M. C. García-Alonso

Corrosion behaviour of AZ31 magnesium alloy with different grain sizes in simulated biological fluids

The corrosion behaviour of AZ31 magnesium alloy with different grain sizes immersed in simulated body fluids was compared in chloride solution (8 gl(-1)) and in phosphate-buffer solution (PBS). The influence of immersion time was also analyzed. Electrochemical techniques such as open circuit potential, polarization curves, transient currents and electrochemical impedance spectroscopy, complemented with scanning electron microscopy and energy dispersive spectroscopy, were used. Immediately after the immersion in the corrosive media the corrosion resistance was similar for both grain sizes of the AZ31 alloy and higher in NaCl solutions than in PBS. However, this corrosion behaviour was reversed after longer periods of immersion due to the stabilizing of the corrosion products of MgO by P-containing compounds. These P-compounds contribute to a higher level of protection by hindering the aggressive action of chloride ions. The best corrosion behaviour of the AZ31 alloy was obtained for the finest grain alloy associated with the highest transfer resistance value, after long periods of immersion in PBS.

Influence of the microstructure and topography on the barrier properties of oxide scales generated on blasted Ti6Al4V surfaces.

The long-term interfacial bond between an implant and bone may be improved by creating a rough surface on the implant in order to increase the surface area available for bone/implant apposition. A natural consequence of surface roughening is an increase in metal ion release, which is itself a surface dominated process. Based on this fact, the aim of this work is to study the influence of the microstructure and topography on the barrier properties of oxide scales thermally generated at 700 °C for 1h on Ti6Al4V surfaces after blasting with Al(2)O(3) particles (coarse) or SiO(2) and ZrO(2) particles (fine). The microstructural and topographical characterization of the thermally treated blasted surfaces has been studied by means of scanning electron microscopy coupled with energy dispersive X-ray analysis, contact profilometry and X-ray diffraction. The barrier properties and corrosion behaviour of the oxide layers have been studied by means of electrochemical impedance spectroscopy (EIS) in Hanks solution. Thermal treatment at 700 °C for 1h promotes the formation of oxide scales with different morphologies and crystalline structures depending on the degree of deformation of the blasted surface. The oxide scale grown on the finely blasted sample has a pine needle-like morphology which is mainly formed of anatase TiO(2). In contrast, the oxide scale grown on the coarsely blasted sample has a globular morphology formed mainly of rutile TiO(2). The differences in morphology, i.e. fine or coarse, of the oxide scales influence the corrosion response of the blasted thermally treated samples in Hanks solution. The EIS results permit evaluation of the different oxide scales from the capacitance and resistance values obtained in the high-frequency region and show a good correlation between the morphology and barrier properties. Oxidation treatment at 700 °C for 1h of Ti6Al4V samples coarsely blasted with Al(2)O(3) improves the corrosion behaviour due to an increase in the thickness of a compact, ordered and more structurally stable oxide scale. This is due to the globular morphology of the rutile (TiO(2)) structure maintaining an average surface roughness suitable for optimal osseo-integration with long-term interfacial bonding between the implant and bone.

Corrosion behaviour of an Fe3Al-type intermetallic in a chloride containing solution

Abstract The corrosion behaviour of an Fe 3 Al-base intermetallic compound with different crystal structures in a chloride containing solution has been investigated. The corrosion current densities of this intermetallic were independent of the material crystal structure showing a passive state stable with time. These corrosion rates were of the same order of magnitude as for 316L stainless steel. The pitting corrosion resistance evaluated by means of cyclic anodic polarization curves was high for all different states. Amongst the different crystal structures of this intermetallic alloy, the two ordered states present the lowest pitting probability. This Fe 3 Al intermetallic shows higher pitting corrosion resistance than the 316L stainless steel but its capacity for repassivation is lower. A damaging factor of influence on the pitting corrosion behaviour is the presence of non-metallic inclusions on the surface which reduce the pitting corrosion resistance by almost a half.

Study of the interaction of inorganic and organic compounds of cell culture medium with a Ti surface.

The interaction between Ti and each component of Dulbeccos modified Eagles medium was studied in depth using different techniques, such as the measurement of the corrosion potential, electrochemical impedance spectroscopy and polarization curves. The characterization of metal surfaces was carried out by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). The adsorption process of each component was studied using the quartz crystal balance (QCM). The QCM and XPS results reveal that the adsorption kinetics for phosphate and calcium ions is slow. However, the bovine serum albumin (BSA) totally covers the Ti surface rapidly. Because the passive film (titanium oxide) has acidic hydroxyl groups, the calcium ions would have a bridging effect on the electrostatic adsorption of phosphate ions as well as that of BSA. The polarization curves reveal that the adsorbed glucose permits the ionic diffusion of the oxygen to the electrode, while the BSA and fetal bovine serum (FBS) adsorbed after 7 days of immersion act as a diffusive barrier. The impedance measurement and data fitting to the electrical equivalent circuit model show that the resistance of the proteins/TiO(2) interface, for Ti immersed in FBS, is higher than those obtained for BSA, due to the proteins present in the solution as well as the fact that the adsorbed proteins on the surface are greater.

In situ cell culture monitoring on a Ti–6Al–4V surface by electrochemical techniques

In this work, the in situ interaction between Ti-6Al-4V alloy and osteoblastic cells has been studied by electrochemical techniques as a function of time. The interaction has been monitored for cell adhesion and growth of human osteoblastic Saos-2 cells on Ti-6Al-4V samples. The study has been carried out by electrochemical techniques, e.g., studying the evolution of corrosion potential with exposure time and by electrochemical impedance spectroscopy. The impedance results have been analyzed by using different equivalent circuit models that simulate the interface state at each testing time. The adhesion of the osteoblastic cells on the Ti-6Al-4V alloy leads to surface areas with different cell coverage rates, thus showing the different responses in the impedance diagrams with time. The effect of the cells on the electrochemical response of Ti-6Al-4V alloy is clearly seen after 4 days of testing, in which two isolated and well-differentiated time constants are clearly observed. One of these is associated with the presence of the cells and the other with a passive film on the Ti-6Al-4V alloy. After 7 days of culture, the system is governed by a resistive component over a wide frequency range which is associated with an increase in the cell coverage rate on the surface due to the extracellular matrix.

Response of MC3T3‐E1 osteoblasts, L929 fibroblasts, and J774 macrophages to fluoride surface‐modified AZ31 magnesium alloy

The present work evaluates the biocompatibility of a fluoride surface-modified AZ31 magnesium alloy (AZ31HF) with different cell lines that coexist in the implant environment to test its potential use as a biodegradable and absorbable biomaterial for bone repair. A clear stimulation of cell proliferation and an enhancement of the mitochondrial respiratory activity were observed when mouse osteoblasts (MC3T3-E1), fibroblasts (L929), and macrophages (J774) cell lines were cultured with the modified alloy. No significant change in apoptosis or viability rates was observed when osteoblasts and fibroblasts cultures were grown in the presence of this alloy. A proteomic analysis of the MC3T3-E1 cell extracts cultured in the presence of AZ31HF showed an overexpression of proteins related with the mineralization process, which is a necessary step for bone repair. An increase in the lactate dehydrogenase activity was observed in the MC3T3-E1 and J774 cell cultures that could be a response of the oxidative stress produced by the presence of the material. This stress could be related to the increase observed in the respiratory mitochondrial activity or respiratory burst measured in theses cultures that indicate damage in the cell membranes and subsequently some cell death. Results reported here, for and against AZ31HF, should be taken into account when considering the potential use of this modified alloy in bone repair applications.

Surface analysis of a heat-treated, Al-containing, iron-based superalloy

This work was supported by the project MAT95-0249-C03-01 of the Spanish Comision Interministerial de Ciencia y Tecnologia (CICYT).

Comparative study of the alumina-scale integrity on MA 956 and PM 2000 alloys

The present work analyzes the oxidation kinetics of MA 956 and PM 2000 alloys at 900 and 1100°C for exposure times up to 1000 hr. Special emphasis was placed on a comparison of the alumina-scale integrity formed at 1100°C by means of electrochemical tests at room temperature, which have been shown to be very reliable methods to detect the presence of microdefects within oxide scales. To check whether a preoxidation treatment makes these materials corrosion resistant against aggressive fluids, an electrolyte containing chloride ions was chosen. The mass gain of MA 956 was found to be slightly lower than that of PM 2000 up to 200 hr exposure at 1100°C and for the whole exposure range at 900°C. A subparabolic time dependence (n=0.3) of the oxide growth rate was determined for both alloys at both temperatures. On the other hand, the electrochemical-impedance spectroscopy (EIS) and anodic-polarization tests performed on preoxidized alloys (1100°C/100 hr) revealed good room-temperature corrosion behavior for both alloys, the corrosion resistance and polarization values being somewhat higher for preoxidized PM 2000. Consideration of these results and those of both surface and cross-section examinations of the scale, the better room-temperature corrosion behavior of preoxidized PM 2000 denotes the formation of a denser and mechanically more stable alumina scale containing a lower number of microdefects. This could result from the higher aluminum content of this alloy and the lower density of chemical heterogeneities within the scale. The higher mass gain of PM 2000 could be related to the higher concentration of oxide nodules on top of the alumina scale, as deduced from SEM examination.

Effect of substrate roughness on the corrosion behaviour of the Al2O3/MA 956 system.

This paper presents the influence of substrate roughness on the corrosion behaviour of the Al2O3/MA 956 system. An alumina layer of thickness 1-5 microm was generated of the MA956 alloy by thermal oxidation at 1100 degrees C using different exposure times. This Al2O3/MA 956 system with a polished substrate has shown excellent corrosion behaviour in a physiological fluid, due to the fact that the alpha-Al2O3 layer formed is dense, continuous and firmly adhered to the substrate, irrespective of the scale thickness. This good adherence allows it to withstand potentials above 1.7 V. Specimens with rough finish substrate and treatment times above 10 h present spallation of the alumina layer at the crests of the roughness profile. In this case a mixed corrosion behaviour between an alumina coated material and one with a passive layer is observed. In both types of specimens, rough and smooth, once the passivation layer is broken the repassivation capacity of the substrate is ensured due to the high chromium content of the alloy, under oxygenation conditions.

Oxidation Behavior of Fine-Grain MA 956 Superalloy

This investigation was undertaken in an attempt to gain a fundamentalunder-standing of the oxidation behavior of a fine-grain MA 956 in thetemperature range 800 to 1200°C, with emphasis placed on the scalemorphology and oxidation kinetics. Oxidation reaction led to the formationof a thin and dense alumina scale, irrespective of the oxidationtemperature. At intermediate temperatures plateletlike oxides of alumina,probably θ-Al2O3, covered the surface, whereas at othertemperatures small nodules of Fe-, Cr-, Ti or Y-rich oxides were observed inthe outer part of the alumina scale. The temperature dependence of theparabolic rate constant revealed a change in the oxidation mechanism with atransition at about 1000°C. The high-temperature mechanism is controlled bythe formation of α-alumina, whereas the low-temperature oxidationmechanism is controlled by formation of metastable alumina. Comparison withthe reported oxidation kinetics for coarse-grain MA 956 is inconclusive,since microstructure (grain size, texture) of the substrate changed duringoxidation in the high-temperature range.