ABSTRACT
Magnesium alloys exhibit an attractive combination of biocompatibility,biodegradability and analogous mechanical properties to natural bone. These properties make them ideal candidates for biodegradable implant applications. In recent years, there is an increasing interest on magnesium alloys used as biodegradable metal implants in potential orthopedic surgery. However, magnesium and its alloys are characterized by low corrosion resistance in the high chloride environment of physiological systems,evolving hydrogen gas and overdosed magnesium ions in the corrosion process at a rate too fast to be dealt with by the tissue, not only causing subcutaneous gas pockets but also leading to the increase of osteoclast activity, which limit their use. So it is very important for orthopaedics clinical application of magnesium alloys to control their degradation rate.
Surface modification is one of the effective solutions to control the degradation rate of magnesium alloys by changing their surface chemical composition and structure,without affecting their strength.
In this paper, the corrosion degradation behavior of AZ31 alloy in Hank’s solution was studied. In order to improve its poor corrosion resistance and promote its osteoconductive, the calcium phosphate coatings were fabricated by chemical deposition and electrodeposition on biodegradable AZ31 alloy. The deposition process and mechanism of calcium phosphate coating was studied through investigation the change of crystalline structure, morphologies and composition of these Ca-P coatings and the change of pH value and electric current of electrolyte. The effect of Ca-P coating on the degradation behavior has also been investigated by potentiodynamic electrochemical technique and immersion test. Furthermore, Fluoride conversion films were synthesized on Mg alloy AZ31 by immersion in hydrofluoric acid (HF) with various concentrations and treatment time. The effects of the concentrations and treatment time of hydrofluoric acid (HF) on the corrosion behavior of the AZ31 alloy were studied by potentiodynamic electrochemical technique and immersion test. Then the process of HF treatment was optimized and the corrosion process of the fluoride conversion film in Hank’s solution was investigated with electrochemical impedance spectroscopy (EIS). Finally, MgF2/Ca-P composite coatings were prepared on AZ31 alloy and their degradation performances were studied. The results are summarized as follows:
①Pitting corrosion and filiform corrosion were the typical corrosion forms of AZ31alloy in Hank’s solution. H2PO4-, HPO42- and Ca2+ ions had been depositing on the sample during the corrosion process. The deposition rate of H2PO4-, HPO42 were higher than that of Ca2+ in the initial corrosion stage, then the Ca/P value increased and the content of calcium phosphate also grew gradually. The results of electrochemical impedance spectroscope (EIS) showed the corrosion failure process of AZ31 alloy had experienced the following three steps, induction of the pitting corrosion, and growth of the pitting corrosion and formation of the corrosion products.
②Any coating on the AZ31 alloy treated by pre-calcification (pre-Ca) and anodized oxidation processes could not be obtained after immersed in Hank's solutions for two days, whereas homogeneous coatings on the same substrates were acquired in a mixed solution of 0.042 mol/L Ca(NO3)2 and 0.025 mol/L NH4H2PO4(Ca-P solutions). It was confirmed by XRD analysis that these coatings were consisted of a flake-like dicalcium phosphate dihydrate (DCPD, CaHPO4·2H2O) crystals and the microstructure of the calcium phosphate coatings were unchanged during the immersion process.
③The coating prepared by electrochemical depositon had two layers. The inner layer was calcium phosphate coating and the outlayer is consisted of flake-shape CaHPO4·2H2O crystallites. The deposition process of calcium phosphate on AZ31 alloy was characterized by high nucleation rate and high crystal growth rate. Raise the temperature could accelerate the rate of deposition, but would lead to no uniform crystals.When the coating was immersed in 0.1mol/L NaOH for 8 h at 80℃，DCPD transformed to HA.
The increase in free corrosion potential and breakdown potential and decrease in corrosion current density, resulted from the coatings, implied the coating enhanced the corrosion resistance of the alloy. At the first 5 days’ immersion in Hank’s solutions, the hydrogen evolution rate of the coated sample was much lower than that of AZ31 substrate,which indicates the coating provides the protection for the Mg alloy substrate. But the corrosion rate of the coated sample was accelerated after pitting corrosion was occurred.
④Fluoride conversion film(MgF2) was synthesized on Mg alloy AZ31 by immersion in hydrofluoric acid (HF) with various concentrations and treatment time.Electrochemical and immersion tests proved that the fluoride conversion coatings significantly improved the corrosion resistance of AZ31. The hydrogen volume of the AZ31 substrate was 41.2 ml, while that of the sample treated with 40 % HF for 10 d was only 0.8 ml, indicating that fluoride conversion film can significantly inhibits the degradation of AZ31 magnesium. There was no bubble or flaking in the interface of fluoride conversion film and substrate during the immersion in Hank’s solution for 15 d.But pitting corrosion and filiform corrosion occurred and the MgF2 dissolved slowly.
Calcium phosphate deposited on the surface of MgF2 film, however, the content of calcium phosphate on the corrosion sites of the film was much more.
⑤Calcium phosphate coatings were formed on AZ31 alloy samples which had been immersed in10%, 20% and 40% HF acid for 72 h by electrodepositon in Ca/P solution for 4 h . The coatings of the first two samples had netted surface and no crystals; the coating of the sample treated by 40% HF was consisted of flake-like DCPD crystals. Compared to the HF-treated samples without calcium phosphate coating, calcium phosphate coating increased the breakdown potential and decreased the volume of hydrogen evolution to 0.1 ml in first 6 days’ immersion, indicating the calcium phosphate coating delayed the degradation of samples. Then the corrosion rate increased slowly. The most improved corrosion protection was achieved using the concentrations of 40 % and 72 h of treatment time and electrodepositing in Ca/P solution for 4 h.
The behavior of the degradation rate increasing slowly of the MgF2/Ca-P coated AZ31 alloy makes it’s a promising degradable bone material.
Keywords: magnesium alloy AZ31, biomedical materials, calcium phosphate coating,Fluoride conversion film, degradation property
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