Physicochemical Characterization of Passive Films and Corrosion Layers by Differential Admittance and Photocurrent Spectroscopy

Two different electrochemical techniques, differential admittance and photocurrent spectroscopy, for the characterization of electronic and solid state properties of passive films and corrosion layers are described and critically evaluated. In order to get information on the electronic properties of passive film and corrosion layers as well as the necessary information to locate the characteristic energy levels of the passive film/electrolyte junction like: flat band potential (Ufb), conduction band edge (EC) or valence band edge (EV), a wide use of Mott-Schottky plots is usually reported in corrosion science and passivity studies. It has been shown, in several papers, that the use of simple M-S theory to get information on the electronic properties and energy levels location at the film/electrolyte interface can be seriously misleading and/or conflicting with the physical basis underlying the M-S theory. A critical appraisal of this approach to the study of very thin and thick anodic passive film grown on base-metals (Cr, Ni, Fe, SS etc..) or on valve metals (Ta, Nb, W etc..) is reported in this work, together with possible alternative approach to overcome some of the mentioned inconsistencies. At this aim the theory of amorphous semiconductor Schottky barrier, introduced several years ago in the study of passive film/electrolyte junction, is reviewed by taking into account some of the more recent results obtained by the present authors. Future developments of the theory appears necessary to get more exact quantitative information on the electronic properties of passive films, specially in the case of very thin film like those formed on base metals and their alloys. The second technique described in this chapter, devoted to the physico-chemical characterization of passive film and corrosion layers, is a more recent technique based on the analysis of the photo-electrochemical answer of passive film/electrolyte junction under illumination with photons having suitable energy. Such a technique usually referred to as Photocurrent Spectroscopy (PCS) has been developed on the basis of the large research effort carried out by several groups in the 1970’s and aimed to investigate the possible conversion of solar energy by means of electrochemical cells. In this work the fundamentals of semiconductor/electrolyte junctions under illumination will be highlighted both for crystalline and amorphous materials. The role of amorphous nature and film thickness on the photo-electrochemical answer of passive film/solution interface is reviewed as well the use of PCS for quantitative analysis of the film composition based on a semi-empirical correlation between optical band gap and difference of electronegativity of film constituents previously suggested by the present authors. In this frame the results of PCS studies on valve metal oxides and valve metal mixed oxides will be discussed in order to show the validity of the proposed method. The results of PCS studies aimed to get information on passive film composition and carried out by different authors on base metals (Fe, Cr, Ni) and their alloys, including stainless steel, will be also compared with compositional analysis carried out by well-established surface analysis techniques

INVESTIGATION OF PASSIVE ELECTRODES USING MODULATION SPECTROSCOPY AND PHOTOPOTENTIAL MEASUREMENTS

La formation et la propriété des produits de surface pour les métaux Cu, Ni et Fe avaient été étudiées par la technique de modulation spectroscopique et du photopotential. Avec la modulation spectroscopique il est possible de déterminer le degré d'oxydation des couches minces. La méthode du photopotential marque le type de la conductibilité et la concentration des lacunes de métal et des lacunes d'anion. L'influence de différents inhibiteurs est étudiée.The formation and properties of surface layers on the transition metal electrodes Cu, Ni and Fe in aqueous solutions were investigated using modulation spectroscopy and photopotential measurements. With modulated reflectivity it is possible to obtain informations concerning the oxidation degree of the surface layer as a function of the electrode potential whereas the second method shows the type of conductivity and the concentration of cation- and aniondefects of the surface coverage. Furthermore the influence of inorganic and organic inhibitors was investigated

Operating map - tool for plating functional layers

The increasing demand for special functional layers and layer systems requires reliable electroplating process control. The use of operating maps is demonstrated covering all aspects of the layer formation starting from nucleation, crystal growth and structure as a function of electrochemical kinetics as well as hydrodynamics and mass transport. In particular, the influence of pulse plating of Zn as an example is demonstrated. Finally, techniques to characterise the deposits such as X-ray fluorescence (layer thickness and distribution), X-ray diffraction (texture and grain size) as well as methods to evaluate hardness, Young's modulus and ductility are applied. The data are used to produce the operating map via a system of neural networks so that the effect of changing electrochemical deposition parameters and resultant deposit characteristics can be determined. This methodology has the advantage of short access time and high reliability

Untersuchung des Korrosionsverhaltens unterschiedlich anodisch oxidierter Aluminiumproben

With 6 figs.Copy held by FIZ Karlsruhe; available from UB/TIB Hannover / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman