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

Abstract

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.