Hydrogen overpotential measurements have been performed on yttrium and 12 rare earth metals in phosphate buffered phosphoric acid. Tafel constants, @, and exchange current densities, iO, have been accurately determined by a graphical analysis technique. Correlations between electrochemical and physical properties are discussed.
R&nn&Des
mesures de surtension d'hydrogene ont 6te effect&es SUP I'yttrium et SIX 12 metaux de terres rares a l'etat de phosphate tampor& d'acide phosphorique.
Les constantes de Tafel, ,8, et les densites de courant d&change, io, ont ttC dCtermin&rs avec precision par une technique d'analyse graphique. Discussion des correlations entre les proprietb tlectrochimiques et physiques.
Zusammenfassmg-An
Yttrium und 12 Metallen aus der Reihe der seltenen Erden wurden Messungen der Wasserstoffiiberspannung in mit Phosphat gepufferter Phosphorslure durchgeftihrt. Mittels einer graph&hen Auswertungsmethode wurden exakte Werte fiir die Tafel-Konstanten, ftir /? und die Austauschstromdichten i. ermittelt. Man diskutiert Zusammenhange zwischen den elektrochemischen und den physikalischen Eigenschaften.
HYDROGEN overvoltage measurements on the rare earth metals are difficult because of their chemical reactivities. Extensive corrosion test9 have shown that only oxalic, hydrofluoric and phosphoric acids are sufficiently noncorrosive for convenient electrochemical studies : all other acids, such as sulphuric and hydrochloric, violently attack the lanthanide elements at low concentrations .l Hydrofluoric acid presents experimental difficulties since it cannot be used in glass containers. Phosphoric acid appears to be the best medium, since it is more ionized than either oxalic or hydrofluoric acid.
Preliminary attempts to determine hydrogen overvoltage parameters in 0.1 N phosphoric acid2 were only partially successful due to interference from concentration polarization and resistance effects. In the present study, these effects were minimized by alterations of the environment.
THEORY
Analysing the rare-earth-metal-phosphoric acid system indicates that considerable deviations from Tafel behaviour are to be expected. Consider the cathodic polarization of a corroding metal in an air-free acid solution. If the reduction process 2H+ + 2e ---f H, is activation-controlled, then the rate of reduction, ired, as a function of overvoltage, E, measured from the corrosion potential is% where B, is the Tafel constant for the cathodic reduction process, and icorr is the corrosion rate expressed as current density. Applied cathodic current density is equal to the difference between oxidation and reduction rates,4 &pp = ired -iox.