A FUNDAMENTAL STUDY OF THE FRICTION AND CORROSIVE WEAR OF AUSTENITIC STAINLESS STEELS IN SODIUM CHLORIDE SOLUTIONS

The interrelation between friction and corrosive wear was investigated in a system composed of AISI 304 stainless steel sliding against a sintered alumina plate in a 3% NaCl aqueous solution. The specially designed corrosion-friction machine was interfaced with a computerized PAR 350-1 corrosion system. The sliding velocity varied between 0.05 and 0.35 m/sec while the load was 4 N. It has been determined that a film potentiostatically grown on the surface of the specimen, at a potential well within the passive range reduces the coefficient of friction with respect to that obtained under dry conditions. The corrosion current increases and the corrosion potential becomes more active as the sliding speed is increased. The friction coefficient, however, is independent of velocity up to about 0.25 m/sec. At higher speed, the coefficient of friction increases, presumably because of direct contact between the bare stainless and the counterface. The corrosive wear is best described by a galvanic couple concept. That is, a galvanic cell is established between the slide and non-slide region of the stainless. The slide area is the active (anode) metal and the non-slide area (cathode) is the passive metal. Any passive film present is continuously thinned, and in some cases, completely removed with increasing sliding velocities. The slide area exhibits no plastic deformation. Pits that repassivate in the non-slide region remain active and subsequently grow in the slide region. The most dramatic observation that supports a galvanic couple concept is the data from the Auger analysis. By theory, the corrosion rate of the non-slide region should be reduced and the corrosion rate of the slide region should be increased. In the absence of sliding the passive film is depleted in Ni. When sliding takes place, the chromium content in the non-slide region increases: in the slide area the chromium is depleted. It is suggested that the sliding encourages the formation of soluble Cr ion complexes (Cr dissolution) rather than the formation of chromium oxides on the surface. Hence, the formation of chromium oxides is encouraged in the passive area.