Utilisation de la reaction de reduction de l'oxygene pour determiner sur electrode tournante la reactivite de quelques alliages dentaires semi-precieux

The reactivity of three dental alloys in artificial saliva has been studied through the diffusion current of oxygen reduction. In order to account for the fluctuations of the potential which can occur in vivo, we have studied the effect of a transient anodic polarization. The less reactive alloy after the anodic treatment appears to be the alloy which contains 10% of Pt and also much less Cu than the other alloys. This conclusion is supported by optical and electron miscroscopic investigation of the samples, which shows that for alloys containing little or no Pt, even with a high Au content, there is a drastic modification of the substrate by the formation of a thick anodic layer which is not sufficiently protective.     

On the corrosion resistance of the submerged arc welded titanium in concentrated hydrochloric acid 8 N

The corrosion behaviour of the submerged arc welded T-35 in concentrated 8 N HCl has been investigated. The weld metal is characterized by a martensite type structure, a hardness superior by 60 HV and an oxygen content about two and a half times higher than the base metal. In order to underline the part played by oxygen and structural transformations intervening during the welding operation, an analysis of the anodic current-potential curves by a reaction model involving four determining steps has been carried out. It has been shown that the two reaction rates which determine the passivation and the passivity of the metal depend on the oxygen content as well as the metallurgical structure. In the active dissolution range, whereas one of the two determining rates depends on the oxygen content, the other one is controlled by the metallurgical structure of the metal.     

Mean value forms in interval analysis

An interval extension of a function written in the centered form or the mean value form offers a second order approximation to the range of values of the function over an interval. However, the two forms differ with respect to inclusion monotonicity; the mean value form is inclusion monotone while the centered form is not. This is demonstrated in the following paper. Further, the mean value form is more generally applicable, and a mean value form for an integral operator is considered. It is shown that this form is also a second order inclusion monotone approximation.     

Anodic behaviour of titanium in acidic chloride containing media (HCl  NaCl). Influence of the constituents of the medium—III. Analysis of the electrochemical impedance. General dissolution — passivation mechanism

Analysing the electrochemical impedance diagrams plotted in a large frequency range (10^?3 Hz10⁴ Hz) enables to complete and support the dissolution—passivation scheme of titanium elaborated from steady state measurements.The analysis of the R_t. I product (charge transfer resistance by stationary current) reveals the presence of two active dissolution paths in tervalen titanium, one of which depends on the presence of the hydride (TiH₂). Moreover it confirms the participation of the tetravalent oxide TiO₂ in the establishment of the stable passivity. A complete reaction model specifying the chemical nature of all the intermediates and final products is proposed.     

An efficient approach for traffic load modelling of long span bridges

Traffic micro-simulation is the newly developed approach for loading calculation of long span bridges. The approach is quite precise, but computationally expensive to consider the full extent of traffic loading scenarios during a bridge lifetime. To address this shortfall, an efficient multi-scale traffic modelling approach is proposed. The proposed approach uses micro- and macro-simulation with different load model varieties (LMVs), or fidelities (levels of detail) of traffic loading in different bridge regions, to achieve optimal computation efficiency while maintaining the precision of loading calculation. Metrics of influence line (IL) characteristics, such as degree of nonlinearity, are proposed to evaluate the appropriateness of the choice of LMV, and standards of the metrics are also investigated to quantify the implementation of LMVs on bridge IL regions in the multi-scale modelling. Finally, two typical ILs are used along with random traffic modelling to study the feasibility of the proposed approach. It is shown that the multi-scale modelling approach proposed here achieves high computational efficiency and accuracy, which is significant for the massive traffic load simulation for lifetime bridge load effect analysis.     

Influence of surface segregation of sulphur on the corrosion resistance of 270 nickel in acidic and alkaline media

270 Nickel has a sulphur content lower than 1 p.p.m. However, surface concentrations close to saturation values can be obtained after several hours of cold-hammering followed by annealing procedures. The effects of this segregation on the electrochemical behaviour of nickel are analysed using current-potential curves and by examining the corrosion features via scanning electron microscopy (SEM). An overall increase in dissolution for the nickel with surface segregation of sulphur with respect to the homogeneous metal is observed in acidic (1m H₂SO₄) as well as in alkaline medium [(K₂SO₄+LiOH) at pH 9.5]. The reaction model proposed here, which depicts the dissolution kinetics, allows specification of the depassivating role of sulphur.     

Behaviour of titanium in concentrated hydrochloric acid: dissolution-passivation mechanism

Titanium dissolved in concentrated hydrochloric acid under valences III and IV. The stationary current-voltage curve is consistent, in a wide potential range, with a reaction model similar to that proposed for the fluorinated sulphuric media. Nevertheless there exist discrepancies between the behaviours in the two types of media. In particular an oxide formation and a particular variation with the potential of the product R_t×I (transfer resistance × stationary current) are specific to the hydrochloric media.     

Anodic behaviour of titanium in acidic chloride containing media (HCl---NaCl). Influence of the constituents of the medium—II. Calculation of the partial reaction orders

Analyzing the stationary current—potential curves with an original simplified reaction scheme of the dissolution—passivation of titanium, we have determined the partial reaction orders of the elementary rate determining steps of the mechanism. The discussion of a negative order with respect to H⁺ ions observed for the passivation electrochemical reaction leads to propose a modified reaction scheme including two passivation paths, one of the two implying the tetravalent oxide TiO₂, responsible of the stable passivity.     

Characteristic traffic load effects from a mixture of loading events on short to medium span bridges

In recent years, highway bridge load assessment has been recognised as an area through which savings can be made by avoiding unnecessary bridge refurbishment and replacement. Load effects in bridges result from single truck crossings or multiple-truck presence events which are, statistically, not identically distributed. Conventional approaches fit statistical distributions to mixtures of non-identically distributed load effects. Inaccuracies in the conventional approach are identified and an alternative approach is developed to find the characteristic load effects. Theoretical and field data are used to show the potential implications of conventional techniques and to demonstrate the application of the new approach.     

Interactions de la reduction du proton solvate et de l'oxygene dissous,sur une electrode de fer en solution sulfurique

Résumé A l'interface Fe/H₂SO₄, 1 N (aéré ou non), et dans le domaine de potentiel (–0·95, –1·2 V/E.S.S.), nous avons trouvé que le courant cathodique mesuré sur une électrode à disque tournant varie avec la vitesse de rotation suivant une loi de la forme:I=A+B ^1/2. A peut être identifié au courant de réduction du proton solvaté mais dépend fortement de la teneur en oxygène de l'électrolyte. La composante diffusionnelleB ^1/2 peut être identifiée à la réduction de l'oxygène dissous mais est très inférieure à celle relative à une surface uniformément réactive. Le blocage résultant est compatible avec l'analyse en fonction du potentiel de la corrélation entreA etB ^1/2 en supposant la réaction suivante: Fe H_adsFe H _ads ^* , où Fe H_ads est un hydrogène adsorbé faiblement lié de courte durée de vie (qq. s) et Fe H _ads ^* est un hydrogène adsorbé fortement lié de longue durée de vie (qq.h). Le déblocage résulte de la réaction chimique H_ads+1/4 O₂1/2 H₂O.Dans le cadre classique du mécanisme de dégagement de l'hydrogène en deux étapes, nous avons montré que notre modèle d'interdépendance implique que l'étape limitant la vitesse soit celle de Tafel à faible surtension et celle d'Horiuti pour des tensions cathodiques plus élevées.

---

We found that at the 1 N Fe/H₂SO₄ (aerated or de-aerated) interface within the potential range (–0d95, –1.2 V/S.S.E.) the cathodic current measured on a rotating disc electrode varies with the rotation speed according to the relation:I=A+B^1/2. A can be assigned to the reduction of H⁺ but depends strongly on the oxygen concentration. On the other hand the diffusional componentB^1/2 can be assigned to the reduction of dissolved oxygen but is much lower than that relative to a uniform reactive surface. The resulting blocking is consistent with the analysis as a function of the potential of the correlation betweenA andB^1/2 by assuming the following reaction: Fe H_adsFe H _ads ^* . Fe H_ads and Fe H _ads ^* are adsorbed hydrogen low bonded with a short life time (a few s) and strong bonded with a long life time (a few h) respectively. The blocked surface is activated by the chemical reaction Fe H_ads+1/4 O₂1/2 H₂O+Fe.In the classic framework of the two-step hydrogen evolution mechanism, we demonstrated that our interdependence model implies that the rate determining step is the Tafel reaction at low overpotentials and the Horiuti reaction for the highest overpotentials.