Corrosion Inhibition of Magnesium Alloy by Dissolved Lithium Carbonate

Abstract

The extent to which dissolved Li2CO3 can inhibit corrosion of lightweight Mg alloy sheet metal in contact with aqueous NaCl solutions was determined. Two Mg alloy sheet metal alloys were studied, which include: AZ31B (3% Al, 1% Zn, 0.5% Mn, balance Mg) and ZEK100 (1.3% Zn, 0.2% Nd, 0.25% Zr, balance Mg). Corrosion inhibition was first determined for each alloy separately when immersed in 0.1 M NaCl (aq), with and without dissolved Li2CO3 added. The addition of 100 mM Li2CO3 (aq) reduces the corrosion rate of AZ31B by a factor of ~10 and ZEK100 by a factor of ~12. Inhibition involves a reduction in both global anodic dissolution and cathode (H2 gas evolution) kinetics. It also involves suppression of localized filament-like corrosion and associated anode/cathode activation. Site specific cross-sectional analysis of the surface film formed during forced anode activation (polarization) revealed the formation of a Li-doped MgO film, akin to what forms, and provides protection to, Mg alloys with Li added as an alloying element. Such film formation was used to explain all corrosion inhibition aspects. Corrosion inhibition was then determined for ZEK100 when immersed in 0.1 M NaCl (aq) with and without a spray-deposited Li2CO3 surface coatings added. A commercial hexafluoro-titanate/zirconate-polymer conversion coating (Bonderite® MNT 5200) also served as the comparative basis. The Li2CO3-coated surface exhibits the lowest relative corrosion, whereas the conversion-coated surface exhibits the highest. Improved corrosion control is attributed to the formation of a compact coating (physical contribution) and the ability of dissolved Li2CO3 to inhibit both the anode and cathode kinetics (electrochemical) contribution. The findings are of interest to automotive industry as a possible means to effectively control corrosion of Mg alloy sheet metal using Li2CO3 as a surface pre-treatment or the inclusion of Li2CO3 to a polymer as an inhibitor additive.