Issue 049, January 31, 2024

Wade A. Jensen, Ph.D., Senior Research Metallurgist

Noble metals make excellent electrical contacts, due to their low contact resistance and corrosion resistance. However, it is common for electrical contacts to be made of base metals such as steel, Ni, or Cu-based alloys. The more abundant base metals have lower intrinsic cost, but are more susceptible to corrosion. There are methods that can deposit a fine layer of noble metal onto the surface of the base metal, providing the benefits of nobility without the expense. One method of creating noble metal plating is through an electroplating process¹, Figure 1. For example, a Ni cathode and a Au anode are placed into an electrolytic solution and an electrical current is passed between them. This causes Au ions to migrate from the anode and towards the cathode, where they lose their charge and deposit themselves onto the cathode. Ideally, this process would create a uniformly thick and continuous Au coating, but imperfections in the process can produce defects that accelerate corrosion and shorten lifetime.

Figure 1: a) Diagram depicting a simple a simple electrolytic cell for coating Au on Ni.

Galvanic Corrosion – Galvanic corrosion occurs when metals, with different electric potentials, are in contact with an electrolytic solution. Because of this, noble metal coatings can accelerate base metal corrosion when gaps in plating are exposed to an electrolyte, Figure 2. This commonly occurs in everyday environments as humidity can condense in pores, scratches, and edges. In this galvanic cell, the base metal acts as the anode, the noble metal acts as the cathode and the electrolyte facilitates the transfer of electrons²,³. This results in accelerated and local corrosion for the anode and can significantly reduce contact lifetime.

Figure 2: Diagram of a galvanic couple caused by a pore in the Au plating and moisture.
Current flows flow from to (anode) Cu to Au (cathode).

Pore Corrosion – The corrosion that occurs in open pores is dependent upon the type of base metal and corrosive environment. There are two main pore corrosion categories; passive pores that form a corrosion barrier or active pores that continually corrode and are not self-limiting. Examples can be found in Figure 3: 1) ambient atmosphere forms a passivating Ni oxide layer that impedes further corrosion, 2) Cu oxide forms under plating and the corrosion spreads under the layer and can go undetected by visual inspection, 2) Fe oxide forms under the Au plating, but due to volumetric changes the oxide pushes the plating outward, and 4) Cu sulfide migrates out of the pore and deposits itself on the plating surface. All forms of active corrosion are deleterious to electrical resistance and eventually result in contact failure.

Figure 3: Examples of Au plating over base metal and resultant corrosion; a) passive oxide barrier forms on Ni plating, b) Cu oxide propagates under Au plating, c) Fe oxide pushes out Au plating, and d) Cu sulfide migrates out of pore and deposits on Au plating surface.

Abrasive Wear – Another common failure mode of plating does not arise from the electroplating process, rather it is caused by the repeated rubbing by another surface. Plate coatings are thin and each abrasive interaction removes a layer of plated material, this will eventually abrade through the plating and expose the base material. This can occur whenever contacts come into abrasive contact with another material, Figure 4.

Figure 4: Example of a sliding contact where the Fe probe tip has worn through the
Au plating and is not in free contact with the underlying Cu.

There are many potential failure modes for plating, and it is imperative that each part receive an equal and dense coating. Skilled Deringer-Ney engineers provide support for the selection of the proper plating process, the development of specifications for third party platers, and inspection procedures on plated parts. Deringer-Ney also provides high performance precious metal alloys that can be used in applications where plating does not provide the necessary performance. Contacts can be designed to utilize high performance Deringer-Ney alloys only where they are needed⁴; providing greater reliability, corrosion resistance, and minimizing precious metal content.

References:

1. Surface Engineering, in: ASM Handbook, 9th ed., 1983.
2. Corrosion, in: ASM Handbook, 9th ed., 1983.
3. W. Callister, 7 Dislocations and Strengthening Mechanisms, in: Materials Science and Engineering
   and Introduction, 7th ed., John Wiley and Sons, Inc., 2007: pp. 194–202.
4. G. Justice, Tech Brief 021 – Pathway to Savings with Precious Metals, in: Deringer-Ney Tech Brief,
5. https://deringerney.com/pathway-to-savings-with-precious-metals/.