Mechanism of Fretting Corrosion

In oxidation wear, most of the metallic surfaces are initially protected from atmospheric oxidation by a thin adherent oxide film. When metals are in contact (under load) and subjected to repetitive weak movements, the oxide layer is broken at the level of asperities and it removes some of the oxide and the resulting metal is oxidized 

A more plausible approach could be a combination of the two approaches cited under wear-oxidation and oxidation wear with relative importance of one or the other depending on the particular system and therefore a function of medium, surface finish and the nature of the materials in contact. It is useful to note that oxygen accelerates corrosion by fretting, in particular, in ferrous alloys (17). 

Wear rate increases with slip amplitude over a range of amplitude. Wear debris can be plate-shaped, ribbon-shaped, spherical, and also irregularly shaped, on the basis of morphology. 

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Although the mechanism of fretting corrosion is only partially understood, it is well-known that fretting proceeds in three stages ... 

The general mechanism of fretting corrosion is shown in Figs 4.76 and 4.77. 

Figure 4.76 The mechanism of fretting corrosion at a steel surface (schematic view)...

Figure 4.77 Sketch illustrating the mechanism of fretting corrosion...

Fretting Corrosion This attack occurs when metals shde over each other and cause mechanical damage to one or both. In such a case, frictional heat oxidizes the metal and this oxide then wears away or the mechanical removal of protective oxides results in exposure of fresh surface for corrosive attack. Fretting corrosion is minimized by using harder materials, minimiziug friction (via lubrication), or designing equipment so that no relative movement of parts takes place. 

Mechanism. Basically, fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion between asperities and oscillatory movement causes ruptures, resulting in wear debris. Most commonly, fretting is combined with corrosion, in which case the wear mode is known as fretting corrosion. For example, in the case of steel particles, the freshly worn nascent surfaces oxidize (corrode) to FejO, and the... 

Modeling fretting corrosion. An equation has been used for steel to evaluate the loss of weight W caused by fretting corrosion based on a model that combines the chemical and mechanical effect of the corrosion by fretting. The chemical factor concerns the oxidation that occurs at the time of wear, corresponding to adsorption of oxygen to form the oxide. The mechanical factor concerns the loss of particles, at the asperities on the opposite surface. 

Fretting corrosion is a specialized form of erosion corrosion where two metal surfaces are in contact and experience very slight relative motion that causes damage to one or both surfaces. Again, in the presence of a corrodent, the movement causes mechanical damage of the protective film, leading to localized corrosion. 

FYom the multitude of intricate corrosion processes in the presence of mechanical action (friction, erosion, vibration, cavitation, fretting and so on) it is justified to touch upon corrosion types joined under a single failure mode induced by mechanical stresses. These are the stresses that govern the corrosion wear rate of metals during friction. Such processes are usually called corrosion stress-induced cracking in the case that the mechanical action is effective only in one definite direction, or otherwise termed corrosion fatigue in the case that compressive and tensile stresses alternate within cycles. In spite of the differences between the appearance of these corrosion types, they have much in common, e.g. fundamental mechanisms, the causes, and they overlap to a certain degree [19]. 

Mechanically assisted degradation can consist of the following types of corrosion erosion-corrosion, water drop impingement corrosion, cavitation erosion, erosive and corrosive wear, fretting corrosion, and corrosion fatigue (CF) (Fig. 1.14). Erosion-corrosion consists of the corrosion process enhanced by erosion or wear. Fretting corrosion consists of the wear process enhanced by corrosion. CF consists of the combined action of fluctuating or cyclic stress and a corrosive environment. 

There are other mechanisms such as fretting, fretting corrosion, and fretting CF, which is a combination of adhesive, corrosive, and abrasive forms of wear. Wear by all mechanisms except fatigue mechanism, occurs by gradual removal of material. One or more of these mechanisms may be operating in a particular machine. In many cases, wear may be initiated by one mechanism but may proceed by other mechanisms and thereby complicate failure analysis (60). 

For systems consisting of common materials (e.g., metals, polymers, ceramics), there are at least four main mechanisms by which wear and surface damage can occur between solids in relative motion (1) abrasive wear, (2) adhesive wear, (3) fatigue wear, and (4) chemical or corrosive wear. A fifth, fretting wear and fretting corrosion, combines elements of more than one mechanism. For complex biological materials such as articular cartilage, most likely other mechanisms are involved. 

Fretting corrosion is another phenomenon that occurs because of mechanical stresses, and, in the extreme, it may lead to failure by fatigue or corrosion fatigue. 

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