Evaluation of Al and Al Alloy Films Formed By Electrochemical Method
Authors: Matsubara Kyohei , Ito Yuya , Yanai Kohei , Fujimori Shunya , Sumiyoshi Rie , Watanabe Nobuaki , Koiwa Ichiro 
Published: February 2020

A zinc plated to prevent corrosion, however, a zinc will be exhausted within twenty years1) 2). An aluminum is one of the most promising candidates as alternative material. An aluminum is the third clarke number material and it is very rich. An aluminum has a great ionic tendency, but it forms very stable oxide film and it suitable as corrosion-resistant materials. However deposition potential of aluminum is less noble than hydrogen evaluation potential. Therefore, an aluminum is difficult to be plated from aqueous solution. An organic solvent and ionic liquid are used as a solvent for Al plating. A dimethyl sulfone (DMSO2) shows low volatility, high stability and wide potential window. Therefore this study used DMSO2as a solvent for Al and Al alloy plating. But the dimethyl sulfone has high viscosity, the plating films are influenced by agitation. We discuss the influence of metallic salts and organic additives on the crystallographic orientation of the films. Electrochemical experiments were carried out in three-electrode cell using a potentiostat (Hokuto Denko HZ-5000) between 110 oC and 150 oC. The reference electrode was an Al/Al3+ electrode (-1.676 V vs. SHE). The counter electrode was an aluminum plate. The working electrodes were 99.99 % pure copper plate. The plating solution was composed of AlCl3 and DMSO2. The ratio between DMSO2 and AlCl3 is 10:3, AlCl3 concentration is 23 mol% and 10:5, AlCl3 mol concentration is 33 mol%. Dehydration was carried out under a nitrogen atmosphere at 150 oC for 30 minutes. As analysis method of the samples, we used X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS).
Figure 1 shows SEM micrographs of Al-Mn alloy films plated from baths whose AlCl3mol concentration was 23 mol% (a,b,c) or 33 mol% (d,e,f,g,h,i,j) by changing deposition potential. The deposition potential region of film formation was different by Al ion concentration. The films were plated at deposition potential region between -1.5 and -3.0 V for 23 mol% Al ion concentration bath and that between -3.0 and -4.0 V for 33 mol% Al ion concentration bath. When the aluminum ion concentration was 23 mol%, grain size was increased by changing deposition potential from -1.5 V to -4.0 V and surface roughness also increased. The manganese content was increased with increasing deposition potential and the maximum Mn content, 26.5 at% at -2.0 V deposition potential. Then the manganese decreased with increasing deposition potential. When the aluminum ion concentration was 33 mol%, surface morphology was changed by chansing deposition potential from -1.5 V to -4.0 V. A clear dependence between deposition potential and surface morphology was not observed. Moreover no clear relationship between deposition potential and Mn content was not also observed. The maximum Mn content, 10.9 at%, was obtained at deposition potential of -3.5 V. The films plated from 23% Al ion concentration bath showed much higher Mn content than those plated from 33% Al ion concentration bath. Ammonium chloride (NH4Cl) is added to the bath in order to decrease the viscosity of the bath as additive. The melting point of the plating solution composed of AlCl3 and DMSO2 was decreased by adding NH4Cl. According to decreasing melting point, the viscosity seems to decrease. The surface morphology of the films is drastically affected by adding of small amount of NH4Cl. By adding of NH4Cl, there is not influenced by stirring, but the particle size is increased. Pinholes occurred increasing additive amount. When ammonium chloride added to the solution, the intensity of the Al(200) peak is decreased. The plating films have no metallic luster. It is found that the surface morphology of the plating films were changed accompanied by metallic salts and additives. The maximum Mn content, 26.5 at% at -2.0 V deposition potential. The films adding NH4Cl is not influenced by stirring, but the crystalline size increased.

Acknowledgements

This work was partially aided by the MEXT-supported Program for the Strategic Research Foundation at Private Universities.

Reference

1) T.Adaniya, J. MMIJ, 128, 57-64 (2012) [in Japanese].
2) K. Halada, M. Shimada and K. Ijima. J. Japan Inst. Metals, 71, 831-839 (2007) [in Japanese].

Figure 1