Effect of Borax Pentahydrate Addition to Acid Bath on the Microstructure and Corrosion Resistance of Zn-Co Coating

In this work, the Zn-Co coatings were synthesized on AISI 4140 steel and aluminum plates by using potentiostatic electrodeposition technique in sulphate-based acidic baths with 0, 20, 40 and 60 gl¯¹ of borax pentahydrate, as additive. The effect of borax pentahydrate on the microstructure of the samples was observed using scanning electron microscopy and X-ray diffractometer. The deposition process was investigated by cyclic voltammetry. The effect of borax pentahydrate on the corrosion resistance of the samples was studied by potentiodynamic polarization technique. The results have demonstrated, that the addition of borax pentahydrate was in favor of the growth of grains. The morphology of pyramidal islands on the surface was changed to a more flat structure. The results have also demonstrated that the effect of borax pentahydrate was not monotonous. With increasing concentration, the corrosion potential was at minimum and the charge transfer resistance $R_{t}$ was at maximum for the sample obtained from the bath with 60 gl¯¹ of borax pentahydate, indicating that this sample showed the best corrosion resistance. It was found that current density first decreased and than increased, due to adsorption of a complex of borax pentahydrate and/or changes in the morphology, however, the initial deposition potential was not affected. The addition of borax pentahydrate to the bath led to formation of the best Zn-Co deposits, composed of coalesced globular fine grains, smaller than ≈2 μm in diameter. In addition, all of the studied Zn-Co deposits consisted of η phases. It is suggested that Zn-Co deposits produced in the bath containing 60 gl¯¹ of borax pentahydrate probably offer sacrificial protection to the steel substrate.