Influence of parameters of electrodeposition Zn - Ni coatings on their structure and composition

Coatings which are anodic covers have had great significance in anticorrosion protection of steel and they are commonly used for a long time. Because of relatively low price of zinc, it's the most widespread metal protecting steel elements from corrosion. However, in some cases, for the sake of rugged conditions of exploitation, application of zinc coatings is limited, e.g. in automotive and aircraft industry. So far in that instances, they are substituted with success by the cadmium layers, which are characterized by good anticorrosive properties. But due to their toxic peculiarities there exists the tendency to replace them by alloy coatings, e.g. Ni - Zn [1]. Electrodepositing of zinc- nickel alloy coatings can be realized both by using alkaline or acid baths. As soon the largest significance found slightly acid chloride baths and minor sulfuric baths. The coatings deposited from acid baths exhibit the greatest anticorrosion resistance in the range of 10 - 15% of nickel in the alloy. Application of alkaline baths permit to receive layer with similar corrosion resistance, but with lower content of Ni (6 - 10%) [2, 3]. It was also found that the coatings exhibit several times better anticorrosive properties than pure zinc layers. [4]. However, during corrosion process zinc undergo preferential digestion, what causing enrichment the layer with nickel, what in consequence lead to increasing corrosion potential in the direction more positive values [2]. In this paper the investigations of electrodeposition of Zn - Ni alloy layers from chloride baths on common steel St3S substrate were presented. The influence of bath composition and cathodic current density on the chemical composition and structure obtained coatings was determinated. The Zn - Ni layers were put to the microhardness tests and corrosion resistance examinations in 5% NaCl solution. There was found that content of alloy-creating metal ions in bath and their mutual molar ratio considerably influence on surface morphology of obtained coatings, effect of used current density is smaller. The content of Ni in layer increases with decreasing of [Zn ²⁺]/[Ni²⁺] ratio in the bath. The hardness of obtained alloy layers is considerably higher that hardness of the substrate (St3S steel) and distinctly depends on amount of nickel in the layer. It was also found that in the range of 13 - 16% at. of nickel content in alloy the best corrosion resistance of layer was obtained.