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Abstract
Electrodeposition of Zn-Ni alloy has been found to be useful in a variety of applications. Such applications generally require a specific coating p’roperties such as corrosion resistance, weldability, formability and paint adhesion. The best corrosion resistance of Zn-Ni deposits were limited in the alloys containing 10 to 18 wt% nickel. However, Zn-Ni alloy coatings in this range of nickel content h-as originally dull, gray and unattractive finish, and then a brightener system has to be developed in order to obtai n bright surface.
This thesis is devoted to study the electrodeposition of the new ternary ZnnNi-Fe alloy deposits from acidic chloride electrolyte. This ternary alloy coatings have better brightness and uniform surface appearance in comparison with Zn-Ni deposits, even without adding any organic brighteners to the plating bath.
A bath developed in this work has been found to be suitable for forming ZnnNi-Fe alloy electrodeposits. It operates at a wide range of pH I to 3, temperature 28
to 52°e and current density from 5 to SO mAcm2-. The ternary Zn-Ni-Fe alloy coatings were electroplated using two operating systems. One is the zinc-nickel electrolyte with pure iron anodes in order to give iron ion into the electrolyte, and the other is Zn-Ni-Fe electrolyte with separated pure anodes of zinc, nickel and iron. Zinc chloride and nickel chloride were used as a source of zinc and nickel ions. The source of ferrous ions is ferrous chloride in the electrolyte. However, the decrease in zinc, nickel and Ferrous ions in the bath, due to the deposition of ternary alloy at the cathode, were supplied by separate dissolution of the three anodes in the second system. The electrolyte contains sodium chloride as a condu”ctive medium, sodium acetate with boric acid as buffering agents and dodecyl sodium sulfate as wetting
agent. The substrate material is a Inild steel plate (low carbun steel). otherwise. III special case. the material is slated.
The electroplated ternary Zn-Ni-Fe alloy exhibited mainly an anomalous type codeposition. The transition from normal to anomalous codeposit ion was observed ;\( low critical current densities. The transition critical current density was not affected by addition of a small amount of FeCl2 (0.06 M) to the Zn-Ni bath. While. the
deposition potential of the alloy was shifted to more negative values with iron codeposition. which acts as an inhibitor to elevate the polarization.
The ternary Zn-Ni-Fe deposits had brighter surface appearance in comparison with binary Zn-Ni deposits. The Hull cell test was used to judge the dependence of Zn-Ni and Zn-Ni-Fe electrodeposits surface brightness on the current density al constant operating conditions. The Zn-Ni deposits were dull or gray on all the plated area, except only at highest current density point. However, the surface brightness area of Zn-Ni-Fe alloy deposits increased with increasing of ferrous chloride concentration in the bath. Approximately all the surface area of Zn-Ni-Fe deposit was bright with addition of 0.1 M ferrous chloride at 45°C and 0.06 to 0.08 M ferrous chloride at 54°C.
The nickel content of deposited alloy is not affected by absence and presence of ferrous chloride (from 0.02 to 0.08 M) at a constant Ni2+/Zn2+ molar ratio if the plating bath. While. the iron content in the deposils was inversely proportional to zinc content, that is. it increases with increasing not only ferrous chloride hut ;Ii”o Ni2+/Zn2+ molar ratio in the bath.
The electrodeposits of Zn-Ni-Fe and Zn-Ni thin films were also plated under the same electrolysis conditions to investigate the following properties: surface
microstructure using X-ray diffraction analysis and scanning electron microscopy (SEM), microhardness, and corrosioh test in 3 wt% neutral sodium chloride media.
The X-ray diffraction analysis shows that the Zn-Ni deposits (containing 14.17 to 19.25 wt% Ni) haveyphase, which has a preferential (41 q and (330) crystal orientation. However, theyphase in Zn-Ni-Fe deposits showed a clear change to prefer (442), (600) and/or (444) crystal.orientation by the codeposition of iron. Also the interplaner distance (d) are increased by codeposition of iron into the lattice of Zn-Ni alloy due to its substitutional codeposition action like an inhibitor.
The deposited Zn-Ni-Fe alloys has a finer grain size in comparison with ZnnNi alloys, as after SEM analysis for the deposits containing approximately the same nickel content, under the same other electrolysis conditions.
It is known that the microhardness of Zn-Ni deposits increased with increasing the nickel content in it. i\. more increase in micro-hardness was observed, when iron codcposited to form the new ternary Zl1-Ni-Fe alloy. The hardest deposit (VHN = 535) was obtained from the bath containing 0.5/0.4 molar ratio of Ni+2/Zn+2and 0.04 M ferrous chloride. The increases in hardness of Zn-Ni-Fe deposits are attributed not only to the increase of nickel content in the alloy as found for Zn-Ni deposits, but also to fines grain size and a widening distortion in the lattice of the deposited alloy due to iron codeposition as observed from SEM and X-ray
The corrosion resistance of electroplating Zn-Ni-Fe and Zn-Ni deposits in 3 wt% neutral sodium chloride medium using potentiostatic and potentiodynamic techniques was investigated. The corrosion resistance of these deposits was evaluated by measuring the corrosion potential after four minutes dipping, and also determination the anodic polarization resistance in the test solution.
In general the ternary Zn-Ni-Fe alloy deposit exhibited better COITO\!OI1 resistance in comparison with Zn-Ni deposit, even when the deposits contained approximately the same nickel content. The best corrosion resistance was observed in the deposit containing 16.74 wt% Ni and 3.95 wt% Fe. The increase in corrosion resistance of ternary deposits is not only attributed to formation of single high nickel y phase, but also to iron codeposition, which causes clearly change of crystal orientation with increase in the crystal lattice spaces and obtains finer grain size.