Research Progress in Effect of Alloying on Electrochemical Corrosion Rates of Mg Alloys

Abstract:

The great potential for applications in automotive, aerospace and electronic industries of magnesium alloys can be attributed to their desirable properties such as low density, high specific strength and stiffness, superior damping capacity and good electromagnetic interference shielding. Meanwhile, the alloys are also finding broad medical applications and receiving extensive scientific research due to their biocompatibility and biodegradability in physiological media. However, the Achilles heel of magnesium is that it corrodes too fast in solutions. Alloying is one of the most important approaches to slow down corrosion rates of magnesium alloys. It facilitates the creation of new Mg alloys by optimizing the composition and content of alloying elements in the stage of materials design, so that the new alloys acquire desired properties to meet the requirements of various applications. During alloying, the elements, secondary phases, gain size and defects greatly influence corrosion kinetics and electrochemistry of magnesium alloys. In terms of corrosion kinetics, most alloying elements can affect the activity of anodic and/or cathodic reactions of Mg, though they have negligible effects on the chemical stability. The secondary phases always serve as cathodes in the micro-corrosion couples, such that the micro-galvanic corrosion is possible and the dissolution of Mg is accelerated. Microstructural homogeneity and grain refinement are responsible for the decrease of corrosion rates of Mg alloys. It is believed that alloys with relatively high grain boundary densities tend to exhibit an analogous relationship between grain size and corrosion rate like the ‘Hall-Petch’ relationship, which can quantitatively reveal the relationship between corrosion rates and grain size. In addition, the defects including dislocations, pores, cracks and stresses are ready to trigger the dissolution of Mg since these locations have high free energy. In this article, the influences of alloying on the corrosion rates of Mg alloys were reviewed from the electrochemical viewpoint, on the basis of the corrosion nature of magnesium metal. Then the possibility of an improved anti-corrosion performance of Mg alloys by some alloying-related methods such as multi-elemental alloying, micro-alloying and alloying control was analyzed. At last, it was pointed out that the future development directions of electrochemical corrosion of magnesium alloys under the influences of alloying, to contribute much broader applications in the future.

Key words:Mg alloys; alloying; microstructure; corrosion electrochemistry