Effect of rare earth (Ce, La) compounds in the electroless bath on the plating rate, bath stability and microstructure of the nickel–phosphorus deposits H. Ashassi-Sorkhabi ⁎ , M. Moradi-Haghighi, M.G. Hosseini Electrochemistry Researches Laboratory, Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran Received 27 May 2007; accepted in revised form 11 July 2007 Available online 20 July 2007 Abstract Effects of added rare earth elements (RE) in the acidic hypophosphite plating bath on the plating rate, bath stability and microstructure of the electroless nickel–phosphorus (EN) deposits were studied. The surface appearance and microstructure were examined under a reflection optical microscope and a scanning electron microscope equipped with an in-situ energy dispersive X-ray spectroscopy, which can evaluate the elemental analysis of deposits. It was demonstrated that the rare earth elements can decrease grain size and refine microstructure. The deposition rate of the Ni–P deposits was estimated by gravimetric, polarization and quartz crystal microbalance (QCM) methods. Results revealed that up to an optimum concentration of rare earth elements, the deposition rate increases. The stability test method was used to determine the stabilization effect of RE on the stability of the bath. It was found that the addition of RE significantly improved the Pd stability of the EN bath. © 2007 Elsevier B.V. All rights reserved. Keywords: Electroless nickel plating; Rare earth elements; Deposition rate; Bath stability; Quartz crystal microbalance 1. Introduction Since the discovery of electroless or autocatalytic nickel plating, it has been widely used in electronics machinery, automobile, aerospace and other industries [1,2]. With excellent properties such as non-magnetic, low internal stress and high corrosion resistance, Ni–P alloy coating with high-P content has been an important undercoat for computer hard disks [3]. Rare earth elements have many special properties, such as magnetic, optical, and electric and hydrogen storage properties and have been successfully used in many fields such as metallurgy, electronics and chemical engineering [4,5]. Researchers have found that the rare earth elements can influence on certain parameters like deposition rate and stability of the bath in Cr, Ni and Cu electroplating process, [6–10]. It is also reported that these elements improve mechanical properties of alloys, like tensile strength, toughness and fatigue resistance in alloys such as Al–Li and Al–Si [11]. In the present study, the effects of added rare earth elements (RE) in acidic hypophosphite plating bath on the properties of the resulting electroless Ni–P deposits were studied. The authors found that the addition of rare earth elements can increase the plating rate and bath stability, and improve the film microstructure by refining the grains of deposits. 2. Experimental details 2.1. Materials and reagents The gold plated quartz crystals and mild steel coupons were used as the substrates. The chemicals used in the experiments, all purchased from Merck, were of reagent grade and used without any further purification. The used EN solution bath was formulated as Table 1. It was chosen because a glycin–citrate combination produces a complex with nickel and makes the plating solution stable [12]. The initial pH value of the plating bath was adjusted by dilute NaOH solutions to 4.5 ± 0.1. De-ionized water was used for solutions preparation and rinsing of glassware. Using a digitally controlled thermostat (Memert), the bath temperature was controlled within 85 ±1 °C under atmospheric environment. The plating was conducted by immersing one piece of the substrate into EN solution (100 ml) in a 150 ml beaker for 1 h. Available online at www.sciencedirect.com Surface & Coatings Technology 202 (2008) 1615 – 1620 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +98 411 3393136; fax: +98 411 3340191. E-mail addresses: habib_ashassi@yahoo.com, ashassi@tabrizu.ac.ir (H. Ashassi-Sorkhabi). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.07.019