Electrochemical behaviour and anodic passivity mechanism of certain metals in electrolyte solutions

Abstract

Abstract The electrochemical behaviour of chromium, nickel and stainless steel (grade 18–8) was studied by the potentiostatic method in a broad range of polarizations embracing the region of the anodic passivity of these metals. The steady dissolution rate ( I st ) as a function of the potential for all the investigated metals was found to be expressed by curves similar in shape. The shifting of the potential towards positive values from the passivity potential is accompanied first by a decrease of the quantity I st according to a Tafel line; afterwards, in a rather broad range of potentials, this rate remains constant. Beyond a certain potential (the overpassivity potential) in the positive direction we again observe a rise of the dissolution rate according to a Tafel line, which has a slope close to the ordinary values for anodic reactions. The overpassivity potential depends on the nature of the metal and the p H of the solution. It was established that the electrochemical behaviour of chromium, nickel and stainless steel rendered passive with respect to the reaction of anodic dissolution is analogous to the electrochemical behaviour of platinum rendered passive with respect to the reaction of H 2 ionization. The measured quantities of differential capacitance and ohmic resistance of the steel electrode, as well as the change experienced by these quantities with the potential, lead to the conclusion that steel passivity does not involve the formation of an oxide film on its surface. This conclusion is in agreement with the observed similarity of the capacitance curve obtained for steel with the corresponding curve for platinum. In the case of platinum the absence of oxide layers on the surface of the metal in the investigated region of polarization is a firmly established fact. It was further established that the obtained experimental data lend themselves to a consistent explanation if we assume that the passivity of the investigated metals is a specific case of the widespread phenomenon of the change of the kinetics (overvoltage) of an electrodic reaction as a result of a change in the state of the electrodic surface, caused in turn by the chemico‐adsorptional interaction of surface atoms of the metal with solution components and reaction products. In the cases examined the kinetic retardation of the anodic reactions appears to set in as a result of the activated adsorption of the oxygen of water. In the light of these considerations an explanation is advanced for the observed dependence of the passivity potential of chromium on the p H of the solution. The retardation of the dissolution process with time and potential continues only until a certain potential is reached. Beyond this potential in the positive direction the process can proceed through another reaction, the formation of ions of a higher degree of oxidation. The view is advanced that in explaining the mechanism of the action of specifically adsorbed anions and the oxygen of water on the kinetics of an electrodic reaction, it is necessary to take into consideration both the change in the nature of the reacting particle as a result of adsorption and the change in its energy state on the surface of the metal and in the volume of the solution.