The reduction of oxygen on rotating ring electrodes made of AISI 420 and AISI 316 stainless steels has been studied in NaCl and Na2SO4 solutions near neutral pH (6 to 10). The reduction rate is strongly dependent on the condition of the surface (specimen preparation and type of oxide), composition of the alloy, and on solution pH, but not on the rotation rate. Reaction rates on AISI 316 SS are approximately half those measure on AISI 420 SS, and much lower than on noble metal electrodes under the same conditions. Little hydrogen peroxide is formed, indicating a strong predominance of the 4-electron reduction path from O2 to HO-. According to the proposed reaction mechanism, the reduction of oxygen, and of other oxyanions, on oxide-covered electrodes is mediated by reduction processes on metal centers in the oxide. The implications of this model for the corrosion and the cathodic protection of corrosion resistant alloys in seawater, and the possible effect of microbial activity on corrosion rates are discussed.
Exposure of corrosion resistant alloys (CRA?s) to aerated brines, e.g., completion fluids, seawater, or brackish waters, may result in localized (pitting or crevice) corrosion. Comparative exposure tests performed in natural and synthetic seawater have shown that the corrosivity of these waters is a function not only of their chemical composition (pH, oxygen concentration, salinity) but also of the biological activity. However, a clear correlation between bacterial levels and corrosion rates has not been observed. The anodic behavior of CRA?s in brines, i.e., the metal dissolution reaction, has been quite extensively studied [see, e.g., ref. 1-3]. Parameters such as corrosion potential, pitting potential, passivation current and repassivation potential have been used to characterize the corrosion resistance of alloys, with mixed results. Several models of the anodic film breakdown and the development of localized corrosion have been proposed [for attention, even though the rate of oxygen reduction plays an important role in determining the corrosion rates and the corrosion control needs of CRA?s, e.g., the cathodic protection current requirements, the rate of formation of calcareous deposits, and the rate of hydrogen generation and absorption under cathodic protection, which may lead to embrittlement. Oxygen reduction kinetics also determines the galvanic corrosion of carbon steel coupled to CRA?s in aerated environments, for example the corrosion of the casing in contact with a CRA tubing when aerated brines are used for well completion. It has also been suggested that one explanation of the effect of microbial activity on the enhanced corrosion of CRA?s is related to an increase in the rate of oxygen reduction.
Studies of the oxygen reduction reaction on passive iron in alkaline environments have providedyielded contradictory results. For example the reaction order with respect to O2 and HO- has variously been reported as being 0.5 and ?1 [6, 7], or 1 and ?0.5 [8- 10]. On of the problems with passive iron is that both the reduction of the oxide and the dissolution of the metal occur over the same potential range as oxygen reduction, thereby making it difficult to clearly distinguish the relative contributions to the overall measured current. In addition, the activity of the electrodes depends very strongly on the method of preparation of the oxide.
Stainless steels, on the other hand, have a much more stable oxide that is spontaneously formed in contact with air or water, and is less affected by changes in potential. Because of the presence of Cr and Ni ions, it can be suspected that the electrochemical b
