Microstructural and Corrosion Properties of Hydroxyapatite Containing PEO Coating Produced on AZ31 Mg Alloy.

Luca Pezzato,Stefano Diodati,Katya Brunelli,Mirko Pigato,Massimiliano Bonesso,Manuele Dabalà

doi:10.3390/ma14061531

Abstract

In this work, the composition of an electrolyte was selected and optimized to induce the formation of hydroxyapatite during Plasma electrolytic oxidation (PEO) treatment on an AZ31 alloy for application in bioabsorbable implants. In detail, the PEO process, called PEO-BIO (Plasma Electrolytic Oxidation-Biocompatible), was performed using a silicate-phosphate-based electrolyte with the addition of calcium oxide in direct-current mode using high current densities and short treatment times. For comparison, a known PEO process for producing anticorrosive coatings, called standard, was applied on the same alloy. The coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and XPS analyses. The corrosion performance was evaluated in simulated body fluid (SBF) at 37 °C. The coating produced on the PEO-BIO sample was porous and thicker than the standard PEO one, with zones enriched in Ca and P. The XRD analysis showed the formation of hydroxyapatite and calcium oxides in addition to magnesium-silicon oxide and magnesium oxide in the PEO-BIO sample. The corrosion resistance of PEO-BIO sample was comparable with that of a traditional PEO treated sample, and higher than that of the untreated alloy.

Highlights

IntroductionMagnesium (Mg) alloys are revolutionary biodegradable metals for orthopedic applications thanks to their good biocompatibility, biodegradability, and acceptable mechanical properties
AZ31 magnesium alloy (nominal composition 3% Al, 0.9% Zn, 0.2% Mn, 0.1% Si, Mg balance (DSM, Beer-Sheva, Israel) was used as substrate for Plasma electrolytic oxidation (PEO) treatment. This alloy is typically used for the studies in the biomedical field [34], even if some specific bioabsorbable magnesium alloys have been recently designed
The presence of hydroxyapatite, verified by both XPS and X-ray diffraction (XRD) analyses, was obtained thanks to the direct addition of Ca and P compounds to the electrolyte and to the interactions between the electrolyte, the substrate and the discharge phenomena

Summary

Introduction

Magnesium (Mg) alloys are revolutionary biodegradable metals for orthopedic applications thanks to their good biocompatibility, biodegradability, and acceptable mechanical properties. Mg is the fourth most abundant cation in the human body and is essential in many metabolic processes. The main advantage in the employment of magnesium alloys in the production of implants is that they degrade in vivo due to the presence of Cl− in the physiological environment, thereby eliminating the need for secondary surgeries to remove the implant. The corrosion products (Mg2+ ) are not harmful for the human body [3,4]. Mg alloys have mechanical properties (45 GPa of elastic modulus) similar to those of bone (3–20 GPa), unlike titanium alloys and stainless steel (110 and 200 GPa, respectively). The stress shielding due to mechanical mismatch between natural bone and metal implants is reduced when Mg alloys are employed [5]

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