Depassivation of some metals by sliding friction

Nickel, TA6V4 alloy and 316L stainless steel were passivated in aerated sulphuric acid solution (0.5 M), and submitted to sliding friction against a loaded alumina ball. The effects of sliding speed and normal applied load on tribological behaviour of the passivating films were analyzed. Friction coefficients were measured and the metal wear rates were deduced from 3D profilometry recordings on the wear tracks. Current variations were also measured during friction tests and the results are discussed in terms of depassivation-repassivation kinetics induced by the passivating film breakdown. The effects of sliding speed and applied load on the depassivation kinetics and thus current enhancement due to friction were particularly studied. The Passivating film formed on nickel surface shows the best resistance to friction and wear, whereas TA6V4 alloy shows the worst behaviour. The Passivating film on this alloy wears easily leading to a high material consumption. 316L stainless steel situates between the two other materials.

Research highlights
► Under low loads (0.5 and 1 N), the alumina ball is in sliding contact with the passivating film which is only partially destroyed.
► The film is fully broken under the highest applied normal load (5 N) leading to increase the coefficient of friction due to a direct contact between the metal and the alumina ball.
► The applied load and the frequency of sliding affect depassivation kinetics and thus current enhancements due to friction.
► The oxide film formed on nickel surface shows an excellent resistance to wear. On the opposite, the oxide film formed on TA6V4 suffers severe wear whereas 316L situates between the two other materials.