J. Creus

Corrosion behavior in artificial seawater of thermal-sprayed WC-CoCr coatings on mild steel by electrochemical impedance spectroscopy

The corrosion behavior in artificial seawater of different as-sprayed ceramic-metallic (cermet) coatings applied on low-alloy steel was studied. Five conditions, associated to modifications of the composition of the powder or deposition parameters were evaluated. The degradation mechanisms were studied during extended immersion tests using conventional electrochemical measurement and electrochemical impedance spectroscopy. The extended immersion tests reveal that these as-thermal-sprayed coatings present a cathodic behavior compared with steel. During the first hours of immersion, the electrolyte infiltrates the defects of the coatings, which then result to the local degradation of the substrate accelerated by the galvanic coupling with the cermet coating. Optical observations and Raman analyses reveal the formation of calcium carbonates like aragonite on the cermet surface, very close to the appearance of local anodic sites. The cross-sectioned views reveal the infiltration of the corrosive solution, and the depth penetration of the degradation of steel substrate probably due to the acidification of the anodic sites.

Electrodeposition of zinc–ceria nanocomposite coatings in alkaline bath

Use of electrodeposited (ED) zinc coatings is a widespread solution to protect steel against corrosion [1]. Many studies were performed in order to enhance the corrosion properties of the ED zinc coatings through alloying elements like nickel, cobalt or iron [2–7]. Mn and Mg additions were also investigated to reduce the release of metals towards the environment [8–12]. However, important discrepancies are reported on how these elements may influence the corrosion properties. Similarly, the improvement of sacrificial properties by reducing the dissolution kinetics of Zn-based coating is still on interest. Metal matrix composites (MMCs) seem to be a promising way to enhance the mechanical properties which are considered as a major drawback of zinc coatings [13]. In such systems, two structures coexist: a soft metallic-based matrix associated with a hard phase. During the past decades, research studies weremainly focused on the electrodeposition of nickel, copper, or cobalt-based composite coatings [14–16] for mechanical and/or corrosion applications, but in a lesser extent on sacrificial zinc matrix [17, 18], subject of our study. Regarding the hardening phase, oxides, like TiO2, SiO2, and Al2O3, are among the most used particles involved in the reinforcement of the metallic matrix [14–16]. Literature presents different mechanisms, based on bi-phased coatings containingmicrometer-sized particles. The most relevant theory is attributable to Guglielmi [19] who proposes an incorporation of particles linked to both the particle concentration in the bath and the current density. However, he does not consider neither hydrodynamic effects nor the particle characteristics. Few years later, Celis et al. improved this model with a more detailed four-step model [20]. Other models have been proposed afterwards but no one can be generalized to all matrix– reinforcement systems. CeO2 nanoparticles, like other rare earth-containing compounds, present promising interests owing to their high hardness [21], low coefficient of thermal expansion, and good corrosion protectiveness. They are attractive in many technological fields for their intrinsic properties such as ionic conduction, reversible oxygen storage, and catalytic activity [22]. CeO2 nanoparticles have already been used to synthesize MMCs by electrodeposition in conventional acidic baths, especially for the nickel [23–25], cobalt [26], and copper systems [27]. However, very few works deal with their incorporation in zinc matrix [28]. Furthermore, according to our knowledge, no studies have been dedicated to the zinc matrix composite coating electrodeposited from an alkaline bath. Deposition of zinc-based composite coatings was described by Azizi et al. [29]. His mechanism implies adsorption of zinc cations on the particle surface. This would favor the development of a positively net charge, susceptible to promote migration towards the negatively polarized substrate, but this approach remains still discussed by other authors [30, 31]. Another important issue to optimize the MMC electrodeposition process concerns the dispersion of particles. Surface L. Exbrayat : C. Rébéré :C. Berziou : C. Savall : J. Creus LaSIE, Université de la Rochelle, Av. Michel Crépeau, 17042 La Rochelle, France

Impact of coherent and incoherent twin boundaries on the microhardness of annealed nanocrystalline Ni–W alloys

Abstract The microstructural evolution of nanocrystalline Ni–W alloys with annealing temperature and more specifically grain boundary (GB) character is investigated through several techniques and correlated with the hardening behaviour. It is shown that two distinct regions can be identified in relation to the annealing temperature and the microstructural evolution. At temperatures below 550 °C (Regime I), a small increase in grain size is observed and is accompanied by a significant hardening and an increase in the fraction of Σ3 incoherent twin boundaries. At temperatures above 550 °C (Regime II), the thermal stability is overcome and important grain growth occurs with a decrease in both the volumic fraction of GBs and the microhardness. It is suggested that the microhardness evolution during heat treatment is influenced by two opposing processes: an increase in the fraction of incoherent twin boundaries (hardening effect) and grain growth (softening effect). Both aspects are directly associated with the mean free path of mobile dislocations.

Dislocations effect on kinetic of passivation of polycrystalline nickel in H2SO4 medium

Abstract Metallurgical state is an important parameter which affects the passivation process. In this study, the first experimental results on the effect of dislocation density and their distribution on the surface on the process of oxide growth are reported for polycrystalline nickel in H 2 SO 4 media at 291 K. It is demonstrated that this effect can not be investigated without tacking into account the grain size and composition of medium. Dislocation density promotes the adsorption of hydroxide and modifies the properties of the oxide layer.