Published by Maney Publishing (c) Institute of Metal Finishing Nanocrystalline zinc–nickel alloy deposition using pulse electrodeposition (PED) technique M. Ilayaraja, S. Mohan*, R. M. Gnanamuthu and G. Saravanan Zinc–nickel alloy was electrodeposited on stainless steel using pulse current deposition (PED) from a chloride–sulphate bath. Duty cycles were varied between 10 and 80% and frequency was changed from 10 to 100 Hz. The deposit characteristics were analysed using SEM, XRD and AFM and the results are presented in this paper. The corrosion resistance of zinc–nickel alloy deposited from direct current deposition (DCD) has been compared with that of the deposit obtained by pulse current using the electrochemical impedance spectroscopy method. Keywords: Zn–Ni, Pulse electrodeposition, XRD, AFM, Potentiodynamic polarisation Introduction Many alternatives are being developed to the well established use of zinc coatings on steel; to improve its corrosion behaviour, zinc–nickel alloy coatings have proved to be very effective because they are far superior to pure zinc deposits for both corrosion resistance and thermal stability. Extensive studies have been carried out on electrodeposition of such alloys from different baths, e.g. chloride–sulphate, 1,2 sulphate and chloride baths. 3,4 Because such zinc alloy deposits provide better corro- sion protection than electrogalvanised coating, 5–8 alloys of zinc with metals such as iron, nickel and cobalt have been considered as prime candidates to replace zinc as protective coating. In particular, studies have shown that the corrosion resistance of electrodeposited Zn–Ni alloy coatings, within a certain composition range (12– 15%) of Ni, can be five to six times better than for pure zinc of equal thickness 9–11 by pulse electrodeposition (PED), which, compared with DCD, can improve the deposition process and some deposit properties in addition to corrosion resistance, namely, porosity, ductility, hardness and surface roughness. 12–14 In the present work, the authors have studied zinc– nickel alloy deposits by PED on stainless steel, with a pre-alloy plating nickel strike: it is difficult to do elec- troplate on stainless steel because of the presence of its passive layer. This passive layer is removed by a suitable pretreatment in acid solution. In this paper, composi- tion, morphology, hardness and corrosion resistance using SEM, AFM, XRD and EIS were investigated. The square-wave cathodic pulse is presently the most widely used method of modulating plating current. In direct current deposition (DCD), only one current- and time-dependent variable, namely, the current density, can be controlled, whereas in PED, three parameters are independently variable, pulse peak current, pulse on time and pulse off time, which determine the physical characteristics of the deposits obtained from a given electrolyte. Pulse electrodeposition improves the current distribution altering the prevailing mass transport conditions 15,16 controlling the microstructure, increasing brightness and decreasing porosity, internal stress and impurities. 17–19 Experimental Surface preparation of substrates An electrolyte consisting of 325 g L 21 ZnSO 4 , 200 g L 21 NiCl 2 , 40 mL L 21 glacial acetic acid and 0?1gL 21 sodium lauryl sulphate were prepared using doubly distilled water and AnalaR grade chemicals. Electrodeposition of zinc–nickel alloy on stainless steel cathodes was carried out after a nickel strike in a solution consisting of nickel chloride (1?009M) and concentrated hydrochloric acid (20%). Graphite was employed as the anode for zinc–nickel alloy deposition. The procedure for alloy plating consisted of the following sequence of operations. The polished substrates were degreased with acetone and then cathodically cleaned in alkaline solution containing sodium hydroxide (35 g L 21 ) and sodium carbonate (25 g L 21 ) for 2 min at 70uC, and rinsed with distilled water. Acid etching was carried out with 20%HCl. Pulse plating of Zn–Ni layers was then carried out on the smooth and clean stainless steel panels. Measurements of pH were made using Testronics 511 digital pH meter. The pH of the bath was maintained at 3?5–4, using 5%H 2 SO 4 . The pulse plating parameters used in the PED technique are given in Table 1. 20 Pulse plating of Zn–Ni Pulse plating was carried out using a Dynatronix model DPR 20-5-10 supply unit. The formulae used for determining various para- meters are given below Duty cycle~ on time on timezoff time |100% Central Electrochemical Research Institute, Karaikudi 630 006, India *Corresponding author, email sanjnamohan@yahoo.com ß 2009 Institute of Metal Finishing Published by Maney on behalf of the Institute Received 3 September 2007; accepted 16 January 2009 DOI 10.1179/174591909X438947 Transactions of the Institute of Metal Finishing 2009 VOL 87 NO 3 145