GMW GMW4700 PDF

This specification covers the basic requirements for barrel or rack electrodeposited zinc alloy plating over a ferrous substrate. Various alloy platings with supplementary treatments are covered by this specification.

Zn- Ni plating is available in two types: Type A and Type B. Zn- Ni Type B plating has higher corrosion resistance performance than Zn- Ni Type A. Zn- Ni Type B must have supplementary sealers on top of the conversion coating. The engineering drawing with GMW4700 Zn- Ni without any type will default to Type A. In case of Type A electrodeposits may be applied with or without supplementary sealers meeting the performance requirements. Table 1 lists the zinc alloy finishes 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 will have to be agreed upon by the purchaser and applicator.

Material Description.

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. This coating has no hexavalent chromate per the requirements of GMW3059.

Plating Composition.

The percentage of alloying element(s) present in the electrodeposits differs for each of the zinc alloys. The concentration limits for the various alloys are shown in Table 1. Caution: The availability of Zn- Co and Zn-Fe-Co are limited. Consult your local Material Engineering (Paint & Corrosion engineering for General Motors Europe (GME)) support to determine availability before specification on part prints.

Accurate control of alloy composition and alloy ratios within the limits given in Table 1 are essential for optimum corrosion resistance and plating performance.

Typical Application.

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. A suffix code G is to be used when specified on fasteners. Use of this finish on fasteners could affect the torque-tension relationship. It is recommended that a torque-tension study of the fastener joint be performed before use of this finish on fasteners.

Zinc Alloy plated parts or their corrosion products must not come in contact with stressed Polyamides 6 or 66.

Coefficient of friction/torque tension values in this specification are determined at ambient temperatures. New applications for this finish should be tested to confirm performance under application specific operating conditions.

Finishes conforming to the requirements of this specification can be used in the application range of -50ºC to 120ºC. For applications outside of this temperature range consult your local Material Engineering (Paint & Corrosion engineering for GME) support.