Corrosion mechanism of nanocrystalline Zn–Ni alloys obtained from a new DMH-based bath as a replacement for Zn and Cd coatings

Abstract

Nanocrystalline Zn–Ni alloys obtained from a newly developed 5,5′-dimethylhydantoin (DMH)-based bath are proposed as a replacement for Zn and Cd coatings due to their excellent corrosion resistance and environmentally friendly properties. However, the mechanism of the superior corrosion performance of the Zn–Ni samples compared with the Zn and Cd coatings has not been greatly investigated. In the present study, the corrosion mechanisms of Zn–Ni alloys and Zn and Cd coatings have been studied. The results show that the corrosion resistance of the deposits is directly dependent on the composition of the products formed during corrosion. The excellent corrosion resistance of the Zn–Ni samples is due to the appearance of simonkolleite in the corrosion products. With increasing current density, the amount of simonkolleite decreases; thus, the corrosion resistance of the Zn–Ni alloys decreases with increasing current density. The XPS studies indicate a higher amount of simonkolleite and an additional protective Ni-rich layer on the Zn–Ni alloys compared to the Zn coating. Moreover, the superior corrosion resistance of the Zn–Ni alloys compared to the Cd coating is associated with the loose structure of CdCl₂·H₂O formed on the Cd coating. Compared with the Zn and Cd coatings, nanocrystalline Zn–Ni alloys display excellent corrosion resistance due to the rapid rate at which zinc is lost from their hydrophilic surfaces at the initial immersion. After a long immersion time, the higher amount of simonkolleite in the corrosion products with hydrophobic surfaces further increases their polarization resistance. The results of Tafel curves and electrochemical impedance spectra (EIS) confirm the above facts that Zn–Ni alloys have better corrosion resistance than Zn and Cd coatings and represent the best alternative to Zn and Cd coatings.