Abstract
This research work aims to develop electrodeposited Zn-Ni alloy coatings with controlled dissolution tendencies on a mild steel substrate. The varying Ni concentration in the electroplating bath, i.e., 10, 15, 20 and 25 g·L−1, affected the surface morphology and electrochemical properties of the deposited Zn-Ni alloy coatings. SEM and EDS analysis revealed the resulting variation in surface morphology and composition. The electrochemical behavior of different coatings was evaluated by measuring the open circuit potential and cyclic polarization trends in 3.5 wt.% NaCl solution. The degradation behavior of the electrodeposited Zn-Ni coatings was estimated by conducting a salt spray test for 96 h. The addition of Ni in the coating influenced the coating thickness and surface morphology of the coatings. The coating thickness decreased from 38.2 ± 0.5 μm to 20.7 ± 0.5 μm with the increase in Ni concentration. Relatively negative corrosion potential (<−1074 ± 10 mV) of the Zn-Ni alloy coatings compared to the steel substrate (−969 mV) indicated the sacrificial dissolution behavior of the Zn-rich coatings. On the other hand, compared to the pure Zn (26.12 mpy), ~4 times lower corrosion rate of the Zn-Ni coating (7.85 mpy) was observed by the addition of 25 g·L−1 Ni+2 in the bath solution. These results highlighted that the dissolution rate of the sacrificial Zn-Ni alloy coatings can effectively be tuned by the addition of Ni in the alloy coating during the electrodeposition process.
Highlights
Cadmium electroplating has been applied to components to enhance the corrosion resistance of the steel substrate
Zn coating on steel surface dissolves sacrificially and its dissolution rate could be controlled by the addition of different alloying elements in the zinc coating
Zn-Ni alloy coatings present better corrosion resistance compared to pure zinc coatings due to the addition of Ni that makes the alloy coatings relatively less active compared to the pure zinc coatings [9,10]
Summary
Cadmium electroplating has been applied to components to enhance the corrosion resistance of the steel substrate. The dissolution rate of zinc coatings is high due to the large potential difference between zinc and steel. Zn coating on steel surface dissolves sacrificially and its dissolution rate could be controlled by the addition of different alloying elements in the zinc coating. Acid and alkali-based bath compositions could produce alloy coatings containing 8–15 wt.% Ni. Different phases are formed during Zn-Ni co-deposition, which highly depend on the nickel contents in the coating. It is reported that the uniformity and corrosion resistance of the Zn-Ni coated steel can be tuned by controlling the Ni contents in the electroplating bath and in the coatings, which differentiates this work from existing studies. With the increase in Ni concentration in the Zn-Ni coatings, the simultaneous decrease in coating thickness and improvement in the corrosion resistance are the unique features of this research work
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