Applying the Powder

T.O. 33B-1-1 3-62 weak leakage field. The surface should be clean, dry, and free of grease. The dry particles will stick to wet or oily surfaces resulting in reduced particle mobility. On surfaces that have just been cleaned of grease by wiping with a rag soaked in a petroleum distillate, a thin film of unevaporated solvent can remain that is sufficient to interfere with the free movement of the powder. This thin film can be removed by dusting the surface with chalk or talc from a shaker can, then wiping the surface with a clean dry cloth. An initial application of the dry magnetic powder followed by wiping often will give a surface over which a second application of powder will move readily. Vapor degreasing, if available; will give a dry, oil-free surface. 3.5.5.4.1 Any loose dirt, paint, rust or scale must be removed. If cleaning is accomplished 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 directed, the inspector should be aware of the danger of missing very fine cracks. A thin, hard, uniform coating of rust or scale will not usually interfere with the detection of any but the smallest defects. The inspector should be aware of the smallest size defect he is to consider, in order to judge whether or not such a coating of rust or scale should be removed. 3.5.5.4.2 Paint or plating on the surface of a part has the effect of making a surface defect appear like a subsurface one. 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 in producing indications through nonmagnetic coatings than the wet method, but if fine cracks are expected, the surface SHALL 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, and in general, should be removed before testing, unless only gross discontinuities are important. Broken or patchy layers of heavy scale also interfere with their tendency to mechanically hold powder around the edges of the breaks or patches, and SHALL be removed if they are extensive enough to seriously interfere with the detection of discontinuities. 3.5.5.5 Applying the Powder. A few rules for the application of dry powder will make the process of testing easier and more effective. The dry particles are heavier and individually have a much greater mass than the very fine particles of the wet method. If they are applied to the surface of a part with any appreciable velocity, the fields at the discontinuities may not be able to stop and retain them. This is especially true when vertical or overhead surfaces are being examined. The powder should reach the surface of parts as a thin cloud with practically zero velocity, drifting to the surface, so that leakage fields have only to hold it in place. For vertical and overhead surfaces, the fields must overcome the pull of gravity, which tends to cause the particles to fall away. Since the dry particles have a wide range of sizes, the finer particles will be held under these conditions, unless the leakage fields are extremely weak. On horizontal surfaces this problem is minimized. The usual mistake is to apply too much powder. Once on the horizontal surface of a part, the powder has no mobility (unless AC or HWDC is being used) and too heavy an application tends to obscure indications. If the part can be lifted and tapped, the excess powder will fall away and indications will be more readily visible. The excess powder can also be gently blown away with an air stream not strong enough to blow off magnetically held particles forming an indication. 3.5.5.6 Applicators. Various devices have been used to make proper powder application easy. One of the most widely used is shown in Figure 3-32. The squeeze bottle is light and easy to use. With some practice, by a combination of shaking as with a salt shaker, and a squeeze on the bottle, powder can be ejected with minimum velocity. Practicing with the bottle on a sheet of white paper will train the inspector to produce an even, gentle overall coverage. A powder gun or blower improves application, especially on vertical and overhead surfaces. The powder gun throws a cloud of powder at low velocity, much like a very thin paint spray. When held about one foot from the surface being inspected, a very light dusting of powder permits easy observation of the formation of indications. On horizontal surfaces the excess of powder is blown away with a gentle air stream from the blower. Two push-button valves on the blower gun control the flow of powder or clean air. Less powder is used with the gun, which helps to assure better inspection. A more elaborate gun-type powder blower has a motor-driven compressor integral with a powder container and air-powder mixer. A multichannel rubber hose connects to the gun. A work light is contained in the gun tip to illuminate the inspection area. A trigger