GMW 4700:2014

Material Description. This specification covers the basic requirements for barrel or rack electrodeposited zinc alloy over a ferrous substrate. Various alloy platings with supplemental treatments are covered by this specification. This specification covers functional requirements only. Cosmetic requirements, such as brightness, shades, tint, etc., are not defined. Material approved to this specification may not meet special cosmetic/appearance requirements and must be agreed upon by the purchaser and applicator. Table 1 lists the zinc alloy finishes covered by this specification.
For parts calling out this specification without a specific alloy (or to ZnNi without a Type A or Type B) but contracted and completing the Production Part Approval Process (PPAP) after January 1, 2014, the finish shall default to Type B. Parts completing PPAP or contracted with GM before 2014 may have ZnNi Type A or Type B.
Due to extremely low availability of applicators, do not specify ZnCo or ZnFeCo on new designs. For existing designs, consult with the responsible Design and Materials Engineers to locate sources or to re-specify the finish.
ZnFe finish applicators are also not widely available. Consult the responsible Materials Engineer before specifying ZnFe.
Symbols. Not applicable.
Typical Applications. Zinc alloy plating is especially suitable for small to medium ferrous parts, such as castings, stampings, clips, fasteners, and other ferrous parts, where levels of corrosion protection exceeding pure zinc are required.
Remarks.
Electroplating may not be suitable for all materials and applications. Each application must be reviewed with the responsible Materials Engineer before usage.
Use of this coating on threaded surfaces and/or bearing surfaces 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 any new application.
Coefficient of friction (CoF) values in this specification are determined at ambient temperatures. New applications for this finish should be tested to confirm performance under applicationspecific operating conditions.
These finishes are suitable for temperatures from -50 °C to 120 °C and to 150 °C for ZnNi. For applications outside this temperature range, consult Materials Engineering.
Zinc and zinc corrosion products may cause localized embrittlement when in contact with stressed Polyamide 6 or 66. Caution must be taken when specifying these finishes on parts in contact with Polyamide 6 or 66.
High-strength or high-hardness steel parts are susceptible to hydrogen embrittlement. While certain precautions and post-plate de-embrittlement baking are generally effective in protecting the parts from hydrogen embrittlement, they are time sensitive and require extra care during processing. Electroplating of steel parts with hardness above 46 HRc is not recommended. See 3.9 for precautions against hydrogen embrittlement.