Quality Control of Wash Temperature and Pressure
The wash temperature, pressure and time are three parameters that are typically controlled in penetrant inspection process specification. A coarse spray or an immersion wash tank with air agitation is often used. When the spray method is used, the water pressure is usually limited to 276 kN/m2 (40 psi). The temperature range of the water is usually specified as a wide range (e.g.. 10 to 38oC (50 to 100oF) in AMS 2647A.) A low-pressure, coarse water spray will force less water into flaws to dilute and/or remove trapped penetrant and weaken the indication. The temperature will have an effect on the surface tension of the water and warmer water will have more wetting action than cold water. Warmer water temperatures may also make emulsifiers and detergent more effective. The wash time should only be as long as necessary to decrease the background to an acceptable level. Frequent visual checks of the part should be made to determine when the part has be adequately rinsed.
Summary of Research on Wash Method Variables
Vaerman evaluated the effect that rinse time had on one high sensitivity water-washable penetrant and two post-emulsifiable penetrants (one medium and one high sensitivity). The evaluation was conducted using TESCO panels with numerous cracks ranging in depth from five to 100 microns deep. A 38% decrease in sensitivity for the water-washable penetrant was seen when the rinse time was increased from 25 to 60 seconds. When the rinse times of two post-emulsifiable penetrants were increased from 20 to 60 seconds, a loss in sensitivity was seen in both cases, although much reduced from the loss seen with the water-washable system. The relative sensitivity loss over the range of crack depths was 13% for the penetrant with medium sensitivity.
-- Vaerman, J., Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations, Proceedings of the 4th European Conference on Non-Destructive Testing, Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, September 1987, pp. 2814-2823.
In a 1972 paper by N.H. Hyam, the effects of the rinse time on the sensitivity of two level 4 water-washable penetrants were examined. It was reported that sensitivity decreased as spray-rinse time increased and that one of the penetrants was more affected by rinse time than the others. Alburger, points out that some conventional fluorescent dyes are slightly soluble in water and can be leached out during the washing processes.
-- Hyam, N. H., Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems, Materials Evaluation, Vol. 30, No. 2, February 1972, pp. 31-38.
Brittian evaluated the effect of wash time on a water-washable, level 4 penetrant (Ardrox 970P25) and found that indication brightness decreases rapidly in the first minute of wash and then slows. The brightness value dropped from a relative value of 1100 to approximately 500 in the first minute and then continued to decrease nearly linearly to a value of 200 after five minutes of wash. Brittian concluded that wash time for water-washable systems should be kept to a minimum.
-- Brittain, P.I., Assessment of Penetrant Systems by Fluorescent Intensity, Proceedings of the 4th European Conference on Nondestructive Testing, Vol. 4, Published by Perganon Press, 1988, pp. 2814-2823.
Robinson and Schmidt used a Turner fluorometer to evaluate the variability that some of the processing steps can produce in the brightness of indications. To find out how much effect the wash procedure had on sensitivity, Tesco cracked, chrome-plated panels, were processed a number of times using the same materials but three different wash methods. The washing methods included spraying the specimens with a handheld nozzle, holding the specimens under a running tap, and using a washing machine that controlled the water pressure, temperature, spray pattern and wash time. The variation in indication brightness readings between five trials was reported. The variation was 16% for the running tap water, 14% for the handheld spray nozzle and 4.5% for the machine wash.
-- Robinson, S. J. and Schmidt, J. T., Fluorescent Penetrant Sensitivity and Removability - What the Eye Can See, a Fluorometer Can Measure, Materials Evaluation, Vol. 42, No. 8, July 1984, pp. 1029-1034.