Federico R. García-Galván

Biocompatibility and Corrosion Protection Behaviour of Hydroxyapatite Sol-Gel-Derived Coatings on Ti6Al4V Alloy

The aim of this work was to prepare hydroxyapatite coatings (HAp) by a sol-gel method on Ti6Al4V alloy and to study the bioactivity, biocompatibility and corrosion protection behaviour of these coatings in presence of simulated body fluids (SBFs). Thermogravimetric/Differential Thermal Analyses (TG/DTA) and X-ray Diffraction (XRD) have been applied to obtain information about the phase transformations, mass loss, identification of the phases developed, crystallite size and degree of crystallinity of the obtained HAp powders. Fourier Transformer Infrared Spectroscopy (FTIR) has been utilized for studying the functional groups of the prepared structures. The surface morphology of the resulting HAp coatings was studied by Scanning Electron Microscopy (SEM). The bioactivity was evaluated by soaking the HAp-coatings/Ti6Al4V system in Kokubo’s Simulated Body Fluid (SBF) applying Inductively Coupled Plasma (ICP) spectrometry. 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) and Alamar blue cell viability assays were used to study the biocompatibility. Finally, the corrosion behaviour of HAp-coatings/Ti6Al4V system was researched by means of Electrochemical Impedance Spectroscopy (EIS). The obtained results showed that the prepared powders were nanocrystalline HAp with little deviations from that present in the human bone. All the prepared HAp coatings deposited on Ti6Al4V showed well-behaved biocompatibility, good bioactivity and corrosion protection properties.

Effect of heat treatment of magnesium alloy substrates on corrosion resistance of a hybrid organic–inorganic sol–gel film

The influence of heat treatment of magnesium alloy substrates on corrosion resistance of a sol–gel coating has been assessed during immersion tests in 0.6 M NaCl aqueous solution. Relative differences in the chemical nature of the layers were quantified by scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX). Corrosion behaviour was evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measurement. Long-term immersion testing show that the sol–gel/heat treated AZ61 substrate exhibits a superior anti-corrosion property in comparison with the sol–gel/non-heated substrate. In contrast, no significant changes have been observed between the heated and non-heated samples in the case of the sol–gel coated AZ31 substrates. A link was found between lower O/Si atomic ratios observed by EDX analysis on the sol–gel coatings after the preparation process and reduced corrosion upon the coated substrates. Heat-treatment increased the protective properties of the passive film on the surface of the AZ61 substrate and hence inhibited magnesium dissolution and hydrophilic group formation during coating preparation.

Native Oxide Films on AZ31 and AZ61 Commercial Magnesium Alloys – Corrosion Behaviour, Effect on Isothermal Oxidation and Sol–gel Thin Film Formation

The authors present a review of their recent research work in an endeavour to interpret the influence of native oxide films on the corrosion behaviour of commercial AZ31 and AZ61 magnesium alloys or on the oxidation kinetics in air at 200°C. The tendency of some of these thin films to be sufficiently protective in mild or weak corrosive environments is examined. For obtaining oxide films with different protective properties, some of the specimens are tested with the surface in the as-received condition, while others are tested immediately after mechanical polishing. The technique applied to characterise thin (thickness of just a few nanometres) oxide films present on the surface of alloys has basically been XPS (X-ray photoelectron spectro‐ scopy) in combination with ion sputtering. Oxidation resistance of the alloys is quantified by thermo gravimetric (TG) curves and their corrosion rate is evaluated by Electrochemical Impedance Spectroscopy (EIS) and hydrogen evolution measure‐ ment in chloride solutions with different aggressivity. Emphasis is placed on the possible effects of: (a) the different thickness of the native oxide films formed on the polished surfaces on the corrosion behaviour of the alloys; and (b) the different film homogeneity and uniformity on the oxidation results. Finally, an attempt will be made to learn more about the influence of the native oxide films that cover the substrate on the subsequent growth and protective behaviour of the sol–gel coatings.

Controlled Rate Thermal Analysis (CRTA) as New Method to Control the Specific Surface in Hydroxyapatite Thin Coatings

The control of the texture in synthetic hydroxyapatite ceramics had limited their appli‐ cation in the field of the materials for bone implantation, even more when it is used as a filling in cements and other formulations in orthopedic surgery. The present article shows preliminary results demonstrating the effectiveness of a modification of the con‐ trolled rate thermal analysis (CRTA), developed by J. Rouquerol, used for the prepara‐ tion of ceramic materials with controlled textural characteristics, during the formation of ceramic powders of synthetic hydroxyapatite at low temperatures. The thermal treat‐ ments of the hydroxyapatite were carried out in a device connected to a computer, to control temperature and pressure system, keeping the decomposition speed constant. Results, reported when preparing ceramic powders of hydroxyapatite at 300 and 850°C under controlled pressure, using synthetic hydroxyapatite with a Ca/P molar ratio equal to 1.64, were checked using IR spectroscopy and X‐ray diffraction, showed that the formed phase corresponds to that of crystalline hydroxyapatite, even at 300°C of maxi‐ mum temperature. Values of specific surface (BET) between 17 and 66 m2/g, with pore size in the range of 50–300 Å in both cases are obtained by N 2 absorption isotherms, when analyzing the isotherms of nitrogen absorption.

A Measuring Approach to Assess the Corrosion Rate of Magnesium Alloys Using Electrochemical Impedance Spectroscopy

An attempt was made to estimate the corrosion rate of AZ31 and AZ61 magnesium alloys immersed in 0.6 M NaCl during long‐term exposure using electrochemical impedance spectroscopy (EIS). The EIS results were compared with the corrosion rate independently assessed by the hydrogen evolution test. A correlation was established between the integration of the polarization resistance (Rp) and charge transfer resistance (Rt) over time, as evaluated by EIS and hydrogen gas measurements. Regardless of the immersion time, a strong link was found between the Rt and Rp values determined by EIS. This relation seems to depend on the composition of the alloy. The influence of immersion time on the estimated corrosion rate reliability was investigated. The typical deviations of the measurement methods are apparently decreasing upon prolonging the immersion time. No significant errors were obtained in the measurement of the corrosion rate when using Rt or Rp determined by EIS with their corresponding “apparent” Stern‐Geary coefficient values compared with the real values determined by gravimetric measurements.