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Introduction to Penetrant Testing

Introduction
History
Improving Detection
—Visual Acuity
—Contrast Sensitivity
—Eye's Response to Light

Principles
Steps for Liquid PI
Common Uses for PI
Pros and Cons of PI

PT Materials
Penetrant Testing Matl's
Penetrants
—Surface Energy
—Specific Gravity
—Viscosity
—Color and Fluorescence
   —Why things Fluoresce
—Dimensional Threshold
—Stability of Penetrants
—Removability
Emulsifiers
Developers

Methods & Techniques
Preparation
—Cleaning Methods
—Metal Smear
Technique Selection
Application Technique
Penetrant Removal
Selecting Developer

Quality & Process Control
Temperature
Penetrant
Dwell
Emulsifier
Wash
Drying
Developer
Lighting
System Performance Check

Other Considerations
Defect Nature
Health & Safety

References

Quizzes
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Quality Control of Emulsifier Bath

Quality control of the emulsifier bath should be performed per the requirements of the applicable specification. The information provided may not reflect the requirement of current specifications and is provided here for general education purposes only.

Lipophilic Emulsifiers

Lipophilic emulsifiers are miscible with penetrants in all concentrations. However, if the concentration of penetrant contamination in the emulsifier becomes too great, the mixture will not function effectively as a remover. AMS 2644 requires that lipophilic emulsifiers be capable of 20% penetrant contamination without a reduction in performance. AMS 2647A requires the emulsifier to be replaced when its cleaning action is less than that of new material.

Since lipophilic emulsifiers are oil-based, they have a limited tolerance for water. When the tolerance level is reached, the emulsifier starts to thicken and will eventually form a gel as more water is added. AMS 2644 requires that lipophilic emulsifiers be formulated to function adequately with at least 5% water contamination and AMS 2647A requires that lipophilic emulsifiers be replaced when the water concentration reaches 5%.

Hydrophilic Emulsifiers

Hydrophilic emulsifiers have less tolerance for penetrant contamination. The penetrant tolerance varies with emulsifier concentration and the type of contaminating penetrant. In some cases, as little as 1% (by volume) penetrant contamination can seriously affect the performance of an emulsifier. One penetrant manufacturer reports that 1 to 1.5% penetrant contamination will affect solutions with a 10% concentration of emulsifier. As the emulsifier concentration increases in the solution, the penetrant contamination tolerance also increases, and a solution with a 30% emulsifier concentration can tolerate from 5 to 8.5% penetrant contamination. The percentage of added penetrant required to destroy washability of the emulsifier can be measured.  An oil tolerance index is commonly used to compare the tolerance of different emulsifiers to contamination by penetrants. AMS 2647A requires that the emulsification bath be discarded if penetrant is noted floating on the surface or adhering to the sides of the tank.

Water contamination is not as much of a concern with hydrophilic emulsifiers, since they are miscible with water. However, it is very important that the emulsifier solution be kept at the proper concentration.

It should also be noted that penetrant dragout, and thus, level of possible emulsifier contamination by the penetrant is dependent on the type of material being processed. Tests have shown that on both polished and grit blasted surfaces, aluminum and stainless steel parts had a greater dragout than titanium parts.

Emulsifier Concentration and Contact Time

The optimal emulsifier contact time is dependent on a number of variables that include the emulsifier used, the emulsifier concentration, the surface roughness of the part being inspected, and other factors. Usually some experimentation is required to select the proper emulsifier contact time. The emulsifier used must be matched to the penetrant material. For method D penetrant systems, the concentration of the emulsifier should not exceed the percentage specified in the controlling specification or the supplier. Since the emulsifier is mixed with water, which is prone to evaporation, it is recommended that the starting concentration be less than that recommended by the supplier. One penetrant manufacturer recommends the following starting concentrations:

  • 20% if the maximum concentration is 30%
  • 13% if the maximum is 20%
  • 6.5% if the maximum is 10%.

Some Research on Emulsifier Concentration and Contact Time

Vaerman reported on the effect of emulsifier concentration on sensitivity. He varied the contact time of a lipophilic emulsifier and compared the results to those from a 5% concentrate hydrophilic emulsifier with a three minute contact time. For a normal contact time of 45 seconds, the lipophilic emulsifier was found to average a nearly 18% drop in sensitive over the range of crack depths (10 to 50 microns). The loss of sensitivity increased rapidly as the lipophilic contact time was increased in steps to five minutes. Also as expected, the decrease in sensitivity increased with increasing crack size.

Vaerman also looked at the effect of hydrophilic emulsifier concentration. It was found that increasing the concentration from 5% to 33% (by volume) resulted in a 15% decrease in sensitivity for a three minute contact time. When a contact time of one minute was used, the decrease in sensitivity was just over 9%.

Ref: -- Vaerman, J., Fluorescent Penetrant Inspection, Quantified Evolution of the Sensitivity Versus Process Deviations, Proceedings of the 4th European Conference on Non- Destructive Testing, September 1987, Pergamon Press, Maxwell House, Fairview Park, Elmsford, New York, Volume 4, pp. 2814-2823.

Hyam also reports on the effect of the emulsifier concentration and contact time. Both hydrophilic and lipophilic removers were tested. The results showed that as the concentration of the emulsifier was increased from 2.5% to 20%, sensitivity decreased. The contact time was shown to have little effect on the hydrophilic system tested (up to 20 minutes) but had a significant effect on the lipophilic system, with sensitivity decreasing as contact time was increased from two to ten minutes.

Ref:-- Hyam, N.H., Quantitative Evaluation of Factors Affecting the Sensitivity of Penetrant Systems, Materials Evaluation, February 1972, pp. 31-38.