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.

Tortuosity but Not Percolation: Design of Exfoliated Graphite Nanocomposite Coatings for Extended Corrosion Protection of Aluminum Alloys

“…The −1.66 V value of the Al 3+ /Al redox couple (versus SHE) implies that zinc and trivalent chrome commonly used to protect steel substrates are ineffective at providing sacrificial cathodic protection to aluminum alloys. Magnesium-based nanocomposite coatings have instead been developed to provide sacrificial protection to aluminum alloy substrates and are effective under certain environments − but are plagued by issues such as the typically high reactivity of Mg particles, the complexity of preparing surface-passivated Mg pigments that can be safely handled, and the need for relatively thick coatings spanning scores of microns. In this article, we demonstrate an alternative approach wherein unfunctionalized exfoliated graphite (UFG) nanocomposite coatings with sub-30 μm thickness dispersed within a polyetherimide matrix show excellent corrosion inhibition of AA7075 substrates upon prolonged exposure to saline environments.…”

A Materials Science Perspective of Midstream Challenges in the Utilization of Heavy Crude Oil

“…At low graphene concentrations, graphene fillers induce greatly increased tortuosity of ion diffusion pathways, enabling excellent corrosion protection of underlying aluminum alloys, whereas at high concentrations, graphene flakes form a percolative network and initiate deleterious galvanic corrosion (Figure C) . As an alternative strategy, highly electroactive Mg nano and microparticles have been embedded within PEI and epoxy/polyamide matrices, wherein they afford sacrificial cathodic protection. ,− …”

Mechanistic origins of corrosion protection of aluminum alloys by graphene/polyetherimide nanocomposite coatings

“…In the case of aluminum alloys, options for more electropositive sacrificial anodes are severely limited. Magnesium is often the metal of choice to imbue cathodic protection but is itself rapidly corroded, and can be exceedingly reactive in challenging environments [24][25][26][27] . An alternative mechanism involves deposition of a less electrochemically active metal, which nevertheless constitutes a dense and conformal passivating layer that is readily transported and precipitated at damage sites as a result of its low solubility oxidation product 17,28 .…”