Influence of trace impurities on the in vitro and in vivo degradation of biodegradable Mg–5Zn–0.3Ca alloys

Abstract

The hydrogen evolution method and animal experiments were deployed to investigate the effect of trace impurity elements on the degradation behavior of high-strength Mg alloys of type ZX50 (Mg–5Zn–0.3Ca). It is shown that trace impurity elements increase the degradation rate, predominantly in the initial period of the tests, and also increase the material’s susceptibility to localized corrosion attack. These effects are explained on the basis of the corrosion potential of the intermetallic phases present in the alloys. The Zn-rich phases present in ZX50 are nobler than the Mg matrix, and thus act as cathodic sites. The impurity elements Fe and Mn in the alloy of conventional purity are incorporated in these Zn-rich intermetallic phases and therefore increase their cathodic efficiency. A design rule for circumventing the formation of noble intermetallic particles and thus avoiding galvanically accelerated dissolution of the Mg matrix is proposed.

Introduction

Magnesium alloys are of special interest in the context of structural lightweight applications in the transport and aerospace industries, and for temporary implants in medicine because of their biocompatibility and biodegradability [1], [2]. One of the major obstacles to widespread application of magnesium alloys, however, is their high corrosion rate [3], [4], which is often attributed to the presence of specific impurity elements. Elements such as Fe, Ni, Cu and Co can significantly accelerate Mg corrosion when their concentrations are above their tolerance limits [5], [6], [7], [8], [9]. Fe, for example, is the most common impurity element; its tolerance limit is reported as 170 ppm in unalloyed as-cast Mg [3] and roughly 5 ppm in wrought Mg [6], [9]. Below this limit, when no Fe-rich particles are formed and thus no electrochemically active cathodic sites exist to accelerate corrosive attack, the corrosion rate decreases dramatically. Recent studies have added silicon (Si) to the above-mentioned reactive impurity elements detrimental to corrosion. Si plays a critical role in promoting the formation and growth of Fe-rich particles and should thus also be considered as corrosion-provoking element [9], [10].

For osteosynthesis implants and stent applications, not only electrochemical and biocompatible requirements are crucial, but also mechanical properties [1]. Recent studies have demonstrated that the alloying system Mg–Zn–Ca offers simultaneous high strength and high ductility, with values of R_p0.2 > 250 MPa, R_m > 300 MPa and A_f > 20%. These attractive large values were achieved within a composition window of 5–6 wt.% Zn and 0.2–0.4 wt.% Ca [11], [12]. Recently the in vivo behavior of an alloy with 5 wt.% Zn and 0.25 wt.% Ca (ZX50) of conventional purity (CP) was investigated, and undesired rapid degradation was observed [13]. In the present study, the alloy ZX50 was chosen again, but this time raw materials of two different purities were used. The first alloy was made of CP Mg identical to that used in [13], but the second was synthesized using ultrahigh-purity (XHP) Mg [14]. The research aim of this study was to compare the in vitro and in vivo behavior of the two alloys CP ZX50 and XHP ZX50, and to clarify the influence of trace elements on degradation characteristics and mechanisms. The in vitro corrosion was determined via the hydrogen evolution method [15], deploying a newly designed testing device [9]. For the in vivo examination pins of each alloy were implanted in the femoral shafts of rats and their degradation was studied over a period of 12 weeks (see also Ref. [13]). A few pins were also explanted after two weeks to investigate their surface topography and elucidate the degradation mechanisms.