ORIGINAL PAPER Corrosion behavior of composition modulated multilayer Zn–Co electrodeposits produced using a single-bath technique V. Thangaraj Æ N. Eliaz Æ A. Chitharanjan Hegde Received: 10 October 2007 / Accepted: 1 October 2008 / Published online: 21 October 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Composition modulated alloy (CMA) electro- deposits of Zn–Co were produced from acid chloride baths by the single-bath technique. Their corrosion behavior was evaluated as a function of the switched cathode current densities and the number of layers. The process was opti- mized with respect to the highest corrosion resistance. Enhanced corrosion resistance was obtained when the outer layer was slightly richer with cobalt. At the optimum switched current densities 40/55 mA cm -2 , a coating with 600 layers showed *6 times higher corrosion resistance than monolithic Zn–Co electrodeposit having the same thickness. The CMA coating exhibited red rust only after 1,130 h in a salt-spray test. The increased corrosion resis- tance of the multilayer alloys was related to their inherent barrier properties, as revealed by Electrochemical Imped- ance Spectroscopy. The corrosion resistance was explained in terms of n-type semiconductor films at the interface as supported by Mott–Schottky plots. Keywords Electrodeposition  Composition modulated alloy (CMA)  Anomalous codeposition  Zn–Co coatings  Corrosion resistance 1 Introduction Codeposition of two metals requires that their individual reversible potentials are reasonably close to each other in the specific bath. This is the case when their standard potentials are close, when the concentration of one of the metals in solution is properly tuned, or when a complexing agent that forms complexes with different stability con- stants is added [1]. Eliaz and Gileadi [1] have recently reviewed the prin- ciples of alloy deposition and the phenomenon of anomalous codeposition (ACD) in the framework of a more comprehensive review of induced codeposition. The term anomalous codeposition was coined by Brenner [2] to describe an electrochemical deposition process in which the less noble metal is deposited preferentially under most plating conditions. This behavior is typically observed in codeposition of iron-group metals (i.e. Fe, Co and Ni), or in codeposition of an iron-group metal with Zn or Cd, with either inhibition or acceleration of the rate of deposition of one of the alloying elements by the other [1, 2]. Even if the concentrations of two metal ions in the plating bath are equal, their concentrations at the surface, which determine the rate of deposition, may be quite different, if their partial current densities are different. During alloy plating, the deposition of the two metals may be under different degrees of mass-transport limitation. Thus, alloy coatings of graded or alternating composition can be produced in the same solution by changing the applied current density [1]. Electrodeposition of composition modulated alloys (CMA’s) has found much interest in recent years because such alloys possess improved corrosion, mechanical, magnetic and electrical properties [2–18]. These properties can be tuned by several factors, including the layer thick- ness, the overall thickness, the chemical composition of V. Thangaraj  A. C. Hegde (&) Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar 575 025, India e-mail: achegde@rediffmail.com N. Eliaz Biomaterials and Corrosion Laboratory, Tel-Aviv University, Ramat-Aviv 69978, Israel 123 J Appl Electrochem (2009) 39:339–345 DOI 10.1007/s10800-008-9677-1