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Simultaneous Thickness and Electrochemical
Potential (STEP) Test
This is a destructive technique designed to measure both the thick-
ness and electrochemical potential of multiplex nickel products.
This test is exceptionally important for nickel-chromium coatings
which are typically subjected to harsh outdoor conditions.
A standard duplex nickel system includes a bright (non-sulphur
baring) and semi-bright (sulphur baring) nickel layer. An adequate
thickness and electrochemical potential between the layers causes
the bright nickel layer to corrode preferentially compared to the
semi-bright layer, improving the corrosion resistance of the
coating. This is why numerous automotive and aerospace specifi-
cations have strict STEP test requirements for duplex nickel. The
STEP test is an electrochemical stripping test, meaning the test
measures the anodic dissolution of the nickel under a constant
current. Based on the varying amount of sulphur content in the dif-
ferent nickel layers (i.e. semi-bright vs. bright), the time and
respective voltage required to dissolve each layer is unique and can
be translated into thickness measurements.
While this test is effective, especially for nickel applications, it is a
destructive test so samples used for this test are scrapped after
testing. This measurement method is restricted to duplex coatings
so the STEP test lacks wide applicability.

Eddy Current
Eddy current is a quick and reliable technique used to measure non-
conductive coatings such as aluminum oxides produced in anodizing
applications and paint layers applied in powder coating shops. An
eddy current thickness gauge is a non-destructive instrument that
measures non-conductive surfaces over non-ferrous substrates like
aluminum or copper. An eddy current probe runs alternating current
through a metallic coil which creates a magnetic field around the
probe. As the probe comes into contact with a nonferrous substrate,
the magnetic field induces a weak electrical current on the surface of
the substrate called an eddy current.
The resulting eddy currents produce their own magnetic fields,
which are detected by the probe. The eddy current gauge correlates the
intensity of the detected eddy currents to the thickness of the noncon-
ductive coating; the thicker the coating, the weaker the intensity. Eddy
current thickness gauges are reliable instruments in addition to being
inexpensive and simple to use.
However, eddy current gauges struggle to report precise measure-
ments when the base substrate is rough. The eddy current
induced on rough surfaces produces varying signals which con-
sequently limits the ability of the gauge to accurately report thick-
nesses. Substrates such as zinc die castings or sandblasted materials

STEP test.

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