RESUMEN
This study investigated the simulated body fluid-assisted stress corrosion cracking (SCC) of an Al-free magnesium alloy (RS66) and a common Al-containing magnesium alloy (AZ91), the former being more suitable for temporary implant applications (however, we used AZ91 for comparison since there are considerable reports on SCC in this alloy). The investigation includes SCC tests under simultaneous conditions of mechanical loading and imposed electrochemical potential that established a combined effect of hydrogen and anodic dissolution as the embrittlement mechanism. Though the RS66 alloy possesses impressive mechanical properties in non-corrosive environments (as a result of its fine grain size), both alloys suffered significant embrittlement when tested in simulated body fluid. The susceptibility of the RS66 alloy to SCC was ~25% greater than that of AZ91, which is attributed to the greater resistance of AZ91 to corrosion/localised corrosion because of its Al content.
RESUMEN
Owing to their suitable mechanical property and biocompatibility as well as the technological possibility of controlling their high corrosion rates, magnesium and its alloys have attracted significant attention as temporary bio-implants. Though the ability of magnesium to harmlessly biodegrade and its inherent biocompatibility make magnesium alloys a suitable choice for a temporary implant, their high corrosion rates limit their practical application, as the implants can potentially corrode away even before the healing process has completed. Different approaches, such as alloying, surface modification, and conversion coatings, have been explored to improve the corrosion resistance of various magnesium alloys. However, the corrosion behavior of magnesium implants with and without a surface modification has been generally investigated under in-vitro conditions, and studies under in-vivo conditions are limited, which has contributed to the lack of translation of magnesium implants in practical applications. This paper comprehensively reviews the prospects of magnesium alloy implants and the current challenges due to their rapid degradation in a physiological environment. This paper also provides a comprehensive review of the corrosion mitigation measures for these temporary implants.
RESUMEN
The complex interaction between physiological stresses and corrosive human body fluid may cause premature failure of metallic biomaterials due to the phenomenon of stress corrosion cracking. In this study, the susceptibility to stress corrosion cracking of biodegradable and aluminium-free magnesium alloys ZX50, WZ21 and WE43 was investigated by slow strain rate tensile testing in a simulated human body fluid. Slow strain rate tensile testing results indicated that each alloy was susceptible to stress corrosion cracking, and this was confirmed by fractographic features of transgranular and/or intergranular cracking. However, the variation in alloy susceptibility to stress corrosion cracking is explained on the basis of their electrochemical and microstructural characteristics.
Asunto(s)
Aleaciones/química , Materiales Biocompatibles/química , Magnesio/química , Ensayo de Materiales/métodos , Prótesis e Implantes , Líquidos Corporales , Corrosión , Electrodos , Humanos , Ensayo de Materiales/instrumentación , Modelos BiológicosRESUMEN
It is essential that a metallic implant material possesses adequate resistance to cracking/fracture under the synergistic action of a corrosive physiological environment and mechanical loading (i.e. stress corrosion cracking (SCC)), before the implant can be put to actual use. This paper presents a critique of the fundamental issues with an assessment of SCC of a rapidly corroding material such as magnesium alloys, and describes an investigation into the mechanism of SCC of a magnesium alloy in a physiological environment. The SCC susceptibility of the alloy in a simulated human body fluid was established by slow strain rate tensile (SSRT) testing using smooth specimens under different electrochemical conditions for understanding the mechanism of SCC. However, to assess the life of the implant devices that often possess fine micro-cracks, SCC susceptibility of notched specimens was investigated by circumferential notch tensile (CNT) testing. CNT tests also produced important design data, i.e. threshold stress intensity for SCC (KISCC) and SCC crack growth rate. Fractographic features of SCC were examined using scanning electron microscopy. The SSRT and CNT results, together with fractographic evidence, confirmed the SCC susceptibility of both smooth and notched specimens of a magnesium alloy in the physiological environment.