T.O. 33B-1-1
2.5.6
Removal of Penetrants with Solvent, Method C.
2.5.6.1
General.
All oil based penetrants are soluble in a large number of organic liquids. However, postemulsifiable
penetrants are most frequently used in Method C. As discussed in the note after paragraph 2.3.7.1, the
majority of solvent removers are Class 2 (non-halogenated), and they can be further subdivided on the basis
of their f lash points or boiling points. For almost all solvent removers, removal of the excess surface
penetrant is accomplished through dissolving and dilution. The exception to this is when an aqueous based
detergent mixture is used as a solvent remover. When higher boiling point solvents are used, care must be
taken to control the amount of solvent applied to the surface. Excess solvent can strip penetrant from defects
or dilute the penetrant in a defect with the result of producing dim, fuzzy indications.
2.5.6.1.1
The selection of a suitable solvent remover depends on a number of factors. The most significant factors are
the evaporation rate (boiling point), f lammability and cost. Solvency is a factor but becomes significant only
when the removal process allows excess solvent to remain on the surface of the part, thus diluting penetrant
that is trapped in defects. For smooth surfaces, high boiling points solvents can be used with minimal
concern since residual solvent can be easily wiped from the surface with a dry cloth. The higher boiling point
solvents are also less f lammable than the lower boiling point materials. For rougher surfaces, caution is
required with the use of the higher boiling point materials; the lower boiling point solvents may be more
appropriate since any residual solvent would evaporate before it could dilute the penetrant in a f law. With
the lower boiling point solvents, however, safety (f lammability) may be a concern. If that is the case, then the
halogenated solvent removers may be used although the cost will be high. HCFC 141b is one approved
halogenated solvent: it is found in several qualified products.
2.5.6.2
Application Procedure.
NOTE
The solvent-cleaner SHALL NOT be applied directly onto the inspection area
to remove excess penetrant.
The use of high sensitivity, postemulsifiable penetrant with the solvent removal method will produce
indications form small, tight f laws. However, improper application procedures will seriously degrade the
indications. The use of excess solvent will remove or dilute entrapped penetrant resulting in a failure to
produce a visible indication.
a. Following the penetrant dwell period, the surface SHALL be wiped with a clean, dry
rag or paper towel to remove the major portion of surface penetrant. The proper
procedure is to make only a single pass and then fold the rag or towel over to provide a
fresh 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 should
only be lightly moistened with the application of a mist of solvent to the cloth. The
cloth SHALL NOT be saturated either by immersion in liquid solvent or spraying on
excessive solvent.
c. A black light SHALL be used to examine the part surface during the intermediate and
final wiping stages. The surface of the rag SHALL also be examined with the black
light after the final solvent wipe. If the rag shows more than a trace of penetrant, it
SHALL be folded to expose a clean surface, remoistened with solvent, and again wiped
across 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 the
surface.
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