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Publié suite au congrès : 15th International Conference on Wear of Materials, July-August 2005International audienceIn the biomedical field, about 6% of hip total joint prostheses have to be replaced after 9 years because of a loosening of the femoral stem. One of the main causes of a new surgical intervention is attributed to the phenomenon of fretting-corrosion, i.e. wear under small movements (lower than 100 µm) in corrosive aqueous medium. To understand this degradation, the fretting between 316L steel and PMMA has been investigated to simulate the fretting between femoral stem and bone cement. First, fretting in air has been studied as a reference environment. No significant wear was observed on the stainless steel. PMMA suffers a wear that exhibits a linear evolution as a function of the cumulated dissipated energy. Third body evolution could explain the particular W shape of the active wear track in PMMA, in dry conditions. In aerated Ringer's solution, at free corrosion potential, stainless steel suffers significant wear damage, and the wear volume increases linearly with time but not with dissipated energy. Moreover the optical observations and three-dimensional (3-D) profilometry show a 'W' shape of the wear track of 316L. A corrosion mechanism involving a crevice effect enhanced by fretting allows to explain the location of the maximum damage zones. Finally, the effect of electrochemical potential on the behaviour of 316L/PMMA contacts has been studied by recording current intensity and cumulated dissipated energy under potentiodynamic conditions. Dissipated energy exhibited a reproducible variation with potential. Lubrication regimes and effect of potential on the surface charges could account for such a behaviour

In the present study alloy 600 was tested in simulated pressurised water reactor (PWR) primary water, at 360°C, under an hydrogen partial pressure of 30 kPa. These testing conditions correspond to the maximum sensitivity of alloy 600 to crack initiation. The resulting oxidised structures (corrosion scale and underlying metal) were characterised. A chromium rich oxide layer was revealed, the underlying metal being chromium depleted. In addition, analysis of the chemical composition of the metal close to the oxide scale had allowed to detect oxygen under the oxide scale and particularly in a triple grain boundary. Implication of such a finding on the crack initiation of alloy 600 is discussed. Significant diminution of the crack initiation time was observed for sample oxidised before stress corrosion tests. In view of these results, a mechanism for stress corrosion crack initiation of alloy 600 in PWR primary water was proposed.

The aim of this work is to characterize the oxidation behaviour of different nickel-base alloys exposed to high-temperature and high-pressure water for short oxidation times. The behaviour of Alloy 600 (74% Ni, 16% Cr, 9% Fe), Alloy 690 (60% Ni, 30% Cr, 9% Fe) and Alloy 800 (47% Fe, 32% Ni, 21% Cr) exposed to simulated, pressurised water reactor primary water at 325• C has been compared. From the combination of chemical and structural data obtained by XPS, nuclear reaction analysis and scanning electron microscopy, the chemical composition and the morphology of the oxide formed on the surfaces have been determined. All alloys present a duplex oxide layer composed of Fe-rich crystals in the external layer and an inner Cr-rich layer that is compact and continuous.

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