T.O. 33B-1-13.3.6 Suspensions.CAUTIONThe use of water suspensions SHALL be carefully controlled to preventcorrosion and provide wetting of ferromagnetic aerospace components. Thisrequires weekly monitoring of corrosion inhibitor and wetting agentconcentrations.3.3.6.1WaterSuspensions.Wetting agents and corrosion inhibitors SHALL be used with water suspensions. Usually the magneticparticle concentrates provided include the correct amount of wetting agent and corrosion inhibitor for initialuse. However, these materials are available separately so the concentrations can be maintained or adjustedto suit the particular conditions. If no corrosion can be tolerated, a higher concentration of corrosion inhibitoris used. Acidity should be checked weekly. The pH of the water bath shall be 6 to 10. If the parts beinginspected have a residual solvent film, more wetting agent is required so the parts’ surfaces will becompletely wetted. Breaking of the bath into rivulets as it is applied over a part is an indication of a need foradditional wetting agent or part cleaning. A water break test shall be conducted daily using a clean specimenor part having the smoothest surface finish to be inspected. The specimen should be f looded with the bathand examined once f looding is stopped. If a smooth continuous film of bath forms over the entire surface,sufficient wetting agent is present. If the film breaks, wetting agent is inadequate if the part is not clean.Before adding wetting agent make certain the specimen or part is clean. Reference shall be made to themanufacturer’s recommendations for the correct quantity of wetting agent to be added.3.3.6.2PetroleumDistillateSuspensions.No additives other than the magnetic particles themselves are used with petroleum distillate suspensions.Petroleum distillate recommendations are included in other publications or specifications.3.3.7MethodsofParticleApplication.3.3.7.1DryParticles.Magnetic particles in dry form may be applied by hand, using rubber squeeze bulbs or plastic squeeze bottlesequipped with perforated caps, similar to an ordinary salt shaker but with smaller holes. The objective is tolay down a light cloud of powder on the part being inspected; this is usually accomplished by using acombination of bulb squeezing and tossing of the powder toward the area being inspected.3.3.7.2WetParticles.Many techniques are used to apply liquid suspension magnetic particles. They range from simple handpouring of the suspension onto a part to large industrial systems in which the suspension is appliedautomatically, either by dumping or spraying. The most common technique for application is through the useof a hand-held nozzle and recirculating pump on stationary units. Occasionally small, hand-held, lever-operated sprayers are used. Aerosol-type containers similar to those used for spray painting are alsoavailable.3.3.8TechniquesforCurrent/ParticleApplication.Two processing techniques are used in magnetic particle inspection. The approach to use in a given casedepends upon the magnetic retentivity of the part being inspected and the desired sensitivity of theinspection to be made. Highly retentive parts may be inspected using what is called the residual technique.The other technique, continuous, must be used on parts having low retentivity. For a given magnetizingcurrent or applied magnetizing field, the continuous approach offers the greatest sensitivity for revealingdiscontinuities.3.3.8.1ResidualTechnique.In the residual inspection technique, magnetic particles, either as a dry powder or in a liquid suspension, areapplied after the parts have been magnetized. This technique is used only when parts are magnetized withDC and when parts have sufficient retentivity to form and retain adequate magnetic particle indications atdiscontinuities. This technique can be used with both longitudinal and circular magnetization with eitherdirect contact or central conductor application. Usually it is limited to the search for discontinuities open tothe surface such as cracks. Residual inspection permits the magnetizing of parts at one time and theapplication of magnetic particle media at some subsequent convenient time. When a central conductor isused, inspection of holes or bores is facilitated since inspection takes place after removal of the centralconductor.3-26Change 1
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