Control of Penetrant
The quality of a penetrant inspection is highly
dependent on the quality of the penetrant materials used. Only
products meeting the requirements of an industry specification,
such as AMS 2644, should be used. Deterioration of new penetrants primarily results from aging and contamination. Virtually all organic dyes deteriorate over time, resulting in a loss of color or fluorescent response, but deterioration can be slowed with proper storage. When possible, keep the materials in a closed container and protect from freezing and exposure to high heat. Freezing can cause separation to occur and exposure to high temperature for a long period of time can affect the brightness of the dyes.
Contamination can occur during storage and use. Of course, open tank systems are much more susceptible to contamination than are spray systems. Contamination by another liquid will change the surface tension and contact angle of the solution. Water is the most common contaminant. Water-washable penetrants have a definite tolerance limit for water, and above this limit they do not function properly. Cloudiness and viscosity both increase with increasing water content. In self-emulsifiable penetrants,
water contamination can produce a gel break or emulsion inversion
when the water concentration becomes high enough. The formation
of the gel is an important feature during the washing processes,
but must be avoided until that stage in the process. Data indicates
that the water contamination must be significant (greater than
10%) for gel formation to occur. Most specifications limit water
contamination to around 5% to be conservative. Water does not readily mix with the oily solution of lipophilic post-emulsifiable systems and it generally settles to the bottom of the tank. However, the inspection of parts that travel to the bottom of the tank and encounter the water could be adversely affected.
Most other common contaminates, such as cleaning solvents, oils, acids, caustics and chromates, must be present in significant quantities to affect the performance of the penetrant. Organic contaminants can dilute the dye and absorb the ultraviolet radiation before it reaches the dye, and also change the viscosity. Acids, caustics, and chromates cause the loss of fluorescence in water-soluble penetrants.
Regular checks must be performed to ensure that the material performance
has not degraded. When the penetrant is first received from the manufacturer,
a sample of the fresh solution should be collected and stored
as a standard for future comparison. The standard specimen should
be stored in a sealed, opaque glass or metal container. Penetrants
that are in-use should be compared regularly to the standard
specimen to detect changes in color, odor and consistency. When
using fluorescent penetrants, a brightness comparison per the
requirements of ASTM E 1417 is also often required. This check
involves placing a drop of the standard and the in-use penetrants
on a piece of Whatman #4 filter paper and making a side by side
comparison of the brightness of the two spots under UV light.
Additionally, the water content of water washable
penetrants must be checked regularly. Water-based, water washable penetrants
are checked with a refractometer. The rejection criteria is different
for different penetrants, so the requirements of the qualifying
specification or the manufacturer's instructions must be consulted. Non-water-based,
water washable penetrants are checked using the procedure specified
in ASTM D95 or ASTM E 1417.
Application of the Penetrant
The application of the penetrant is the step of
the process that requires the least amount of control. As long
as the surface being inspected receives a generous coating of
penetrant, it really doesn't matter how the penetrant is applied.
Generally, the application method is an economic or convenience
It is important that the part be thoroughly cleaned
and dried. Any contaminates or moisture on the surface of the
part or within a flaw can prevent the penetrant material from
entering the defect. The part should also be cool to the touch.
The recommended range of temperature is 4 to 52oC (39