T.O. 33B-1-1
3-19
Residues from cleaning processes can remain on the part surface and become
contaminants. Paint removers can leave sticky residues that either trap particles or
contaminate recirculating baths.
Section 3 of Chapter 6 contains a detailed discussion of cleaning methods. The following is a brief summary.
a. Alkaline cleaners are non-flammable water solutions containing alkaline detergents that can remove
certain types of oils by saponifying (converting the oil to soap) or displacement. They can be used hot
or cold, as a dip or as a spray.
b. Solvent cleaners dissolve oil, wax, grease and some other contaminants. They can be applied by
spraying, wiping, or dipping. Solvent cleaners are an efficient and practical means of removing light
preservatives and soil from parts taken out of storage for magnetic particle inspection prior to use. This
also includes light soils that accumulate during transit and handling from the cleaning shop, but prior
to being subjected to the inspection process.
c. Mechanical methods, such a wire brushing or abrasive blasting, can be used to remove rust or other
corrosion deposits. These methods, if used improperly, can damage parts and conceal discontinuities
and should only be used as directed.
d. Paint removers can be a solvent, bond release agent, softening agent or combination.
e. Steam cleaning is a form of alkaline or detergent cleaning and can remove loosely bound inorganic
contamination and many organic contaminants from the test surfaces.
f. Ultrasonic cleaning combines solvent or detergent cleaning with very vigorous mechanical action to
loosen contaminants.
3.2.4
Surface Preparation.
3.2.4.1
Considerations when using The Dry Powder MPI Technique.
In general, the smoother the surface of the part to be tested and the more uniform its color, the more favorable are the
conditions for the formation and the observation of the powder pattern. This applies particularly to inspections being
made on horizontal surfaces. For sloping and vertical surfaces, the dry powder may not be held on a very smooth
surface by a weak leakage field. The surface should be clean and dry and free of oil and grease. The dry particles will
stick to wet or oily surfaces and not be free to move over the surface to form indications. This may completely prevent
the detection of significant discontinuities by obscuring the flaw indications with a heavy background. On surfaces that
have been cleaned of grease by wiping with a rag soaked in a high boiling point solvent, such as naphtha, a thin film of
unevaporated solvent often remains that is sufficient to interfere with the free movement of the magnetic particles.
This film can be removed by wiping the surface with a clean, dry cloth, flushing with a low boiling point solvent, or
dusting the surface with chalk or talc from a shaker can, and then wiping the surface with a clean dry cloth. An initial
application of the dry magnetic powder itself, followed by wiping, can also provide a surface over which a second
application of powder will move readily.
3.2.4.1.1
Any loose dirt, paint, corrosion or scale can be removed with a wire brush, by shot or grit blasting, or other means. If
cleaning is done with shot or grit blasting, there is a peening effect, especially on softer steels, which may close up fine
surface discontinuities. The effect is more pronounced with shot than with grit, but if these cleaning methods are used
the operator should be aware of the danger of missing very fine cracks. A thin, hard, uniform coating of corrosion or
scale will not usually interfere with the detection of any but the smallest defects.
