A degradation kinetics model of Mg–Zn–Mn–Ca alloys in Kokubo solution

Abstract

In this work, a model of the degradation of the Mg–Zn–Mn–Ca alloy system in SBF (Simulated Body Fluid) is proposed by applying the isoconversion method. Alloy ingots with nominal compositions of Mg–2Zn–0.2Mn–0.2Ca (wt.%) designed A.2 alloy and Mg–4Zn–0.2Mn–0.2Ca (wt.%), A.4 alloy, was induction melted at 680 °C. In a eudiometer, the alloys were immersed for 600 min in Kokubo solution at temperatures of 23.5 °C, 36.5 °C, 49.5 °C, and the evolution of hydrogen was monitored. In the isoconversion method, the Arrhenius' equations and the kinetic mechanism models of chemical and diffusion control were used to construct the model. The degradation rate in A.2 alloy varied from 6.66 mg of Mg/cm2 to 9.40 mg of Mg/cm2, and in A.4 alloy, from 5.48 mg to 6.70 mg of Mg/cm2 with a test temperature variation of 23.5–49.5 °C. The control mechanism observed was the mixed one, being predominantly of chemical control. The activation energy found for the isoconversion model that used the kinetic mechanism model of chemical control was 3.24 kJ/mol for A.2 alloy and 3.45 kJ/mol for A.4 alloy, the pre-exponential factor of 4.34 × 10−3 mm−1 for A.2 alloy and 4.21 × 10−3 mm−1 for A.4 alloy. It is concluded that the isoconversion model that best represents the degradation of A.2 and A.4 alloys in Kokubo solution is the model of the kinetics mechanism of chemical control associated with the Arrhenius equation, which presents good correlations results comparing the experimental results with the proposed model.