Surface morphology, crystal structure and orientation of aluminium coatings electrodeposited on mild steel in ionic liquid
Introduction
Interest in the preparation of aluminium coatings has steadily increased due to its excellent corrosion resistance, decorativeness and physicochemical properties. Several procedures can be employed for coating of Al on various substrates—mainly on steels—such as hot dipping [1], [2], thermal spraying [3], sputter deposition [4], [5], vapor deposition [6], [7], etc. These techniques are rather expensive and often impractical, and the main disadvantages of them are the uneven surfaces, impossibility of controlling the thickness and quality of the layer, and possible damage of the specimen at employed high temperatures. On the contrary, the electrodeposition method is a valuable technology due to its advantages such as mild conditions, easy operation, uniform thickness distribution, adjustable microstructure of the deposited layers and unlimited substrates (including arbitrary shapes or complex geometries).
Electrodeposition of Al from aqueous solutions is impossible owing to a massive hydrogen evolution at the cathode caused by the rather negative standard potential of Al/Al(III) couple (−1.67 V vs. NHE). Essentially, two types of aprotic electrolytes are eligible: organic solvents and molten salts. There are three typical kinds of organic solvents used for electrodeposition of Al: aromatic hydrocarbons [8], [9], dimethylsulfone [10], [11] and ethers [12], [13]. At present, only two commercial processes based on organic solvents are available for electroplating of Al: SIGAL [8] and REAl [14], [15]. The main disadvantages of such processes are the flammability and volatility of the electrolytes. High temperature inorganic molten salts, such as NaCl–KCl [16], AlCl3–NaCl [17] and AlCl3–NaCl–KCl [18] have been extensively studied for the electrodeposition of Al. However, the high temperature and their highly corrosive natures bring great difficulties for finding container materials that can withstand chemical attack by the melts.
Over the last decades, aluminium plating in the room temperature molten salts (also called room temperature ionic liquids—RTILs), has received considerable attention. As a new and novel generation of solvents, RTILs exhibit many attractive properties, including excellent chemical and thermal stability, low melting points with negligible vapor pressure, high electrical conductivity and solvent transport properties, wide range of operational liquid temperature, ability to dissolve various organic, inorganic, and organometallic compounds and large electrochemical window of about 4.0 V. These properties give RTILs a certain potential to play a vital role in the electrochemistry field. Electrodeposition of Al from AlCl3 based ILs were intensively and systematically studied from the 1980s, seeing for example [19], [20], [21], [22], [23], [24], [25], [26]. These ILs exhibit adjustable Lewis acidity depending on the molar ratio of AlCl3/IL [27]. Electrodeposition of Al can only be performed under the Lewis acidic condition, in which the Al2Cl7− precursor, the dominant species in the electrolyte, can be electrochemically reduced to the metallic form according to the following reaction [28]:4Al2Cl7− + 3e− → Al + 7AlCl4−A lot of progresses have been made and the electrodeposition was performed on various substrates such as platinum [20], [29], tungsten [20], [21], gold [30], [31], copper [32], [33], glass carbon [20] and mild steel [34]. However, little information has been reported in the literature about the systematic study of the relationships between deposit appearance, surface morphology and current efficiency with current density and temperature, respectively. Moreover, there is a lack of published reports describing the effects of current density and temperature on the crystal structure and orientation of Al electrodeposited from RTILs. In this work, we describe comprehensive and detailed investigation of surface morphology and crystal orientation of Al coatings electroplated on mild steel substrates in Lewis acidic AlCl3/[bmim]Cl ILs by galvano-static technique. Temperatures were determined in the range from 308 K to 328 K based on our parallel study, where smoother, denser and brighter deposits were obtained.
Section snippets
Experimental
The electrolyte preparation and subsequent eletrodeposition were both conducted in an electrolytic cell with a jacket under a dry nitrogen atmosphere. [bmim]Cl was synthesized in our laboratory. Anhydrous AlCl3 (powder) obtained from Beijing Chemical Reagents Company, without further purification, was used as the initial source of Al. The Lewis acidic electrolyte (with 2.0:1 molar ratio of AlCl3/[bmim]Cl) was carefully prepared by mixing precise quantities of AlCl3 and [bmim]Cl. The obtained
Effect of current density on the surface morphology
Current density usually has an important effect on the deposit brightness, thickness distribution, current efficiency and microstructure of the electrodeposits. Herein the effect of current density was investigated from 8 mA/cm2 to 24 mA/cm2 at 308 K, from 8 mA/cm2 to 26 mA/cm2 at 318 K and from 8 mA/cm2 to 32 mA/cm2 at 328 K in 2.0:1 AlCl3/[bmim]Cl for 0.5 h in galvano-static mode (the supreme current density is enhanced with the temperature increasing). All of the Al-deposited samples were dense,
Conclusions
It can be concluded that surface morphology and crystal orientation of aluminium deposits on mild steel substrates from Lewis acidic AlCl3/[bmim]Cl ILs are affected markedly by variations in current density and temperature. It appears that the morphology of deposits under lower temperature (308 K) was relatively independent of current density. Deposit microstructure obtained at higher temperature (318 K and 328 K) revealed obvious changes as a function of current density. It was shown that the
Acknowledgements
This work was supported financially by National Natural Science Foundation of China (No. 20776140), National Science Fund for Distinguished Young Scholars of China (No. 20625618) and National 863 Program of China (No. 2006AA06Z317 and No. 2006AA06Z371).
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