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Caractérisation des mécanismes d'usure par tribocorrosion d'alliages modèles Ni-Cr

Abstract : Some components of nuclear power plants, as steam generator tubes are made from Ni base alloys. These components are exposed to severe environment of high temperature and high pressure and submitted to contact mechanical stresses. These Ni – based alloys properties are determined by their ability to form on their surface an inner protective barrier film mainly composed of Cr2O3. The steam generator tubes are among the most difficult components to maintain, on the hand, because of their safety importance and secondly, the exchange tubes are subject to various degradation mechanisms, because of the harsh conditions of work. Wear by tribocorrosion is a physicochemical aging mechanism which occurs in the management of the nuclear power plants life time.Tribocorrosion is an irreversible process which involves mechanical and chemical / electrochemical interactions between surfaces in relative motion, in the presence of a corrosive environment. The goal of this study was to quantify in terms of quantity and quality the wear generated by tribocorrosion process on Ni – Cr model alloys. Two model alloys: Ni -15Cr and Ni -30Cr were used to highlight, evaluate and compare the influence of the chromium content on the formation of the protective oxide layer and the role played by the latter one on the kinetics and mechanisms of wear by tribocorrosion. The tribocorrosion experiments were performed by using a pin -on–disc tribometer under controlled electrochemical conditions in LiOH – H3BO3 solution. The corrosion – wear degradation of the protective layer during continuous and intermittent unidirectional sliding tests was investigated by a three-stage tribocorrosion protocol. In the first stage, electrochemical techniques (open circuit potential measurements and electrochemical impedance measurements) were used without applying unidirectional sliding to monitor and evaluate the characteristics of protective oxide layer formed on the surface of the two model alloys. In the second stage, the same electrochemical techniques were employed during the application of the unidirectional sliding test to evaluate the total material loss for different applied contact pressures. In the last stage of the experimental procedure the self healing process was investigated by intermittent unidirectional sliding tests.The total material loss after a tribocorrosion experiment, determined by profilometric measurements, is the sum of two components: the material loss due to corrosion of active material in the wear track, which can be calculated fro, the corrosion current values given by impedance measurements; applying Faraday’s law, and the material loss due to mechanical wear, the difference between total material loss and the material loss due to corrosive wear. Experiments proved that the total material loss increases with the increase of the contact pressure in both cases of Ni – Cr model alloys. The prevailing contribution in the total volumetric material loss is the material loss due to mechanical wear of active material in the sliding track. Nevertheless, the increase of the chromium content improves the resistance to the corrosive wear of active material in the sliding track. From qualitative viewpoint, a mechanism of abrasive wear by micro-ploughing was revealed in the sliding tracks.
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Claudiu Constantin Ionescu. Caractérisation des mécanismes d'usure par tribocorrosion d'alliages modèles Ni-Cr. Autre. Ecole Centrale Paris, 2012. Français. ⟨NNT : 2012ECAP0044⟩. ⟨tel-00782644⟩

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