François Wenger

Preparation and investigation of nanostructured SiC-nickel layers by electrodeposition

Nanocomposite layers have been obtained by electrochemical deposition of silicon carbide nanoparticles (mean diameter 20 nm) in a nickel plating bath. The influence of nanoparticles on nickel electroplating is studied by electrochemical impedance spectroscopy (EIS) at cathodic potential. Cathodic polarization diagrams have been determined in the electrolyte with and without nanoparticles. We show that incorporation of 20 nm SiC particles into nickel deposition modifies the impedance and cathodic polarization diagrams in correlation with the modification of the surface morphology and crystallinity of nickel matrix.

Analysis of local corrosion of large metallic structures or reinforced concrete structures by electrochemical impedance spectroscopy (EIS)

Abstract The problem of monitoring of corrosion of large metallic structures by EIS is approached in the simple case of a long locally corroded bar, in contact with an electrolyte or embedded in concrete. The influence of the shapes, sizes and arrangement of the electrodes on the interpretation of impedance measurements is shown to be very important. A model is proposed to describe the influence of the large size of the working electrode which induces a spatial distribution of the impedance. This effect of spatial distribution may allow accurate location of the corroded areas and quantitative analysis of corrosion of these areas.

Electrochemical noise analysis of cathodically polarised AISI 4140 steel. I. Characterisation of hydrogen evolution on vertical unstressed electrodes

For investigating stressed carbon-steel electrode (AISI 4140) cracking induced by hydrogen embrittlement with the electrochemical noise (EN) technique, the contribution of the evolving hydrogen bubbles to the EN has to be determined. Under cathodic current control, various reasons may explain the fluctuations of the electrode potential. In the first paper of this series the electrolyte resistance (ER) fluctuations, which yield ohmic-drop fluctuations, were analysed for unstressed vertical tensile specimens. A simplified model was proposed to tentatively derive the characteristic parameters of the gas evolution, such as the bubble mean size and evolution mean rate, from the power spectral density (PSD) of the ER fluctuations. However, results in only qualitative agreement with optical observations were obtained, indicating that the complicated screening and dragging effects of rising bubbles have to be taken into account in the modelling for vertical electrodes. The electrode potential fluctuations of unstressed tensile specimens were investigated in the second paper of this series and the influence of a stress was examined in the third paper.

Tribocorrosion: Material Behavior Under Combined Conditions of Corrosion and Mechanical Loading

Oxide particles, referred to as ‘debris”, are released from the contacting materials. Then, the debris can be removed from the contact zone or on the contrary trapped in it. In the case of removal, the debris dissolve chemically or are dragged out by a hydraulic flow along the material surface. In this case, the tribocorrosion mechanism is based on a repeated tearing off of the oxide after each contact and eventually a removal of some of the underlying material depending on the intensity of mechanical stress acting on the contacting materials. The major concern is then to quantify and eventually to model the kinetics of repassivation as accurately as possible. This type of tribocorrosion process can be classified as an oxidative wear mechanism as, for example, the ‘mild oxidative wear model’ (Quinn, 1992). In the case of debris trapping, one has to consider that under appropriate hydrodynamical, chemical, and thermal contact conditions and relative speed of the two contacting bodies, the debris will remain temporarily in the contact zone mainly as colloids with a diameter usually in the range of a few hundred nanometers. Two cases may then be distinguished: (a) the debris accelerates the wear in comparison to the case of debris-free contacts is accelerated by an abrasive effect, or (b) the debris slows down the wear compared to the case where the contact zone is free of any debris, resulting in a protective effect.

Comparison between tribocorrosion mechanisms of Stellite 6 and Zircaloy 4 in LiOH-H3BO3 solutions

Tribocorrosion mechanisms of Stellite 6 and Zircaloy 4 alloys were studied by means of tribocorrosion tests carried out in conditions of unidirectional sliding friction in a solution of boric acid and lithium hydroxide, with same chemical composition as water in nuclear Pressurized Water Reactors. With Stellite 6, a “soft wear” process is obtained, controlled by mechanical removal and subsequent restoration of a very thin passive film. With Zircaloy 4, in the same tribological conditions, a major influence of abrasion by zirconia particles, pulled out from the zirconia surface layer, was observed. These major differences in the tribocorrosion mechanisms explain the differences found in wear laws.