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SECTION V PT PROCESS CONTROL | Up
TM-1-1500-335-23 Nondestructive Inspection Methods Manual | Next
Frequency of Process Control Checks |
T.O. 33B-1-1
1.5.1.3
Causes of Materials Degradation.
1.5.1.3.1
Materials Contamination.
Materials contamination is a primary source of degrading the performance of a penetrant system. There is a
number of contaminating materials and their effect on performance depends upon the type of material. Some
of the common contaminants frequently encountered are:
a.
Water is probably the most common type of contaminant. It can occur by careless or improper rinsing
or carry over from other parts.
b.
Organic materials such as paint, lubricants, oils, greases, and sealants are another source of
contamination. These materials, if not removed from parts during precleaning, can dissolve in the
penetrant and react with or dilute it, so that it loses some or all of its ability to function.
c.
Organic solvents such as degreaser f luid; cleaning solvent, gasoline, and antifreeze solution are
common types of contaminants. These materials dissolve in the penetrant and reduce its effective-
ness in proportion to the amount present. A small change in performance is usually not noticeable
(5% or less of the total volume). The method of entry into penetrant is usually carry-over on the inside
interior cavities of the part.
d.
Dirt, soil and other insoluble solids are carried into the penetrant, emulsifier, and developer as a
result of improper precleaning and carry-over from other parts. Another common source of soil
contamination occurs when the dwell stations are used to store parts. Most dwell stations have drain
pans, which return the eff luent back to the immersion tanks. Any soil falling from unclean parts into
the drain pan will be washed into the tank with the drain eff luent.
e.
Acid and alkaline materials are serious contaminants of penetrant solutions. They react with the
penetrant to destroy f luorescence brightness even when present in fairly small quantities. They are
usually residues from etching; plating or the cleaning processes.
f.
Penetrant is a normal contaminant of emulsifier in the postemulsifiable process. It can be carried in
on penetrant covered parts during the penetrant dwell step. As the penetrant builds up in volume, it
will gradually slow the emulsifying action, and if the level becomes high enough, the emulsification
process will stop.
1.5.1.3.2
Evaporation Losses.
Penetrant materials used in open tanks are continuously undergoing evaporation. The rate of evaporation is
increased with warmer temperatures and large tank surfaces. Evaporation losses of penetrant result in an
increase in viscosity, thus slowing penetration and emulsification. Evaporation of water washable penetrant
may slow or speed washability, depending on the penetrant formula. Evaporation losses in developer
solutions increase the concentration, which produces a heavier coating that may mask smaller indications.
Since evaporation losses take place very gradually, performance change may become significant before it is
noticed.
1.5.1.3.3
Heat Degradation.
Penetrants, especially f luorescent penetrants, are sensitive to elevated temperatures. Temperatures over
140oF (60oC) can reduce the f luorescence; and temperatures over 250oF (121oC) may destroy it completely.
High temperatures also speed evaporation of the volatile components of penetrants, causing undesired
performance changes. High temperatures in penetrants can occur from the following:
a.
Immersion of heated or hot parts.
b.
Inspection of surfaces exposed to the sun, such as f light line aircraft.
c.
Improper storage before being placed in use, such as storage in direct sunlight.
1.5.1.4
Process Degradation.
Not only do materials degrade, but equipment and procedures (other elements of the process) can deteriorate
also. Black light bulbs age and become dirty, reducing their output. Drying oven thermostats can be
improperly set or may malfunction, resulting in excessive temperatures which may cause critical procedures
to be performed incorrectly. Materials, equipment and procedures SHALL be periodically audited during
their service life to assure satisfactory process performance.
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