Electrodeposition of zinc–nickel alloy by pulse plating using non-cyanide bath S. Mohan*, V. Ravindran, B. Subramanian and G. Saravanan In this study, zinc–nickel electrodeposition was carried out in a sulphamate bath at pH 3–4 by pulse plating and the deposits obtained were characterised by measuring microhardness, surface roughness and by employing SEM, XRD, AFM techniques. The corrosion behaviour of the deposits was evaluated by potentiodynamic polarisation. The deposits obtained by pulse plating have an increased Ni content, thought to be responsible for an improved corrosion resistance. Keywords: Zn–Ni, Pulse plating, XRD, AFM, Potentiodynamic polarisation Introduction In recent times, the most widely used corrosion pro- tective coating for steel has been zinc which is coated on steel either by hot dipping or electroplating. For good corrosion protection, the coating thickness should be 25 mm. Owing to certain disadvantages of thicker coat- ings, e.g. weldability and formability are poor when thick coatings are used, the current interest is in the development of thin coatings. Alloys of zinc with nickel have five to six times better corrosion resistance than zinc. 1–4 It has been further shown that the formability 5 and weldability 6 of zinc–nickel alloy coated steel are good. Although zinc–nickel alloy electrodeposits are mainly used as coatings to improve the corrosion resistance of automobile steel bodies, these coatings have been considered for several other applications such as for electrocatalytic water electrolysis, 6,7 coating for steel cord reinforcement of tyres 8 and in the electronics industry. 9 Zinc–nickel alloys have also been considered as alternative coatings to cadmium. 10,11 Since cadmium is a toxic metal leading to health hazards and risk of pollution, environmental regulations are encouraging the use of alternative systems. 12 Pulse plating has several advantages over conven- tional plating. Current density, on time, off time and frequency can all be varied. Because of this, pulse plating has received considerable interest over the past 20 years and has proved to be a successful route for materials processing. 13–18 Pulse plating improves deposit properties such as porosity, ductility, hardness and surface roughness. Pulse electrodeposition can be used as a means to produce a unique structure, i.e. coatings with properties not obtained by direct current plating. Pulse electrodeposition also yields a finer grained homogeneous surface appearance of the deposit, because a higher instantaneous current density is possible during deposition. Pulse plating can also yield uniformity in alloy composition and grain structure, smoother and denser deposits with negligible porosity. For alloy codeposition, pulse plating can produce com- positions and structures that are not obtainable in dc plating. 19 With pulse current, the average current density can be increased to 8 A dm 22 , whereas in direct current, only up to 4 A dm 22 can be achieved. 20 Because of this, the percentage of nickel can be increased to 20% in zinc–nickel alloy. Even though a large volume of literature is available on the electrodeposition of zinc– nickel alloys, no systematic work is believed to have been carried out for this using pulse plating. It is generally accepted that the highest corrosion resistance of zinc–nickel alloys can be obtained with nickel content in the 10–15% range. A search for a non- cyanide zinc plating bath resulted in the development of a zinc–nickel sulphamate bath, 21 yielding grey, uniform and semibright deposits in the presence of boric acid, sodium lauryl sulphate and b-napthol. Experimental Surface preparation of substrates Mild steel having the following composition in wt-%, was used: Fe–0?003C–0?23Mn–0?03S–0?011P. The Zn–Ni coatings were deposited on mild steel substrates using pulse plating. Coupons of the substrate were cut to an approximate size of 75625 mm and polished well mechanically using 1/0, 2/0, 3/0 and 4/0 emery papers successively. The polished substrates were degreased with acetone and then cathodically cleaned in alkali solution containing sodium hydroxide and sodium carbonate for 2 min at 70uC, and rinsed with distilled water. Pulse plating of Zn–Ni layers was then carried out on the smooth and clean mild steel panels as described below. pH measurements were made using a TESTRONICS 511 Digital pH meter. The pH of the bath was maintained at 3?5–4, using 5%H 2 SO 4 . Pulse plating of Zn–Ni Pulse plating was carried out using a DYNATRONIX, model DPR 20-5-10 current generator. The formulae used for determining various para- meters are given below 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 8 July 2008; accepted 20 August 2008 DOI 10.1179/174591909X423600 Transactions of the Institute of Metal Finishing 2009 VOL 87 NO 2 85