T.O. 33B-1-13-19Residues from cleaning processes can remain on the part surface and becomecontaminants. Paint removers can leave sticky residues that either trap particles orcontaminate 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 removecertain types of oils by saponifying (converting the oil to soap) or displacement. They can be used hotor cold, as a dip or as a spray.b. Solvent cleaners dissolve oil, wax, grease and some other contaminants. They can be applied byspraying, wiping, or dipping. Solvent cleaners are an efficient and practical means of removing lightpreservatives and soil from parts taken out of storage for magnetic particle inspection prior to use. Thisalso includes light soils that accumulate during transit and handling from the cleaning shop, but priorto being subjected to the inspection process.c. Mechanical methods, such a wire brushing or abrasive blasting, can be used to remove rust or othercorrosion deposits. These methods, if used improperly, can damage parts and conceal discontinuitiesand 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 inorganiccontamination and many organic contaminants from the test surfaces.f. Ultrasonic cleaning combines solvent or detergent cleaning with very vigorous mechanical action toloosen contaminants.3.2.4 SurfacePreparation.3.2.4.1 ConsiderationswhenusingTheDryPowderMPITechnique.In general, the smoother the surface of the part to be tested and the more uniform its color, the more favorable are theconditions for the formation and the observation of the powder pattern. This applies particularly to inspections beingmade on horizontal surfaces. For sloping and vertical surfaces, the dry powder may not be held on a very smoothsurface by a weak leakage field. The surface should be clean and dry and free of oil and grease. The dry particles willstick to wet or oily surfaces and not be free to move overthe surface to form indications. This may completely preventthe detection of significant discontinuities by obscuring the flaw indications with a heavy background. On surfaces thathave been cleaned of grease by wiping with a rag soaked in a high boiling point solvent, such as naphtha, a thin film ofunevaporated 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, ordusting the surface with chalk or talc from a shaker can, and then wiping the surface with a clean dry cloth. An initialapplication of the dry magnetic powder itself, followed by wiping, can also provide a surface over which a secondapplication of powder will move readily.3.2.4.1.1Any loose dirt, paint, corrosion or scale can be removed with a wire brush, by shot or grit blasting, or other means. Ifcleaning is done with shot or grit blasting, there is a peening effect, especially on softer steels, which may close up finesurface discontinuities. The effect is more pronounced with shot than with grit, but if these cleaning methods are usedthe operator should be aware of the danger of missing very fine cracks. A thin, hard, uniform coating of corrosion orscale will not usually interfere with the detection of any but the smallest defects.
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