Texture and Surface Morphology Development in Zinc and Zinc-Cobalt Electrodeposits K. Raeissi, a,z A. Tufani, a A. Saatchi, a M. A. Golozar, a and J. A. Szpunar b a Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b Department of Mining, Metals and Materials Engineering, McGill University, Montreal, QC H3A 2B2, Canada The morphology and texture formation in zinc and zinc-cobalt coatings, electrodeposited onto low carbon steel substrate in an acidic sulfate bath, was studied. The predominant texture component of zinc coating at low current density was pyramidal 11.5 and 11.6 nonfiber, while, for zinc-cobalt deposition, a nonfiber 11.0 prism was found as the predominant texture component. Hydrogen adsorption, during the zinc electrodeposition process, inhibited lateral bunching growth and produced a low-angle pyramidal texture component, which developed ridge morphology. Adsorption of cobalt or cobalt-containing species was the reason for promoting a “field-oriented texture”–type growth in zinc-cobalt deposition, which resulted in a coating morphology, consisting of numerous fibers grown almost normal to the substrate surface. At higher overpotentials, the adsorption was hindered. This led to the progression of lateral growth and the development of a sharp 00.2 fiber texture component in zinc and zinc-cobalt electrodeposits. © 2008 The Electrochemical Society. DOI: 10.1149/1.2999353 All rights reserved. Manuscript submitted August 5, 2008; revised manuscript received September 18, 2008. Published October 20, 2008. Electrodeposited zinc coatings have attracted increased attention because of the flexibility in their properties. By sacrificing itself, zinc offers protection to steel. Studies to improve the corrosion re- sistance of the zinc coatings are desirable because they are not so resistant to corrosion in marine atmospheres. 1 Under some circum- stances, zinc becomes passivated and the protective property is hindered. 2 Hence, the electrodeposition of zinc, alloyed with group eight metals Ni, Co, and Fe, has attracted considerable interest. 1,3,4 This can be attributed, mainly, to excellent corrosion resistance, paintability, and good formability. 5 These properties are controlled by chemical composition, phase composition, and the microstructure of the deposit. According to recent findings, some of the coating properties, in particular, the corrosion resistance and paintability, are influenced by the morphology and crystallographic texture of the deposit. 6,7 Various morphologies and textures can be obtained simply by applying different electrochemical conditions to the electrodeposi- tion bath and preparing the steel substrates differently. The proper- ties of the coating, such as corrosion resistance, paintability, and formability, are closely related to the morphology and texture of the coating. 8-14 Although numerous research works have been under- taken to study the electrochemical dependence of the texture and morphology of zinc and zinc alloy electrodeposited coatings, the development of the texture and the morphology and the reasons for their variations, using electrochemical parameters and surface prepa- rations, are still not completely understood. In zinc electrodeposi- tion, Park and Szpunar 13,15 found that the major texture components, developed onto the electropolished steel surface at low current den- sities, were high-intensity basal 00.2 and low-intensity pyramidal 10.X planes. Similar results were reported by Vasilakopoulos et al., 16 using chemically polished steel substrate. A previous study on the zinc electrodeposition by Vasilakopoulos et al. showed that the predominant texture of a nonfiber pyramidal 11.5 and 11.6 type was developed onto an electropolished steel surface at low overpo- tentials, while at higher overpotentials, a 00.2 fiber texture com- ponent dominated. 16 The former texture component produced ridge morphology in the deposit, but platelet morphology was a result of the latter. Although there have been numerous studies on the electrochemi- cal and morphological aspects of zinc-cobalt electrodeposits, there has not been any significant work on the effect of electrochemical variables on the texture. Carpenter and Farr 17 studied morphology development in a zinc-cobalt alloy deposition from alkaline and acid solutions, systematically. They reported the morphological variation of the resulted coating with increasing cobalt content. 17 The aim of this work is the comparison of texture development and morphology in zinc and zinc-cobalt electrodeposits onto elec- tropolished low carbon steel from an acidic sulfate bath. For this purpose, an electrochemical impedance spectroscopy was used to study the electrochemical phenomenon occurring at different over- potentials. Orientation distribution function ODF and scanning electron microscope SEM methods were also applied to evaluate the texture and morphology of the coatings. Experimental The substrate was prepared from a commercial cold-rolled low carbon steel sheet, with a thickness of 1 mm. The specimens were disk shaped, with a surface area of 0.85 cm 2 . They were mechani- cally ground down to 800 grit abrasive SiC paper and subsequently electropolished in a solution of 95% acetic acid and 5% percholoric acid for about 2.5–3 min. After electropolishing, the specimens were washed with distilled water and soaked in 10% sulfuric acid for 20 s. Then, the specimens were washed again with distilled water and immediately placed in the electroplating bath. The bath composition was ZnSO 4 ·7H 2 O  620 g L -1  plus Na 2 SO 4  75 g L -1  for zinc deposition and ZnSO 4 ·7H 2 O  620 g L -1 , CoSO 4 ·7H 2 O  125 g L -1 , and Na 2 SO 4  75 g L -1  for zinc-cobalt deposition. The pH of the bath was adjusted to 2 in both baths with dilute sulfuric acid. Deposition was conducted in a stan- dard corrosion cell with two graphite counter electrodes and a satu- rated calomel electrode SCE used as the reference electrode. This electrode was placed close to the cathode surface via a luggin cap- illary filled with bath solution. The temperature of the cell was maintained at 25°C. An EG&G model no. 263A computer- controlled potentiostat/galvanostat was used to maintain the current density at 10, 100, and 200 mA cm -2 . The plating time was set to 1100, 110, and 55 s, in order for the above current densities to produce a constant coating thickness of 5 m, according to Fara- day’s law. Potential vs time curves were plotted during the deposition pro- cess. Cathodic polarization tests were run with a scan rate of 40 mV s -1 . An EG&G AC responser model no. 1025 was coupled with the potentiostat/galvanostat to measure impedances. AC imped- ance tests were undertaken in a conventional cell, with platinum counter electrodes and SCE as the reference electrode, with an ar- rangement similar to the standard corrosion cell. The ac measure- ments were carried out at 1.05, -1.3, and -1.45 V for zinc and -1.08, -1.57, and -2.02 V for zinc-cobalt deposition. A Philips XL30 SEM was used to observe the morphology of the z E-mail: kraeissi@cc.iut.ac.ir Journal of The Electrochemical Society, 155 12 D783-D790 2008 0013-4651/2008/15512/D783/8/$23.00 © The Electrochemical Society D783 Downloaded 30 Nov 2008 to 155.69.4.4. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp