T.O. 33B-1-12.5.6RemovalofPenetrantswithSolvent,MethodC.2.5.6.1General.All oil based penetrants are soluble in a large number of organic liquids. However, postemulsifiablepenetrants are most frequently used in Method C. As discussed in the note after paragraph 2.3.7.1, themajority of solvent removers are Class 2 (non-halogenated), and they can be further subdivided on the basisof their f lash points or boiling points. For almost all solvent removers, removal of the excess surfacepenetrant is accomplished through dissolving and dilution. The exception to this is when an aqueous baseddetergent mixture is used as a solvent remover. When higher boiling point solvents are used, care must betaken to control the amount of solvent applied to the surface. Excess solvent can strip penetrant from defectsor dilute the penetrant in a defect with the result of producing dim, fuzzy indications.2.5.6.1.1The selection of a suitable solvent remover depends on a number of factors. The most significant factors arethe evaporation rate (boiling point), f lammability and cost. Solvency is a factor but becomes significant onlywhen the removal process allows excess solvent to remain on the surface of the part, thus diluting penetrantthat is trapped in defects. For smooth surfaces, high boiling points solvents can be used with minimalconcern since residual solvent can be easily wiped from the surface with a dry cloth. The higher boiling pointsolvents are also less f lammable than the lower boiling point materials. For rougher surfaces, caution isrequired with the use of the higher boiling point materials; the lower boiling point solvents may be moreappropriate since any residual solvent would evaporate before it could dilute the penetrant in a f law. Withthe lower boiling point solvents, however, safety (f lammability) may be a concern. If that is the case, then thehalogenated solvent removers may be used although the cost will be high. HCFC 141b is one approvedhalogenated solvent: it is found in several qualified products.2.5.6.2ApplicationProcedure.NOTEThe solvent-cleaner SHALL NOT be applied directly onto the inspection areato remove excess penetrant.The use of high sensitivity, postemulsifiable penetrant with the solvent removal method will produceindications form small, tight f laws. However, improper application procedures will seriously degrade theindications. The use of excess solvent will remove or dilute entrapped penetrant resulting in a failure toproduce a visible indication.a. Following the penetrant dwell period, the surface SHALL be wiped with a clean, dryrag or paper towel to remove the major portion of surface penetrant. The properprocedure is to make only a single pass and then fold the rag or towel over to provide afresh surface for each succeeding wipe.b. When the surface penetrant has been reduced to a minimum with dry rags or towels,any residual penetrant is removed with a fresh rag or towel moistened with solvent.The amount of solvent applied to the rag or towel is critical. The cloth or towel shouldonly be lightly moistened with the application of a mist of solvent to the cloth. Thecloth SHALL NOT be saturated either by immersion in liquid solvent or spraying onexcessive solvent.c. A black light SHALL be used to examine the part surface during the intermediate andfinal wiping stages. The surface of the rag SHALL also be examined with the blacklight after the final solvent wipe. If the rag shows more than a trace of penetrant, itSHALL be folded to expose a clean surface, remoistened with solvent, and again wipedacross the part.d. This procedure SHALL be repeated until the rag shows little or no trace of penetrant.e. Finally the part be wiped with a clean dry rag to remove any residual solvent on thesurface.Change 12-61
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