of the most critical steps in the penetrant inspection process
is preparing the part for inspection. All coatings, such as paints,
varnishes, plating, and heavy oxides must be removed to ensure
that defects are open to the surface of the part. If the parts have
been machined, sanded, or blasted prior to the penetrant inspection,
it is possible that a thin layer of metal may have smeared across
the surface and closed off defects. It is even possible for metal
smearing to occur as a result of cleaning operations such as grit
or vapor blasting. This layer of metal smearing must be removed
Coatings, such as paint, are much more elastic than metal and
will not fracture even though a large defect may be present just
below the coating. The part must be thoroughly cleaned as surface
contaminates can prevent the penetrant from entering a defect.
Surface contaminants can also lead to a higher level of background
noise since the excess penetrant may be more difficult to remove.
Common coatings and contaminates that must be removed include:
paint, dirt, flux, scale, varnish, oil, etchant, smut, plating,
grease, oxide, wax, decals, machining fluid, rust, and residue
from previous penetrant inspections.
Some of these contaminants would obviously prevent
penetrant from entering defects, so it is clear they must be removed. However, the impact of other contaminants
such as the residue from previous penetrant inspections is less
clear, but they can have a disastrous effect on the inspection.
Take the link below to review some of the research that has been
done to evaluate the effects of contaminants on LPI sensitivity.
Click here to learn more about
possible problems with Cleaning Practices.
good cleaning procedure will remove all contamination from the
part and not leave any residue that may interfere with the inspection
process. It has been found that some alkaline cleaners can be
detrimental to the penetrant inspection process if they have silicates
in concentrations above 0.5 percent. Sodium metasilicate, sodium
silicate, and related compounds can adhere to the surface of parts
and form a coating that prevents penetrant entry into cracks.
Researchers in Russia have also found that some domestic soaps
and commercial detergents can clog flaw cavities and reduce the
wettability of the metal surface, thus reducing the sensitivity
of the penetrant. Conrad and Caudill found that media from plastic
media blasting was partially responsible for loss of LPI indication
strength. Microphotographs of cracks after plastic media blasting
showed media entrapment in addition to metal smearing.
It is very important that the material being inspected has not been smeared across its own surface during machining or cleaning operations. It is well recognized that machining, honing, lapping, hand sanding, hand scraping, grit blasting, tumble deburring, and peening operations can cause some materials to smear. It is perhaps less recognized that some cleaning operations, such as steam cleaning, can also cause metal smearing in the softer materials. Take the link below to learn more about metal smearing and its affects on LPI
Click here to learn more about metal
Robinson, Sam J., Here Today, Gone Tomorrow! Replacing Methyl
Chloroform in the Penetrant Process, Materials Evaluation, Vol.
50, No. 8, August 1992, pp. 936-946.
Rummel, W., Cautions on the Use of Commercial Aqueous Precleaners
for Penetrant Inspection, Materials Evaluation, Vol. 16, No. 5,
August 1998, pp. 950-952.
Glazkov, Y.A., Some Technological Mistakes in the Application
of Capillary Inspection to Repairs of Gas Turbin Engines, translation
from Defektoskopiya - The Soviet Journal of Nondestructive Testing,
Vol. 26, No. 3, New York, NY Plenum/Consultants Bureau, January
1990, pp. 361-367.
Glazkov, Yu . A., Bruevich, E.P., and Samokhin, N.L, Special
Features of Application of Aqueous Solutions of Commercial Detergents
in Capillary Flaw Inspection, Defektoskopiya - The Soviet Journal
of Nondestructive Testing, Vol. 19, No. 8, August 1982, pp. 83-87.