Mihai V. Popa

Corrosion resistance improvement of titanium base alloys

The corrosion resistance of the new Ti-6Al-4V-1Zr alloy in comparison with ternary Ti-6Al-4V alloy in Ringer-Brown solution and artificial Carter-Brugirard saliva of different pH values was studied. In Ringer-Brown solution, the new alloy presented an improvement of all electrochemical parameters due to the alloying with Zr; also, impedance spectra revealed better protective properties of its passive layer. In Carter-Brugirard artificial saliva, an increase of the passive film thickness was proved. Fluoride ions had a slight negative influence on the corrosion and ion release rates, without to affect the very good stability of the new Ti-6Al-4V-1Zr alloy.

Characterisation of anodic oxide films formed on titanium and two ternary titanium alloys in hydrochloric acid solutions

The purpose of this paper is to characterise the anodic oxide films formed on titanium and two ternary titanium alloys, Ti-15Mo-5Al and Ti-10Mo-10Al in 1%, 10% and 20% hydrochloric acid solutions at 25, 50, and 75°C. The anodic film on titanium in hydrochloric acid is stable between + 0.6 V (SCE) and + 2.0 V (SCE). For our new ternary titanium alloys, the passive film is formed at about + 0.6 V (SCE) and is stable to + 2.0 V (SCE). The anodic polarization curves for alloys differ from the base metal curve, presenting two peaks for the critical passivation current density in the active-passive potential range. At the first current peak (the first critical passivation potential E cr1 ) a porous titanium pentaoxide (Ti 3 O 5 ) is formed. When the potential reaches the second current peak (the second critical passivation potential E cr2 ) the compact and protective titanium dioxide (TiO 2 ) is formed. The impedance spectra exhibit the typical behaviour for a passive film i.e. a near capacitive response illustrated by a phase angle close to - 90 over a wide frequency range. The oxide film on titanium and its alloys in hydrochloric acid solutions exhibits a high resistance and a low capacitance (with the increase of the potential) attributed to the surface roughness decrease as the oxide layer thickens.

Anodic passivity of some titanium base alloys in aggressive environments

The passivity of titanium, binary Ti-15Mo and ternary Ti-15Mo-5Al alloys in hydrochloric acid solutions was studied by potentiostatic, potentiodynamic, linear polarization and electrochemical impedance spectroscopy (EIS) techniques. The anodic passivity of binary Ti-15Mo and ternary Ti-15Mo-5Al titanium alloys differs from that of the base metal in hydrochloric acid solutions. The corrosion potentials of both alloys are nobler than of the titanium because the beneficial effect of molybdenum. The critical passivation current density for binary Ti-15Mo alloy is higher than of titanium; this fact can be explained by the instability of the constituent phases in hydrochloric acid solutions. Ternary Ti-15Mo-5Al alloy exhibits two critical passivation current densities (icr1 and icr2) with higher values than of the base metal and two critical passivation potentials (Ecr1 and Ecr2); at the first critical passivation potential (Ecr1) the porous titanium trioxide (Ti3O5) is formed and at the second critical passivation potential (Ecr2) this oxide is converted to a still higher valence oxide, the compact and protective titanium dioxide (TiO2). The dissolution current densities in the passive range of alloys are higher than of the base metal due the dissolution of the alloying elements in this potential range. The alloys are more resistant than titanium presenting lower corrosion rates. A three time constants equivalent circuit was fitted: one time constant is for the double layer capacity (Cdl) and for the passive film (Rp); another time constant is for the charge transfer reactions visualised by a constant phase element (CPE) and a resistance (R1); the third time constant is for diffusion processes through the passive film represented by a resistance (R2) and a Warburg element (W). Anodische Passivitat von verschiedenen Titanbasislegierungen in aggressiven Umgebungen Die Passivitat von Titan sowie der binaren Legierung Ti-15Mo und der ternaren Legierung Ti-15Mo-5Al wurde in Salzsaurelosungen mittels potentiostatischer, potentiodynamischer und linearer Polarisation sowie elektrochemischer Impedanzspektroskopie (EIS) untersucht. Die anodische Passivitat der binaren Ti-15Mo und der ternaren Ti-15Mo-5Al Legierungen unterscheidet sich in Salzsaurelosungen von dem reinen Titan. Die Korrosionspotentiale beider Legierungen sind edler als das des Titans aufgrund des positiven Einflusses des Molybdans. Die kritische Passivierungsstromdichte fur die binare Ti-15Mo-Legierung ist hoher als die von Titan, was durch die Instabilitat der Bestandteile in den Salzsaurelosungen erklart werden kann. Die ternare Ti-15Mo-5Al-Legierung weist zwei kritische Passivierungsstromdichten (icr1 und icr2) auf, die hoher liegen als fur das reine Metall, und zwei kritische Passivierungspotentiale (Ecr1 und Ecr2); beim ersten kritischen Passivierungspotential (Ecr1) wird das porose Titantrioxid (Ti3O5) gebildet und beim zweiten kritischen Passivierungspotential (Ecr2) wird dieses Oxid in ein Oxid mit noch hoherer Wertigkeit, das kompakte und schutzende Titandioxid (TiO2), umgewandelt. Die Auflosungsstromdichten im Passivbereich der Legierungen sind hoher als beim reinen Metall aufgrund der Auflosung der Legierungselemente in diesem Potentialbereich. Die Legierungen sind widerstandsfahiger als das Titan, was sich in niedrigeren Korrosionsgeschwindigkeiten bemerkbar macht. Ein elektrischer Aquivalentschaltkreis wurde angepasst: eine Zeitkonstante steht fur die Doppelschichtkapazitat (Cdl) und fur den Passivfilm (Rp); eine weitere Zeitkonstante steht fur die Ladungsubergangsreaktionen, die durch ein Konstantphasenelement (CPE) und einen Widerstand (R1) dargestellt werden; die dritte Zeitkonstante steht fur den Diffusionsprozess durch den Passivfilm, was durch einen Widerstand (R2) und ein Warburgelement (W) reprasentiert wird.

Microstructure, mechanical properties, and corrosion resistance of Ti-20Zr alloy in undoped and NaF doped artificial saliva

The corrosion behavior of a new, advanced Ti-20Zr alloy with α+β microstructure (determined by optical microscopy, XRD, and SEM) and very good mechanical properties (obtained from the stress-strain curve) is studied in this paper. The composition of the alloy native passive film was determined from a XPS analysis and the long-term corrosion resistance in undoped and doped states with 0.05M NaF artificial Carter-Brugirard saliva of different pH values, simulating the severe functional conditions of a dental implant, was analyzed by electrochemical methods. This alloy possesses an advantageous balance between good mechanical resistance and plasticity and Young’s modulus and exhibits more favorable electrochemical parameters and corrosion resistance than CP Ti due to its more resistant passive layer containing Ti2O3, TiO2, and ZrO2 protective oxides. After 1000 h of immersion in saliva, the protective properties of the alloy were enhanced due to the deposited surface layer that incorporated protective phosphates (shown by SEM and XPS).

CORROSION BEHAVIOR OF SOME TITANIUM BASE ALLOYS IN ACID SOLUTIONS

Abstract Corrosion and passivation behavior of new, ternary titanium alloys Ti-10Mo-10Al and Ti-15Mo-5Al in nitric acid solutions of different concentrations (20% and 60%) and temperatures (25○, 50○, and 75○C) is presented in this paper. Electrochemical impedance spectroscopy (EIS) data were obtained at different potentials as Bode plots. The spectra show some deviations from the ideal behavior, which demonstrate the necessity to consider a complex equivalent circuit with two time constants. This equivalent circuit is composed of the uncompensated ohmic resistance of the solution (R Ω ), the double layer capacity (C dl ) in parallel with the polarization resistance (R p ) for a passive film, and a Warburg element (W) associated with a resistance (R 1 ) for the diffusion processes. The anodic polarization curves present the self-passivation of these alloys. Both titanium and its ternary alloys directly pass into a passive state, due to the fact that, in strong oxidizing nitric acid solutions, the titanium directly oxidizes to Ti 4+ ions and spontaneously forms titanium dioxide TiO 2 . The passivation of the titanium alloys is due to the compact and protective film containing titanium dioxide, TiO 2 , and some insoluble molybdenum oxide, MoO 3 .

Influence of deposition technique on the protective properties of epoxy films

The electrochemical impedance spectra, the anodic polarization curves and the time dependence of paint capacitance and paint resistance have been used in this paper to evaluate the protective properties of epoxy films on carbon steel substrate. The coatings were formed using four deposition techniques (brushing, immersion, cathodic and anodic electrodeposition) with the aim to determine the effect of deposition type on the anticorrosive performances of epoxy paint. Interpretation of Nyquist and Bode impedance spectra with the immitance analysis Equivcrt. programme has established an electrical equivalent circuit with two time constants fitted to describe the electrodeposited epoxy/carbon steel system in the 3 % sodium chloride solution and an electrical equivalent circuit with four time constants fitted in case of epoxy films applied by brush or immersion. The anodic polarisation measurements show nobler corrosion potentials and smaller dissolution current densities for the carbon steel in the presence of the electrodeposited films in comparison with epoxy coatings applied by brush or with the immersion technique. The values of the porosity, water uptake, and ionic transport through the film emphasize the higher performances of the electrodeposited films, characterized by uniformity, porosity absence, low water permeability and few conductive pathways.

Biomimetic Hybrid Inorganic/Organic Coatings in the Increasing of the Integration Capacity of TiAlVZr Bioalloy

This paper will address key issues related to the synthesis, characterization and utilization of inorganic/organic hybrid coatings on TiAlVZr bioalloy to control adsorption at interface and the integration capacity of the implant. To enhance the integration capacity function of the biomaterial is necessary to control the composition and structure via the use of hybrid materials consisting of organic and inorganic biomimetic phases [1,2]. Such composite coatings mimic the structure of natural materials of tissues, in our case hydroxyapatite (HA) by association of bovine serum albumin (BSA) with calcium phosphates matrices. Calcium phosphate coating have been deposited on metal substrates by electrochemical method.

Electrochemical characterisation of electrodeposited organic coatings

The electrochemical impedance spectroscopy and the potentiostatic polarization techniques have been used to characterize the organic coatings electrodeposited on carbon steel. The coatings were formed during the cathodic and anodic electrodeposition of the modified epoxy resins on carbon steel samples previously polished and degreased (no primer was used). The experiments were conducted in stagnant, natural aerated 3% NaCl solution under ambient conditions. Analysis of the impedance spectra has established that an electrical equivalent circuit with two time constants fits to describe the electrodeposited epoxy coating/carbon steel system in the electrolyte solution. For the characterisation of the electrodeposited epoxy coatings the physical properties obtained from the principal elements of the equivalent electrical circuit and the electrochemical parameters obtained from the anodic polarization curves were used. It was established that these coatings present good performances characterized by low porosity, low water uptake and few conductive pathways and therefore a high efficiency. The coatings present a good adhesion to the carbon steels substrate; during the exposure period (60 days), no degradation or delamination process could be observed. Elektrochemische Charakterisierung von elektrolytisch abgeschiedenen organischen Beschichtungen Die elektrochemische Impedanzspektroskopie und potentiostatische Polarisationstechniken wurden eingesetzt, um organische Beschichtungen, die elektrolytisch auf unlegiertem Stahl abgeschieden wurden, zu charakterisieren. Die Beschichtungen wurden wahrend kathodischer und anodischer Elektrolytabscheidung von modifizierten Epoxidharzen auf Proben aus unlegiertem Stahl, die vorher poliert und entfettet wurden (es wurde keine Grundierung verwendet), gebildet. Die Versuche wurden in stagnierender, naturlich belufteter 3% NaCl Losung unter Raumbedingungen durchgefuhrt. Die Analyse der Impedanzspektren ergab, dass ein elektrischer Aquivalentschaltkreis mit zwei Zeitkonstanten geeignet ist, um das System elektrolytisch abgeschiedene Epoxidbeschichtung/unlegierter Stahl in der Elektrolytlosung zu beschreiben. Zur Charakterisierung der elektrolytisch abgeschiedenen Epoxidbeschichtungen wurden die physikalischen Eigenschaften verwendet, die aus den Elementen des elektrischen Aquivalentschaltkreises und den elektrochemischen Parametern aus den anodischen Polarisationskurven ermittelt wurden. Es wurde festgestellt, dass diese Beschichtungen ein gutes Verhalten zeigen, was durch geringe Porositat, geringe Wasseraufnahme und wenige leitfahige Pfade und damit einer hohen Wirksamkeit gekennzeichnet ist. Die Beschichtungen zeigten eine gute Haftung zu dem Substrat aus unlegiertem Stahl; wahrend der Auslagerung (60 Tage) konnte kein Schadigungs- oder Enthaftungsprozess beobachtet werden.

Performance Assessment of Electrodeposited Thin Epoxy Films

The paper presents the experimental results concerning the performance assessment of electrodeposited epoxy films using electrochemical techniques (electrochemical impedance spectroscopy–EIS and anodic polarization) assisted by microscopical studies. The films were formed during the cathodic and anodic electrodeposition of a modified epoxy resin on carbon steel substrate. The specimens were tested in a normal aerated 3% NaCl solution at room temperature for a total period up to 1500 hours. Analysis of the impedance spectra has established an equivalent electric circuit with two time constants fitted to describe the electrodeposited epoxy film/carbon steel/electrolyte system. The time monitoring of the principal elements from the equivalent electric circuit were used to determine protective properties of the electrodeposited epoxy films. Results show a high performance of electrodeposited epoxy films, characterized by uniformity, insignificant porosity, very low water and ions permeability.