Summary 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 the specimens.
-- 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. 83-87.