Improvement of Al coating adhesive strength on the AZ91D magnesium alloy electrodeposited from ionic liquid Jinwei Tang ⁎, Kazuhisa Azumi Graduate School of Engineering, Hokkaido University, N13W8, Kitaku, Sapporo 060‐8628, Japan abstract article info Article history: Received 11 February 2012 Accepted in revised form 7 June 2012 Available online 15 June 2012 Keywords: AZ91D Mg alloy Aluminum Zincate pretreatment Adhesion strength Adhesive strength of Al coatings electrodeposited on AZ91D in aluminum chloride/1-ethyl-3-methylimidazolium chloride was improved by the development of a zincate pretreatment and optimized current pulse electrodepo- sition method. A dense Zn layer was deposited on both the α- and β-phase surfaces of AZ91D substrate pre- treated in a Cu 2+ added activation bath prior to the zincate process. On this zincated substrate an Al coating was electrodeposited by bipolar current pulse polarization and the adhesive strength of the Al coating to the substrate was better than 11 MPa. The Al coating obtained under the optimal condition also showed good cor- rosion resistance in 3.5 wt.% NaCl solutions. © 2012 Published by Elsevier B.V. 1. Introduction Magnesium based alloys have a number of attractive properties such as low density and high strength-to-weight ratio. These proper- ties make them ideal candidates for many applications. However, Mg alloys are susceptible to galvanic corrosion which may cause severe degradation including deterioration of mechanical properties and un- attractive appearances and as a result restricted the use of Mg alloys. Aluminum coating for corrosion protection of Mg alloys has received extensive attention because of its light weight and high corrosion re- sistance. The less-noble properties of Al coating are also a desirable quality for the Mg substrate to avoid severe galvanic corrosion be- tween the coating and substrate at defects in the coating. Recyclability of Al coated Mg alloys is a further favorable property since Al is a pri- mary alloying element of the AZ series of Mg alloys. Several methods for producing Al coatings on Mg alloys have been developed, including thermal spray coating [1], electroless plating [2,3], physical or chemi- cal vapor deposition [4], and hot dipping [5]. Compared with these methods, electrodeposition is a relatively economical and convenient technique. It has been reported that Al and its alloys can be electro- deposited on Mg alloy from acidic aluminum chloride 1-ethyl-3- methylimidazolium chloride (AlCl 3 –EMIC) ionic liquids [6–16]. De- spite the improvement of corrosion resistance due to these coatings, poor adhesion strength of Al coatings to the Mg substrate has also been reported in the previous work [6,8,11]. Yang et al. [11] performed a post-plating heat treatment of Al coatings on AZ91D to improve the adhesive strength and hardness, and to maintain corrosion resistance. They found that heat treatment at 473 K was the most effective, as an intermediate layer rich in γ-phase (Mg 2 Al 3 ) was formed between the Al coating and substrate. Direct electroplating on Mg alloys is difficult because a loose MgO and/or Mg(OH) 2 film easily forms on the substrate even at conditions where the content of oxygen or water in the atmosphere or in the ionic liquids is low. These films disturb the successive electrodeposi- tion of metal on the substrate and thus lessen the adhesion strength of the electrodeposited layer. Therefore, a suitable pretreatment is necessary to remove this surface film and to suppress further forma- tion of such layers prior to the electrodeposition [17–26]. Zincate pre- treatment for Mg alloys has been investigated to provide a protective and electrically conductive layer desirable for the subsequent metal electroplating [27–30]. It is also important to obtain uniform Zn de- posits on AZ91D alloys in the zincate pretreatment for the successive plating. Recently, the authors have established that the addition of small amounts of Cu 2+ ions to the traditional activation bath im- proved the uniformity of Zn layer deposited on Mg alloys in the follow- ing zincate pretreatment because highly dense fine Cu is deposited in the activation bath and acts as nucleation sites for the Zn deposition [31,32]. It was also found that the current pulse polarization method could improve smoothness and density of Al coatings by adding a small anodic current to the pulse cycle resulting a slight dissolution of the already formed film [33,34]. Based on this previous work, the pre- sent study electrodeposited Al coatings on AZ91D from AlCl 3 –EMIC ionic liquid using a current pulse technique and the improved zincate pretreatment. The morphology of the surface and cross-sections, adhe- sion strength, and corrosion protection performance were evaluated as a function of deposition condition. Surface & Coatings Technology 208 (2012) 1–6 ⁎ Corresponding author. Tel.: + 81 11706 6749; fax: + 81 11706 7897. E-mail address: tangjinwei@tpri.com.cn (J. Tang). 0257-8972/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2012.06.024 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat