J. Llopis

Radiochemical study of the anodic behaviour of palladium

Radiochemical measurements carried out with 103Pd allow us to study the electrochemical corrosion of this metal even under conditions of passivation. Different anode potentials have been used to study the corrosion rate, electrical yield and nature of the solution products. Work has been done on aqueous solutions of HClO4 and KOH.

Passivation of ruthenium in hydrochloric acid solution

Abstract Anodic and cathodic charging curves are used to investigate the passivation of ruthenium in HClO4 and HCl solutions. The surface oxidation determining the passivation of ruthenium is the formation of an oxide film several molecules thick. In HClO4 solution films of Ru2O3 and/or RuO2 are formed on the electrode; at higher potentials there is oxidation to RuO4. In HCl solution, the formation of Ru2O3 is the most probable process.

Anodic corrosion of ruthenium in hydrochloric acid solution

Abstract The electrochemical corrosion of ruthenium in HCl solution both by the action of dc and ac leads to the formation of chloro-complexes of Ru(III) and/or Ru(IV), depending on potential. With solutions of HClO 4 anodic corrosion by dc may occur with formation of RuO 4 , which partially dissolves in the electrolyte. In HCl solution anodic corrosion by dc leads to the formation of as yet undefined chloro-complexes. The superimposition of square-wave ac notably increases the attack if the frequency is below 4 c/s. Under these conditions the complexes [RuCl 6 ] 3− and/or [RuCl 6 ] 2− are formed in proportions depending on the experimental conditions. The corrosion resistance of ruthenium in HCl solution is higher than that of rhodium and platinum and of the same order than that of iridium.

Radiochemical study of the anodic corrosion of ruthenium

Neutron irradiation of ruthenium provides a radiochemical method of high sensitivity to study its corrosion and passivation. The metal passivates in HCl solutions at potentials > 950 mV (sce), although some corrosion remains. Comparison of the results with those for Pt shows that Pt passivates more completely. A scheme is proposed to explain the influence of Cl− on the mechanism of anodic corrosion of metals of the Pt group.

Passivation of rhodium in hydrochloric acid solutions

Abstract Conclusions about the passivation of rhodium have been obtained by studying the anodic and cathodic charging curves with solutions of HClO 4 and/or HCl as electrolytes. The electrochemical surface oxidation of rhodium causes passivation. In HClO 4 solutions this oxidation leads to the formation of monomolecular films of Rh 2 O 3 and/or RhO 2 , according to the polarizing potential. In solutions of HCl only the formation of Rh 2 O 3 and adsorbed Cl atoms takes place.

Study of the impedance of a platinum electrode in the system Br2/Br− (HClO4, aq.)—II. Mechanism of the electrode reaction☆

Abstract The Randles circuit well represents impedance measurements carried out with activated Pt electrodes. This enables us to study the variation of j o for redox reactions with concentration of the reactants, at constant potential, and also the variation of j o with potential, keeping constant the concentration of one of the reactants. The results thus obtained indicate that the step Br 2 + e ⇌ Br 2 − is rate-determining; it is followed or preceded by the rapid equilibria Br 2 − ↽ Br − + Br 2Br ↽ Br 2 . The mechanisms proposed hitherto for the electrochemical behaviour of the halogen/halide systems at inert electrodes are discussed, and it is reasoned that the ‘reversibility’ of these systems increases in the order Cl 2 /Cl − 2 /Br − 2 /I.

Surface reactions of iron with hydrocarbon solutions of organic sulphides

Compounds labelled with 35S have been used to study the reactivity of iron with a series of butyl sulphides and polysulphides up to the tetrasulphide. Experiments carried out in the absence of air allowed us to establish the following order of increasing reactivity to iron: RSR < RSSR 《RSH<RS3 < RS4R A mechanism to explain this surface reaction has been postulated, taking into account the lability of the S atoms in the molecules. The influence of the oxygen on the kinetics of this reaction is made evident when the results of experiments carried out in the presence or absence of air are compared. It is apparent that the presence of oxygen increases the reactivity of mono- and disulphides, but decreases that of the mercaptans, this diminution being more important in the case of tri- and tetrasulphides. A mechanism to explain the action of oxygen in the sulphuration reaction has been proposed.

Study of the impedance of a platinum electrode in the system Br2/Br− (HClO4, aq.). I. Influence of the surface state☆

Abstract Impedance measurements, over a wide range of frequencies, of a platinum electrode immersed in a solution of Br2/Br− (HClO4, aq.), allow us to obtain the values of the exchange current jo, it being also possible to study its variation with the experimental conditions. The results thus obtained depend on the surface state of the electrode. With activated electrodes, the Randles circuit explains well enough the electrochemical behaviour of this system. With poisoned electrodes the circuit is inadequate, but it can be shown that the exchange current is decreased by the poisoning effect. Decrease of the exchange current with increase of surface oxidation of the electrode has also been observed. As a first approximation the poisoning effects can be explained by a diminution of the effective area concerned with electron transfer, but the inadequacy of the Randles circuit indicates that adsorption processes have to be taken into account.

Anodic corrosion of rhodium in hydrochloric acid solutions

Abstract The anodic corrosion of rhodium in HCl solutions by the action of dc occurs with a high overpotential and there is formation of Rh(III) complexes (yell

Passivation of iridium in hydrochloric acid solutions

Abstract Anodic corrosion of Ir in solutions of HCl proceeds through the formation of IrCl62− and IrCl63− ions; the formation of Ir(III) tends to be more important at less positive potentials. This process requires a high overpotential and Ir passivates easily by adsorbed oxygen. The polarization curves obtained with HCl of increasing concentration show that for concentrations lower than 3 N Ir passivates even for the lower current densities. For concentrations higher than 4 N these curves show two parts; the first corresponds to the anodic corrosion of the metal and the second to evolution of Cl2 and O2 on the passivated metal. Study of the anodic and cathodic charging curves allows one to estimate the “oxidized” surface fraction after passivation of the electrode. This surface “oxidation” decreases as the concentration of HCl increases. The influence of the specific adsorption of Cl− is considered in the discussion of the results.