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

Abstract

Dense and adhesive pure aluminum layers were electroplated on Cu and mild carbon steel plates from the Lewis acidic aluminum chloride-1-ethyl-3-methylimidazolium chloride [60 mole percent (m/o) AlCl3] ionic liquid by using current-controlled DC methods. The influence of the current density on the surface morphology and grain size of the electrodeposited Al was investigated by optical microscopy. The corrosion resistance of Al-coated steel was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization measurements for deposits prepared with and without toluene as a co-solvent. Both deposition current density and deposit thickness were crucial factors affecting the structure, integrity, and corrosion resistance of electrodeposited Al layers on steel substrates. Aluminum films prepared in the ionic liquid containing toluene exhibited better overall durability, brightness, and practical corrosion resistance than films prepared from the neat ionic liquid. IntroductionAluminum 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 oxide film, which is formed spontaneously upon exposure to air at ambient temperature. Unfortunately, aluminum cannot be plated from aqueous solutions because hydrogen is evolved before most Al(III) species are reduced to Al. Among the many nonaqueous solvent systems that have been used to plate Al are those based on room-temperature haloaluminate ionic liquids (ILs).The first system, aluminum chloride-1-ethylpyridinium bromide, was reported in 1951. (1) Improved ionic liquids were obtained by replacing the pyridinium bromide salt component first with 1-alkylpyridinium chloride (2) and eventually the now ubiquitous 1,3-dialkylimidazolium chloride salts. (3) Although slow to reach commercial development (BASF ® SE), plating baths based on the latter IL offer numerous safety and convenience advantages compared to those based on pyrophoric organoaluminum compounds. (4) It was recognized during the inception of Al plating baths based on haloaluminate ionic liquids that the addition of an aromatic co-solvent, e.g., benzene or toluene, leads to brighter, more adherent coatings. (5, 6) Several research groups have examined or reviewed the electrodeposition of Al from room-temperature chloroaluminates containing at least one of these aromatic solvents. (7-12) The addition of co-solvents appears to 10.1149/06404.0549ecst ©The Electrochemical Society ECS Transactions, 64 (4) 549-561 (2014) 549 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-04 to IPimprove the appearance of Al coatings prepared in chloroaluminate ILs, but there is little information about the effects of bath co-solvents on the corrosion resistance of these platings. In this work, we report the direct current plating of...