beneath the layer. For solvent removable penetrants, this can be done only by minimizing the amount of solvent used
during the removal process.
When penetrants are used in open tanks, it is inevitable that some water contamination will occur. Postemulsifiable
penetrants are inherently tolerant to water intrusion. Since they are oil based materials, any extraneous water will settle
to the bottom of the tank. Although their performance is not degraded, corrosion of the tank can occur. However,
water washable penetrants contain emulsifiers and will combine with water. They can tolerate the addition of small
amounts of water without losing their properties. The military procurement specification, MIL-1-25135E, requires
Method A penetrants to tolerate the addition of 5 percent of water, based on volume, without gelling, separating,
clouding, coagulating, or floating of water on the surface.
Mechanism of Fluorescence.
The mechanism of fluorescence involves two factors: the atomic structure of the fluorescent material and the energy
level or wave length of the radiation source. The basic component of all matter is the atom that consists of protons,
neutrons and electrons. The protons and neutrons form a positively charged nucleus or core, while the negatively
charged electrons circulate in orbits around the nucleus. The orbits are actually shells or rings of discrete energy levels
with a definite number of electrons in each shell. A material will fluoresce only if it has a certain atomic structure: the
energy holding the electrons in orbit in the outer shells must be low, and there must be vacant electron space in the
outermost shell. When a photon of electromagnetic radiation from an X-ray or ultraviolet light impacts an electron in
an atom of fluorescent material, the electron absorbs some of the photon energy and jumps from its natural shell to a
higher energy shell. The electron is unstable in this condition and immediately returns to its natural shell or orbit. In
returning to equilibrium, the electron releases its excess energy as electromagnetic radiation. The released
electromagnetic energy always has a longer wavelength than the exciting radiation. Thus, ultraviolet radiation with a
wavelength of 365 nm (nanometer, a unit of length) causes some fluorescing materials to release energy that has a
longer wavelength of 400 to 700 nm. This is the wavelength range of visible light. The human eye is most sensitive to
yellow-green light at approximately 510-560 nm in darkness. Most dyes are made to emit this range.
One of the more important factors responsible for the effectiveness of the penetrant process is the visibility of the
indication. Penetrants containing fluorescent dyes are not especially visible under white light. However, when
subjected to near ultraviolet (365 nm) radiation (UV-A or black light), the dyes emit visible light. Some dyes emit
more visible light per unit of black light energy than others. In addition, the amount of light given off is proportional
to the amount of dye in the penetrant. Brightness is a measure of the amount of visible light given off when fluorescent
dye is exposed to black light. It is controlled by the particular dyes efficiency in converting black light into visible
light and by the quantity of dye dissolved in the penetrant. High efficiency dyes are brighter than low efficiency dyes.
Fluorescent dyes lose their ability to fluoresce after prolonged exposure to black light. Resistance to this loss is termed
ultraviolet stability. ASM 2644, requires a diluted sample of fluorescent penetrant to retain a minimum brightness
after a one-hour exposure to 800 µW/cm2 (microwatts per square centimeter) of black light.
The term sensitivity, when used to describe a penetrant performance characteristic, is the ability to produce
indications from very small, tight cracks. This ability involves both penetrating ability and brightness. The flaw
opening in discontinuities is usually restricted, and the void volume is such that only a very small amount of penetrant
can be entrapped. The penetrant must enter and exit the flaw with enough dye to produce a noticeable indication.
The qualification test for sensitivity involves a comparison of the brightness of indications produced by a candidate
penetrant system (penetrant and emulsifier) versus the indications produced by a penetrant system designated as a
reference standard. The test panels for visible-dye penetrants are thermally cracked aluminum blocks. The test panels
for fluorescent-dye penetrants are a series of titanium or nickel alloy panels containing various sizes of laboratory
generated fatigue cracks. There is only one set of the latter qualification test panels, and it is not presently possible to