B. Vuillemin

SVET, AFM and AES study of pitting corrosion initiated on MnS inclusions by microinjection

As pitting is a random phenomenon, it is difficult to predict where a pit will appear on the surface and consequently the use of local probes is rendered difficult. In this work, a new method to study pitting corrosion on a MnS inclusion on 316L stainless steel is proposed. It consists in modifying locally the chemistry in its vicinity by injecting with a microcapillary an aggressive solution of NaCl, H2SO4 or HCl. Once a pit appears, scanning vibrating electrode technique (SVET) is used to follow the current fluctuations over and around the pit when the metal is polarized at a passive potential. In another series of experiments the effect of local activation of MnS inclusion was studied ex-situ using Auger electron spectroscopy (AES) and atomic force microscopy. It is observed that a single pit can be initiated only when hydrochloric acid is injected, whereas sulphuric acid only partially dissolved the inclusion. On another hand, the surface morphology is not affected when a sodium chloride solution is injected. A significant enrichment in sulphur is detected around the inclusion by AES, and micropits are observed in the metal at the edge of the inclusion after HCl activation. Anodic zones are detected by SVET around the inclusion, whereas a cathodic current flows from the inclusion. The anodic current is clearly ascribed to the breakdown of passivity induced by adsorbed sulphur coming from the MnS dissolution, whereas various assumptions can be proposed for the origin of the cathodic current.

Influence of the Chemical Composition and Electronic Structure of Passive Films Grown on 316L SS on Their Transient Electrochemical Behavior

Modifications of the passive film formed on 316L stainless steel (SS) in acidic media were performed under dc or ac polarization. The effects of these ageing treatments on the chemical composition and the electronic structure of the passive film were studied. Chemical analysis with angle resolved X-ray photoelectron spectroscopy revealed that ageing induces a net enrichment in chromium oxide in the inner part of the passive film and an increase of the thickness of the inner oxide. Semiconducting behavior of the passive films was revealed by a Mott-Schottky analysis. The passive film was considered as a dielectric with different trapping levels lying in the bandgap. The observed n- or p-type semiconductivity can be assumed to be the result of the ionization of these trap levels under polarization. The most important modification of the electronic structure observed after ageing was a net decrease of the density of acceptor levels, which is associated with a less disordered inner chromium oxide. In the light of these results, the effect of ageing on the transient electrical coupling of the passive film with an active surface was investigated in free potential conditions. The capacitive response revealed by the potential and current transients was related to the localized states in the bandgap of the passive film. The current transient during the potential drop was related to the repassivation of the active surface. At the same time, electrons from the active site are transiently stored in the capacity formed by trap levels in the inner oxide of the passive film before being consumed by the cathodic reaction. The kinetics of the overall process should be limited by the heights of the potential barriers at the different interfaces and by the energy level of the acceptor-type traps.

An electrochemical quartz crystal microbalance study of lithium insertion into thin films of tungsten trioxide I. Modeling of the ionic insertion mechanism

A theoretical description of the mechanism of lithium insertion into amorphous thin films of tungsten trioxide (a-WO3) prepared by thermal vacuum evaporation of WO3 powder is presented. The model developed is based on the experimental results obtained by chronoamperometry and ac impedance spectroscopy associated with electrochemical quartz crystal microbalance (EQCM). The electrode mass change and the current flowing through the electrochemical cell during cathodic polarization are simulta neously recorded. As expected, it can be observed that the insertion process is associated with a gain of mass of the inserted electrode at long times (t > 1 s). On the other hand at short times (t < 1 s) a net loss of mass is observed. The existence of a non faradaic surface process involving the expulsion of anions from the electrode surface and occuring simultaneously with the faradaic insertion of non-solvated lithium ions into the oxide layer is discussed.

Quantitative Analysis of Iron Dissolution during Repassivation of Freshly Generated Metallic Surfaces

A channel flow double electrode was coupled with the pulsed laser depassivation technique to investigate the repassivation kinetics on iron in sulfuric acid (pH 1 to 3). The amount of ferrous ion dissolved from the activated iron electrode during repassivation was measured. The total charge for repassivation is separated into partial charges for dissolution of ions, Q{sub d}, and formation of adsorbed species, Q{sub f}. The charge Q{sub d} decreases with the electrode potential. The charge Q{sub f} does not depend on the electrode potential. Its value corresponds to the formation of a monolayer of trivalent oxide film. A reaction model for repassivation of iron involving two adsorbed species was simulated numerically in good agreement with the experimental results. The actual rate of repassivation was characterized by the fractional coverage of passivating species.

Relation between grain size and hydrogen diffusion coefficient in an industrial Pd-23% Ag alloy

Abstract The diffusion mechanism of hydrogen in an industrial cold worked Pd–23% Ag alloy is investigated according to its grain size. The microstructural parameters are determined by X-ray diffraction analysis and the diffusion parameters are determined using the galvanostatic electrochemical permeation technique. Two diffusion mechanisms are displayed. For apparent grain size below 40 nm the hydrogen atoms diffuse in the grain boundaries. For grain size above 100 nm, an intergranular diffusion mechanism occurs. For intermediate grain size both mechanisms coexist. Furthermore, the diffusion in the grain boundaries is not faster than the diffusion in the bulk, as generally observed for pure samples. This difference is due to interactions between diffusing hydrogen and impurities in the grain boundaries.

An electrochemical quartz crystal microbalance study of lithium insertion into thin films of tungsten trioxide II. Experimental results and comparison with model calculations

Abstract Lithium insertion into amorphous thin films of tungsten trioxide (a-WO 3 ) prepared by thermal vacuum evaporation of WO 3 powder has been studied experimentally by chronoamperometry, cyclic voltammetry and ac impedance spectroscopy associated with electrochemical quartz crystal microbalance (EQCM). During cathodic polarization of the electrode and at short times two antagonistic processes occur. One is a non faradaic process and is associated with the expulsion of anions from the electrode surface under the effect of the electric field built in the electrolyte when a potential difference is imposed between the electrodes. The other one is the faradaic insertion of non-solvated lithium ions into the oxide. These two processes occur simultaneously. Under application of a cathodic overvoltage the former one leads to a loss of mass of the electrode and the latter one to a gain of mass. These two processes may explain the experimental data. From these data the diffusion coefficient of lithium ions into the oxide is determined and it is shown that it varies as insertion proceeds. A thermodynamic study of lithium insertion into a-WO 3 is also performed to solve the equations of the theoretical model proposed in Part I of this paper. This thermodynamic study allowed us to determine the repulsive and/or attractive interactions between the inserted species and the host matrix as insertion occurs and the influence of the electronic term on the potential.

Effect of Aeration on the Microelectrochemical Characterization of Al2Cu Intermetallic Phases

Local probing of the galvanic coupling between micrometer-sized Al 2 Cu phases and the surrounding aluminum-based matrix of a specially heat-treated 2011 alloy is investigated using a microcapillary electrochemical cell. The aging of an isolated Al 2 Cu particle, i.e., selective dissolution of aluminum, is controlled by the aeration condition at the tip of the capillary due to the nature of the silicon membrane used as a gasket. The local electrochemical behavior of an isolated particle after aging confirms the mass transport control of the oxygen reduction on the galvanic coupling of the particle with the surrounding matrix.

Investigations of Cut-edge Corrosion of Galvanized Steels by the Scanning Vibrating Electrode Technique

Corrosion of galvanized steel sheets often starts at cutedge, leading to steel corrosion or paint delamination during the life time. The knowledge of protection mechanisms is important to improve the corrosion resistance at cut-edge; in particular the influence of zinc corrosion products on corrosion activity. Local electrochemical methods, as pH and potential sensor or the scanning vibrating electrode technique (SVET), are useful to describe cut-edge corrosion 3, . SVET has been used to measure corrosion activity of galvanized steel cut-edge, mounted in an epoxy-resin and immersed in a 0.03 M NaCl solution. Current density locations (SVET) above the coated steel cut-edge has been studied and compared with simulation. A numerical model has been developed; Fig. 1 shows schematically its geometry, which consists in a band of steel juxtaposed with a band of zinc. Three electrochemical reactions are considered at electrode boundaries; Eq. 1-3 for the oxidation of zinc and steel and reduction of oxygen, respectively (Vm is the metal potential). ( ) 0 2 exp( ) m Zn Zn Zn Zn V V E J F k α − − = [1]

Bipolar electrolysis for tritium recovery from weakly active tritiated water

Abstract Detritiation facilities produce low activity tritiated water from which tritium cannot be recovered. Bipolar electrolysis, based on the electrochemical permeation of hydrogen and its isotopes through Pd–Ag alloy membranes, allows tritiated water enrichment together with negligible gaseous tritium release. Our purpose is to enrich water from 500 Ci/l (1.85×1013 Bq/l) to more than 2000 Ci/l (7.40×1013 Bq/l). We first describe the principle of bipolar electrolysis and its application to isotopic enrichment. The experimental part of this work consists of the determination of the isotopic separation factors. From these experimental values, we simulated the working of an operational cell and we demonstrate the feasibility of the process.

Predictive Model for Cut-Edge Corrosion of Galvanized Steels

A numerical model for the electrochemical behavior of cut-edge of galvanized steels is proposed. Some experimental data of current densities above cut-edge immersed in a 0.03M NaCl solution have been measured, using a scanning vibrating electrode technique, and compared with some simulated ones. A good fit has been obtained. The model geometry has been modified by decreasing the electrolyte thickness in order to tend towards an atmospheric corrosion case; such situation that is not easily accessible by electrochemical studies. Three regions can be distinguished according to the efficiency of the galvanic coupling to protect steel.