Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L31/00—Materials for other surgical articles, e.g. stents, stent-grafts, shunts, surgical drapes, guide wires, materials for adhesion prevention, occluding devices, surgical gloves, tissue fixation devices
  • A61L31/02—Inorganic materials
  • A61L31/022—Metals or alloys
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L27/00—Materials for grafts or prostheses or for coating grafts or prostheses
  • A61L27/02—Inorganic materials
  • A61L27/04—Metals or alloys
  • A61L27/047—Other specific metals or alloys not covered by A61L27/042 - A61L27/045 or A61L27/06
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L27/00—Materials for grafts or prostheses or for coating grafts or prostheses
  • A61L27/28—Materials for coating prostheses
  • A61L27/30—Inorganic materials
  • A61L27/32—Phosphorus-containing materials, e.g. apatite
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L31/00—Materials for other surgical articles, e.g. stents, stent-grafts, shunts, surgical drapes, guide wires, materials for adhesion prevention, occluding devices, surgical gloves, tissue fixation devices
  • A61L31/08—Materials for coatings
  • A61L31/082—Inorganic materials
  • A61L31/086—Phosphorus-containing materials, e.g. apatite
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/30—Anodisation of magnesium or alloys based thereon
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L2420/00—Materials or methods for coatings medical devices
  • A61L2420/02—Methods for coating medical devices

Definitions

• the present invention relates to the field of chemicals and more precisely to that of biocompatible alloys, as it concerns a process for covering biocompatible magnesium-based alloys, to be used for biomedical applications, with a coating which controls the corrosion and degradation thereof and makes the amount of hydrogen developed tolerable by the human body.
• Magnesium alloys and in particular AZ31 magnesium alloys are used for the production of biomedical devices such as prostheses and stents which are subject to corrosion phenomena inside the human body which can compromise the functioning thereof [B. Heublein, R. Rohde, V. Kaese, M. Niemeyer, W. Hartung, and A. Haverich. "Biocorrosion of magnesium alloys: a new principle in cardiovascular implant technology?" Heart 89, no. 6 (2003): 651-656; P. Zartner, R. Cesnjevar, H. Singer, and M. Weyand. "First successful implantation of a biodegradable metal stent into the left pulmonary artery of a preterm baby" Characterization and Cardiovascular Interventions 66, no.
• Said magnesium alloys have excellent mechanical properties, in particular the elastic modulus of the AZ31 magnesium alloys is comparable with that of human bone and they are biodegradable. Furthermore, they do not need to be removed because they perform their function of mechanical support for the immediate period necessary and then degrade releasing non-toxic substances, which can therefore be assimilated without problems by the human body [D.A. Robinson, R.W. Griffith, D. Shechtman, R.B. Evans, and M.G. Conzemius. "In vitro antibacterial properties of magnesium metal against Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus" Acta Biomaterialia 6, no.
• Chinese patent no. CN109758605 discloses a coating of acile hydroxyapatite on the surface of a magnesium alloy and the preparation process by means of electrochemical deposition.
• Chinese patent no. CN109646717 discloses a nano-hydroxyapatite coating on the surface of a magnesium alloy and the ultrasonic-based preparation process.
• Chinese patent no. CN109440160 discloses a process for modifying the surface of a magnesium alloy with a composite coating of hydroxyapatite modified with dopamine.
• Chinese patent no. CN109295438 discloses a process for preparing a surface micro structure hydroxyapatite coating on a magnesium alloy by means of a hydrothermal process in which the magnesium alloy is subjected to surface polishing, grinding and ultrasonic cleaning sequentially in acetone, deionized water and ethyl alcohol, thereafter drying; then the magnesium alloy thus treated is immersed in a Na Oh solution at a temperature between 60 and 90°C, then rinsed with deionized water and dried, and placed in a high pressure reactor containing the hydrothermal reaction solution. The reactor is heated up to a temperature between 100 and 140°C.
• the hydroxyapatite coating consists of a two-layer structure: the upper layer is composed of a flower shaped cluster structure composed of nanobars, and through the spaces between the clusters, it can be seen that the lower layer is uniform and with compact nanobar structure.
• the coating has a high resistance to corrosion and is of great commercial distribution value.
• Chinese patent no. CN109183127 discloses a process for preparing a composite coating of surface hydroxyapatite on a magnesium alloy which comprises the steps in which the surface of the magnesium alloy is subjected to a treatment of honing, polishing and oxidation in sequence, subsequently immersed in the hydroxyapatite dispersion liquid modified with polydopamine at 100-120°C for 0.5-1 hours, then the surface is extracted, cleaned with distilled water, dried and immersed again in the hydroxyapatite dispersion liquid modified with polydopamine, and the operation is repeated 3-5 times.
• Chinese patent no. CN109161955 discloses a preparation process for the electro-deposition of a hydroxyapatite and graphene oxide coating on the surface of a magnesium alloy by immersion in a hydrolyzed solution of aminopropyl triethoxysilane to obtain the silanization of magnesium, and the electrodeposition of hydroxyapatite and graphene oxide.
• Chinese patent no. CN108166036 discloses a process for preparing a nano-hydroxyapatite coating containing fluorine on the surface of a biomedical magnesium alloy.
• Chinese patent no. CN108004527 discloses a process for preparing a zinc-doped hydroxyapatite coating used for a magnesium alloy material.
• Chinese patent no. CN106756925 discloses a hydroxyapatite coating containing silver on the surface of a magnesium alloy.
• Chinese patent no. CN105457099 discloses a two-layer fluorine- doped hydroxyapatite coating on a magnesium alloy and a microwave preparation process thereof.
• Chinese patent no. CN104888271 discloses a process for preparing a hydroxyapatite coating containing strontium on the surface of a biodegradable magnesium alloy.
• Chinese patent no. CN104789957 discloses a microwave preparation process of a flower-shaped hydroxyapatite coating layer on the surface of the magnesium alloy.
• Chinese patent no. CN104436301 discloses a process for preparing an organic-inorganic hybrid coating on the surface of a magnesium alloy.
• Chinese patent no. CN104404480 discloses a process for preparing a composite coating of hydroxyapatite and bone collagen on the surface of a magnesium alloy.
• Chinese patent no. CN103933611 discloses a process for preparing a composite hydroxyapatite/polylactic acid coating on the surface of the medical magnesium alloy.
• Chinese patent no. CN103463681 discloses a process for preparing a biodegradable coating of fluorohydroxyapatite (FHA).
• FHA fluorohydroxyapatite
• Chinese patent no. CN103446627 discloses a process for preparing a hydroxyapatite coating with a modified surface on a biodegradable magnesium alloy using heat and ultrasonic treatment.
• Chinese patent no. CN101643929 discloses a preparation process by pulse electrodeposition of a hydroxyapatite coating on the surface of pure magnesium or a magnesium alloy and comprises the use of pure magnesium or a magnesium alloy as the material (substrate) for preparing the electrolyte, in which the concentration of Ca 2+ is 2.0-42.0 mmol * If 1 ; the concentration of H 2 P0 4 ⁇ is 1.0-26.2 mmol * If 1 ; the molar ratio of Ca/P is 1.6-2.0; the support electrolyte concentration is 0.1-1.0 mol * If 1 ; a pH value is 4.0-6.0; using the substrate material as the cathode and graphite as the anode; heating to 50-90°C; keeping the temperature constant; electrodeposition in a one way or two-way pulse mode to control the deposition current value, where the one-way pulse electrodeposition parameters comprise: 1-40 mA/cm 2 peak current density; pulse frequency 10-2000Hz; service factor 10-30 percent; the parameters of the bidirectional
• the inventors of the present invention have perfected a process, based on anodization, for covering magnesium alloys with a coating, in particular AZ31 magnesium alloys, to obtain a coating which has a protective function for the alloy in an aggressive environment.
• the process of the present invention allows to control the morphology, the thickness and the composition of the coating by varying the parameters of the anodization process, where the morphological features are able to predetermine the useful life of the product.
• the process of the present invention thanks to the appropriate selection of the composition of the bath and of the electrical operating parameters, allows the growth of efficient coatings with an excellent corrosion resistance.
• the technical problem is therefore solved by providing a process for covering magnesium alloys with a coating based on magnesium phosphates, magnesium carbonates and hydroxyapatite comprising the following steps: a)anodization in galvanostatic conditions of a magnesium alloy in an electrolytic solution based on potassium dihydrogen phosphate and potassium phosphate in a non- aqueous solvent at a current density between 0.5 mA cm -2 and 8 mA cm -2 , at a temperature between 100°C and 200°C and a time between 100 seconds and 60 minutes until a magnesium alloy is obtained with a coating of magnesium oxide and phosphates, in which the magnesium alloy is the anode and the cathode is an inert electrode; b)annealing the magnesium alloy having a coating of magnesium phosphates and magnesium carbonates as obtained at the end of step a) at a temperature between 350°C and 450°C for a time between 20 and 30 hours until a magnesium alloy is obtained with a coating
• the object of the present invention is also the magnesium alloy with a coating based on magnesium phosphates, magnesium carbonates and hydroxyapatite directly obtained from the process of the present invention.
• Figure 2 shows the impedance spectra (Nyquist diagrams) recorded at the OCP related to the AZ31 alloy samples anodized at different temperatures.
• Figure 4 shows the impedance spectra (Nyquist diagrams) recorded at the OCP related to the AZ31 alloy samples anodized at different current densities.
• Figure 5 shows the polarization curves related to the anodized AZ31 alloy samples at different current densities.
• Figure 6 shows the impedance spectra (Nyquist diagrams) recorded at the OCP related to the AZ31 alloy samples anodized at different times.
• Figure 7 shows the impedance spectra (Nyquist diagrams) recorded at the OCP related to the samples of uncoated AZ31 alloy, anodized alloy and anodized and heat-treated alloy.
• Figure 8 shows the volume of hydrogen produced (per surface unit) as a function of the immersion time for an untreated AZ31 alloy and an anodized one.
• Figure 9 shows the Raman spectrum of anodized and heat-treated AZ31 after 14 days of immersion in buffer SBF.
• the magnesium alloy AZ31 is defined by the following chemical composition: 2.89% wt Al, 0.92% wt Zn, 0.05% wt Mn, 0.01% wt Si, 0.002% wt Cu, 0.001% wt Ni, 0.004 % wt Fe, balance Mg.
• galvanostatic conditions mean conditions of constant electric current density between the electrodes.
• Simulated Body Fluid solution or Hank's solution means a solution having the following composition: NaCl 8 g/L, KC1 0.4 g/L, NaHCOs 0.35 g/L, NaH 2 P0 4 -H 2 0, Na 2 HP0 4 -H 2 0, CaCl 2 -2H 2 0, MgCl 2 , MgS0 4 -7H 2 0, Glucose.
• inert electrode means a platinum or steel electrode.
• OCP values means the recorded values of open circuit potential.
• annealing means the heat treatment which can alter the micro structure of a material.
• the present invention relates to a process for covering magnesium alloys with a coating based on magnesium phosphates, magnesium carbonates and hydroxyapatite comprising the following steps: a)anodization in galvanostatic conditions of a magnesium alloy in an electrolytic solution based on potassium dihydrogen phosphate and potassium phosphate in a non- aqueous solvent at a current density between 0.5 mA cm -2 and 8 mA cm -2 , at a temperature between 100°C and 200°C and a time between 100 seconds and 60 minutes until a magnesium alloy is obtained with a coating of magnesium oxide and phosphates, in which the magnesium alloy is the anode and the cathode is an inert electrode; b)annealing the magnesium alloy having a coating of magnesium phosphates and magnesium carbonates as obtained at the end of step a) at a temperature between 350°C and 450°C for a time between 20 and 30 hours until a magnesium alloy is obtained with a coating of magnesium phosphates and magnesium
• step a) the magnesium alloy is subjected to a cleaning step, preferably manual mechanical cleaning.
• K 2 HP0 4 is present in a concentration between 0.3 and 0.9 M and K 3 P0 4 is present in a concentration between 0.1 and 0.5 M.
• K 2 HP0 4 is present in a concentration equal to 0.6 M and K 3 P0 4 is in a concentration equal to 0.2 M.
• the non-aqueous solvent is non-toxic.
• the non-aqueous solvent is selected from the group consisting of: 1,4-butanediol, 1-decanol, dodecanol, glycerol.
• the non-aqueous solvent is glycerol.
• the electrolyte solution is 0.6 M of K 2 HP0 4 and 0.2 M of K 3 P0 4 in glycerol.
• the current density is 2 mA cm -2 .
• the temperature is 160°C.
• the time is 30 minutes.
• the temperature is 400°C.
• the time is 24 hours.
• the simulated body fluid solution consists of NaCl 8 g/L, KC1 0.4 g/L, NaHC0 3 0.35 g/L, NaH 2 P0 4 ⁇ H 2 0, Na 2 HP0 4 -H 2 0, CaCl 2 -2H 2 0, MgCl 2 , MgS0 4 -7H 2 0, Glucose.
• the temperature is 37°C.
• the coating based on magnesium phosphates, magnesium carbonates and hydroxyapatite obtained by means of the process of the present invention has a thickness between 5 and 20 pm, more preferably 10 pm.
• the magnesium alloy with a coating based on magnesium phosphates, magnesium carbonates and hydroxyapatite directly obtained from the process of the present invention can be used for the preparation of biomedical devices such as prostheses, stents.
• an AZ31 magnesium alloy sheet was used (chemical composition: 2.89% wt Al, 0.92% wt Zn, 0.05% wt Mn, 0.01% wt Si, 0.002% wt Cu, 0.001% wt Ni, 0.004% wt Fe balance Mg) subjected to manual mechanical cleaning with the aid of a lapping machine and silicon carbide abrasive papers with decreasing grain size (P1000, P2400 and velvety cloth), then cleaned in acetone through ultrasound, rinsed with distilled water and dried with absorbent paper.
• the electrolyte is a 0.6 M solution of K 2 HP0 4 and 0.2 M of K 3 P0 4 in glycerol; the anodization is carried out in a galvanostatic regime at 160°C for 30 minutes; the electrodes are AZ31 (anode) and a Dimensionally Stable Anode (DSA) (cathode) connected to a galvanostat, the current density is 2 mA cm -2 .
• AZ31 anode
• DSA Dimensionally Stable Anode
• SBF Simulated Body Fluid solution
• AZ31 samples were used (chemical composition: 2.89% wt Al, 0.92% wt Zn, 0.05% wt Mn, 0.01% wt Si, 0.002% wt Cu, 0.001% wt Ni, 0.004 % wt Fe, balance Mg).
• This magnesium alloy combines high hot ductility with high level mechanical performance.
• the samples are represented by thin sheets whose area was carefully measured before the tests were carried out. The sheets were subjected to manual mechanical cleaning with the aid of a lapping machine and silicon carbide abrasive papers with decreasing grain size (P1000, P2400 and velvety cloth), then they were cleaned in acetone through ultrasound, rinsed with distilled water and dried with absorbent paper.
• the electrolyte used for the growth of the protective coatings is a 0.6 M solution of K 2 HP0 4 and 0.2 M of K 3 P0 4 in glycerol suitably stirred and thermostated with the aid of a heating plate provided with a thermocouple.
• the anodizations were carried out in a galvanostatic regime; the electrodes, having the AZ31 sample as anode and a Dimensionally Stable Anode (DSA) as cathode, were connected to a galvanostat.
• An electrochemical characterization was carried out to study the performance of the coatings produced.
• the impedance spectra were recorded in a frequency range between 100 kHz and 100 mHz, with an AC potential signal of amplitude 10 mV, at the previously measured OCP potential.
• the electrochemical cell used for the characterization includes a three-electrode configuration in which the AZ31 sheet constitutes the working electrode, the counter electrode (used exclusively to close the circuit) is formed by a platinum retina and an Ag/AgCl electrode was used (0.2 V vs. Standard Hydrogen Electrode) as reference electrode. Finally, the electrodes were connected to a PARSTAT 2263 potentiostat. The polarization curves were recorded in a ⁇ 250 mV interval with respect to the OCP measured with an electrode potential scan rate of 1 mV/s.
• the electrolytic anodization solution consists of a bath of 0.6 M K 2 HP0 4 and 0.2 M of K 3 P0 4 in glycerol.
• the use of phosphate salts provides species which incorporate into the protective film and coat the Mg alloy in a compact manner.
• the magnesium phosphate, Mg 3( P0 4)2 has a PBR of 2.29 [R.-C. Zeng, L. Sun, Y.-F. Zheng, H.-Z. Cui, and E.-H. Han. "Corrosion and characterisation of dual phase Mg-Li-Ca alloy in Hank's solution: The influence of microstructural features.” Corrosion Science 79 (2014): 69-82].
• the parameters of the anodization process such as electrolyte temperature, anodization current density and anodization time have been varied.
• Figure 1 shows the growth curves, cell voltage with respect to time, related to the anodization process with an imposed current density, i, equal to 2 mA cm -2 and for an anodization time of 30 minutes, by varying the temperature of the anodization bath (100°C, 160°C and 200°C).
• two zones can be distinguished in the growth curves of the coatings on AZ31 alloy: the first zone (up to about 200 s) in which the cell voltage varies linearly with time, and the second in which the cell voltage remains constant (on average).
• a linear variation of the voltage with time indicates the increase of a barrier film [M. Santamaria, F. Di Quarto, S. Zanna, and P. Marcus. "The influence of surface treatment on the anodizing of magnesium in alkaline solution.” Electrochimica Acta 56, (2011): 10533-10542.], i.e., a film with uniform thickness and composition.
• the different slope dV/dt of the initial section of the three curves obtained at different temperatures therefore indicates a different increase of the barrier film in the three conditions.
• the curve recorded at 200°C differs, compared to the other two, also in the section in which the cell voltage remains constant on average, with a decrease in the recorded voltage at about 700 s.
• This second part of the growth curve is instead usually indicative of the formation of a porous film, due to "sparking" phenomena, i.e., damage and consequent reconstitutions of the film, above the barrier film which, in the case of the growth of the coating at 200°C, breaks causing a sudden decrease in the cell voltage.
• Table 1 shows the OCP values measured after the anodization process.
• the most anodic OCP value is that related to the sample anodized at 160°C while the OCP values of the anodized samples at the other two temperatures were found to be coincident and more cathodic.
• Figure 2 instead shows the Nyquist diagrams related to the impedance spectra recorded at the OCP after the anodization process.
• the graph shows how the global impedance of the system comprising the anodized alloy at 160°C is the highest, thus showing how the coating has a higher protective action against the underlying alloy.
• all the tests will concern coatings increased by anodization conducted at 160°C.
• Figure 3 shows the growth curves recorded at three different current densities, 0.5 mA cm-2, 2 mA cm-2 and 8 mA cm-2 respectively.
• dV/dt is that measured during the growth curve (in the linear section)
• PM is the molecular weight
• p the density of the increased coating, which strictly depend on the composition of the coating itself. From the electric field, it is possible to derive the so-called anodization ratio, AR, which gives an indication of how much the coating increases per volt of voltage applied during growth, and therefore of the thickness of the increased layer. This only applies to the barrier layer which increases during the first step, i.e., until the voltage varies linearly over time.
• the colouring of the coating increased at a lower current density is similar to that of the alloy without coating synonymous with a thin and not very protective coating, instead the coating increased at 2 mA cm-2 is clearer and more homogeneous with respect to that increased at 8 mA cm-2.
• Table 2 shows the anodization ratios for the three different coatings, considering the formation of MgO and Mg3(P04)2 respectively.
• Table 2 Figure 4 shows the Nyquist diagrams related to the impedance spectra recorded at the OCP after the anodization process at different current densities. It can be seen from the graph that the global impedance of the system comprising the alloy anodized at 2 mA cm-2 is the highest, thus showing how the coating has a higher protective action against the underlying alloy than the coatings increased at 0.5 mA cm-2 and 8 mA cm- 2. Finally, the polarization curves related to the three coatings (figure 5) were recorded. Although the corrosion potential recorded for the coating increased at 0.5 mA cm-2 is the most anodic (-1.34 V vs. Ag/AgCl), the lower current density related to the cathode process (i.e., the development of hydrogen gas) was recorded for the coating increased at 2 mA cm-2.
• the third parameter which was optimized is the anodization time.
• the 100-second test aims to show the quality of the barrier film while the 60-minute test aims to thicken the porous film as much as possible.
• the overall impedance is higher in the case of coating increased for 30 minutes (figure 6).
• the presence of a porous layer is necessary to ensure a good quality of the film (which does not occur for the coating increased for 100 seconds) while, after a certain growth time, the breakdown phenomena tend to destroy the film and therefore reduce the protective capacity thereof.
• the anodization process was therefore optimized as a function of the current density, time and temperature of the electrolyte.
• the anodized samples were placed in an oven at a temperature of 400°C for a period of 24 hours. From the images acquired by SEM microscopy of the anodized sample and the anodized and heat-treated sample, it was possible to notice how the heat-treated coating has a more compact structure and provides greater protection to the metal substrate.
• the Nyquist diagrams related to the impedance spectra acquired at the open circuit potential value for a non-anodized AZ31 alloy, for an anodized alloy and finally for an anodized and heat-treated alloy were then compared (Figure 7).
• the anodized and heat-treated sample shows an overall impedance one order of magnitude greater than the simply anodized sample, and two orders of magnitude greater than the "bare" AZ31 alloy.
• the downward force measured by the scale is the result of various forces: the total mass of the beaker, sample and wire, mbeak, is assumed to be constant; the other force directed downwards is the one acting due to the solution which is located above the beaker containing the sample.
• the upward force is that of buoyancy resulting from the accumulation of hydrogen in the inverted beaker.
• the net weight force measured by the scale can be expressed using the following formula: where pe and pH2 are the densities of the electrolyte and hydrogen respectively, g is the gravitational acceleration, A is the section of the inverted beaker, h2 and hi are the heights of the free-surface beaker and of the H2/solution interface respectively.
• FIG. 8 shows the experimental curves related to the produced hydrogen resulting from the corrosion of a "bare" AZ31 alloy and an anodized and heat-treated AZ31 alloy, immersed in Hank's solution.
• the slopes of the linear sections of the curves shown in Figure 8 are 0.189 and 0.0056 ml cm-2 day-1 for an AZ31 alloy without coating and for an anodized AZ31 alloy, respectively.
• the results obtained show a notable decrease in hydrogen produced in the case of the anodized and heat-treated alloy.
• the value of 0.0056 ml cm-2 day-1 appears to be lower than the tolerable limits for the in vivo applications tested on guinea pigs for biodegradable orthopaedic implants (0.01 ml cm-2 day- 1) [G. Song. "Control of biodegradation of biocompatible magnesium alloys" Corrosion Science 49, no. 4 (2007): 1696- 1701].
• the anodized and heat-treated AZ31 alloy samples were then immersed in SBF thermostatic buffer at 37°C for different immersion times (3, 7 and 14 days) to evaluate the bioactivity thereof and the consequent increase in biocompatibility. 10 ml of buffer solution was added per surface cm2 and it was replaced on a daily basis in order to avoid excessive increases in pH. Table 3 shows the compositions of the samples, obtained by EDX survey, for different immersion times.
• the percentages of P and Ca (constituent elements of hydroxyapatite) present in the samples increase with increasing immersion time.
• the peak present at 950 cm-1 confirms the presence of hydroxyapatite on the sample surface, thus increasing the biocompatibility of the anodized and heat- treated AZ31 alloy sample (figure 9).

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