S. Maximovitch

Characterisation of the passive layer and of hydroxide deposits of nickel by impedance spectroscopy

Abstract The capacitance of the nickel passive layer and hydroxide deposits, studied in 1 M KOH, depends on frequency and potential. Close to the Ni(OH) 2 NiOOH equilibrium potential, Nyquist and Bode diagrams show a characteristic frequency which can be used to determine the hydrogen diffusion coefficient in the solid phase for one of the studied oxides. Outside this domain and on all electrodes, capacitance only involves surface processes depending both on the oxidation state and the nature of the layer. For NiOOH, high capacitances are obtained, similar to those on metals. They are proportional to the layer thickness, and certainly related to the large real area. Conversely, Ni(OH)2 capacitance decreases with increasing thickness and always exhibits the semiconductive p-type behaviour of the passive layer. This is explained by a difference in the solid state stoichiometry of the deposited layers and of the passive layer. When NiOOH is reduced to Ni(OH)2 with electronic transfer coupled with proton insertion, the layer composition changes with simultaneous diminution of electron sites (NiIII). When the number of these sites is higher than the number of proton sites, which is assumed to be the case for passive layers, the remaining NiIII act as electronic acceptors and p-type semiconductivity is observed. For deposited layers the number of proton sites is assumed to be equal to that of the electron sites. They therefore exhibit high impedance intrinsic semiconductivity. The Mott-Schottky plot maximums are explained by a reversible change variation of surface states over a limited potential range. Passive layer reduction occurs in the lower potential range explored.

Study of electrochemically formed Ni(OH)2 layers by EIS

Passive layers formed in KOH and Ni(OH) 2 layers deposited under various conditions in Ni(NO 3 ) 2 neutral solutions are studied by EIS in 1 M KOH. They act as a cpe considered in first approximation as a capacitance in a frequency range depending both on potential and on the formation conditions of the layer. Influence of formation conditions and ageing with time or potential on the p-semiconductive properties of passive layers are examined in relation to the stabilisation of the film. The capacitance of α-Ni(OH) 2 deposits decreases with thickness corresponding to insulating properties. Impedance results are consistant with an insulating porous layer covering the inner semiconductive passive layer. The difference in the electronic conductivity of deposits and passive layers may result from their different ionic conductivity : ionic species that easily diffuse into deposits, react with the electronic acceptor defects and the intrinsic semiconductivity is reached. On some thick deposits at Ni(OH) 2 /NiOOH equilibrium potential, the diffusion coefficient is deduced from the characteristic frequency in the low frequency range.

The electrochemical incorporation of molybdenum in the passive layer of a 17% Cr ferritic stainless steel. Its influence on film stability in sulphuric acid and on pitting corrosion in chloride media

The behaviour of acidified aged molybdate solutions has been studied. These solutions, which can be reduced to blue molybdenum deposits close to −200 mV(SSE) for 1 < pH < 2, have strong inhibiting effects on the steel active dissolution current. Both deposition and inhibition seem to be related to reduced polymer species formed in aged acid solutions. The electrochemical molybdenum incorporation treatment is studied on a Fe-Cr ferritic steel in molybdate acid solutions. The sample is first depassivated in 0. l M H2SO4, then a low concentration molybdate solution is added and the passive layer is built by a slow increasing voltammetric sweep. Incorporation of molybdenum is confirmed by AES and XPS surface analysis. Acid stability and pitting corrosion resistance are studied by means of corrosion tests and molybdenum modified layers are compared to passive films formed in various conditions and to literature data. Electrochemical polarisation has a preponderent effect on acid stability measured by the depassivation time and molybdenum has no effect or rather a negative effect. The presence of molybdenum increases the pitting potential in neutral and acidified 0.5 M NaCl solutions. The effect of redox and polymerisation reactions of molybdate in acid media on pitting corrosion is discussed.

Insertion of lithium in vanadium and mixed vanadium titanium oxide films

Abstract Numerous authors have suggested the use of vanadium–titanium mixed oxides as counter-electrodes for lithium conducting electrochromic devices. In this paper insertion of Li in sol–gel made (V 2 O 5 ) x –(TiO 2 ) 1− x films with x values in the range 0.5–1 were electrochemically studied by coulometric titration, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Raman spectroscopy was also performed on these films and has shown that the vanadium oxide films are well crystallised. TiO 2 containing films are totally amorphous and consequently the absence of γ-V 2 O 5 phase allows the insertion beyond 1 Li per oxide mole in the films. The lithium diffusion in TiO 2 containing films could be explained by the participation of the titanium through the V–O–Ti groups. The decrease of the diffusion rate observed on EIS data is probably due to the difference between the VO and TiO bonds and their respective numbers.

Growth of a nickel oxide layer on a rotating nickel electrode in a non-buffered solution of sodium sulfate

The electrochemical reactions which occur on a rotating disk nickel electrode in a nonbuffered aqueous solution of 0.5 M Na2SO4 (5 < pH < 13) are: water oxidation; formation of Ni (II) and low life time Ni (III or IV) soluble species; formation of a black electrochrome NiOOH layer from acid base reaction of Ni (III) or redox reaction of Ni (IV) with water. The oxidation reaction of Ni occurs probably on reactivated parts of the passive electrode as a consequence of a significant local decrease of pH due to high O2 evolution rate localized on some surface defects. Difficulties of reactivation of nickel explain why NiOOH layer grows lower and lower as the solutions pH are higher. In 1 M KOH solutions, the voltammetric characteristics of the compound synthetized are similar to those of the passive layer of nickel. The oxidized form (NiOOH) of the passive layer of Ni in 1 M KOH forbids further growth of the layer. This behavior can be explained by the layer compacity and by fixed positions of water molecules in the NiOOH lattice. The continuous growth observed for the synthetized layers is due to their porosity, which permits permanent solution-Ni contact.