Alloys in the Presence of Organic Additives

Linear sweep voltammetry has been used as an “in situ” technique to characterize thin layers of ZnNi alloys. The zinc dissolution peak potentials from ZnNi deposits were found to be more noble [El,= -0.7V (SCE) and Ez,= -0.575 V (SCE)] than those of the pure metal ions [-OX3 V (SCE)] due to a formation of two ZnNi intermediate phases 7 and 6 respectively. The effect of d c and pulse parameters on deposition of ZnNi alloys from sulfate electrolytes in the presence and absence of organic compounds have been studied in unagitated solutions and with a rotating disc electrode The additive had a major effect on crystal size and the brightness of the deposits. At overpotentials of -0.75 V ( S C E ) and current density of 2 A/dm2 or less, Ni deposition and hydrogen evolution occurs and on the stripping voltammograms only the anodic nickel peak is observed. At sufficient high cathodic overpotentials Zn deposition reaction is mass-transpon control. Ni deposition reaction is inhibited when Zn is codeposited. The mechanism of ZnNi codeposition is discussed. INTRODUCTION Anomalous codeposition has been recognized in connection with several alloy deposition systems such as N f e , NiCo and NiZn (1-11). In the case of ZnNi. zinc deposits at a faster rate which leads to a higher concentration of the alloy composition in Zn, different than one wouId expect based on the composition of the plating solution, where the concentration of Ni(I1) ions are higher than those of Zn(1I). Kurachi et al (12-18) have deposited successfully NiZn layers using only low current densities up to 5 A/dm2. They found that the cathode deposits have a complex structure corresponding to y phase which behaves like a monometallic electrode and has a higher overpotential than pure zinc. Several authors (19-22) considered this alloy to be a possible substitute to cadmium. According to Noumi et al (23), ZnNi alloys may be prepared at high current densities from sulphate solutions and pH in range of 1.5 to 2.5. More recently, the same alloy was prepared using chloride electrolytes (24-25). According to these authors NiZn alloy with y phase is obtained which shows the best corrosion resistance at 12-13 7% nickel content. The alloy was found to have excellent coating adhesion and weldability but poor phosphatability and paintability (26). The effects of pulse plating on Ni-Zn alloy StNCNre in unagitated solutions were studied by Nishimura et al. (27). They found that the pulse period has a major effect on crystal size. Off time also affected the Ni content and the structure of the ZnNi electrodeposits. Existing limitations to deposit ZnNi layers on the pH and the concentration of Zn(1I) ions in the electrolyte indicate that both hydrogen evolution and Zn deposition are mass-transport controlled. Accordingly, the agitation and the presence of organic additives which may suppress the hydrogen evolution (28-33) should play a significant role in the Ni-Zn deposition system. Systematic studies of these effects have not been reported in the literature. This paper examines the d c plating and pulse plating (PP) of Ni-Zn alloys on stationary and rotating disc electrode from different electrolytes and in