ABSTRACT

The newest generation of aluminum, high strength steel and magnesium alloys show significant potential for light weight applications in aerospace, transportation and consumer electronic applications. However, these alloys suffer from significant corrosion problems. To address this issue, a series of nanostructured binary aluminum alloys (AlCr, AlMn, AlZr) have been prepared and tested for corrosion performance. The composition of these single-phase alloys, created via electrodeposition from ionic liquids, can be tuned to adjust microstructure and modify properties such as hardness and pitting potential in order to target application specific requirements. Here we present data on alloy composition, hardness and corrosion performance (ASTM B117 & potentiodynamic polarization) relative to the base aluminum alloys 6061 & 2070. Results show that the thermodynamically stable nanocrystalline aluminum alloys provide improved corrosion resistance as compared to the base alloys.

INTRODUCTION

Technological innovation in the field of lightweight materials has been driven primarily by the transportation industry’s need to lower fuel costs and reduce emissions. As a result, advances in design and production methods have led to the development of novel bulk metal alloys with improved strength to weight ratios.1,2 These materials include aluminum, magnesium and high strength steel alloys capable of extended use as components in the automotive, aerospace, and maritime industries.

Even with the strength improvements of these materials and despite of the formation of natural oxide coatings, corrosion issues3 continue to limit potential applications and slow the timeline for implementation. For aluminum alloys, pitting, and galvanic discrepancies are the primary mechanisms leading to corrosion.4 Pitting corrosion is facilitated by exposure to halide ions that enhance breakdown of the natural oxide layer, representing a significant risk factor for transportation applications. Galvanic corrosion can manifest as a result of corrosion potential differences within the alloy (i.e. secondary phases) or from physical mating to more noble metals.

Coating solutions have been developed that attempt to address these issues, including conversion coatings, paints and corrosion resistant metal films, however, these solutions often suffer from their own pitfalls.5 Here we present corrosion performance data relating to a new class of nanostructured aluminum alloy (AlCr, AlMn, AlZr) coatings. These metal films are electrodeposited from ionic liquid electrolytes and are formed as thermodynamically stabilized supersaturated solid solutions.6,7 The addition of high levels of alloying elements serve to reduce the grain size of the alloy, drastically improving strength and hardness (up to 450 HV). Furthermore, the ability to modify alloy composition of the coatings presents a straightforward means of engineering coating corrosion potentials. Coupled with a lack of secondary phases, corrosion behavior of the coatings is significantly improved leading to a durable, environmentally friendly and corrosion resistant coating solution.

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