Surface tension and wetting action are only two requirements of a penetrant. In addition to penetrating ability, a
satisfactory penetrant must resist removal from discontinuities when excess surface penetrant is removed, produce a
noticeable indication, and be practical and economical to use. Formulation, selection, and application of penetrant
materials require consideration of many physical and chemical properties. Some of these properties, other than surface
tension and wetting ability, are discussed in the following paragraphs.
Viscosity is a measure of a liquids resistance to a change in physical shape and is related to internal friction. Viscosity
varies with temperature, decreasing as the temperature is raised and increasing with lower temperatures. Viscosity has
no effect on penetrating ability. Some highly viscous fluids, such as molasses, have very good penetrating ability, while
some low viscosity liquids, such as pure water, have very poor penetrating ability. However, from an application
viewpoint, viscosity affects the speed with which a penetrant enters a discontinuity. Viscosity also determines how
much penetrant will remain on a part surface during the dwell period. High viscosity penetrants cling to the surface,
requiring increased effort for removal. Very thin penetrants (low viscosity) may drain from the part surface so quickly
that insufficient penetrant remains to enter into discontinuities.
Specific gravity is the ratio of the density of a substance to the density of distilled water at 40°F (4°C). This is also the
ratio of the weight of the substance to an equal volume of water. Specific gravity has no direct effect on the
performance of a penetrant. Most commercial penetrants have a specific gravity of less than one, primarily because
they are made up of organic materials having low specific gravities. For this reason, water contamination sinks to the
bottom of the penetrant tank.
Flash point is the temperature at which sufficient flammable vapor is given off a liquid to form an explosive mixture in
air over the liquid. The flash point does not affect the performance of a penetrant. High flash points are desirable to
reduce the hazard of fire. Penetrants and lipophilic emulsifiers meeting the requirements of ASM 2644 have a
minimum flash point of 200°F (93°C) if they are to be used in open tanks.
Volatility is characterized by the vapor pressure or boiling point of a liquid. It is associated with the evaporation rate of
liquids and it is desirable for penetrant materials to have a low volatility, i.e., a high boiling point. However, in the
case of petroleum products, viscosity increases as the boiling point goes up. In this group of materials, the lower
viscosity is preferred because they require less penetrating time. Still, for practical purposes, high volatility can be
avoided before viscosity becomes a problem. High volatility results in a loss of penetrant in open tanks. In addition, a
highly volatile material will dry on the part during the penetrant dwell, leaving a film that is difficult to remove.
Entrapped penetrant would also have a tendency to dry or lose its liquid properties, resulting in failure to bleed back out
of a discontinuity to produce an indication.
Fluorescent Dye Thermal Stability.
The dyes used in fluorescent-dye penetrants lose their brightness or color when subjected to elevated temperature. Loss
of brightness or color also occurs at moderate temperatures, but at a slower rate. This loss is termed heat fade. ASM
2644 specifies the maximum allowable brightness loss as a function of penetrant sensitivity. This test is performed
after a penetrant has been subjected to an elevated temperature. Thermal stability is an important consideration during
hot air drying before or after developer application.
Water Washable Penetrant Thermal Stability.
Thermal stability is the ability of water washable penetrants to resist physical changes under normal operating
conditions. ASM 2644 requires water washable penetrants submitted for qualification to be thermally cycled between
0°F and 150°F for 8 hours without separation or precipitation of constituents or major degradation in performance.