 0.003 inch, and in general
should be removed before inspections. Broken or patchy layers of heavy scale or paint also tend to interfere by holding
powder around the edges of the breaks or patches and should be removed if they are extensive enough to interfere with
the detection of discontinuities.
3.2.4.1.3
When preparing for contact testing, nonconductive coatings shall be removed from the contact areas.
3.2.4.2
Considerations when using The Wet Suspension MPI Technique.
In general the same requirements apply for the wet method as for the dry technique. Dirt, corrosion, loose scale or
paint and oil or grease should be removed. When preparing for contact testing, nonconductive coatings shall be
removed from the contact areas. The test surface should be free of contaminants that can dissolve into the inspection
bath. Such dissolved contaminants can become concentrated in a recirculating test bath and increase its viscosity, or its
background fluorescence, or both. This will gradually retard the forming of indications, and make fluorescent
indications increasingly hard to see. Insoluble particulate contaminants on the surface, such as corrosion, sand, and
grit can accumulate in a recirculating wet bath to the point where they interfere with the formation and visibility of
indications, and force the bath to be discarded sooner than normal.
3.2.4.2.1
Further problems can arise with the wet method of inspection if surface contaminants are present. Moisture on the test
surface can be emulsified into an oil bath and cause the magnetic particles to coagulate and settle out of the bath, where
they are no longer available to form indications. Oil or grease can harm aqueous inspection baths in two ways. First,
their presence on the test surface can either prevent the bath from wetting and covering the entire surface, or can cause
the bath to peel off the surface, stripping any indications off with it. Second, the oil can be emulsified in an aqueous
bath, and again coagulate the magnetic particles.
3.2.4.3
Plugging and Masking.
When it is possible for the inspection media to become entrapped or to damage components, plugging and/or masking
SHALL be employed. Plug small openings and holes with hard grease or similar nonabrasive readily soluble material.
This prevents the accumulation of the magnetic particles and carrier liquid where it cannot be completely and readily
removed by conventional cleaning and air blasting.
3.2.5
Post Inspection Cleaning.
CAUTION
All plugs and masks shall be removed after post-inspection cleaning.
3.2.5.1
Necessity for Post Inspection Cleaning.
3.2.5.1.1
Particles.
The magnetic particle inspection process leaves behind at least a scattering of magnetic particles that are abrasive.
This may or may not be harmful to the later reuse of the part subjected to the inspection process. Where this slight
residue cannot be tolerated, it must be removed. When its presence makes no difference, post-inspection cleaning can
be eliminated. Dry magnetic particle inspection leaves only the particles behind. These particles are fairly coarse,
quite abrasive, and probably magnetically bonded to the test surface. The wet method magnetic particles are much
finer than the dry method magnetic particles (0.0002 inch instead of 0.002 inch to 0.006 inch in diameter), and they are
