of Research on Cleaning Prior to LPI
The following is a summary of some of the articles that have
been published on the effects that some contaminants can have
on penetrant inspection and also what effects improper cleaning
methods and materials can have on the inspection.
Klein showed that when a test specimen was contaminated with cutting
oil, there was a reduction in sensitivity even when the specimen
was vapor degreased before inspection. The specimens used for
this study were quenched cracked 2024 aluminum blocks. The reduction
in sensitivity was believed to be the result of incomplete removal
of the cutting oil from the defects.
Klein warns that acid entrapment from a prepenetrant etch can
have disastrous effects on the penetrant inspection. The article
states that the sodium hydroxide caustic often used to etch aluminum
parts does not affect penetrants but that acids used to etch parts
of other materials do have an effect. Experts in the penetrant
field warn that caustics can in fact reduce penetrant brightness.
Careful cleaning of both acid and caustic etches before penetrant
inspection is highly recommended.
-- Kleint, R. E., Deception by Penetrants, Materials Evaluation,
Vol. 45, No. 7, July 1987, pp. 845-847, 850.
Residue from Previous Penetrant Inspections
Klein also found that the effectiveness of a penetrant was significantly
reduced if the part had been previously inspected with a different
penetrant, although proper pre- and post- cleaning (degreasing)
operations had been performed. This was the case when the previous
inspection was performed with a different fluorescent penetrant
or a visible dye penetrant. When the same penetrant system was
used for both inspections, there was no extensive loss in sensitivity
when the specimens received the required pre- and post-cleaning.
Klein's study also showed that even the most careful post-cleaning
operations leave some penetrant in the defects.
-- Klein, R. E., An Evaluation of the Effectiveness of Penetrants,
Nondestructive Testing, September- October, 1958, pp. 421-429.
Researchers from the Canadian Armed Forces studied the effects
of residual entrapped penetrants. The researchers concluded that
repetitive inspections produce greatly reduced indications when
pre- and post-cleaning operations are not performed properly.
-- Nielson, D. C. and Thompson, J. G. H., Evaluation of Liquid
Penetrant Systems, Materials Evaluation, Vol. 33, No. 12, December
1975, pp. 284-292.
Amos Sherwin revisits the issue in a 1990 Back to Basics
article in Materials Evaluation. The focus of this article
is on the effect of a previous visible penetrant inspection on
a fluorescent penetrant inspection. To illustrate the degrading
effect of type II penetrant on type I penetrant, Sherwin suggests
a simple experiment. He instructs to mix one percent visible and
99 percent fluorescent penetrant together and note the almost
complete lack of fluorescence under black light. Apparently, the
red dye acts as an UV filter and stops nearly all fluorescence.
The article also notes that soaking the test piece in isopropanol
for 10 minutes between the two inspections did provide some improvement
but did not result in acceptable performance.
-- Sherwin, A., Still a Good Rule: Visible Penetrant Inspection
Not to Precede Fluorescent, Materials Evaluation, Vol. 48, No.
12, December 1990, pp. 1457-1458.
Tanner, Ustruck, and Packman developed a procedure to accurately
measure the amount of penetrant absorbed into the cracks of a
chrome plated panel specimen. The procedure they used involved
applying penetrant to the sample and letting it dwell for a set
time. They then degreased the specimen using toluene in a closed
flask and used a colorimeter to measure the fluorescence of the
used toluene. Then, by using a very accurate pipette, they added
drops of penetrant to fresh toluene until the colorimeter value
matched that of the toluene used to degrease the sample. With
this very accurate method of measuring the amount of penetrant
absorbed, they showed that a small amount of solvent from the
precleaning operation, if left trapped in a flaw, can have a drastic
effect on the performance of a penetrant.
-- Tanner, R.D., Ustruck, R.E., and Packman, P.F., Adsorption
and Hysteresis Behavior of Crack-Detecting Liquid Penetrants
on Steel Plates, Materials Evaluation, September 1980, pp. 41-46.
In terms of cleaning penetrant system
performance check specimens and nonproduction parts, researchers
in the Netherlands evaluated the length of time required to clean
test specimens using an organic solvent bath with ultrasonic agitation.
Using specimens with fatigue cracks, five organic solvents were
tested. The specimens were considered cleaned of the penetrant
when no bleedout was detectable when the specimens were evaluated
12 hours after the cleaning operation. The solvents tested were
acetone, Freon, Chlorotene NU, Toluol, and MEK. For all solvents,
at least two hours of processing was required to properly clean
-- De Graaf, E. and De Rijk, P., Comparison Between Reliability,
Sensitivity, and Accuracy of Nondestructive Inspection Methods,
13th Symposium on Nondestructive Evaluation Proceedings, San
Antonio, TX, published by NTIAC, Southwest Research Institute,
San Antonio, TX, April 1981, pp. 311-322.
It is also important that the cleaning process itself does not
cause a reduction in penetrant sensitivity. Some chemical cleaning
processes have been found to plug defects. In many cases, chemical
cleaning alone does not adequately prepare the surface of a part
for inspection and mechanical cleaning methods must be employed.
These mechanical cleaning methods, such as grit, or other media
blasting, sanding, and even steam cleaning, have been shown to
cause metal smearing in some alloys.
Sam Robinson of Sherwin Inc. discusses an important cleaning consideration
in a paper titled "1,1,1-Trichloroethane Here Today, Gone
Tomorrow! Replacing 1,1,1-Trichloroethane in the Penetrant Process."
He cautions that some mild alkaline cleaners include sodium metasilicate
as an ingredient. Sodium metasilicate, sodium silicate, and related
compounds can adhere to the surface of parts and form a coating
that prevents penetrant entry into cracks.
-- 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.
Ward Rummel states that, based on his conversations with industry
experts, "silicates in concentrations above 0.5 percent may
be detrimental to subsequent penetrant inspection."
-- Rummel, W., Cautions on the Use of Commercial Aqueous Precleaners
for Penetrant Inspection, Materials Evaluation, Vol. 16, No.
5, August 1998, pp. 950-952.
Russian researchers have also found that the cleaning solution
can have an effect on the inspection results. They report that
after parts have been washed with cleaning liquids containing
a solution of domestic soap or oleic potash soap, some cracks are
no longer detectable. They attribute this reduction in sensitivity
to a clogging of the cavities and a reduction in wettability of
the metal surface by the penetrant. Several photographs are offered
that supports these claims.
-- Glazkov, Y.A., Some Technological Mistakes in the Application
of Capillary Inspection to Repairs of Gas Turbine 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.
In another article, the Russian researchers further investigated
the effects of cleaning and rinsing components with aqueous solutions
of commercial detergents (CDs) on the detectability of cracks.
They reported that some CD solutions improved crack detectability
while others impaired detectability. Some of the cleaning solutions
formed deposits in the cracks that were difficult to remove and
could prevent the formation of penetrant indications. To ensure
efficient capillary inspection of fatigue cracks in the vanes
of gas turbine engines, they recommended that components be thoroughly
rinsed in water with the aid of ultrasound and, if possible, dried
at 350-400C (661-751F) for components made of creep-resistant
nickel alloys, or at 140-170C (283-337F) for other alloys.
-- 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.