Electrodeposited aluminum and aluminum-transition metal alloys are promising materials for use as corrosion-protective coatings for steel.   In fact, it is thought that electrodeposited aluminum might be a replacement for cadmium in some applications.  This substitution is important because the process used for plating Cd is associated with significant health and environmental concerns.  From health and environmental perspectives, the chromium plating process is also problematic, and substitutes for chromium are eagerly sought.   Among the possible alternatives are aluminum-transition metal alloys such as Al-Mn and Al-W.  These alloys are sufficiently durable to replace electroplated chromium in at least some applications.

Aluminum plating is not feasible from aqueous solutions because water is reduced and hydrogen is evolved prior to aluminum deposition.  Thus, the plating process must be carried out in a nonaqueous environment.  Most commercial aluminum plating processes utilize mixtures containing pyrophoric alkylaluminum fluorides dissolved in toluene, e.g., the Siemens SIGAL® process.  This very effective technology has been purchased and improved by AlumiPlate, Inc., but the newer process is still based on similar pyrophoric plating solutions and continues to invoke significant health and safety concerns. 

Aluminum electrodeposition from ionic liquid electrolytes, particularly organic salt-based chloroaluminates, such as aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl₃-EtMeImCl), is an attractive alternative to the classical methods for preparing aluminum coatings described above. These well known ionic liquids have low vapor pressures, low viscosities, high electrical conductivities and thermal stabilities, and so far as known, relatively low toxicities.  Commercially marketed plating solutions based on these ionic liquids with the addition of proprietary additives are now marketed under the BASIONICS trademark by BASF. 

In our work, we have developed a Portable Aluminum Deposition System (PADS) for plating aluminum and its alloys from Lewis acidic AlCl₃-EtMeImCl and related chloroaluminate ionic liquids.  This technology was developed as an alternative to typical open bath electrodeposition processes and is suitable for field applications.  The utility of the PADS device was demonstrated by electrodepositing Al and Al-Mn films on copper and carbon steel.  These electrodeposition experiments were performed using constant potential and constant current plating methods.  The deposits were evaluated based on alloy composition, compactness, surface morphology, durability, and corrosion resistance.  The Al-Mn films produced with the PADS device were specular, dense, and adherent, and they rivaled the quality of similar deposits prepared in an open bath (see figure below).  As expected, the Al-Mn films demonstrated significantly better resistance to chloride-pitting corrosion and much greater hardness compared to pure Al films.  In this presentation, we will describe the relationship between the operational parameters of the PADS device and the composition and overall quality of the resulting Al-Mn deposits.

This work was funded by the Strategic Environmental Research and Development Program (SERDP) through contract DE-AC05-00OR22725 to Oak Ridge National Laboratory with subcontract to the University of Mississippi