Lithium Carbonate as a Corrosion Inhibitor for Mg Alloy Sheet Metal Product

Reducing the overall mass of transportation vehicles by the use of strong lightweight materials is arguably the best enabling means to improve fuels efficiency and, thus reduce harmful emissions. Wrought Mg alloys with high specific strength, stiffness and warm formability (ZE alloys) provide enabling opportunities as an integral part of lightweight multi-material structural assemblies to achieve this goal. One key technical issue preventing widespread implementation of Mg alloy components is the high rate of dissolution (corrosion) in aqueous environments. This is driven in large part by the inherent reactivity of Mg (lowest standard reduction potential among industrial engineering metals), the high rate of the hydrogen evolution reaction that dominates the cathodic process and the tendency of the native oxide surface film to breakdown and be replaced with a significantly thicker and much less protective corrosion product film. Dissolved Li2CO3 (aq) is being evaluated as an effective corrosion inhibitor for bare Mg alloy sheet metal product (AZ31B and ZEK100) when immersed in 0.1 M NaCl (aq) as a precursor to the development of a conversion coating technology. The extent of corrosion protection is being determined at various Li2CO3 levels (mM) by making volumetric H2 gas evolution and mass loss measurements as a function of immersion time. Complementary potentiodynamic polarization measurements are being conducted to determine the manner in which dissolved Li2CO3 (aq) affects the global anodic and/or cathodic processes that control corrosion. SVP measurements are being made to resolve and track changes in the localized filament corrosion and associated cathodic activation resulting from effective inhibition of dissolved Li2CO3 (aq). Both surface analysis by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) examination of FIB-prepared cross-sections are being utilized to determine the film chemistry and local fine-scale structure involved in the filament corrosion and associated cathodic activation if and when it develops with time. A similar set of measurements is being made with Li(OH)2 (aq) additions (at equivalent pHs) to determine the specific role of Li+ (aq) and CO3 2− (aq) ions in providing improved corrosion control. This paper presents and discusses the results within the context of developing a self-healing conversion coating layer for inclusion in automotive paint schemes.