Short Communication Electroless deposition of Ni–P–B 4 C composite coating on AZ91D magnesium alloy and investigation on its wear and corrosion resistance A. Araghi, M.H. Paydar * Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran article info Article history: Received 31 August 2009 Accepted 27 December 2009 Available online 15 January 2010 abstract In the present work, the effect of applying ternary Ni–P–B 4 C composite coating from an electroless plat- ing bath containing sulfate nickel, sodium hypophosphate and suspended B 4 C particles, on the corrosion and wear resistance of an AZ91D, high aluminum cast magnesium alloy, was investigated. Regarding low corrosion resistance of magnesium alloys, chromium oxide plus HF (Hydro Fluoric Acid) pretreatment was applied to prepare the substrate for coating treatment in electroless bath. The pH value and temper- ature of the electroless bath were 9 and 82 °C, respectively. The coating was characterized for its micro structure, morphology, microhardness, wear and corrosion resistance. SEM (Scanning Electron Micro- scope) observation showed dense and coarse nodules in the ternary composite coating and the cross sec- tion of Ni–P–B 4 C coating offered presence of well dispersed B 4 C particles in the coating. The hardness of the Ni–P–B 4 C composite coatings was around 1200 MPa, more than what can be obtained for Ni–P coat- ings (about 700 MPa). The wear test which was carried out by using pin on disc method, showed that ter- nary Ni–P–B 4 C composite coating had a good wear resistance and more superior than Ni-P coating. The polarization test results for ternary Ni–P–B 4 C composite coating exhibited good corrosion resistance properties in protecting the AZ91D magnesium alloy, but not better than Ni–P coating. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Owing to high strength to weight ratio, magnesium and its al- loys considered as light alloys, have found a wide range of applica- tions, from aerospace to the electronics and automotive industries. However, their usages may be limited as a result of low wear and corrosion resistance [1–3]. Application of suitable coatings can solve this problem to some extent. Coatings of high wear resistance and considerable hardness can protect the substrate alloy from aforementioned phenomena. Furthermore, considering corrosion problem, applied coating shall be capable of acting as a barrier layer to separate the alloy from the corrosive media, resulting in higher corrosion resistance. The coating therefore, should be homogeneous and free from porosity and voids [3]. Many researchers have focused on applying Ni–P and composite coatings on plain substrates, such as steel and copper, using elec- troless coating method. However, a few researches have been allo- cated to applying such a protective coating on magnesium and its alloys due to their high reactivity and corrosion rate in aqueous solutions [4]. Application of electroless composite coating of Ni– P–B 4 C on AZ91D magnesium alloy is thus a new subject which is carried out in this research. There are several advantages for electroless coating over other coating methods, particularly electrodeposition, such as homoge- neous deposition over irregular substrates, simplicity and no need for special equipment [5]. In the present work electroless deposi- tion of Ni–P–B 4 C composite coating on AZ91D alloy was studied. Furthermore wear and corrosion resistance of the applied compos- ite coating and the effect of presence of B 4 C particles on the men- tioned properties were investigated. 2. Method Cast magnesium alloy AZ91D specimens with dimension of 20 mm  20 mm  5 mm, and B 4 C powder, with an average size of 1.5 lm, were used as substrate and reinforcement particles, respectively. Table 1 shows the chemical composition of the Mg al- loy was used. To prepare the substrates and make them ready for pretreatment and coating process, they were polished up to No. 4000 SiC paper. As mentioned before, magnesium alloys demonstrate great reactivity and low corrosion resistance in aqueous solutions. Thus, they require a suitable pretreatment operation prior to electroless deposition. The pretreatment carried out in this study, called CH + HFP (Chromium + Hydro Fluoric Acid Pretreatment) [2,3], prepares the alloy to be electrolessly deposited. In this operation, specimens were degreased and immersed in a solution of chromic and nitric acids for a maximum duration of 2 min, followed by 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.12.042 * Corresponding author. Tel.: +98 711 2307293; fax: +98 711 6287294. E-mail address: paaydar@shirazu.ac.ir (M.H. Paydar). Materials and Design 31 (2010) 3095–3099 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes