Synergistic effect of ninhydrin and iodide ions during electrodeposition of zinc at steel electrodes Magdy A.M. Ibrahim a, b, ⁎, Enam M.A. Omar c a Chemistry Department, Faculty of Science, Taibah University, Al Maddinah Al Mounwara, Saudi Arabia b Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt c Applied Chemistry Department, Faculty of Applied Science, Taibah University, Al Maddinah Al Mounwara, Saudi Arabia abstract article info Article history: Received 8 November 2012 Accepted in revised form 20 March 2013 Available online 29 March 2013 Keywords: Ninhydrin Cyclic voltammetry Potentiodynamic polarization Electrodeposition Iodine ions Wagner number The addition of Nin and /or iodide ions to the sulfate bath for zinc electrodeposition onto steel substrates has been examined as a possible means of improving the quality and uniformity of deposit distribution. The investigation was carried out using potentiodynamic polarization, cyclic voltammetry and anodic linear stripping voltammetry tech- niques, complemented with XRD analysis and SEM measurements. The synergistic effect between Nin and iodide ions is proved using the data obtained from the polarization curves, throwing power, throwing index, Wagner number, cyclic voltammetry and anodic stripping voltammetry. On the other hand, addition of Nin or iodide ions increases the TP% by about five times, however, addition of the combination of both of them increases TP% by more than seven times. The inhibition of zinc reduction in the presence of Nin and/or iodide ions was assumed to occur via adsorption which followed the Langmuir adsorption isotherm. The initial nucleation and growth of zinc on steel followed the model of 3D instantaneous nucleation. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Zinc coatings provide the most effective and economical way of protecting steel against corrosion [1]. Moreover, it's excellent corro- sion resistance in most environments accounts for its successful use as a protective coating on a variety of products and in many exposure conditions [2]. Several plating electrolytes for zinc deposition have been used in industry, including cyanide, alkaline non-cyanide, and acid (chloride and sulfate) solution. Cyanide process is being prohibited due to its health and environmental pollution hazards as well as high effluent treatment costs. Alkaline non-cyanide process, in spite of its non-toxicity, has several disadvantages such as low efficiency and strong corrosive attack. Comparatively, sulfate baths increased current efficien- cy, reduced power consumption, and improved the surface morphology for zinc electrodeposition in acidic solution [3]. Adding organic compounds to an electroplating bath is one of the most effective and most frequently used methods to improve the quality of the deposits, controls the surface morphology and improves the throwing power of the baths [4]. The additives usually make the electroplated surface more durable, of uniform grain size, luster, and compact for better corrosion resistance [3,4]. In addition, additives affect the deposition and crystal building processes as adsorbate at the cathode surface [5–8]. Numerous investigations were carried out using different organic additives during zinc electrodeposition by several authors [9–23]. The halide ions are usually added to the plating baths to promote anode dissolution by dissolving the passive film on its sur- face [24]. Therefore, the aim of the present work was to study the effect of Nin as a new organic additive for zinc deposition from a sulfate bath in the presence and absence of iodide ions, and to understand the mechanism of zinc deposition in the presence of these additives. The throwing power and throwing index in the presence and absence of these addi- tives were also investigated. 2. Experimental Electrodeposition of zinc was carried out using an acid sulfate bath containing: 0.15 M ZnSO 4 ⋅ 6H 2 O, 0.15 M Na 2 SO 4 at pH 3.5 and at 25 °C. The pH of the electrolyte was adjusted by addition of 1:1 H 2 SO 4 . The additive tested was ninhydrin (Nin) in the presence and absence of KI. The Nin solution was prepared by dissolving the appropriate amount in 250 ml doubly distilled water, and then added the required amount to the electrolyte. ninhydrin (2,2-Dihydroxyindane-1,3-dione) Surface & Coatings Technology 226 (2013) 7–16 ⁎ Corresponding author. Tel.: +966 501221667; fax: +966 8470235. E-mail address: imagdy1963@hotmail.com (M.A.M. Ibrahim). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.03.026 Contents lists available at SciVerse ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat