Corrosion Resistance of Aluminum Films Electroplated on Steel from Chloroaluminate Ionic Liquids Containing Toluene as a Co-Solvent

Aluminum is a widely used material for corrosion resistant coatings.  From a thermodynamic perspective, it is a very active metal.  However, its excellent corrosion resistance stems from the formation of a robust, self-healing ~50 nm thick barrier film of Al₂O₃, which is formed spontaneously upon exposure to air at ambient temperature. Unfortunately, it is established that aluminum cannot be plated from aqueous solutions under normal conditions because hydrogen is evolved before most Al(III) species are reduced to Al.  A number of commercial Al plating processes have been developed.  Typically, these processes are based on pyrophoric mixtures of AlCl₃ or AlBr₃ with LiAlH₄ dissolved in suitable organic solvents, such as ethers or mixtures of substituted benzenes.

The first report in which a room-temperature haloaluminate ionic liquid was used to plate Al appeared in 1948.¹  Although moisture-sensitive themselves, such ionic liquids, particularly those based on mixtures of AlCl₃ and quaternary ammonium chloride salts, have many safety and convenience advantages over the currently used commercial Al plating baths.  In this work, we report the pulsed-current plating of Al on steel substrates from the Lewis acidic AlCl₃-1-ethyl-3-methylimidazolium chloride (EtMeImCl) ionic liquid with and without organic co-solvents.  Compared to DC methods, pulsed-current techniques can produce electrodeposited metals with smaller grain size and greater brightness, as well as less internal stress and correspondingly greater corrosion resistance.

Some preliminary results for Al films electrodeposited on steel from this ionic liquid are shown in Figs. 1 and 2.  The surface morphology and corrosion resistance of these films were interrogated with optical microscopy and electrochemical impedance spectroscopy (EIS) in aqueous NaCl, respectively.  Micrographs of typical Al electrodeposits (Fig. 1) indicate that their surface morphologies changed significantly with changes in t_on and t_off.  For example, the grain size of the Al film prepared with t_on = 0.25 s is much smaller than that for deposits prepared with t_on =1 s. Figure 2 shows complex plane impedance plots resulting from EIS experiments.  These plots show at least one time constant.  A circuit model for an electrode coated with an inert porous layer² was fitted to the data in Fig. 2 to extract information about the pore resistance (R_p) of the coating.  The pore resistance reflects the corrosion resistance of the electrodeposited films.  Figure 2 shows that the best results were obtained for t_on = 0.25 s and t_off = 0.50 s in agreement with the results obtained by microscopy.  It was generally found that films with the best corrosion resistance were obtained with smaller values of t_on.  Optimization of the parameters for the pulse plating of Al will be discussed with respect to the surface structure, grain size, corrosion resistance, and brightness of the deposits.

References

 

¹F. H. Hurley, U.S. Pat. 2,446,331 (1948).

²M. E. Orazem and B. Tribollet, Electrochemical Impedance Spectroscopy, (New Jersey: John Wiley and Sons, 2008).

Acknowledgement

This work was funded by the Strategic Environmental research and Development Program (SERDP) through contract DE-AC05-00OR22725 to Oak Ridge National Laboratory.