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