General Motors Company

Tin Zinc Plating for Grounding Applications - Issue 4; English
 N GMW3200

Annotation

This specification covers the requirements for tin-zinc electroplate on steel substrates, with provisions for supplementary treatment. This finish is intended for corrosion resistance and electrical grounding purposes.

Material Description. The material approved to this specification is a tin-zinc electroplate layer with passivation layer, plus a subsequent topcoat providing additional corrosion protection and consistent coefficient of friction.

Symbols. Not applicable.

Applicability. This finish is suitable for fasteners and other small parts designated for electrical grounding purposes. The applicable temperatures are from -50 °C to 150 °C, above which, the corrosion protection performance will degrade. Consult Materials Engineering for higher temperature applications.

Note: This coating shall not be used for applications exceeding 195 °C during manufacture or in service.

Remarks.

The global availability of this finish is low. GMW4700 ZnNi Type B Code G is a good alternative for grounding and corrosion protection.

Note: GMW4700 is not a drop-in replacement for GMW3200. Consult Materials Engineering for design and validation requirements.

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 new application.

This coating contains a protective topcoat for friction control and additional corrosion protection. For threaded fasteners, the assembly (tightening) process overcomes this topcoat and result in good electrical conductivity on approved finish materials. However, for non-threaded parts, the topcoat may decrease electrical conductivity. It is recommended that a study of the electrical conductivity of the part/joint be performed before releasing this finish on new applications. For special applications where the topcoat is undesirable, the finish may be specified as “NO TOPCOAT”.