Ultraviolet and Thermal Stability of Penetrant Indications
Exposure to intense ultraviolet light and elevated temperatures can have a negative effect on fluorescent penetrant indications. Fluorescent materials can lose their brightness after a period of exposure to high intensity UV light. One study measured the intensity of fluorescent penetrant indications on a sample that was subjected to multiple UV exposure cycles. Each cycle consisted of 15 minutes of 800 microwatt/cm² UV light and 2.5 minutes of 1500 microwatt/cm² UV light. Two penetrants were tested in the study, water washable, level 3 and a post emulsifiable, level 4. The results from the study showed that the indications from both penetrants faded with increased UV exposure. After eight exposure cycles, the brightnesses of the indications were less than one half their original values.
At an elevated temperature, penetrants can experience heat degradation or "heat fade." Excessive heat:
1. evaporates the more volatile constituents which increases viscosity and adversely affects the rate of penetration.
2. alters wash characteristics.
3. "boils off" chemicals that prevent separation and gelling of water soluble penetrants.
4. kills the fluorescence of tracer dyes.
This fourth degradation mechanism involves the molecules of the penetrant materials. The phenomenon of fluorescence involves electrons that are delocalized in a molecule. These electrons are not specifically associated with a given bond between two atoms. When a molecule takes up sufficient energy for the excitation source, the delocalized bonding electrons rise to a higher electronic state. After excitation, the electrons will normally lose energy and return to the lowest energy state. This loss of energy can involve a "radiative" process such as fluorescence or "non-radiative" processes. Non-radiative processes include relaxation by molecular collisions, thermal relaxation, and chemical reaction. Heat causes the number of molecular collisions to increase, which results in more collision relaxation and less fluorescence.
This explanation is only valid when the part and the penetrant are at an elevated temperature. When the materials cool, the fluorescence will return. However, while exposed to elevated temperatures, penetrant solutions dry faster. As the molecules become more closely packed in the dehydrated solution, collision relaxation increases and fluorescence decreases. This effect has been called "concentration quenching" and experimental data shows that as the dye concentration is increased, fluorescent brightness initially increases but reaches a peak and then begins to decrease. Airflow over the surface on the part will also speed evaporation of the liquid carrier, so it should be kept to a minimum to prevent a loss of brightness.
Generally, thermal damage occurs when fluorescent penetrant materials are heated above 71oC (160oF). It should be noted that the sensitivity of an FPI inspection can be improved if a part is heated prior to applying the penetrant material, but the temperature should be kept below 71oC (160oF). Some high temperature penetrants in use today are formulated with dyes with high melting points, greatly reducing heat related problems. The penetrants also have high boiling points and the heat related problems are greatly reduced. However, a loss of brightness can still take place when the penetrant is exposed to elevated temperatures over an extended period of time. When one heat resistant formulation was tested, a 20 % reduction was measured after the material was subjected to 163oC (325oF) for 273 hours. The various types of fluorescent dyes commonly employed in today's penetrant materials begin decomposition at 71oC (160oF). When the temperature approaches 94oC (200oF), there is almost total attenuation of fluorescent brightness of the composition and sublimation of the fluorescent dyestuffs.
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.
Muller, F.A. and Fantozzi, F.F., Advances in Improving the Heat-Fade Resistance of Fluorescent Penetrants, Materials Evaluation, July 1987, pp. 848-850.
Sherwin, A. G. and Holden, W. O., Heat Assisted Fluorescent Penetrant Inspection, Materials Evaluation, Sept. 1979, pp. 52-56, 61.
Robertson, A.J., Heat Stable Fluorescent Penetrants, Paper S2, 4th Pan Pacific Conference on Nondestructive Testing, Vol. 1, Parkville, Victoria, Australia, Australian Institute for Non-Destructive Testing, November 1983.
Lovejoy, D.J., The Importance of the Physical Nature of Fluorescence in Penetrant Testing, Reliability in Non-Destructive Testing: Proceedings of the 27th Annual British Conference on Non-Destructive Testing, London, UK, Pergamon Press, 1989, pp. 483-491.