T.O. 33B-1-1
4-89
f. Check all measured values against the tolerances specified by the written procedure. All abnormal
values should be reported as required by the procedure.
4.8.2.18
Plating Thickness Reference Standards.
Reference standards for plating thickness measurements must have the same electrical conductivity, magnetic
permeability, and geometry as the part. These requirements apply to both the base material and the plating. Electrical
conductivity and magnetic permeability for the base material are usually obtained by using the same alloy and temper
for the standards as used in the part. Particular care should be taken in processing the materials to ensure that similar
properties are obtained. The surface finishes of the part and standard should also be alike. To obtain the same
electrical conductivity, magnetic properties, and surface finish for plating on the parts and reference standards; the
plating must be performed in baths of similar composition and subject to similar controls. If the plating on the part is
stress-relieved prior to thickness measurement, the references should receive the same treatment. Several methods of
determining plating thickness on reference standards can be used. One of these is to carefully measure the thickness
prior to plating and again after plating. The difference represents the thickness of the plating (plating is applied to one
side only). A second method is to measure the plating on an adjacent section of the standard by removal of a
rnetallographic specimen. The total thickness of the plating plus substrate must exceed the effective depth of
penetration in the part. A total thickness of 2.5 to 3 combined standard depths of penetration is usually considered
sufficiently thick. This thickness may be determined by adding the standard depth of penetration in the plating and the
substrate at the frequency used. For example, if approximately 0.003-inch thick sliver plating on aluminum is to be
measured at 200 KHz, the minimum total thickness can be determined as follows:
a. The standard depth of penetration of silver at a frequency of 200 KHz is 0.007 inch. Therefore, the
0.003-inch of silver in the plating represents 0.4 standard depth of penetration.
b. The 2024-T3 aluminum base material must be at least 2.5 - 0.4 = 2.1 standard depths of penetration.
If the conductivity and magnetic permeability of a metal are known, the standard depth of penetration can be
determined.
4.8.3
Measurement of Nonconductive Coatings.
4.8.3.1
Nonconductive Coatings.
A wide variety of nonconductive coatings are applied to military hardware. Primers, paints, and plastics and sealants
are widely used to protect metals from corrosion. Anodic coatings are used on metals, particularly aluminum, to
prevent surface deterioration. Other oxide coatings provide protection against heat or wear. Boron epoxy laminates
increase stiffness and strength. To control the thickness of such nonconductive coatings or to measure their loss during
service, eddy current inspection techniques have been employed with a high degree of accuracy.
4.8.3.2
Basis for Measurement of Nonconductive Coatings.
The determination of thickness of nonconductive layers or materials is a relative measure of the magnetic coupling
between the probe and the underlying conductive material. In other terms, the thickness of a nonconductor is a direct
measurement of lift-off or the spacing between the probe and the conductor. Because the properties (electrical
conductivity, magnetic permeability, and geometry) of the underlying materials affect the signal detected by the probe,
they must be constant or their variation minimized by instrument adjustment. Three requirements for measurement of
nonconductive coatings by eddy current techniques are: (1) the nonconductive coating must be in intimate contact with
a conductive material; (2) the thickness of the coating must be less than the effective range of the varying magnetic
field generated by the probe; and (3) the thickness of the substrate must be at least 2.5 times the standard depth of
penetration at the frequency employed.
