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
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 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
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
- 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
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.