Electrical and electronic devices have become integral components of modern living and convenience. They come in many shapes and forms and can be found in households, industrial plants and vehicles, to name just a few. Unfortunately, electronic devices that experience cyclic vibrations or thermal variations are susceptible to fretting corrosion where two metallic surfaces touch, such as in electrical terminals and contacts. Preventing the fretting motion is the best solution, but there are methods to prevent the onset of fretting corrosion where motion is unavoidable. (A discussion about preventing corrosion during the design and manufacturing stages can be found in the article Controlling Corrosion in Electronic Devices.)
Connectors and Terminals in Electrical Devices
Connectors and terminals are used to provide discrete connections between separate electrical subsystems. They allow for the flow of current within the system and are necessary to complete the electrical circuit. Discrete connections are necessary for a number of reasons including maintenance, upgrades, replacement part substitution and manufacturing convenience. However, when mated, connections should not produce unwanted or excessive electrical resistance that can result in power loss, signal distortion or component malfunction.
Connections between components naturally produce resistance called constriction resistance. Constriction resistance occurs because, on a microscale, all contact surfaces are inherently rough. This means that rather than a smooth surface, electrical current is transferred between contacts via very small contact spots (typically microns in diameter) called asperities. These small areas of contact through the asperity points are what give rise to the inherent resistance in the system. Although constriction resistance cannot be totally eliminated, it can be made worse by corrosion in and around the connection interface at the asperity points. The most common type of corrosion found in electrical contacts is fretting corrosion.
What is Fretting Corrosion?
Fretting corrosion is a type of motion-induced corrosion that results in a build-up of oxidized wear and debris when metallic surfaces in contact with each other are subjected to small (in the order of a few tens of microns) repetitive motions. These microscopic fretting movements can occur as a result of vibration, thermal expansion/contraction or mechanical and thermal shock.
Let us consider, for illustration purposes, two tin-plated electrical connectors in direct contact. When exposed to moisture and oxygen, tin will naturally form a protective oxide layer that is very hard and brittle. When contact is initially made, the tin oxide layer will crack under the applied loading due to its brittle nature, exposing the fresh underlying tin. Under the applied load, the underlying tin will force its way through the cracks in the tin-oxide layer to make contact at the surface, resulting in a stable electrical connection.
Should the contact interface move the new surface layer will become exposed to the atmosphere and quickly form another tin-oxide layer. Meanwhile, at the new contact location, the tin-oxide layer cracks to expose the underlying tin to make another stable connection. This cycle repeats each time there is movement in the contact location. Continued microscopic movement (fretting) will eventually result in the accumulation of insulating oxide debris, which can significantly reduce the number of effective contact areas (or asperity spots) and increase the contact resistance.
Increased contact resistance due to corrosion build-up can lead to poor performance and failure in a variety of electrical devices. In low current circuits, increased resistance can cause frequent drops and even interrupt the electrical signals. However, in high current circuits, these small increases can result in overheating, which can eventually lead to device failure.
Methods to Control Fretting Corrosion in Electrical Devices
There are a number of measures that can be taken to mitigate the effects of fretting corrosion, which fall into two approaches: fretting movement prevention and corrosion prevention.
Preventing Fretting Motions
One of the components required for the formation of fretting corrosion is movement. Restricting movement prevents exposing intact connector metal to the atmosphere and hence reduces oxide debris build-up. One of the most effective ways to achieve this is through the use of high contact forces between the contacts, which provides mechanical stability by increasing the friction between them and helps to counteract the fretting motions.
However, it should be noted that increasing the contact force could lead to enhanced wear and tribocorrosion of the contacts over time and effectively reduce the number of mating cycles before the connector is completely worn away. In addition, if the high contact forces are insufficient to restrict fretting movement, the rate of fretting corrosion and contact degradation may be increased. Therefore, it is important that the contact force being used is appropriate for the fretting movement expected.
Other means of preventing or minimizing fretting motions include using extra clamps or brackets to reduce connector movement, using seals to dampen vibrations and prevent air/oxygen intrusion, or by using engineering methods to adjust the relative movements between the contacting surfaces.
Preventing Fretting Corrosion between Electrical Connections
If fretting movements cannot be restricted, measures can be taken to prevent corrosion associated with these microscopic movements. The most common and effective method is to apply a lubricant between the contacting surfaces. Lubricants provide protection in two ways: first, by reducing the friction between the contacting surfaces, thus minimizing the rate of contact wear over time, and second, by acting as a barrier that prevents air and moisture from coming into contact with the electrical terminal or interface.
When using lubricants, potential issues may arise in areas with high concentrations of dirt or dust. Some lubricants have a tendency to retain dust particles that can add to the “roughness” of the interface and ultimately contribute to fretting corrosion.
Corrosion prevention can also be achieved through the use of gold or silver plated contacts and terminals. Gold and silver are both noble metals, which mean that they are naturally resistant to oxidation and corrosion. (For more information about noble metals, see An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.) Though highly effective, plated contacts can become worn after numerous mating cycles, once again leaving the connector susceptible to corrosion. Therefore plated contacts may require regular maintenance to ensure maximum serviceability.
Fretting corrosion is a common problem found in electrical terminals and contacts. There are a number of cost-effective preventative solutions that greatly outweigh the cost of replacement or repair. Determining the fretting driving force is vital; however it is important to choose an appropriate solution because an incorrect one may be ineffective or in some cases actually increase the rate of fretting corrosion.