The fretting corrosion of mild steel

Abstract

A quantitative investigation of the fretting corrosion of mild-steel specimens is described. Measurements have been made of the frictional forces, the degree of damage and the variations in the electrical contact resistance for a wide range of applied loads, vibration amplitudes and number of cycles of motion. In addition, the nature of the fretting corrosion scars and debris has been examined using optical and electron microscopy and electron diffraction. The same sequence of phenomena is observed under all conditions, namely, (i) the formation of intermetallic welds, (ii) the production of black α-Fe 2 O 3 particles and ultimately (iii) the production of fine red-brown α-Fe 2 O 3 particles. However, the magnitude of the frictional forces, the wear rates and the contact resistances are greatly dependent upon the amplitude of vibration. At large amplitudes large intermetallic junctions form soon after the onset of motion and the friction rapidly rises above its initial value. Subsequently the friction drops to a very low value, μ ~ 0·05; this is due to the presence of loose oxidized debris which accumulates and tends to roll between the rubbing contacts. At small amplitudes the scale of the welding is so reduced that no perceptible rise in friction occurs before the friction falls to its final low value. Measurements of the depths of damage in the scars show that the holes which form arise from the original welding mechanism and that they subsequently disappear. At large amplitudes the wear rates obey the same simple rules of wear as are obeyed in unidirectional motion, namely, the wear is proportional to the distance of sliding; the wear rate is proportional to the load and independent of the apparent area of contact. Further-more, there is close agreement in the magnitude of the wear rates in unidirectional motion and during fretting at large amplitudes. At small amplitudes, however, much smaller rates of damage are obtained.