Mechanism of Fretting Corrosion

A review of the facts suggests that the mechanism of fretting corrosion includes a chemical factor and a mechanical factor, with observed damage, in general, resulting from both. An asperity rubbing on a metal surface is considered to produce a track of clean metal which immediately oxidizes, or upon which gas rapidly adsorbs. The next asperity wipes off the oxide or initiates reaction of metal with adsorbed gas to form oxide. This is the so-called chemical factor of fretting. In addition, asperities dig below the surface to cause a certain amount of wear by welding or shearing action in which metal particles are dislodged. This is the mechanical factor of fretting. Metallic debris produced by fretting is thought not to oxidize spontaneously, as proposed by the molecular-attrition theory, but instead converts partially to iron oxide by secondary fretting action of particles rubbing against themselves or adjacent surfaces. This accounts for the fact that Fe2O3 is found by x ray to be the major corrosion product and metallic iron is present, if at all, in only very small amount. The quantitative expression for fretting corrosion derived on the basis of the foregoing model for relatively large values of load, frequency, and slip is W ( total ) = ( k 0 L 1 ⁄ 2 - k 1 L ) C f + k 2 l L C where W is the specimen weight loss, L is the load, C is the number of cycles, f is the frequency, l the slip, and k0, k1, and k2 are constants. The first two terms are the chemical factor of fretting corrosion and the third term is the mechanical factor. Accordingly, the equation predicts that fretting-corrosion weight loss is a hyperbolic function of frequency, is parabolic with load and linear with number of cycles or magnitude of slip. Furthermore, the terms concerned with a frequency effect disappear when the chemical factor is suppressed, as is observed when fretting tests are conducted in nitrogen. These conclusions are confirmed by the reported data. In addition, the calculated reaction-rate constant for oxidation of a freshly formed iron surface obtained from fretting data is reasonable and falls between two independently observed values. In our experiments, the chemical or corrosion factor falls within 6 to 78 per cent of the total observed fretting damage, depending on conditions of test. Remedial measures are outlined in light of the data and the proposed mechanism.