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This specification covers the basic requirements for a black corrosion
protective coating which is
produced by the use of the bulk electrodeposition process or, if the
parts do not lend themselves
to barrel processing, the parts may be racked prior to the
electrodeposition process. The process
includes the application of a zinc phosphate coating, non-chromic acid
rinse or seal,
electrodeposited black organic coating, oven bake, water/oil emulsion
post treatment and air dry.
GM6047 Code G process includes a zinc phosphate coating, non- chrome
rinse or seal, cathodic
electrodeposited black organic with a torque modified post dip.
This process is particularly effective where thread build up of paint
in recessed areas is
undesirable and also where elongated recessed areas must be protected.
This coating is suitable for
bolts, nuts, clips and other small ferrous or nonferrous parts where
above normal corrosion
protection or adhesion, freedom from hydrogen embrittlement and
decorative black color are desired.
Parts that are not suitable for barrel processing may be racked but
must be processed through the
same black organic coating employed in the barrel process.
The coating system to this specification shall be free of hexavalent
chromate as per GMW3059
requirements. Refer www.gmw3059.com
Note: Only sources with hexavalent chromate free finishes will be
listed in the approved source
list for this specification in MATSPC database effective December
2005. The sources (chemical
suppliers and applicators) should contact the GMNA Finish Committee to
be approved for the
hexavalent chromate free finish.
Application. The organic coating is applied to properly zinc
phosphated small parts via a
rotatable, porous, electrodeposition barrel or, if the parts are not
suitable for barrel
processing, the parts may be racked. Parts from either coating process
are oven cured at 200°C
for a minimum of 18 minutes. Immediately following the cure, the parts
are immersed in a GM
approved protective oil compound in concentrations, temperatures and
immersion times specified by
the approved chemical manufacturer. GM6047M Code G will not use
protective oil for this
application. The process is completed when packaged or placed in
suitable confined containers.
Code G of this specification is recommended for fastener applications.
GM6047M Code G has separate
approved sources. Non-fastener applications will specify GM6047M
without any code, which is oily so
not recommended against plastic surface.
Use of this finish on threaded surface and/or bearing surface of
joints could affect the
torque-tension relationship. It is recommended that a torque-tension
study of the fastener joint be
performed before releasing this finish on a new application
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