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In the present study, to understand the mechanism of Mn on inhibiting Fe-caused Mg corrosion, the corrosion behaviour of commercial pure Mg and Mg-6Mn alloy in 0.6 M NaCl solution is investigated. It is found that in Mg-6Mn alloy, Fe impurity is incorporated into Mn to form Mn (Fe) phase with Fe as solid solute. The initial galvanic corrosion cannot be reduced through converting Fe-rich phase to Mn (Fe) phase, since Mn (Fe) phase also has relatively strong cathodic activity and has much larger volume fraction than Fe-rich phase. However, the cathodic activation behaviour of pure Mg is inhibited. The cathodic activity even decreases for Mg-Mn alloy with increased exposure time, due to the reduced cathodic HER at the Mn (Fe) particles. Mn can be oxidized at the OCP of Mg-6Mn alloy, resulting in relatively dense Mn-rich corrosion film on particle surface, which separates the particle from the electrolyte and, consequently, inhibits HER. [ABSTRACT FROM AUTHOR]

Abstract: The biocorrosion behavior of a magnesium alloy in two simulated body solutions, Hank''s solution and simulated blood plasma (SBP) solution was investigated by electrochemical and weight loss testing for biomedical application. The solution volume/surface area (SV/SA) ratio was changed to reveal the effect of immersion condition on the biocorrosion behavior. A same tendency was observed in the corrosion rate of magnesium alloy in all testing conditions: a high corrosion rate at the initial stage, and rapid decrease in the first 2–3 days followed by a stable corrosion rate in the following stage. A higher corrosion rate was observed in Hank''s solution than in SBP solution due to high Cl−, low Ca2+ and PO4 3− concentration in Hank''s solution. However, no difference in the surface reaction product was observed between the samples immersed in Hank''s solution and in SBP solution. It was found that the SV/SA ratio significantly affected the corrosion rate of magnesium alloy. Low ratio resulted in a high pH, which resisted the corrosion. But when the ratio was high enough, 6.7 for example, the influence was negligible. By changing the ratio, the biocorrosion behavior of magnesium implant in different implantation sites can be simulated, for example, low ratio for the case of in muscle and high ratio for the case of in marrow cavity. It is suggested that selection of the simulated solution and the SV/SA ratio would be very necessary to simulate different the in-vivo biodegradation behavior of magnesium in different implantation environment. [Copyright &y& Elsevier]

• Porous Ag-enriched phase forms during corrosion via de-alloying of Mg-Ag phase. • Dissolved Ag mainly redeposits to Mg surface forming Ag nanoparticles. • Both of the formed porous Ag-enriched phase and re-deposited Ag nanoparticles can result in evident cathodic activation. • The cathodic activation ability is significantly affected by Ag content and existing form of Ag. The effect of Ag on cathodic activation and corrosion behaviour of Mg-Mn-Ag alloys are investigated in the present work. During corrosion, Mg-Ag intermetallic phase undergoes dealloying to form porous Ag-rich remnant whereas Ag in the alloy matrix is dissolved to form Ag+ ions that subsequently redeposit on the alloy surface via displacement reaction with Mg to form Ag nanoparticles, which lead to significant cathodic activation. The cathodic activation is a self-catalysed process since enhanced cathodic activity promotes anodic dissolution which, consequently, results in further enhancement of cathodic activity. Thus, the corrosion rate of Mg-2 Mn-4Ag alloy increases significantly with immersion time. [ABSTRACT FROM AUTHOR]