Polarization (original) (raw)
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Electrochemical Polarization I. A Theoretical Analysis of the Shape of Polarization Curves
aqueous reaction to form the hydrated fluorides which are subsequently dehydrated at elevated temperatures in an atmosphere of HF gas. Extensive studies were made of the conditions for the preparation of high-purity ZrF4, and pilot plant equipment is described which was used to prepare 100 lb batches of the fluorides. The reduction step was investigated thoroughly, particularly for Zr, and those factors which affect metal quality and yield were determined. Reduction yields of 96 % were obtained with both Zr and Hf. After are-melting, the sponge Zr had a hardness of 40-45 Rockwell A and was readily cold-rolled into sheet. Zr metal thus prepared had a purity of about 99.8%. Hf metal, similarly prepared, had a hardness of 69 Rockwell A and was hot-rolled but was too brittle to be easily cold worked. The Hf was low in metallic impurities, but contained considerable amounts of C, N, and oxygen. ACXNOWLI~DGMENTS The authors are especially grateful to J. W. Starbuek for his valuable contribution in the experimental redue-tion studies, to B. A. LaMont and co-workers for the chemical analyses, and to C. Lentz and associates for the spectrographic analyses. ABSTRACT At low overvoltage values, deviations from Tafel behavior for a noncorroding electrode are due primarily to the reverse reaction of the oxidation-reduction system, and at high overvoltages to concentration and/or resistance polarization. It is shown further that the practice of placing straight lines through a few experimental points is extremely hazardous, while the indiscriminate introduction of "breaks" is contrary to the electrode kinetics described. Further complexities arising from a corroding electrode are described. In this instance , the forward and reverse reactions of both of the oxidation-reduction systems forming the corrosion couple must be considered. This representation of the local polarization diagram of a corroding metal is more fundamental than that used previously in the literature, and thus provides a clearer picture of the various factors which affect the corrosion rate and the shape of polarization curves. A region of linear dependence of potential on applied current is described for a corroding electrode by treating it in a manner analogous to that for a noncorroding electrode. An equation is derived relating the slope of this linear region to the corrosion rate and Tafel slopes. This relation provides an important new experimental approach to the study of the electrochemistry of corroding metals since, in some instances, interfering reactions prevent determination of T~fel slopes at higher current densities. Polarization measurements are an important research tool in investigations of a variety of electrochemical phenomena. Such measurements pernfit studies of the reaction mechanism and the kinetics of corrosion phenomena and metal deposition. In spite of their wide applicability and extensive use, considerable uncertainty in the interpretation of polarization measurements still exists. Some of the uncertainties include the proper method of plotting data and the correct interpretation of "breaks" in polarization curves. Abrupt changes in slope of overvoltage vs. log current have been given considerable significance in the past few years. Logan (1) examined various methods of plotting cathodic polarization measurements to evaluate the correspondence between current required for complete cathodic protection of a system and current flow at the potential break. He reported that the potential break) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.88.117.172 Downloaded on 2016-01-28 to IP
The simulated shapes of the polarization curves were correlated with the type of metal electrodeposition process control as a function of the ratio of the exchange current density to the limiting diffusion current density (j 0 /j L). Diagnostic criteria based on the j 0 /j L ratios were established. For j 0 /j L > 100, the system is under the ohmic control. In the range 1 < j 0 /j L ≤ 100, there is mixed ohmic–diffusion control. Pure diffusion control appears in the range 0.1 < j 0 /j L ≤ 1. For j 0 /j L ≤ 0.1, the system is activation controlled at low overpot-entials. The proposed diagnostic criteria were verified by comparison of the simulated curves with experimentally recorded ones and by morphological analysis of deposits obtained under the different types of control of the metal electrodeposition process.
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