The influence of dissolved oxygen in seawater on the fretting corrosion of roping steel

Abstract Results are presented which support a previous proposal that the rate of damage caused to a roping steel by fretting in seawater is strongly influenced by electrochemical corrosion. An interpretation of these results implies a rate of damage controlled by the rate of an associated cathodic reaction, oxygen depolarization, occurring on the surface immediately outside the fretting scar. The rate of fretting corrosion is then related, particularly at low concentration, to the amount of dissolved oxygen gas in the electrolyte. A model, presented, permits the summation of polarization curves for individual anode (scar) and cathode (surrounding) areas, in order to calculate corrosive wear rates. The model predicts a linear increase in fretting corrosion rate with increasing dissolved oxygen in the electrolyte. The wear rates observed experimentally, however, are not simply proportional to the limiting rates of oxygen depolarization in the cathodic area, and a plausible explanation for limiting wear rates relies upon the assumption that at high rates of oxygen depolarization (at high dissolved oxygen concentration) high surface alkalinity in the cathode area will cause the precipitation of insulating corrosion products there. There is experimental evidence for these solids. The model assumes that passive films in the fretting scar are continually being removed and reformed between each half cycle of fretting, though the inability for Fe2+ ions to diffuse away from the anode (scar), and/or the incomplete removal of wear products there, may equally give rise to wear rates lower than those predicted by the model. The mechanisms are discussed.