This specification covers the basic requirements for barrel or rack electrodeposited zinc alloy plating over a ferrous substrate. Four (4) alloy platings are covered by this specification: zinc-iron, zinc-cobalt, zinc-nickel, and tin-zinc. The alloy platings are especially suitable for small to medium ferrous parts such as castings, fasteners, stampings, clips and other ferrous parts where levels of corrosion protection exceeding pure zinc are required.
Table 1 lists the alloy finishes covered by this specification.
Note 1: This finish is not recommended for applications where white corrosion is a cosmetic specification.
Material Description. The plating to this specification contains hexavalent chromate. This plating will be discontinued effective December 2005 or Model year 2006 parts whichever is earlier.
Note 2. This specification shall not be used on new designs. Use GMW4700 for zinc-nickel, zinc-cobalt and zinc-iron platings and GMW3200 for the tin-zinc plating on new designs or the substitute finish on carry-over parts. The surface lubricity of GMW4700 and GMW3200 finishes is different than that of GM6280M
Note 3: For GMNA conformance to the hexavalent chromate free requirements per GMW3059 is mandatory. For other GM regions the responsible Engineering group should be contacted if deviations are required for local programs
To distinguish between alloys, the chemical symbol of the alloying element must be added as a suffix to the specification number to designate the alloy desired. The zinc alloy electrodeposits covered by this specification require, and must be supplied with, a chromate conversion coating that is specifically formulated for each alloy. Exception, chromating is optional for parts that will ultimately be painted.
Example: GM6280M (Zn-Co) yellow designates zinc cobalt alloy with yellow chromate.
Yellow designates the desired chromate color. A chromate color shall be specified since the corrosion performance of the alloy is significantly influenced by the chromate color and type.