T.O. 33B-1-13-203.2.4.1.2Paint or plating on the surface of a part has the effect of making a surface defect behave like a subsurface defect. Therelative thickness of the plating or paint film and the size of the defects sought, determine whether or not the coatingsshould 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 exceeds0.003 inch. Most coatings of cadmium, nickel or chromium are usually thinner than this, and the plating makes anexcellent background for viewing indications. Hot galvanized coatings are thicker than 0.003 inch, and in generalshould be removed before inspections. Broken or patchy layers of heavy scale or paint also tend to interfere by holdingpowder around the edges of the breaks or patches and should be removed if they are extensive enough to interfere withthe detection of discontinuities.3.2.4.1.3When preparing for contact testing, nonconductive coatings shall be removed from the contact areas.3.2.4.2 ConsiderationswhenusingTheWetSuspensionMPITechnique.In general the same requirements apply for the wet method as for the dry technique. Dirt, corrosion, loose scale orpaint and oil or grease should be removed. When preparing for contact testing, nonconductive coatings shall beremoved from the contact areas. The test surface should be free of contaminants that can dissolve into the inspectionbath. Such dissolved contaminants can become concentrated in a recirculating test bath and increase its viscosity, or itsbackground fluorescence, or both. This will gradually retard the forming of indications, and make fluorescentindications increasingly hard to see. Insoluble particulate contaminants on the surface, such as corrosion, sand, andgrit can accumulate in a recirculating wet bath to the point where they interfere with the formation and visibility ofindications, and force the bath to be discarded sooner than normal.3.2.4.2.1Further problems can arise with the wet method of inspection if surface contaminants are present. Moisture on the testsurface can be emulsified into an oil bath and cause the magnetic particles to coagulate and settle out of the bath, wherethey 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 causethe bath to peel off the surface, stripping any indications off with it. Second, the oil can be emulsified in an aqueousbath, and again coagulate the magnetic particles.3.2.4.3 PluggingandMasking.When it is possible for the inspection media to become entrapped or to damage components, plugging and/or maskingSHALL 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 readilyremoved by conventional cleaning and air blasting.3.2.5 PostInspectionCleaning.CAUTIONAll plugs and masks shall be removed after post-inspection cleaning.3.2.5.1 NecessityforPostInspectionCleaning.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 slightresidue cannot be tolerated, it must be removed. When its presence makes no difference, post-inspection cleaning canbe 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 muchfiner than the dry method magnetic particles (0.0002 inch instead of 0.002 inch to 0.006 inch in diameter), and they are
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