Effect of additives on the corrosion mechanism of nanocrystalline zinc-nickel alloys in an alkaline bath

The surface properties and wear resistance of dull Zn-Ni coatings are significantly improved by adding additives in the studied DMH-based bath. However, as the anti-corrosion coatings, the influence of the studied additives on corrosion resistance of Zn-Ni alloys has not been well investigated. In this study, the effect of additives on the corrosion mechanism of Zn-Ni alloys is investigated by the XPS technique and electrochemical measurements. The formation mechanism of the corrosion product film is also studied. It is found that simonkolleite, hydrozincite and zinc oxide are the main corrosion products for both dull and bright Zn-Ni samples. The XPS analysis reveals that the amount of simonkolleite is over 64.15% after 3 h of immersion, whereas the share of zinc oxide as a more stable corrosion product increases to above 35.89% after 24 h of exposure. Better corrosion resistance is observed for bright Zn-Ni alloys compared to that of dull Zn-Ni alloys. This behavior could be due to the smaller grain size and hydrophilic surface of bright Zn-Ni alloys during the initial stage of immersion. After the long immersion time, the better corrosion resistance of bright Zn-Ni samples is related to their increased amount of simonkolleite in the corrosion products. The corrosion resistance of the coatings decreases with increasing of the immersion time, which is associated with the increased share of zinc oxide and the decreased contact angle (CA). This could also be due to the increase of the porosity of corrosion products with the increase of immersion time. According to the above analysis, it is concluded that the corrosion products of bright Zn-Ni alloys have an increased amount of simonkolleite, which is formed at a quicker rate, resulting in the better corrosion resistance of bright Zn-Ni alloys compared to dull Zn-Ni alloys.