The Galvanic Coupling Approach to Crevice Corrosion

摘要

The galvanic nature of crevice corrosion is a generally accepted concept but the coupled electrochemical behaviour and its role in crevice corrosion has not been studied until recently. This work has shown that the galvanic coupling effectiveness, i.e. how much the anodic dissolution is enhanced respective to the open-circuit dissolution rate, depends on the kinetics on both electrodes, the latter being functions of the aggressiveness of the environment of interest. A dimensionless parameter, the coupling coefficient η, is proposed which can be used as a measure of the effectiveness of the enhancement in anodic dissolution due to coupling. This work demonstrates the application of the coupling effectiveness approach to a real corroding crevice. A real corroding crevice is considered to be an array of anode/cathode element pairs in different chemical environments, each element of the crevice interior being coupled to an element on the exterior cathode with different coupling effectiveness. By (i) determining i the local solution aggressiveness at different locations inside a corroding crevice, (ii) obtaining the thermodynamic and kinetic information for both the anodes and cathode in corresponding environments, and (iii) calculating the coupled dissolution rates at these locations, it is possible to predict the shape of the crevice attack and its evolution with time. Using simulated crevices comprising the stainless steels, UNS S30500 and S31603 the chloride ion concentration is measured using an array of microelectrodes. The corresponding coupling coefficient is then determined and used to predict the actual shape of the crevice attack. Further applications of the galvanic coupling approach to quantify the crevice corrosion resistance of materials are discussed.