Electroplating zinc-nickel alloys on mild steel

Electroplated zinc coatings are considered to be one of the basic methods for the protection of a steel against corrosion.

Recently the interest has increased to zinc-nickel coatings thanks to their good corrosion and mechanical properties, in comparison with pure zinc coatings. Besides, coatings from zinc-nickel alloy are the most promising substituents for cadmium coatings.

The purpose of the given work is the deposition of coatings which possess greater hardness and corrosion resistance.

For electrodeposition sulphate, chloride, cyncate, oxalate, pyrophosphate and sulphamate electrolytes were used.

The protective ability against corrosion is increased for the alloys containing 12-14 % of nickel. In addition, this composition of an alloy provides anode character of steel products protection against corrosion. Therefore Zn-Ni alloys can be used instead of cadmium coatings. By the way, those cadmium coatings are toxic and expensive.

The percentage of zinc and nickel in an alloy has been studied by atomic absorbtion spectrometry method. Electrodeposition with cathodic current density equal 1 a/dm2 allows to obtain light, compact, homogeneous, pores free deposits of a zinc-nickel alloy. Nickel content in these alloys from pyrophosphate electrolytes is 14-16 % , from chloride electrolytes it is 7-9 %, from oxalate it is 17-19 % and from sulphamate electrolytes it is 14-16 %. In the interval of current density from 1 to 3 А/dm2 high-quality platings are obtained.

To characterize polarization the polarizing curves from each electrolyte have been determined. Polarizable curves were taken with the help of PI-50-1 potentiometer. It appeared, that sulphamate and oxalate electrolytes possess the highest polarization. Therefore, the smoothest and very small grain deposits are obtained from these electrolytes. Ions of zinc in these electrolytes can form oxalate and ammonia or sulphamate complexes. Therefore, the introduction of oxalate ammonia or sulfamate nickel into electrolytes increased polarization. The throwing power is also increased with the introduction of these additives into electrolytes for zinc and zinc-nickel coatings. The current efficiency was defined with the help of coulometric analysis. The current efficiency is high enough for all electrolytes and is approximately 100 %.

Microhardness of coatings has been estimated by means of PMT-3 device. The hardness of alloys is greater as compared with pure zinc coatings.

Corrosion resistance has been estimated in two ways.

The first method is based on the investigation of corrosion potential and the definition of a corrosion current.

The second method is in keeping samples in a desiccator with a salt fog.

Both methods have shown the greatest corrosion resistance for Zn-Ni alloy coatings.

Dogadkina E.