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Electroplated coatings |
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An electrolytically applied
coating shall be defined as a protective
metallic layer being deposited onto the surface
of metal articles by immersing these parts in an
aqueous solution through which an electrical
current is passed. |
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Layer thickness,
measuring point and measuring method |
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Because of the variations in
layer thickness on electroplated surfaces on
mechanical fasteners, the local layer thickness
is measured at a given spot considered
significant for the purpose of assessing the
protection against corrosion. |
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The layer thickness
can be measured by:
- direct determination in accordance with DIN
50933
- the jet method is accordance with DIN 50951
- the coulometric method in accordance with DIN
50955
- microscopic determination in accordance with
DIN 50950
In borderline cases the last method is
governing. |
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Screw thread
tolerances |
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The basis for the layer
thickness of electroplated coatings is given by
the tolerances for ISO-metric screw threads in
accordance with DIN 13 resp. ISO-unified screw
threads in accordance with ISO 5864 (ANSI B1.1)
prior to the electroplating that means tolerance
field g resp. 2A for bolts and screws and H resp.
2B for nuts.
The coating must not cause the zero line to be
exceeded, so bolts and screws have to meet the
go-gauge with tolerance field h resp 3A and a
measurable layer thickness can only be applied
to nuts on condition that the tolerance field H
resp 2B is not being fully utilized down to the
zero line. |
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Hydrogen
embrittlement |
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Due to the risk of
hydrogen-induced delayed brittle fracture bolts
and screws with a tensile strength Rm >= 1000
N/mm or a hardness >= 300 HV (F >= 98N) have to
be baked on 200 ± 100C as soon as possible but
within 4 hours after the coating process. This
is also mandatory for resilient (springy)
fasteners with hardness >= 400 HV (F >= 98N) |
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Passivation by
chromate treatment |
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This after-treatment has to
be carried out in accordance with DIN 50941 and
after baking. The protection against corrosion
is considerably increased by chromatizing. Out
of the different colours from bluish (white) to
black , yellow passivation is preferred. |
