Francis Dalard

Influence of fluoride content and pH on the corrosion resistance of titanium and its alloys.

In the last decade, new titanium alloys have been developed in different areas of dentistry, such as TiAl6V4 and NiTi. Concurrently, treatments using fluoride supplementation, such as odontology fluoride containing gels, have also been widely used in odontology. The aim of this study was to compare the NiTi, NiTiCo and TiAl6V4 alloys with the titanium, regarding the corrosion resistance in Fusayama Meyer artificial saliva with different pH and fluoride contents. In the experimental methods, the corrosion potential was measured over time, then a linear sweep voltametric analysis measured the polarization resistances and corrosion currents. Our results have shown that TiAl6V4 alloys have a good corrosion resistance as good as that of titanium in Fusayama Meyer saliva and acid salivary solution, except NiTi and NiTiCo alloys. Conversely, we noticed, as for the titanium, a remarkable localized corrosion phenomenon of those new alloys in fluoride and acid-fluoride salivary solutions. The fluoride ions could cause the breakdown of the protective passivation layer that normally exists on the titanium and its alloys, leading to pit corrosion.

Measurement and evaluation of galvanic corrosion between titanium/Ti6Al4V implants and dental alloys by electrochemical techniques and auger spectrometry

The purpose of this study was to investigate, in different experimental conditions, the galvanic corrosion phenomena which can exist between a dental suprastructure and a dental implant. The electrochemical behavior of 7 alloy superstructures with titanium and titanium alloy (Ti6A14V) implants was investigated by electrochemical means in Fusayama-Meyer de-aerated saliva and Carter-Brugirard (AFNOR) non de-aerated saliva. Different techniques were used to obtain the value of the galvanic coupling current and potential for each couple. All showed very low corrosion rates, ranging from 10(-6) to 10(-8) A. Surface analysis confirmed these results.

Comparison of corrosion behaviour in presence of oral bacteria

The aim of this study was to compare the resistance of the corrosion of dental alloys in a solution containing oral bacteria named Actinomyces viscosus (ATCC19246). In this paper, we explain the choice of this precise species of bacteria, then specify its culture in artificial saliva and the experimental precautions needed to avoid the pollution by other bacteria. The electrochemical behaviour of two dental alloys (Ni-Cr alloy and gold-based alloy) was investigated by electrochemical means in sterile Fusayama artificial saliva (AS), AS enriched with sterile yeast extract (YE) and YE modified by introducing bacteria (AV). Open-circuit potentials, potentiodynamic curves, polarization resistance and impedance spectroscopy are the electrochemical procedures selected for this work. It has thus been shown that the open-circuit potential of the non-precious alloy is always lower than that of the gold precious alloy, and the colonization of metal surface by bacteria caused a drop in open circuit potential. The electrochemical impedance spectroscopy results have shown that the electrolyte resistance decreased between the AS, YE and AV milieu, in the presence of bacteria a slight decrease in polarization resistance was observed with the precious alloy and an increase with the non-precious alloy. The drop in the electrolyte resistance cannot explain the change in polarization resistance. The influence of Actinomyces viscosus might be essentially due to the consumption of oxygen at the metal/electrolyte interface of the specimen. For the non-precious alloy, the absence of oxygen (instigator of corrosion) led to an increase in polarization resistance whereas the slight decrease for the precious alloys might be justified by the organic and inorganic metabolites released by bacteria in to the electrolyte. The scanning electron micrography after electrochemical analysis, confirmed the absence of contaminants. These preliminary results demonstrate the unquestionable influence of this bacteria on the corrosion behaviour of the alloys studied, however, further studies are necessary.

Chloride Removal From Archaeological Cast Iron by Pulsating Current

Abstract The electrochemical stabilization of archaeological cast iron, currently used in restoration and conservation, is designed to extract chlorides by applying an electrical field between the metal (object containing a metal core), which is cathodically polarized, and a stainless steel cage, which is anodically polarized. However, stabilization of artifacts in potentiostatic mode is a relatively long procedure and weakens the artifacts because of the hydrogen evolution when the applied voltage is too negative. In an effort to reduce the secondary effects caused by hydrogen, we decided to study the use of pulsating currents. The aim is to be selective with respect to the reactions produced and to increase the rate of dechlorination treatment. By applying a pulsating signal with a constant current and a limit potential (Elim, Elim <E), the same quantity of chloride (100%) was extracted in the same treatment time as in potentiostatic mode. After optimizing the pulse parameters (i, ton, toff, Elim), the extraction ratio (QCl/Qtotal) increased from 0·17 (potentiostatic mode) to 0·49 (pulsating mode) and side-effects due to hydrogen were avoided.

Electrochemical behaviour of zinc-iron intermetallic compounds in an aqueous solution containing NaCl and ZnSO4

Abstract FeZn alloys have widespread technological applications, especially in the automotive industry with galvannealed steel sheets. Pure and homogeneous intermetallic zinc-iron phases were fabricated and characterized for their metallurgical and physical properties. The electrochemical behaviour of these phases was studied in a chloride-sulfate solution generally used to characterize galvannealed coatings on steel. Selective dissolution occurred for the zinc-rich ζ and δ phases. This mechanism induced an iron enrichment of the surface and caused a modification of the electrochemical behaviour of the sample. A film of corrosion products, developed during the coulometric anodic dissolution of galvannealed steel sheets, seems to be related to this surface change. The study confirmed the previous results and showed that electrochemical stripping of the galvannealed coating is not convenient, in this electrolyte, for accurate determination of the structure of the coatings.

Benzotriazole as inhibitor for copper with and without corrosion products in aqueous polyethylene glycol

Electrochemical methods, including polarization experiments and impedance spectroscopy, were used to evaluate the effectiveness of benzotriazole (BTA) in an aqueous solution of polyethylene glycol (PEG) in protecting polished archaeological copper or archaeological copper covered with corrosion products. The adsorption of PEG on the polished copper significantly limited the corrosion current. The presence of benzotriazole enhanced the protection of the polished copper, giving maximum protection at a concentration of 10−2 mol l−1 of BTA in 20 vol% PEG 400 solution. On the other hand, PEG solutions caused degradation of the corrosion products of the copper. This degradation increased with time. When BTA was added, the corrosion products were preserved and, the higher the BTA concentration, the more the corrosion current decreased. In PEG 400 solution protection of the corrosion products of the copper by BTA improved over time.

A study of galvanic corrosion during coulometric dissolution of galvannealed steel

Abstract Galvanostatic stripping is used as a coating structure analysis technique for galvannealed steel sheet. The thickness of the coating determined in this way is underestimated by about 20% compared to the thickness measured by chemical dissolution. This study has shown that pH, electrolyte composition and current density affect coulometric measurements. Because of additional galvanic current between zinc rich zinc-iron phases and more noble phases (iron rich zinc-iron phases), hydrogen evolves during anodic stripping. This is especially observed at low current density. Also the duration of coulometric anodic dissolution cannot give the actual coating thickness. Potentiodynamic analysis tends to show that potentials observed on galvanostatic stripping curves are the result of different iron-zinc phase layers.

Study of sacrificial anode cathodic protection of buried tanks : Numerical modelling

A finite element numerical model was set up to calculate the secondary distribution of potential and current density at the surface of a buried tank. The steel gas tank of interest was protected by both coating and two sacrificial anodes (magnesium alloy or zinc). The dispersion of actual soil properties was taken into account by use of three typical soils. The comparison of two dimensional and three-dimensional models shows that the 2D model is obviously both convenient and time saving. The numerical model allows the calculation of the cathodic protection current and of the local potential in every point of the tank. The model intends to compare the relative influence of coating quality, electric conductivity of soil and position, size and type of the sacrificial anodes (magnesium or zinc). Soil conductivity and coating porosity appear as the two most influential parameters. This model justifies the interest of the tank experimental potential and current measurements.List of symbols

Contribution of the Acoustic Emission Technique to Study Aluminum Behavior in Aqueous Alkaline Solution

In situ acoustic emission (AE) was coupled with linear-sweep voltammetry from anodic to cathodic potentials to study 5N5-aluminum behavior in 4 M aqueous potassium hydroxide solution. We clearly highlight a correlation between the AE and electrochemical signals. Moreover, the different types (e.g., in terms of frequency) of AE signals detected, render possible the discrimination between the different electrochemical phenomena that can take place simultaneously on the electrode surface during the voltammetry, e.g., hydrogen evolution, oxide formation, etc. Such phenomenological discrimination was not achievable only using electrochemical measurements.

AISI 304 L stainless steel decontamination by a corrosion process using cerium IV regenerated by ozone. Part I: Study of the accelerated corrosion process

This paper describes the study of a new decontamination process of AISI 304L stainless steel from dismantled nuclear power plants. A very thin active contaminated surface layer was stripped from the underlying metal by corrosion in a solution of nitric acid with the addition of cerium nitrate. The Ce4+/Ce3+ concentration ratio was initially equal to unity and ozone/oxygen bubbles were used to regenerate Ce3+ ions into Ce4+ ions. The study was performed in a laboratory cell prior to preliminary optimization in a three-litre reactor. The objective was to obtain a corrosion rate of about 10 micrometers per day. This target was reached in 10−2 mol l−1 of cerium nitrate with bubbling of a 1.56 g h−1 ozone flow in a 60 l h−1 total gas flow. The corrosion rate depended essentially on the Ce4+ concentration. The stainless steel exhibited intergranular corrosion. The corrosion rate was monitored by measuring the solution oxidizing potential using a precious metal electrode.