Electrodeposition of Al-Ni-Cr alloys from chloroaluminate ionic liquid for bond coat on gamma-TiAl

Abstract

In order to expand the utilization of γ-TiAl for high temperature structural applications, constant efforts have been dedicated to the development of coating systems to provide adequate oxidation resistance above 800oC. However, most coating methods currently employed, such as magnetron sputtering, plasma spray, and EBPVD are regarded as line-of-sight techniques, which limit the possible geometries to be coated. In this study, the possibility of electrodeposition, which emerges as a cost-effective and non-line-of-sight method, of metallic bond coats on TiAl was explored. Based on the practical feasibility, this research was targeted on Al-Ni-Cr alloy coatings. Lewis acidic chloroaluminate ionic liquid, consisting of 66.7 at.% AlCl3 and 33.3 at.% 1-ethyl-3-methylimidazolium chloride, was adopted as the baseline plating bath. The alloying elements, such as Cr and Ni were introduced by anodic dissolution of pure metals. The concentrations of Ni(II) and Cr(II) ions as a function of applied charge were measured by ICP-AES. With the conditions adopted for the experiments, the highest concentration of Ni(II) ions was 141 mM, while the concentration of Cr(II) ions was around 30 mM due to its limited solubility. The electronic absorption spectrum indicated that Cr(II) ions were tetrahedrally-coordinated, while Ni(II) ions were octahedrally-coordinated. The diffusion coefficient of Cr(II) ions was almost 10 times that of the Al(III) ions. With the large difference between the work functions of Al and Ni, Al(III) ions could be reduced by under-potential deposition in Ni solution, where the highest reduction potential was at least 0.55 V above Al(III)/Al reversible potential. In Cr solution, Al(III) ions could also be reduced under-potentially, but the under-potential was less than 0.1 V. However, in the Ni-Cr solution, no under-potential reduction of Cr(II) was observed at the presence of Ni(II). The morphologies and microstructures of the electrodeposits differ significantly as potential goes from under-potential through equilibrium potential to over-potential, and accordingly, similar microstructures can be obtained at corresponding current densities. The in-situ phase composition of Al-Cr deposits was measured by anodic stripping voltammetry as a function of deposition potential and convection. The addition of toluene as a co-solvent for Al-Cr electrodeposition was also studied, which led to the effect of decrystallization. The potentiostatic current transient for Al-Ni-Cr electrodeposition demonstrated three stages of the deposition process including nucleation, diffusion controlled growth and steady state growth, and illustrated the effect of convection on the last two stages. The pitting corrosion behaviors of the coatings were tested in both Na2SO4 and NaCl solutions by cyclic polarization. The oxidation tests were conducted on electrodeposits obtained in the over-potential region, which contains a duplex structure of Al matrix and embedded AlNiCr amorphous particles. An under coat of Ni was introduced between the TiAl substrate and Al-Ni-Cr top coat by nickel strike and nickel plating to provide adequate adhesion. Severe inter-diffusion occurs between Ni and TiAl. A continuous Al2O3 was formed in the top coat which provided good oxidation resistance.