Magnesium Alloy Corrosion Under Thin Electrolyte Layer Using Electrochemical Impedance Spectroscopy and Polarization Curve

Abstract

Here, we reported some simulated atmospheric corrosion studies of magnesium alloy (including AZ91D and AM60) based on electrochemical impedance spectroscopy and polarization technology, and the effect of rare earth element Ce or La on corrosion behavior of magnesium alloy. In the case of AZ91D magnesium alloy, the electrochemical results reveal that the corrosion rate of AZ91D under thin electrolyte layers (TELs) is slower than that in bulk solution and decreases with the TEL thickness thinning. Bothe the anodic and cathodic reactions are restrained under TELs, especially in thickness thinning to around 200 μm and lower. For AM60 magnesium alloy and pure magnesium, the corrosion processes are also inhibited by decreasing thickness of TEL. All those corrosion rates and behavior of magnesium and its alloy are different from that of aluminum, copper and zinc alloy, which are due to the special cathodic reaction for magnesium alloy. Rare earth elements Ce or La have a strong effect on the dissolution of AM60 magnesium alloy under TEL. Typical impedance diagrams consist of two capacitive arcs at high and intermediate frequencies in all cases. At low frequencies, an arc or tail in the Nyquist plots appears sometimes disappears due to the complex corrosion reaction. Comparing to AM60, AM60 with rare earth element addition presents the 3rd capacitive loop sometimes instead of an inductive tail under the TEL thicknesses of 200 and 300 μm, which is associated with reaction taking place on the cathodic intermetallic compounds formed with rare earth elements. The EIS and polarization measurements indicate the rare earth element alloying can reduce corrosion rate of AM60 and AZ91D both under TEL and in bulk electrolyte.