Storage Temperature Stability.
Storage temperature stability is the ability of a penetrant to resist physical and chemical changes when stored in sealed
containers at appropriate temperatures. Penetrant materials, excluding dry developer, SHALL NOT be stored in direct
sunlight or at temperatures above 140°F (60°C) or below 0°F (-18°C).
Penetrant materials may cause deterioration and damage to materials that react to
Penetrant materials should not react with the materials to be inspected. It is necessary that the penetrant, emulsifier
and developer material be chemically inert relative to the parts being inspected. Most oil base materials meet this
requirement. However, water contamination of many oils may cause the mixture to become alkaline. This is one of the
reasons why water contamination must be avoided. While oily penetrant materials are generally inert to most metals,
there is no one material that can be formulated for all parts. Chemical reactivity of penetrant materials must be
considered whenever a new application is encountered. Some rubber (natural and synthetic) and plastic (transparent
and opaque) parts are susceptible to attack by the solvents and oils in the penetrant materials. Some metals can be
degraded at elevated temperatures by the trace amounts of sulfur or chlorine in conventional penetrants. Special low
sulfur and low chlorine materials are available and are discussed in Section 2.8, Special Purpose Materials, paragraph
Toxicity is the measure of adverse effects on humans resulting from contact with the material. It applies to any
abnormal effects ranging from nausea and dermatitis through dysfunction of major organs, such as the liver or kidneys.
It is essential that penetrant materials be nontoxic. In qualifying penetrant materials for the QPL, the manufacturer
must submit a certified statement identifying each ingredient in the product by a recognizable chemical or trade name.
This information is evaluated for toxicity by the USAF Occupational and Environmental Health Laboratory before the
material is listed as a Qualified Product.
The visibility of indications depends upon the fluorescent or visible dye dissolved in the penetrant oils. The oils used in
penetrants must have good solvent properties to dissolve and hold the dye in solution. It must maintain the dye in
solution under the wide range of temperatures encountered during transit and storage of the penetrant. If even a small
amount of separation occurs, recombination may be very difficult or not possible, resulting in decreased penetrant
This term describes two conflicting requirements for a penetrant: a) the ability to be removed from a surface leaving
little or no residual background and b) resistance to being removed from discontinuities. In order to meet the first
requirement, the penetrant must maintain the dyes in solution even when in the form of a thin film on the surface of a
part and without its more volatile components that have been lost during the dwell time. This requirement is more
difficult for water washable penetrants than postemulsifiable penetrants because the water washable penetrant does not
receive the additional solvent or surfactant of the emulsifier/remover during the removal process. The second
requirement is met by the penetrant resisting the removal process. For water washable penetrants and postemulsifiable
penetrants used with a lipophilic emulsifier, this is accomplished by the formation of a gel with the penetrant/water
mixture during washing that protects the penetrant in discontinuities from removal. For postemulsifiable penetrants
used with a hydrophilic emulsifier (Method D), the resistance is due to the lack of diffusion of the surfactants into the
surface penetrant layer, thus making only the thin surface layer emulsifiable and not the penetrant in discontinuities