The frequency response of a model mechanism for acceptor-limited electrochemical polishing is investigated. The mechanism consists of charge-transfer and surface-reaction steps involving adsorbed intermediates in the absence of precipitate films with electropolishing resulting from mass-transport limitations for a solvating acceptor species arriving at the surface from the bulk electrolyte. Important qualitative differences in the impedance diagrams between the aeceptor mechanism and the more commonly observed salt-film precipitation mechanisms permit a clear distinction to be made concerning the likely origin of the polishing phenomenon in a given experimental system. Whereas in salt-film mechanisms the position and size of the high-frequency loop in the impedance diagrams can depend strongly on convection and applied potential, for the aeeeptor mechanism the high-frequency limit is unaffected by potential and convection, and the diameter of the first impedance loop is independent of potential aIong the limiting-current plateau. At low frequencies as well the two models differ considerably. In particular, systems operating under an aeceptor-limited mechanism can exhibit features influenced by Warburg-Nernst impedances, while salt-filmsystems cannot. A comparison of the shapes of the impedance diagrams and the characteristic behavior of the high-frequency loops with recent experimental measurements for the anodic dissolution of iron-chromium alloys in phosphoric and sulfuric acid supports the hypothesis that the iron-chromium system is governed by aeceptor transport in the limiting-current region.Electropolishing, the creation of a bright surface finish by anodic dissolution of a metal workpieee, is frequently employed in industrial applications where more traditional mechanical buffing procedures would prove either too difficult or too costly. The polishing phenomenon is characterized by the disappearance of specific crystallographic or grain-boundary attack on the micron or submicron scale, resulting in a smooth surface with a mirror-like luster.It has been suggested that the disappearance of oriented dissolution during polishing is caused by a shift in dissolution mechanism from surface-kinetic control, which may vary from one crystallographic orientation to another, to diffusion control, which should be insensitive to crystallographic features. 1 This hypothesis has been demonstrated for a number of systems known or suspected to form precipitate salt fiIms at the electrode surface at high anodic current densities. In such cases, the metal-ion concentration is fixed at its saturation value at the film-electrolyte interface, and the dissolution process is limited by mass transport of the ions away from the surface. Experimental studies of salt films formed during high-rate dissoIution of iron in chloride solutions 2 indicate that such a film is indeed responsible for poIishing. Moreover, alternating-current impedance diagrams measured during polishing indicate that the precipitate consists of a complex structur...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.