Metal Deposition‐Dissolution in Molten Halides: On the Question of Measurability of Very Fast Electrode Reaction Rates

Abstract

Metal deposition‐dissolution reactions in molten salts are very fast and difficult to investigate, as evidenced by large discrepancies in the reported exchange current densities. An error analysis of the dc relaxation techniques was carried out, and it was determined that the measurement of the exchange current density can have a systematic error if the surface reaction rate is much faster than the diffusion rate. This systematic error will result in measured exchange current densities approximately equal to the largest exchange current density measurable under the given conditions. This systematic error can also result in fortuitously linear vs. and vs. plots, even when the results are grossly in error. A computer curve fitting data evaluation coupled with a statistical sensitivity analysis is suggested to avoid these problems. The exchange current densities of iron, nickel, and molybdenum were measured in eutectic melt at 450°C using an improved double‐pulse galvanostatic technique. Only that of iron could be measured reliably ( at ion concentration). The nickel reaction is too fast to be measured (at least 5 A‐cm−2 at ion concentration), and the molybdenum results are unreliable because of melt decomposition. The large discrepancies between these results and some results reported by earlier workers can be explained on the basis of the above‐described error analysis. It is concluded that past experiments resulting in large exchange current densities should be reexamined with improved measuring and data‐evaluation techniques because it is possible that the reported values represent only the applicability limit of the measuring technique rather than the true exchange current density.