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,