Study of electronic-ionic conducting transformation of temporarily protective oil coating in salt solution

摘要

In this paper, a multiple microelectrode array was developed to study the electronic-ionic conducting transition of temporarily protective oil coating in salt solution. It was pointed out that there existed electronic-ionic transition of temporarily protective oil coating in salt solution varying with immersion time. At the early stage of immersion, the oil coating was an electronic conductor, which having very low conductivity. With increasing of immersion time, the oil coating transformed from electronic conductor to ionic conductor, where ionic diffusion in the coating was predominant. Based upon Bailey and Ritchie's general electrochemical theory for the oxidation of metals, an electrochemical analysis of potential inhomogeneity of the oil coating was presented. It was assumed that only one step could be rate-determining in the overall reaction in the metal/oil film/solution system. It was predicted that there were two kinds of film in the temporarily protective oil coating, ionic and electronic conducting film. For the ionic conducting film, potentials of oil coated electrode were equal to equilibrium potential of metal oxidation at the metal/coating interface (E{sub}a{sup}(eq)), and it had the most negative value. For the electronic conducting film, potentials obtained were equal to the equilibrium potential of the oxidant reduction at the coating/electrolyte interface E{sub}c{sup}(eq), and it had the most positive value. Potential measurements in this paper verified the results of Wormvell and Brasher's research in 1949; the distribution of potential on oil-painted wire beam electrodes being heterogeneous and following a discontinuous binomial probability distribution were also elucidated.