signature=6bec5c9a121db355c0e386f99012a67b,A mechanistic in vitro study of the microgalvanic degrada...

A mechanistic in vitro study of the microgalvanic degradation of secondary phase particles in magnesium alloys

R.Walter,M.Bobby Kannan

Biomaterials and Engineering Materials(BEM)Laboratory,School of Engineering and Physical Sciences,

James Cook University,Townsville,Queensland4811,Australia

Received5March2014;revised15May2014;accepted28May2014

Published online7June2014in Wiley Online Library(http://www.doczj.com/doc/d0be9cc24b35eefdc9d33353.html).DOI:10.1002/jbm.a.35247

Abstract:The aim of this work was to understand the effect of microgalvanic degradation on secondary phase particles in magnesium alloys under in vitro condition.Pure magne-sium and Mg17Al12(b-phase)were galvanically coupled in simulated body fluid and the degradation behavior was studied using electrochemical impedance spectroscopy. The galvanic coupling produced a phosphate/carbonate layer on the b-phase,which initially increased the degradation resistance.However,the deposited phosphate/carbonate layer rapidly degraded once the galvanic coupling was removed,and b-phase exhibited similar deg-radation resistance to that of pure magnesium.A phenom-enological model has been presented,demonstrating the galvanic coupling effect.V C2014Wiley Periodicals,Inc.J Biomed Mater Res Part A:103A:990–1000,2015.

Key Words:magnesium,biodegradation,galvanic effect,in vitro test

How to cite this article:Walter R,Kannan MB.2015.A mechanistic in vitro study of the microgalvanic degradation of second-ary phase particles in magnesium alloys.J Biomed Mater Res Part A2015:103A:990–1000.

INTRODUCTION

In recent years,there has been a growing interest in magnesium-based alloys for potential applications in biode-gradable implants.1–3Table I is a list of the widely studied magnesium alloys under in vitro and/or in vivo conditions.4–24 All the studies have focussed on the overall degradation behavior of magnesium alloys.However,it can be noticed that the magnesium alloys used in the studies contain sec-ondary phase particles.Due to the difference in chemistry, these secondary phase particles will have different dissolu-tion behavior as compared to that of magnesium matrix.

Secondary phase particles in magnesium alloys are gen-erally cathodic to the magnesium matrix and are more deg-radation resistant than the magnesium matrix.25–27In chloride-containing solution,the secondary phase particles play a dual role on the degradation behavior of these alloys.28,29If the volume fraction of these secondary phase particles is relatively high,they improve the degradation resistance of the alloy by acting as a stable barrier against degradation,that is,dissolution of the magnesium matrix can leave behind a continuous network of the stable second-ary phase particles.Conversely,if the volume fraction is low, the secondary phase particles act as cathodic sites for microgalvanic degradation of the anodic magnesium matrix and consequently increase the rate of dissolution of the magnesium matrix.30

Recently,Kannan8studied the in?uence of microstruc-ture on the in vitro corrosion behavior of AZ91magnesium alloy,and found that the b-phase(Mg17Al12)was stable in simulated body?uid(SBF)as compared to the magnesium matrix.The die-cast AZ91magnesium alloy,which contains a large volume fraction of b-phase,clearly revealed a stable network of b-phase after potentiodynamic polarization test. This stable network of secondary particles has been widely reported in AZ91magnesium alloy immersed in chloride-containing solutions.29,31–33In a more recent study,Kannan et al.34examined the short-term biodegradability of b-phase alone in SBF.It was found that the b-phase was noble to pure magnesium by680mV,and the authors reported80% higher degradation resistance for b-phase than that of pure magnesium.

Since there is a signi?cant difference in the electrochemi-cal potential between b-phase and magnesium,the magne-sium alloys containing b-phase would undergo microgalvanic degradation.The corrosion effects of secondary phase distri-bution and re?nement in AZ80magnesium alloy in 3.5% NaCl were reported by Sun et al.,35but to the authors’best knowledge there has been no work on this form of degrada-tion in physiological conditions.Recently,Kalb et al.13 reported deposits of Mg(OH)2around microcathodes,that is, zirconium and iron rich particles in WE43alloy.While it is understood that secondary phase particles are cathodically protected by the magnesium matrix,it is not clear how the degradation behavior of the secondary phase particles changes once microgalvanic effects are reduced,that is,via undermining or dissolution of the magnesium matrix.This

Correspondence to:M.B.Kannan;e-mail:bobby.mathan@http://www.doczj.com/doc/d0be9cc24b35eefdc9d33353.html.au

990V C2014WILEY PERIODICALS,INC.