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Abstract
In this study, introduction of additives to the acidic and the alkaline Zn-Ni electroplating baths changed strongly and slightly the coatings performance, respectively. The nickel content of zinc-nickel coatings plated from acidic baths was more sensitive to the presence of additives while, that of the coatings obtained from alkaline baths, was not.
Mononphasic deposits were obtained in alkaline medium and no significant changes of the peak intensity with different additives. Mixed phases;y and n phases appeared in presence of the additives in the acidic bath. Aniline produced the same influence; high intensity of the y-phase in both alkaline and acidic baths. The alloys deposited in presence of saccharin or sodium hypophosphite in the acidic baths and aniline or sodium tungstate in the alkaline baths exhibited the highest corrosion resistance.
The effect of the additives on the alloys deposited from acidic medium depended mainly on the formation of nanostructured coatings with uniform crystal size of the two phases while in alkaline medium it depended mainly on the intensity of y-phase. The acidic medium showed an inverse relationship between the crystal size of n-phase and the corrosion resistance of the deposits. On the other hand, the alkaline baths presented a proportional relationship between the intensity of y-phase and the corrosion resistance of the deposits.
The Zn-Ni-Al2O3 nanocomposite coatings have been successfully deposited in both acidic and alkline bath . The high corrosion resistance of the nanocomposite coatings deposited from acidic medium was related to uniform distribution of the nano- AL2O3 in the coatings. The nanocomposite coatings deposited from alkaline medium with homogeneous structure (circular morphology ) exhibited excellent corrosion resistance, while the spongy structure (tubes morphology ) impaired the corrosion resistance.