MIL-HDBK-1110
10.3.13.2 High Voltage Holiday Detectors. High voltage (up to
30,000 volts or more) holiday detectors are normally used on
coatings greater than 20 mils in thickness. The rule of thumb is
to use 100 volts per mil of coating. The exploring electrode may
consist of a conductive brush or coil spring. It should be moved
at a rate not to exceed the pulse rate of the detector. If a
holiday or thin spot in the coating is detected, a spark will
jump from the electrode through the air space or a thin area of
the coating to the metal. The resultant hole in the coating will
locate the holiday or thin spot that requires corrective action.
10.3.14
Adhesion Tester. There are two basic types of testing
for determining adhesion of coatings: the tape and the pull-off
test. The tape test is mostly used in the field, and the pull-
off test, in the laboratory. The tape test is most useful when
adhesion is low. Thus, it is often used to determine whether an
old coating has adequate adhesion to support another layer of
paint, or whether there is compatibility between coating layers.
This test cannot distinguish among good adhesion levels. The
pull-off test is more time consuming to perform since a "dolly"
or fixture must be glued to the surface of the coating. The test
measures the tensile force needed to remove the fixture. Pull-
off forces up to several thousand pounds per square inch can be
measured.
10.3.14.1 Tape Adhesion Test. In the tape test, ASTM D 3359, an
X or a lattice pattern is cut through the coating to the
substrate. Special pressure-sensitive tape is applied over the
cut and rapidly pulled off at an angle of 180 degrees. The cut
area is then examined for extent of deterioration. A kit is
available with a knife, chrome-plated steel template and tape for
performing the test.
10.3.14.2 Pull-Off Adhesion Test. In the pull-off test, ASTM D
4541, a metal dolly is bonded to a coated surface at a
perpendicular angle with an adhesive, usually a two-component
epoxy. After the adhesive has fully cured, a force is gradually
and uniformly applied to the dolly until it is detached from the
coating (or until the desired pull-off level is reached). One
type of pull-off tester has a hand wheel that is turned to apply
the force. The hand wheel/ratchet spanner is tightened until the
dolly is detached or a prescribed force is applied. Another type
applies the pull force pneumatically with compressed gas.
Machine application of pull produces more accurate results than
manual application. In both cases, care must be taken to make
sure the dolly and instrument are both aligned perpendicular to
the coated surface. A horizontal surface is preferred.
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