Controlling Corrosion of Lightweight Magnesium Alloys through Improved Surface Stability

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Reducing the overall mass of transportation vehicles by the use of lightweight alloys is one of the means by which improved fuel efficiency and reduced harmful emissions can be attained. Wrought magnesium (Mg) alloy components, with their high specific strength and stiffness, provide significant opportunities to achieve these goals. Arguably the 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 (cathode activation) that dominates the cathodic process on corroded surfaces and the tendency of the native protective oxide surface film to breakdown and be replaced with a relatively thick partially-protective corrosion product film. This proposal serves to to elucidate and control the film-based cathode activation that drives the localized corrosion upon Mg surfaces. Specific short-term goals include: (i) identify the controlling film composition and/or structure factors responsible for the cathode activation by synthesizing and studying surrogate filmed Mg surfaces, (ii) resolve the evolution of the air-formed oxide film on corroding Mg surfaces in-situ using laser Raman spectroscopy (LRS) to validate the film composition findings of the ex-situ transmission electron microscopy (TEM) method, and (iii) control (reduce) cathode activation by ion implanting H2 evolution poisons within the film of pre-oxidized Mg surfaces as an initial step towards developing an enabling surface modification corrosion control technology. Conventional electrochemical polarization techniques will be employed to characterize the global electrochemical response of corroding Mg surfaces in aqueous chloride solutions and to accelerate breakdown processes under controlled current (galvanostatic polarization) conditions. The scanning vibrating probe technique will be used to resolve and track the anodic and cathodic processes as they move across freely corroding and anodically polarized magnesium surfaces. High resolution electron microscopy techniques will be used to study ex-situ the site-specific film structure and chemistry involved in initiating and sustaining the cathode activation in time. The ex-situ examinations rely on the premise that the films will not be significantly altered upon removal from solution. This issue will be specifically addressed by conducting a comparative in-situ characterization of the composition of the surface films formed using laser Raman spectroscopy. The is critical to illuminate pathways for new discovery opportunities for the design and development of improved corrosion-resistant pre-treatment coating processes for structural wrought Mg alloys. **