Electrochemical Behaviors and Properties of Zn-Ni Alloys Obtained from Alkaline Non-Cyanide Bath Using 5,5^′-Dimethylhydantoin as Complexing Agent

Abstract

Zn-Ni alloys were prepared in an alkaline bath with 5,5 -dimethylhydantoin (DMH) and Na 4 P 2 O 7 · 10H 2 O as the complexing agents. The electrochemical behaviors of Zn-Ni alloy and Zn only on Pt and glassy carbon (GC) electrodes were investigated by cyclic voltammetry (CV), chromoamperometry (CA) and cathode polarization. Results show that DMH is the main complexing agent in the bath. Different behaviors of CV curves on Pt and GC electrodes were observed. The effects of Ni 2+ /Zn 2+ ratio and bath temperature on CV curves were investigated. The transfer coefficient (α) and diffusion coefficient (D) were also calculated. The nucleation growth processes of Zn-Ni alloy and Zn only show an agreement with three-dimensional progressive growth. Zn-Ni alloy coatings were also characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The morphology of deposits is rectangular pyramid. The grain size decreases with the extension of the electrodepositing time and the decrease in the deposition potential can also lead to the decrease of grain size in deposits. The phase structure changes from the mixture of η and γ phases to single γ phase with (411) Zn-Ni alloy coatings are widely used to provide good protection for steel as replacements for Cd coatings in anticorrosion applications. 1,2They also have attracted great interests in relatively aggressive environment, i.e. high temperatures and severe oxidizing conditions.3 It has been reported that the processes of Zn-Ni alloy deposition can be divided into two types, the acid type and alkaline type. Most of Zn-Ni alloys are electrodeposited from the acid bath, 4,5 which displays a higher current density but poor distribution of the plated metal. However, the commercial alkaline bath used for Zn-Ni alloy deposition contains cyanide, which is toxic and carcinogenic to human health. In recent years, many attempts were done to investigate the alkaline non-cyanide bath and a number of complexing agents, such as tartrate, 6 sodium acetate, 7,8 amine, 9 triethanolamine, 10 glycinate, 11,12 ethylenediamine, 13 citrate and urea have been proposed. Among them, most of the complexing agents are used to complex Ni 2+ in the alkaline zincate bath which has a relatively low current efficiency and few effects have focused on investigating the alkaline non-zincate electrolyte due to its poor stability. Therefore, it is very urgent to find an alkaline non-zincate bath with high current efficiency and good stability to replace the cyanide bath.It is expensive and time-consuming to study a new complexing agent. Our research group has successfully used 5,5 -dimethylhydantoin (DMH) as the main complexing agent to electrodeposit Au, 14 Ag, 15 and Cu 16 in alkaline non-cyanide bath. According to their results, it can be concluded that DMH mainly has two different roles in metal electrodeposition. One is to complex the metal ions and the other is to adsorb on surface of the cathode. It is pos...

Electrochemical behaviour and analysis of Zn and Zn–Ni alloy anti-corrosive coatings deposited from citrate baths

“…Moreover, the peak value of the g-phase with the (330) plane orientation was higher than that of other g-phase orientations such as g-NiZn 3 , demonstrating that the g-phase with the (330) plane orientation plays an assertive role in the Zn-Ni alloy coatings. 28,31,36 The intensity of the g-phase (g-Ni 2 Zn 11 ) of sample (b) was high in comparison to sample (c). The crystal peaks intensities illustrated the information on the number of phases and also the relative amounts of Zn-Ni in the deposits.…”

“…CPE 1 corresponds to the electric double layer capacitance at the interface of the coating/electrolyte solution. 28 The mathematical equation for the impedance of constant phase element (CPE) is given below: 45…”

Role of the newly synthesized brightener in modification of surface properties of Zn-Ni alloy electrodeposited on steel substrate

“…In eqn. (4), N o is the number density of substrate active sites, a is the steady state nucleation rate constant per site, while k' is the numerical constant and is given by: Figure 2 shows the current versus time (I-t) transients, recorded during metal reduction in the potential range from -1.12 to -1.18 V in absence of SC and -1.34 to -1.46 V in presence of SC in the plating bath. The transients exhibit the typical shape for a nucleation process with threedimensional growth of nuclei limited by diffusion of the electroactive species [16].…”

“…Electrodeposition of Zn-Ni alloys is interesting, because these alloys exhibit significantly higher corrosion resistance and better surface morphology than pure zinc [3]. Although nickel is more noble metal than zinc, the co-deposition of zinc and nickel in Zn-Ni alloy is anomalous [4][5] and a higher percentage of zinc is present in the final deposit. According to Lin's conclusion, anomalous electrodeposition of zinc-nickel alloy is caused by the slow kinetics and hydrogen evolution at the nickel deposit.…”

Studies on the enhanced properties of nanocrystalline Zn–Ni coatings from a new alkaline bath due to electrolyte additives

“…49 5,5'-dimethylhydantoin (DMH) has been successfully used in Au, 17 Ag 18 and Cu 19 50 electrodeposition and it is a promising complexing agent in Zn-Ni alloy deposition 51 owing to the large stability constants of DMH with Zn 2+ and Ni 2+ . 20 According to our 52 previous studies, 6,21 the investigated new alkaline Zn-Ni alloy bath has a high 53 stability and the aged baths have little effect on the electrochemical behaviors of the 54 investigated electrolyte. Furthermore, the rate controlling step of the bath is a mixed 55 controlled process and the actual nucleus growth of Zn-Ni alloys corresponds to the 56 progressive nucleation process.…”