Synergistic Effect of Gelatin and Glycerol on Electrodeposition of Zn-Ni Alloy
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THE field of electroplating is experiencing rapid change due to its applications in new areas of research where deposition is required to be made in micron/ submicron patterns. The electrocrystallization process was a very phenomenological field of electrochemistry for a long time, and now it has reached a molecular level. With the new available investigation methods, detailed information about the surface structure of the electroplated coatings can be gained. A variety of additives can be used as leveling/brightening agents for imparting the better appearance to the coatings. The success of electroplating depends to a large extent on the type of addition agents introduced in plating electrolytes.[1] Many scientists have tried a variety of additives, mainly organic compounds that impart good leveling and brightening effect on deposits by hindering or even inhibiting the electrodeposition process. However, a detailed study of their actions is still lacking. The use of Zn-Ni, Zn-Co, and Zn-Fe alloy coatings to improve the corrosion resistance of mild steel (MS) sheets offers an alternative to pure Zn coating because of VAISHAKA R. RAO, Research Scholar, and AMPAR CHITHARANJAN HEGDE, Professor, are with the Department of Chemistry Electrochemistry Research Laboratory, National Institute of Technology Karnataka, Surathkal, Srinivasnagar 575025, India. Contact e-mail: hegdeac@rediffmail.com Manuscript submitted October 4, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
improved mechanical properties and corrosion resistance.[2–4]. Generally Zn-Ni deposit is more corrosion resistant and significantly harder. The presence of a high percentage of nickel provides higher barrier resistance to the deposit. Although the standard electrode potential of zinc, E0 = –0.76 V, is less than nickel E0 = –0.25 V, the concentration of the nickel in the deposit appeared to be less than in the bath due to characteristic ‘‘anomalous codeposition’’ of Zn-Fe group metals.[5] The deposition of more noble metal (Ni) has been inhibited by a less noble (Zn) metal.[6,7] Attempts were made to decrease the anomalous deposition of Zn-Ni alloy either by introducing some inert species in the deposits or by building ternary alloy deposits.[8–10] The usage of certain additives is another approach to alter the mechanism of deposition by changing the grain size and composition of the alloy components.[11] Soares et al. studied Zn-Ni electrodeposition using gelatin as an additive and observed that the efficiency of galvanostatic deposition increased with gelatin.[12] Cyclic voltammetry (CV) study demonstrated that a combination of gelatin and sulfanilic acid improved the homogeneity and brightness of the Zn-Ni coating. It was reported that preferential adsorption of these compounds resulted in a change in phase structure of the deposit.[13] The effect of addition of glycerol on cathode current efficiency (CCE) of Zn-Ni alloy was studied by Oliveira et al.[14] It was reported that CCE increased even up to 90 pct with glycerol addition, in the potential range of 1.3 V
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