What is fretting corrosion? The symptoms are often thought of as ‘gremlins’ in the circuitry. In automotive and passenger cars, an engine management warning light appearing on the the dashboard can be the result of open circuit resistance, caused by fretting corrosion. The electrical connectors in a vehicle appear static to the naked eye but are subject to micro motion/vibration abrasion due factors such as engine vibration (combustion engine), thermo cycling (thermal expansion and contraction) and/or road conditions. These micro-movements wear down the connector/contact metal coatings down to the base material that then becomes oxidised. As this oxide layer builds up and increases, the oxide film acts as an insulator between the contacts which creates an open circuit, resulting in voltage drop across the terminal and ultimately, power failure or signal loss.
Minimising fretting wear and keeping oxidation at bay in very important and this is where the application of a connector grease is key. Connector grease has two primary benefits:
- The grease reduces physical wear between the connecting surfaces as they undergo micro-motions and fretting wear. This helps to preserve the layered coatings on the connectors, designed to prevent oxidation and minimise resistance.
- They insulate the system from the surrounding environment, preventing the build-up of insulative oxide layers which are ultimately responsible for resistance increase and signal loss.
More about the automotive engine management warning light example…
It’s a classic example of fretting corrosion and/or oxidation issues. Typically you’ll take your vehicle to a dealership (if under warranty) or to a garage and the technician will plug in his diagnostic equipment. If the engine management warning light is displaying intermittently, you are at the mercy of fortune…is the alarm currently displaying? If not, no fault will be found. If the warning light is active then it is likely a circuitry problem will be identified. The technician will then begin a process of elimination to see which connector/terminal is causing the problem. Mostly likely a physical inspection is required which involves un-mating connectors to find the fault. Sometimes fretting is clearly seen, but quite often fretting corrosion is hard to see or not visible to the naked eye.
If the later is correct then the technician may simply un-mate connectors and re-mate after a negative inspection. In this situation the act of un-mating and re-mating the connector has temporarily cleared the fault; this is most likely to confuse the technician but the fault has been cleared, which is his/her only real concern. It is expected that the engine management warning light will reappear after a number of miles, which could be 500 miles or could be 5000 miles, it’s impossible to give a perfect example as the severity of fretting is variable due to a plethora of factors.
Consider that fretting corrosion in one vehicle or one device is annoying but manageable, but consider this kind of warranty issue in a high volume or mass produced piece of equipment? The warranty costs could be enormous.
Some examples of oxidation
Oxidation is a factor in fretting corrosion but can also be a stand alone problem, which is worth bearing in mind when devices are not in constant use.
Think of a time when you have not used a TV remote control for some weeks. When you first try to use the remote control it will not work. The average person will either knock the remote control on a hard surface or maybe even uncover the batteries, rotate them or pull the batteries out and re-insert them. All of these actions have the same effect of breaking up the oxide layer that has built up between the batteries and the battery contact…now the remote control will work again!
What about that torch that you haven’t used for ages that lives in the garage or kitchen draw?….Just like the remote control example, if not initially working, a sharp knock/bang on a hard surface usually helps. This is another example of oxide layers being scratched away, allowing current to flow again between batteries and contact surfaces.
Unlubricated vs. Lubricated Connectors
The difference in fretting wear between lubricated and unlubricated connectors is obvious. Our engineering partners built a custom fretting wear test apparatus that monitors the fretting cycles (micro-motion) of terminals until resistance increases 100 milliohms over the static baseline.
Why not just ‘crank up’ the contact force in the connector design to prevent fretting corrosion occurring?
It’s a fine balance that needs to be struck and can frustrate the most experienced engineer. Have the contact forces too high and the connector design tolerances too tight?…then you get problems with initial mating forces:
- The initial mating force is so high that the contact surface scars created are too deep and the oxidation cycles is stimulated – see below paragraph about the oxidation cycle. Which can also increase the chances of more minor vibration to develop into fretting corrosion.
- The contact forces are so high that it presents an assembly problem in terms of repetitive strain injury for production process individuals. We have seen this issue before. A well meaning design chance means the introduction of a lubricant to facilitate the initial connector mating, especially if the connector is physically large or delicate.
- The tolerances between connector plastics/connector housing can become too tight and can become a physical problem to mate, especially given a higher likelihood of ‘tolerance stack’ issues. If the connector expects to be separated during functional life, damage caused by un-mating and re-mating could be expected.
The possible above complications only add to the frustration of trying to negate fretting corrosion occurring in the first place. For the above reasons and other factors, fretting corrosion is often described as fretting wear phenomena.
Take a look at our article that goes into more detail – Electrical fretting corrosion can be prevented with a synthetic connector grease or go ahead and look at the article on the subject of choosing an electrical connector grease/lubricant.