This article considers the problem of galvanic corrosion on a connector system/contact interface and how it can be prevented or at least reduced.
What is galvanic corrosion?
A galvanic reaction is actually the creation of electrical current via chemical reaction. The result is the transfer/combining/corrosion of materials via the same reaction as in a battery.
During this process, ions of metal move from metal A (anode) to metal C (cathode) while electrons move from metal C to metal A, balancing the system/circuit. So a galvanic reaction requires two dissimilar metals, an electrolyte and a common electrical connection (a path for the ions and electrons to flow).
For electrical contacts, the typical scenario is:
1. Dissimilar metals are mated (tin plug mated to a gold receptacle or vice versa).
2. Waiting for a reactive environment.
3. An electrolyte is introduced (moisture, salt spray, flux, etc.).
4. A tiny electrical current begins to flow through the unpowered contacts.
5. Galvanic reaction begins in the contact interface (and electrolytic reaction once power is applied).
6. The materials corrode, forming resistive compounds.
7. Contact resistance increases as the materials corrode, eventually leading to contact failure.
Although the main thrust of this article is a discussion of the galvanic/electrolytic reaction in electrical contacts, the same principles hold true for fasteners, guide hardware, cable conductors and wherever dissimilar metals are in close proximity.
What is a galvanic cell?
A galvanic cell is similar to a battery and a plating cell containing an anode, cathode and electrolyte. The definition of a galvanic cell is a chemical reaction that produces electricity, like in a battery, while an electrolytic cell is a chemical reaction resulting from electrical stimulation like electroplating. One material is giving up electrons, while the other material is giving up ions.
So in an electrical circuit where there is current supplied, both reactions are taking place, both resulting in similar damage. Also called metal to metal corrosion or dissimilar metal corrosion, galvanic corrosion can also be compared to plating whereby one metal is transferred onto the surface of another (dissimilar) metal.
Examples from history
Back in the 1970s, an automotive manufacturer began using an aluminium bracket against a steel component. It was not long before they learned about putting steel against aluminium and then not being able to get them apart because they had galvanically corroded, and thus bonded together. The industry now uses an ‘anti-seize type of goop’ between the aluminium and steel. Mixing metallurgies was found to be unacceptable in this environment.
The aluminium wire implementation in houses during the 1960s and 1970s caused a number of house fires and was eventually done away with until the 1990s when a suitable process was finally developed for aluminium wire application. Aluminium and copper do not mix well without taking the appropriate steps to mitigate the galvanic reaction.
General susceptibility rules
The accepted limits as called out by NASA (NASA-STD-6012 and NASA-STD-6016) and the US Department of Defense (MIL-STD-889) have been established for many years and are repeated in many other documents.
In Table 1, some materials that are typically not encountered in the life of a connector system have been removed. If one is considering mating dissimilar metals, then the following classifications can be used in conjunction with the table to assess the system susceptibility to galvanic corrosion for various applications.
The numbers next to the environments represent the maximum safe difference between the voltages of the two materials being considered. The Anodic Index (AI) convention is used, where gold’s AI is zero volts.
As an example, when gold (0 V) is mated to Tin (0,65 V), the AI voltage difference is 0,65 V. As can be seen when comparing this gold-tin delta voltage to the application-specific limits, this combination would not be recommended for any applications according to the accepted standard criteria.
Application susceptibility classifications
The maximum delta AI voltage recommended for typical applications is as follows:
* Harsh environments (outdoor, military, etc.): 0,15 V.
* Normal environments (typical consumer products, uncontrolled indoor use): 0,25 V.
* Controlled environments (indoor, temperature and humidity controlled): 0,50 V.
Some of the metals listed in Table 1 may not be used directly as electrical contact material, but may be encountered during processing or during the life of the product.
How to stop galvanic corrosion
The most effective way of avoiding galvanic corrosion is to simply avoid mixing metallurgies. Failing that, the following methods can be employed:
Use an anti-seize compound
While anti-seize compounds are an effective way of preventing galvanic corrosion, many of the substances used may interfere with the electrical performance of the contact system and as such, this method of mitigation is not viable in some cases. Also, due to the wet nature of some of these substances, they may have the propensity to hold particulate contaminants such as dust, wear debris, etc.
Test the system
When mixing metallurgies, a proper testing protocol (aligned with the application) should be performed consisting of temperature/humidity cycling, steady state humidity with polarising voltage or possibly other environments that will exacerbate this reaction such as salt fog or even mixed flowing gas. The testing will depend on the application.
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