Study of Cathodic Metal Transfer to Magnesium Surfaces in Aqueous Environments and Engine Coolant Formulations by Surface Analytic Methods

摘要

Engine blocks and associated powertrain components are among the applications of magnesium alloys being considered for automotive weight reduction. A research project sponsored by the U.S. Department of Energy and the U.S. Automotive Materials Partnership, demonstrated a potential weight reduction up to 25% relative to the cast components of an all-aluminum engine. The corrosion performance of a magnesium engine block in the presence of aqueous engine coolants is one of the primary technical challenges in the development of such a technology. In addition to the fundamental challenge of a principally aqueous environment is that of the coolant loop being a source of dissolved metal ions such as Fe and Cu, which can potentially "plate out" on nascent magnesium surfaces, thereby aggravating further corrosion as localized cathodes. The mechanism and severity of this "autocatalytic" aggravation of magnesium corrosion is highly speculative and analogous to mechanisms hypothesized for aluminum alloys in similar situations. The present study considers the composition and growth of the surface film produced on pure magnesium and AM-SC1 magnesium alloy due to transition metal transfer in ethylene glycol-water mixtures containing Fe~(2+) and Cu~(2+) ions in solution at 20 and 80°C. Surface compositional analysis was conducted using Rutherford Backscattering Spectroscopy (RBS) and Auger Electron Spectroscopy (AES). It was observed that metal ion transfer from the electrolyte to the magnesium surface created local galvanic corrosion cells on the magnesium surface and promoted higher rates of corrosion in localized areas.