The in vivo and in vitro corrosion of high-purity magnesium and magnesium alloys WZ21 and AZ91

Abstract

abstract Corrosion was studied in vitro in Nor’s solution (CO 2 – bicarbonate buffered Hank’s solution) at 37 !C, andin vivo implanted in the lower back muscle of rats. Nor’s solution is a good model for HP Mg and WZ21,because (i) the pH is maintained by the same buffer as in blood and (ii) concentrations of corrosive chlo-ride ions, and other inorganic constituents, are similar to those in blood. The higher in vitro corrosion rateof AZ91 was caused by micro-galvanic from second phases. The lower in vivo corrosion rate of AZ91 wastentatively attributed to suppression of micro-galvanic corrosion by tissue encapsulation. 2013 Elsevier Ltd. All rights reserved. 1. Introduction1.1. Mg alloy corrosionMg alloys are gaining interest for biodegradable medical im-plant applications [1–13] due to a good combination of load bear-ing mechanical properties, and controllable corrosion rates. Theirpoor corrosion resistance, however, has limited their applicationswhere they might be exposed to corrosive solutions containingchloride ions [1,2,14–20]. In addition there are known issues[1,14,21] with the measurement of the Mg corrosion rate usingTafel extrapolation of polarisation curves.The poor corrosion resistance of Mg alloys [1,2,14–17,22–27]results (i) because Mg is an active metal and has a high drivingforce for corrosion, whereas only weak protection is provided bynaturally-occurring surface films and (ii) micro-galvanic corrosionacceleration is caused by second phases. Mg alloys typically cor-rode faster than high-purity (HP) Mg. No alloying element has pro-duced a corrosion rate for a solid–solution Mg alloy lower thanthatof HP Mg in a technically relevant testing solution like 3% NaCl, un-like Cr in stainless steels, nickel and cobalt base alloys, where Cralloying produces low corrosion rates [28–36] above a critical Crconcentration by means of a more stable passive film.HP Mg means that the impurity elements (Fe, Ni, Cu and Co) areeach below their (alloy dependent) tolerance limit [1,14,15,26].Above the tolerance limit, the corrosion rate is high. Some studies[26,37,38] have been dominated by the Fe impurity element, eventhough they aimed to study completely different phenomena.Fishing-line specimens, and plug-in specimen, were developed[17] in response to the known issues [1,14,21] with the measure-ment of Mg corrosion using Tafel extrapolation of polarisationcurves. Fishing-line specimens were designed as the most-mini-malist-possible specimen mount,and identified the issue of crevicecorrosion for Mg during immersion tests [17].Plug-in specimens were subsequently designed [17] to haveno crevice, and to allow reliable polarisation curves to be mea-sured. Three reasons were identified [17] why Tafel extrapola-tion had previously not yielded corrosion rate measurements inagreement with independent measurements. (1) Crevice corro-sion can occur in the specimen mount when the specimen ismounted in a metallurgical mount, or similar, as widely usedin Mg corrosion studies. (2) The evolving hydrogen and corrosionproducts can cause decoupling of the corrosion and electrochem-ical measurements. (3) Often the polarisation curves, used forTafel extrapolation, have been measured soon after specimenimmersion. The initial corrosion behaviour may not correlatewith steady state corrosion behaviour, either in vivo or in vitro,particularly as Mg alloy corrosion can have a substantialincubation period.