Controlling Corrosion of Lightweight Magnesium Alloys through Improved Surface Stability

通过使用轻质合金来减少运输车辆的总质量是提高燃油效率和减少有害排放的方法之一。锻造镁(镁)合金部件具有较高的比强度和硬度，为实现这些目标提供了重要机会。可以说，阻碍镁合金组件广泛应用的关键技术问题是在水环境中的高溶解(腐蚀)速度。这在很大程度上是由于镁(工业工程金属中最低的标准还原电位)固有的反应性，在腐蚀表面上主导阴极过程的高析氢反应(阴极激活)，以及天然保护性氧化物表面膜破裂并被相对较厚的部分保护性腐蚀产物膜取代的倾向。这一建议旨在阐明和控制驱动镁表面局部腐蚀的膜基阴极激活。具体的短期目标包括：(I)通过合成和研究代理镀膜的镁表面，确定负责控制阴极激活的膜成分和/或结构因素，(Ii)利用激光拉曼光谱(LRS)原位解决腐蚀镁表面上空气形成的氧化膜的演化，以验证非原位透射电子显微镜(TEM)方法的膜成分结果，以及(Iii)通过在预氧化的镁表面的膜内注入析氢毒物来控制(减少)阴极激活，作为开发能够实现表面改性腐蚀控制技术的第一步。传统的电化学极化技术将被用来表征腐蚀的镁表面在氯化物水溶液中的整体电化学响应，并在受控电流(恒电流极化)条件下加速击穿过程。扫描振动探头技术将用于分辨和跟踪阳极和阴极过程，因为它们在自由腐蚀和阳极极化的镁表面上移动。高分辨电子显微镜技术将用于异地研究特定位置的薄膜结构和化学成分，这些结构和化学涉及到及时启动和维持阴极激活。非原位检查的前提是薄膜在从溶液中移出时不会发生明显的变化。这一问题将通过对使用激光拉曼光谱形成的表面膜的组成进行比较原位表征来具体解决。这对于阐明为设计和开发结构变形镁合金改进的耐腐蚀前处理涂层工艺提供新的发现机会是至关重要的。**

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. **