Corrosion resistance of compositionally modulated multilayered Zn-Ni alloys deposited from a single bath

By consecutive deposition at two different current densities from a single sulfate - chloride bath, compositionally modulated multilayered (CMM) coatings of Zn - Ni alloys, with different number, thickness and sequence of the sublayers were obtained. The corrosion resistance of the coatings was studied by potentiodynamic dissolution and by corrosion potential measurement. In the current - potential ( stripping) curves two well-defined peaks were observed. With increase in the number of sublayers, regardless of their individual thickness, the correlation between the amount of Zn, dissolved at more negative potentials and the whole amount of the metal in the CMM coatings, decreases. The corrosion potentials of CMM coatings are most positive (-0.940 V vs SSE) when they end with a Zn-Ni18% oversublayer. As a result of the alternation of Zn - Ni alloy sublayers with different Ni content the obtained CMM coatings have increased corrosion resistance in comparison with the monolayer coatings of the composing alloys.