Fretting Corrosion

Fretting Corrosion is corrosion damage at the interface between two contacting surfaces at least one of which is a metal that is caused by relative often oscillatory slip between the two surfaces. Vibration tends to be the cause of the oscillations. Continuous slip can also lead to fretting corrosion. Additional descriptive terms are wear corrosion and friction oxidation. The resulting surface is often discolored and, in the extreme, exhibits pits with fatigue cracks nucleating from them.

Mechanism

On a microscopic scale, each surface that composes the interface usually has peaks or gently sloping hills known as asperities and valleys as shown on this figure.

These asperities on each surface form the points at which the interfaces touch. When the surfaces move past each other, these asperities rub against the opposite surface. If there is a difference in hardness, the harder surface will tend to cause metal loss (flaking) on the softer surface resulting in fresh, non-oxidized metal to appear. If both surface have about equal hardness then flaking can occur on both surfaces. Most metals will re-oxidize and create an metal oxidation product. When an asperity rubs the same area as the surfaces move past each other, that oxidation product is removed, newly exposed metal from deeper in the metal reoxidizes. The pattern repeats itself as motion persists. In addition, asperities penetrating into the surface cause wear. Local temperatures can become high enough that metal particles stick to a surface (akin to a weld). Further motion causes these particles to dislodge. The asperities themselves can suffer elastic or plastic deformation depending on whether the local pressure on the asperity exceeds the plastic flow pressure. The result is often a groove in the softer metal accompanied by pits and possibly cracks.

Possible Circumvention

A number of measures have been found to decrease the risk of fretting corrosion. Some of these are as follows.

• Coating one of the contact surfaces with a very soft metal such as tin, silver, or indium
• Design of the contact so that slip between surfaces does not occur, load between surfaces is minimized, and oscillatory vibrations are minimized
• Use of elastomeric type of cements
• Use of water-based or oil-based lubricants especially those containing extreme pressure additives such as organic phosphates or organic sulfonates