Zhiming Shi

In vitro biodegradation behavior of magnesium and magnesium alloy.

Magnesium has the potential to be used as degradable metallic biomaterial. For magnesium and its alloys to be used as biodegradable implant materials, their degradation rates should be consistent with the rate of healing of the affected tissue, and the release of the degradation products should be within the bodys acceptable absorption levels. Conventional magnesium degrades rapidly, which is undesirable. In this study, biodegradation behaviors of high purity magnesium and commercial purity magnesium alloy AZ31 in both static and dynamic Hanks solution are systematically investigated. The in vitro test results show that magnesium purification and selective alloying are effective approaches to reduce the degradation rate of magnesium. In the static condition, the corrosion products accumulate on the materials surface as a protective layer, which results in a lower degradation rate than the dynamic condition. Anodized coating can significantly further reduce the degradation rate of magnesium. This study strongly indicates that magnesium can be used as degradable implant material as long as the degradation is controlled at a low rate. Magnesium purification, selective alloying, and anodized coating are three effective approaches to reduce the rate of degradation.

Influence of the chloride ion concentration on the corrosion of high-purity Mg, ZE41 and AZ91 in buffered Hank's solution

This research studied the influence of the chloride ion concentration on the corrosion behaviour of high-purity magnesium (Mg) and two Mg alloys in Hank’s solution, using hydrogen evolution and weight loss. A buffer based on CO2 and NaHCO3 was used to maintain the pH constant. The corrosion behaviour was governed by a partially protective surface film, and film breakdown by the chloride ions. The carbonated calcium phosphate layer that formed in Hank’s solution was important in determining the protective properties of the surface film.

Magnesium and Magnesium Alloys as Degradable Metallic Biomaterials

Drawbacks associated with permanent metallic implants lead to the search for degradable metallic biomaterials. Magnesium alloys have been highly considered as Mg has a high biocorrosion potential and is essential to bodies. In this study, corrosion behaviour of pure magnesium and magnesium alloy AZ31 in both static and dynamic physiological conditions (Hank’s solution) has been investigated. It is found that the materials degrade fast at beginning, then stabilize after 5 days of immersion. High purity in the materials reduces the corrosion rate while the dynamic condition accelerates the degradation process. In order to slow down the degradation process to meet the requirement for their bio-applications, an anodized coating is applied and is proved as effective in controlling the biodegradation rate.

An Innovative Specimen for Mg Corrosion Studies

Plug-in specimens enable measurement of reliable Mg polarization curves. Cathodic polarization curves were measured for high purity Mg in 3.5% NaCl saturated with Mg(OH)2 using (i) mounted specimens and (ii) plug-in specimens. Polarization curves yielded the corrosion current density icorr and the corresponding corrosion rate Pi, which was compared with corrosion rates evaluated from hydrogen evolution, PH, and weight loss, PW. Mounted specimens produce Pi values three times larger than plug-in specimens, due to crevice corrosion in the mounted specimens. Plug-in specimens had no crevice and allow simultaneous measurement of PH and Pi. Pi was less than PH and indicated an apparent valence of 1.45 in support of the existence of the uni-positive Mg+ ion.

Corrosion of Mg for biomedical applications

This chapter reviews our understanding of Mg corrosion and the measurement of Mg corrosion. There is some emphasis on the use of Mg alloys for biodegradable medical applications. Mg melt purification using Zr has been shown to provide the opportunity to produce ultra-high-purity Mg alloys, which could lead to stainless Mg. Nors solution may be a good start model for the study of Mg for biodegradable medical implant applications. The uni-positive Mg+ corrosion mechanism is consistent with the know corrosion data, although it is clear that self-corrosion is also important.

Understanding the Corrosion of Mg and Mg Alloys

This article summarizes current, fundamental knowledge of the corrosion of magnesium, and highlights some latest developments. Mg is a most reactive metal. The high available reaction energy, and because the simultaneous exchange of two electrons is forbidden by quantum mechanics, means that there is a highly reactive intermediate that can chemically split water. Thus, part of the corrosion rate cannot be measured with electrochemical techniques. Furthermore, the evolving hydrogen can insulate part of the corroding surface from electrochemical measurement. It is best practice to simultaneously use several measurement techniques, and to compare the measured corrosion rates in the same units.

Anodization and corrosion of magnesium (Mg) alloys

Anodization is one of the most important and effective surface pre-treatments for Mg alloys. This chapter systematically summarizes Mg alloy anodizing behavior, the compositions and microstructures of anodized films on Mg alloys and the anodization-influencing factors. Based on the anodizing voltage variation, gas evolution and sparking behavior in a typical anodizing process and the characteristic composition and microstructure of an anodized coating, a four-stage anodizing mechanism is postulated. Moreover, the corrosion performance of anodized Mg alloys is systematically reviewed and a corrosion model is proposed to explain the corrosion performance and electrochemical behavior. It is believed that some of the measured electrochemical features can be utilized to rapidly evaluate or compare the corrosion resistance of anodized Mg alloys.

Electrochemical potential noise of AISI type 321 stainless steel under stress in acid sulphate solutions

Abstract An investigation was carried out on the features of potential fluctuations of AISI type 321 stainless steel under various constant loads in acidic sodium sulphate solutions with or without the addition of chloride ions. The probability of occurrence of potential noise was found to be affected by the stress level, the concentration of Cl− and the acidity of the solution. It increased with increasing stress and concentration of Cl−. However, increasing acidity led to a decrease in the potential noise level. The probability of potential noise fluctuations decayed gradually with increasing time after loading. Based on a mechanism of the breakdown and repassivation, a statistical analysis has been undertaken of the potential fluctuations recorded in the present study.

Porous Titanium Scaffolds Fabricated by Metal Injection Moulding for Biomedical Applications

Biocompatible titanium scaffolds with up to 40% interconnected porosity were manufactured through the metal injection moulding process and the space holder technique. The mechanical properties of the manufactured scaffold showed a high level of compatibility with those of the cortical human bone. Sintering at 1250 °C produced scaffolds with 36% porosity and more than 90% interconnected pores, a compressive yield stress of 220 MPa and a Young’s modulus of 7.80 GPa, all suitable for bone tissue engineering. Increasing the sintering temperature to 1300 °C increased the Young’s modulus to 22.0 GPa due to reduced porosity, while reducing the sintering temperature to 1150 °C lowered the yield stress to 120 MPa, indicative of insufficient sintering. Electrochemical studies revealed that samples sintered at 1150 °C have a higher corrosion rate compared with those at a sintering temperature of 1250 °C. Overall, it was concluded that sintering at 1250 °C yielded the most desirable results.

Mechanical properties, in vitro corrosion resistance and biocompatibility of metal injection molded Ti-12Mo alloy for dental applications

A biocompatible Ti-12Mo alloy was fabricated by metal injection moulding (MIM) using non-spherical titanium, molybdenum powders and a purposely designed binder. The density, microstructure and tensile properties were characterized. This was followed by a detailed assessment of its in vitro corrosion and biocompatibility performances, compared with that of two commonly used titanium-based materials extra low interstitial (ELI) Ti-6Al-4V and commercially pure (CP) titanium. The MIM-fabricated Ti-12Mo alloy can achieve a wide range of mechanical properties through controlling sintering process. Specimens sintered at 1400 °C are characterized by fairly uniform near-β microstructure and high relative density of 97.6%, leading to the highest tensile strength of 845.3 ± 21 MPa and elongation of 4.15 ± 0.2% while the highest elastic modulus of 73.2 ± 5.1 GPa. Owing to the formation of protective TiO2-MoO3 passive film, the MIM-fabricated Ti-12Mo alloy exhibits the highest corrosion resistance including the noblest corrosion potential, the lowest corrosion current density and the highest pitting potential in four different electrolytes. The in vitro cytotoxicity test suggests that the MIM-fabricated Ti-12Mo alloy displays no adverse effect on MC3T3-E1 cells with cytotoxicity ranking of 0 grade, which is nearly close to ELI Ti-6Al-4V or CP Ti. These properties together with its easy net-shape manufacturability make Ti-12Mo an attractive new dental implant alloy.