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of the surface layers of titanium alloys. The absorption of carbon into the surface layers of steel effectively lowers the
magnetic permeability. The solution of hydrogen and oxygen in the surface of the titanium alloy lowers the
conductivity of the surface. The amount of surface contamination can be measured by measuring the changes in
permeability and conductivity.
4.8.2.15
Effects Of Material Properties on Plating Thickness Measurements.
Although the depth of penetration of eddy currents in metals decreases with increasing electrical conductivity, lack of
penetration for measuring plating thickness is seldom a problem. Plating and coating thicknesses rarely exceed 0.005-
0.010 inch and in many instances are less than 0.003 inch thick. The sensitivity of inspection is controlled to a large
measure by the difference in conductivity and/or magnetic permeability between the base metal and the plating.
Coating or plating thickness measurement is considered feasible if the product of conductivity and permeability for the
base metal and the coating have a ratio of 1.5 or greater or 0.67 or less. Sensitivity increases as the difference in the
conductivity or permeability value between coating and substrate increases. Therefore, a rough determination of
sensitivity can be obtained from an impedance curve, which shows the positions of substrates and coating at the
frequency and probe size used for inspection.
4.8.2.16
Effect Of Test Conditions on Plating Thickness Measurement.
Normally, the frequencies employed for plating thickness measurement are relatively high, 100 KHz and greater. In
specialized equipment, frequencies as high as 6 MHz are available. These frequencies provide high sensitivities for
very thin coatings. As the Conductivity differences between plating and base metal decrease, the frequency may be
either increased or decreased as necessary, to obtain equivalent sensitivity for the thickness to be measured.
Considerable latitude from these approximate values may be exercised in choosing the actual operating frequency. If
doubt exists, a trial calibration curve should be prepared. To reduce the effects of surface roughness and variations in
nonconductive coatings, lift-off compensation (intermediate layer technique) should be employed. Generally, 0.002 to
0.003-inch lift-off compensation is sufficient unless very rough surfaces are present in the test area. An increase in
probe diameter and the use of air cores rather than ferrite cores has the effect of increasing measuring sensitivity and
extending the depth to which accurate plating thickness measurement can be performed.
4.8.2.17
Procedures for Plating Thickness Measurement.
A written procedure should be established for each application of eddy current inspection techniques for plating
thickness measurement. Each procedure should include the following steps:
a. Define the objective of the plating or coating thickness measurement. The type of base metal and
plating should be included in the procedure.
b. Clean any foreign material from the inspection area. Even though lift-off compensation is employed,
excessive build-up of foreign material in excess of lift-off adjustment could lead to significant errors.
c. Select the test system, instrumentation, and probe that will perform the thickness measurement to the
required accuracy.
d. Develop or verify calibration curve and standardize the test system using the specified standards. A
calibration curve must be available for each combination of instrument and probe
e. Perform plating thickness measurements at the designated points. The probe should be held against the
part with constant pressure. When available, spring loaded probes can be used to aid in maintaining
constant pressure. For curved surfaces, a fixture may be used to maintain the probe normal to the
surface. Plating thickness measurements should be made in areas where the readings are not affected
by adjoining structures, edges, or variations in total plating plus substrate thickness that are within the
effective limit of penetration. At least three readings should be taken at each measurement position to
ensure accurate and repeatable values. The calibration of the instrument should be periodically checked
against the standards to guard against instrument drift.
