Processing Steps of a Liquid Penetrant Inspection
Surface Preparation: One of the most critical steps
of a liquid penetrant inspection is the surface preparation.
The surface must be free of oil, grease, water, or other contaminants
that may prevent penetrant from entering flaws. The sample may
also require etching if mechanical operations such as machining,
sanding, or grit blasting have been performed. These and other
mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.
Penetrant Application: Once the surface has been thoroughly
cleaned and dried, the penetrant material is applied by spraying,
brushing, or immersing the part in a penetrant bath.
Penetrant Dwell: The penetrant is left on the surface
for a sufficient time to allow as much penetrant as possible
to be drawn from or to seep into a defect. Penetrant dwell time
is the total time that the penetrant is in contact with the
part surface. Dwell times are usually recommended by the penetrant
producers or required by the specification being followed. The
times vary depending on the application, penetrant materials
used, the material, the form of the material being inspected,
and the type of defect being inspected for. Minimum dwell times
typically range from five to 60 minutes. Generally, there is no
harm in using a longer penetrant dwell time as long as the penetrant
is not allowed to dry. The ideal dwell time is often determined
by experimentation and may be very specific to a particular
Excess Penetrant Removal: This is the most delicate part
of the inspection procedure because the excess penetrant must
be removed from the surface of the sample while removing as
little penetrant as possible from defects. Depending on the
penetrant system used, this step may involve cleaning with a
solvent, direct rinsing with water, or first treating the part with an
emulsifier and then rinsing with water.
Developer Application: A thin layer of developer is
then applied to the sample to draw penetrant trapped in flaws
back to the surface where it will be visible. Developers come
in a variety of forms that may be applied by dusting (dry powdered),
dipping, or spraying (wet developers).
Indication Development: The developer is allowed to
stand on the part surface for a period of time sufficient to
permit the extraction of the trapped penetrant out of any surface
flaws. This development time is usually a minimum of 10 minutes. Significantly longer times may be necessary for tight cracks.
Inspection: Inspection is then performed under appropriate
lighting to detect indications from any flaws which may be present.
Clean Surface: The final step in the process is to
thoroughly clean the part surface to remove the developer from
the parts that were found to be acceptable.