Magnesium Nanocomposite Coatings for Protection of a Lightweight Al Alloy: Modes of Corrosion Protection, Mechanisms of Failure

In light of the increased emphasis on lightweighting of vehicular components and continued use of high-performance aluminum alloys in the aerospace industry, designing alternatives to carcinogenic chromium-based corrosion control systems has emerged as an urgent imperative. The high activity of aluminum and the heterogeneous surface structure of Al alloys renders effective corrosion inhibition a formidable challenge. Here, the effective corrosion protection of AA7075 alloys by Mg/polyetherimide nanocomposite coatings prepared by dispersing solution-grown Mg nanocrystals within a polyamic acid matrix followed by imidization on the substrate are demonstrated. The active nanocrystal filler and nanocomposite are characterized using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and cross-sectional electron microscopy. Electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) responses of coatings are evaluated over the course of 100 days of exposure to a 3.5 wt% aqueous solution of NaCl. These results suggest that the nanocomposite coatings endow efficacious cathodic and barrier protection to the underlying alloy substrate. The Mg/PEI nanocomposite coatings endow immediate cathodic protection to AA7075 substrates upon salt water immersion with rapid mobilization of the active filler. The nanocomposites represent a vital addition to the sparse set of chrome-free options for corrosion protection of lightweight alloys.