Effect of plasma electrolytic fluorination and sacrificial anode design on durability enhancement of biodegradable magnesium alloy

Abstract

Magnesium (Mg) and its alloys have attracted fundamental research in the fields of orthopedics, cardiovascular, and dentistry as biodegradable metallic materials. However, Mg exhibits poor corrosion resistance, especially in a physiological environment, which limits its applicability in medical applications. This study uses a type of Mg alloy called AZ31. The corrosion resistance of AZ31 was improved using plasma electrolytic fluorination (PEF) as a surface treatment. Then, the surface morphology, coating thickness, and composition were observed, and the corrosion behaviors of PEF-coated AZ31 were evaluated via electrochemical and weight loss tests. The tensile strength of PEF-coated AZ31 after immersion in simulated body fluid (SBF) was investigated after 4, 8, and 12 weeks and compared with Bare AZ31 which underwent the same treatment. Pitting corrosion caused a rapid decline in the mechanical strength of AZ31. We verified pitting corrosion using the hardness tester to indent the surface of the samples and performed pitting corrosion analysis. In addition, we prevented the rapid corrosion of AZ31 by designing a “corrosion guide design (CGD)” for the unprecedented Mg implant according to the potential difference between Bare and PEF. The result of coating with PEF showed that a porosity structure of pore size 600–900 nm and thickness 1–14 µm was generated on the AZ31 substrate. MgF2 was covered on the coated surface. In electrochemical corrosion and immersion corrosion tests, the PEF-coated AZ31 exhibited efficiently improved corrosion resistance compared to Bare AZ31 in SBF. In the results of tensile strength through corrosion, the growth trend of weight loss percentage for PEF-coated AZ31 was less than that of Bare AZ31. However, the tensile strength of the PEF-coated AZ31 after immersion was lower than that of Bare AZ31. We verified that the pitting corrosion was the main reason why the strength of PEF-coated AZ31 was lost rapidly. As seen in the results, the CGD can fully protect the strength of the AZ31, but only for a period time. Although the PEF coating has a structure that favors biological growth and it can increase the corrosion resistance of AZ31, there is a limitation in its clinical use because of its generation of pitting corrosion. The result in the PEF-coated AZ31 could lead to mechanical fracture through biodegradation. However, after surface-treatment, Mg could have increased corrosion resistance but the pitting corrosion might rapidly reduce the mechanical strength of an Mg implant, resulting in serious consequences for patients. Perhaps the CGD would remedy the disadvantages of surface coating technology on applying biomedical Mg implants.