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

Les alliages à base nickel sont utilisés dans les centrales nucléaires pour la fabrication des tubes générateurs de vapeur, parce qu ils présentent une bonne résistance aux sollicitations mécaniques élevées dans un milieu corrosif à haute température. La résistance au milieu corrosif est attribuée à une couche protectrice très fine de Cr2O3 qui se forme à la surface de l alliage. Les tubes générateurs de vapeur comptent parmi les composants les plus délicats à entretenir, d une part, du fait de leur importance pour la sûreté et, d autre part, parce que les tubes d échange sont soumis à de nombreux mécanismes de dégradation à cause de conditions sévères de fonctionnement.. L objectif de cette étude a été d évaluer et de comparer le comportement en tribocorrosion de deux alliages modèles Ni 15%Cr et Ni 30%Cr. Ces alliages ont été utilisés pour mettre en évidence, évaluer et comparer l influence de la teneur en chrome sur la formation de la couche d'oxyde superficielle et le rôle de cette dernière sur les mécanismes et cinétiques d'usure par tribocorrosion. L étude de la tribocorrosion a été effectuée à l aide d un tribomètre pion disque par application d un frottement unidirectionnel, en régime continu et intermittent en milieu aqueux boré lithié à la température ambiante. La dégradation de la couche protectrice formée à la surface des deux alliages a été suivie en mettant en œuvre un protocole expérimental en trois étapes. Dans la première étape du protocole, le suivi de l évolution du potentiel libre et l enregistrement des diagrammes d impédance électrochimique ont été utilisés pour analyser le comportement électrochimique des deux alliages en absence de frottement. Lors de la deuxième étape, les mêmes techniques électrochimiques ont été employées en présence d un frottement continu et pour différentes pressions de contact. Durant l étape trois du protocole, des essais de tribocorrosion en régime de frottement intermittent ont été réalisés pour mettre en évidence la capacité d auto cicatrisation de la surface des deux alliages modèles. Pour les deux alliages Ni - Cr, les essais de tribocorrosion ont mis en évidence une augmentation de l usure totale avec la pression de contact appliquée. Quel que soit la force normale appliquée, la principale composante de l usure totale est l usure mécanique du substrat mis à nu. L usure purement corrosive du substrat diminue avec l augmentation de la teneur en chrome. Du point de vue qualitatif, le principal mécanisme d usure identifié a été l usure abrasive par micro - labourage. Dans cette étude a été effectuée une étude de l usure d alliages modèles Ni Cr, dans les conditions de tribocorosion, ainsi que une analyse du mécanisme et la cinétique d usure. Le comportement de la couche superficielle d oxyde de chrome et son effet sur la résistance à la tribocorrosion ont été évalués. Ces résultats peuvent servir de base à la compréhension de l origine des problèmes qui peuvent apparaître pendant la vie d un alliage inoxydable à base nickel soumis à des efforts mécaniques en milieu agressifs et permettre une sélection plus pertinente des matériaux métalliques pour diverses applications industrielles, particulièrement dans le domaine nucléaire.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.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Effect of Mean Diameter Size of Disperse Phase on Morphology and Corrosion Resistance of Phenol – Formaldehyde Resin/Zn Coatings

The present work has the purpose of obtaining composite coatings using phenol – formaldehyde (PF) resin electrodeposited with zinc. The phenol – formaldehyde resin/Zn coatings were electrodeposited from a suspension of PF resin particles with two dimensions for mean diameter size of particles (0.1-5.0 μm and 6-10 μm) in aqueous zinc sulphate electrolyte. Suspension was prepared by adding 10 g/L PF resin particles into solution. The thickness and morphology of the coatings were investigated by SEM method. By adding PF resin in zinc electrolyte for electrodepositing we obtained a very good distribution of PF resin particles on zinc surface. The electrochemical behavior of the layers in the corrosive solution was investigated by electrochemical methods. As electrochemical test solution 0.5 M sodium chloride was used in a three electrode open cell. It was observed that by adding PF resin particles in zinc electrolyte for electrodeposition  were obtained composite layers more resistant to corrosive attack of 0.5 M NaCl than pure zinc obtained from electrodeposition at the same parameters for electrodeposition. Corrosion rate for pure zinc coatings was 72.05 µm/year versus 15.34 µm/year for coatings with mean diameter size of particles 0.1-5.0 μm, respectively 10.11 µm/year for coatings with mean diameter size of particles 6-10μm. Values of polarization resistance obtained with both electrochemical methods (potentiodynamic polarization and electrochemical impedance spectroscopy) were in a very good agreement

Improved Hardness and Tribocorrosion Properties of Nickel Coatings by Co-Depositing ZrO2 MicroSized Dispersed Phase During Electroplating Process

The tribocorrosion properties of ZrO2 – nickel microstrucured composite coatings have been studied in the following conditions: - Solution: 0.5 M K2SO4; - Tribo corrosimeter: pin on disc connected with electrochemical cell; - Normal Force: 10 N; - Rotation Speed: 120 tours/min. The objectives of our study in principal are to fully understanding the tribocorrosion process kinetic and mechanism of composite coatings materials. The samples with coating on a top of a cylinder were installed in a cell, containing the electrolyte and electrodes, and mounted on a pin-on-disc tribometer, with the working surface of the specimen facing upwards. The counterbody (pin) was a corrundum cylinder (7 mm in diameter), mounted vertically on a rotating head, above the specimen. The lower spherical end (radius = 100 mm) of the pin, was then applied against the composite surface (disc) with an adjustable normal force. When rotation was applied, the end of the pin draws a circular wear track (16 mm in diameter) on the working composite surface. Both continuous and intermittent friction tests were carried out. In the intermittent tests, friction was applied periodically: during each cycle, friction was first applied for 2 seconds at 120 rpm (sliding speed 100 mm/second) under 10 N (average pressure 120 MPa for hertzian contact conditions) and then stopped during a latency time (20 or 200 s.). This mechanical solicitation was repeated over 2500 cycles. Some features of these tests reproduce the wear conditions of composite coatings in the intermittent friction, K2SO4 0.5 M was used as corrosive and passivating electrolyte for tribocorrosion tests

Study of Zro2/Co-Deposition with Cobalt from Chloride Electrolyte

The metal-matrix composites are materials in which the properties of a metallic host material are modified with addition of a second phase (ceramics) by electrodeposition process. The second phase can be hard oxides or carbides particles or diamond, or solid lubricants or even liquid containing microcapsules. Most composite coatings contain micron-sized particles. The major challenges with the codeposition of second phase particles are the achievement of a high level of codeposition and avoiding the agglomeration of particles suspended in the electrolytes. The poorly homogeneous distribution of second phase particles in the metallic matrix can be detrimental to the mechanical properties. This work shows the most recent results regarding the influence of ZrO2 bioceramic dispersed in the cobalt matrix during electroplating process from a chloride cobalt electrolyte. The ZrO2/Co composite coatings were electrodeposited from a suspension of ZrO2 particles (mean diameter 10 μm) in aqueous cobalt chloride electrolyte. Suspension was prepared by adding 20 g/L ZrO2 particles into solution. Co-deposition of dispersed micro sized particles with cobalt during electroplating process influences the structure and surface morphology of composite coatings obtained. The pure cobalt coating has a rather regular surface, whereas the composite coatings surface has fine different disturbed structure. The composite coating thickness increases with increasing the current density, whereas the content of ZrO2 particles inside the cobalt matrix decreases with current density

Electrodeposition Obtaining and Corrosion Behaviour of Phenol Formaldehyde Resin/Zinc Composite Coatings

The present work has the purpose of obtaining composite coatings in zinc matrix by using phenol - formaldehyde resin type NOVOLAC (RESITAL 6358/1) in the electrodeposition process of zinc. The PF resin/Zn composite coatings were electrodeposited from a suspension of PF resin particles (diameter < 56 μm) in aqueous zinc sulphate electrolyte. Suspension was prepared by adding 10 g/L and 25 g/L PF resin particles into solution. The pure zinc coating has a rather regular surface, whereas the composite coatings surface has fine surface structure. For layers obtained at 3 A/dm2 the polarization resistance is higher for composite coatings with 10 g/L PF resin (Rp = 238.75 Ω cm2) than pure zinc (Rp = 34.54 Ω cm2). For layers obtained at 5 A/dm2 the polarization resistance is higher for composite coatings with 25 g/L PF resin (Rp = 430.19 Ω cm2) than pure zinc (Rp = 127.34Ω cm2). As test solution 0.5 M sodium chloride was used, it was observed that by adding PF resin particles in zinc electrolyte for electrodeposition it was obtained a very good distribution of resin particles on zinc surface and PF resin/Zn composite coatings obtained are most resistant at corrosive attack than pure zinc obtained from electrodeposition under the same conditions

Corrosion behaviour of titanium in the presence of Streptococcus mutans

Objective The main aim of this in vitro study was to evaluate the influence of Streptococcus mutans on the corrosion of titanium. Methods S. mutans biofilms were formed on commercially pure titanium (CP-Ti) square samples (10 mm × 10 mm × 1 mm) using a culture medium enriched with sucrose. Open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements were used to evaluate the corrosion behaviour of CP-Ti in the presence of S. mutans in Fusayama's artificial saliva. The corrosion of biofilm-free CP-Ti samples was also evaluated in artificial saliva. Biofilms biomass was measured by spectrophotometry, using crystal violet staining, after 1, 2 and 7 days. Results The OCP values recorded on CP-Ti in the presence of S. mutans (−0.3 ± 0.02 V vs. SCE) was lower than those on biofilm-free CP-Ti (−0.1 ± 0.01 V vs. SCE) after 2 h of immersion in artificial saliva (p < 0.05). That reveals a high reactivity of titanium in presence of S. mutans. Impedance spectra revealed the formation of a compact passive film on titanium in artificial saliva or in the presence of a 2 days old S. mutans biofilm even though the corrosion resistance of CP-Ti has decreased in presence of a S. mutans biofilm. Conclusion The presence of bacterial colonies, such as S. mutans, negatively affected the corrosion resistance of the titanium.The authors acknowledge the financial support provided by Alban Programme (cod. E06D103407BR), the Erasmus Student Exchange Programme of the CEC, FCT (PTDC/CTM/67500/2006) and the Scientific Research Community on Surface Modification of Materials funded by the Flemish Science Foundation (WOG-FWO-Vlaanderen)

Morphological Aspects and Corrosion Behaviour of Phenol Formaldehyde/Zn Composite Coatings

The present work has the purpose of studying morphological aspects and corrosion behaviour of pure zinc and composite coatings having zinc as metal matrix and phenol – formaldehyde resin type Novolac (RESITAL 6358/1) as dispersed phase obtained during the electrodeposition process of zinc. The phenol – formaldehyde resin/Zn composite coatings were electrodeposited from a suspension of phenol – formaldehyde resin particles in aqueous zinc sulphate electrolyte by adding 10g/L and 25g/L of phenol – formaldehyde resin particles in the electrolyte solution. The dimensions of resin particles were 0.1 - 5μm. The morphological aspects of the coatings were investigated by scanning electron microscopy method. While the pure zinc coatings have a rather regular surface, the composite coatings surfaces have finer grains and different morphologies. The electrochemical behaviour of the coatings in the corrosive solution was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy methods. As electrochemical test solution, 0.5M NaCl was used in a three electrode open cell. The polarization resistances of phenol – formaldehyde resin/Zn composite coatings are bigger than that of pure zinc coatings obtained under the same condition for electrodeposition

Introduction in "Testing tribocorrosion of passivating materials supporting research and industrial innovation: Handbook"

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Tribocorrosion: Material behavior under combined conditions of corrosion and mechanical loading

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