If the post-inspection cleaning is inadequate, the residues must be considered as contaminants during a subsequent
reinspection. Developer residues on the part surface will retain penetrant causing a high residual background that can
obscure valid indications. When retained in crevices, joints or faying surfaces, developer residues will cause false
indications. Developer residues also absorb and retain moisture and, if not dried, may cause corrosion of the part.
Penetrant residues, if not removed from discontinuities, will dry forming a varnish-like material in the flaw. This
entrapped residue may not fluoresce and will reduce or prohibit entry of penetrant during future tests of the part.
DOD prohibits the use of visible-dye penetrant on aircraft, engine, and missile
parts, except for those with specific engineering approval.
The red dye in visible-dye penetrant acts as a filter to black light radiation. When red dye residues mix with fluorescent
penetrant in a discontinuity, the fluorescent brightness can be reduced or destroyed. Visible-dye penetrant should not
be used if the part may be inspected with fluorescent penetrant at some later time. If a part has been previously
inspected with visible penetrant and requires reinspection, the reinspection should be done with visible-dye penetrant.
If fluorescent penetrant inspection must be performed to achieve the required sensitivity, special cleaning processes
should be used to insure removal of the visible penetrant residue of previous tests.
Introduction to Cleaning Processes.
The success of any penetrant inspection procedure depends upon the part surface and discontinuities being free of any
contaminants or soils that might interfere with the penetrant process. There are a variety of cleaning methods which
may be utilized. The methods are generic and are used principally for corrosion prevention and preparation of items
for surface treatments. There are no special methods used exclusively to prepare parts for penetrant inspection. Some
of the cleaning methods are discussed in the following paragraphs.
Some alkaline cleaning compounds will attack aluminum parts and components.
Care must be used in selecting the proper cleaning process for the materials to be
Alkaline cleaners are water solutions of chemicals which remove soils by a chemical action such as saponifying
(converting chemicals into soap) or displacement rather than dissolving the soils. Cleaners of this type usually have
components to aid in lifting the soils from the part surface. After displacement, the soil may be carried as a suspension
in the cleaner; it may separate; or, in the case of fatty soils, it may react with the cleaner to form water soluble soaps.
Alkaline cleaning is usually accomplished in immersion tanks with the solution at or near its boiling point. The
cleaning action is expedited by some type of agitation. Following alkaline cleaning, parts and components must be
thoroughly rinsed to remove any traces of the cleaning compound prior to penetrant inspection.