THE PECULIARITIES OF ELECTROCHEMICAL DEPOSITION AND MORPHOLOGY OF Zn-Mn ALLOY COATINGS OBTAINED FROM PYROPHOSPHATE ELECTROLYTE

The first successful attempt to electrodeposit Zn-Mn alloy coatings from alkaline bath was made only a few years ago. In this kind of solution, potassium pyrophosphate (K4P2O7) serves both as a complexing agent and as the basic electrolyte. The aim of this work was to study the electrodeposition process and properties of Zn-Mn alloy coatings deposited from pyrophosphate solution, with a new kind of alkaline pyrophosphate bath. Namely, chloride salts were used as the source of metal ions and ascorbic acid was used as reducing agent. The composition of the plating solution was as follows: 1 mol dm(-3) K4P2O7 + 0.017 mol dm(-3) ascorbic acid + 0.05 mol dm(-3) ZnCl2 + 0.05 mol dm(-3) MnCl2 center dot 4H(2)O. Cathodic processes during the alloy electrodeposition were investigated using linear voltammetry. The influence of addition of small amounts of ascorbic acid on the cathodic processes was established. It was shown that this substance inhibits hydrogen evolution and increases the current efficiency of alloy deposition. The current efficiency in the plating bath examined was in the range of 25 and 30%, which was quite higher as compared to the results reported in the literature for electrodeposition of Zn-Mn alloy from pyrophosphate bath. Electrodeposition of Zn-Mn alloys was performed galvanostatically on steel panels, at current densities of 20-120 mA cm(-2). The coatings with the best appearance were obtained at current densities between 30 and 80 mA cm(-2). The surface morphology studies, based on atomic force microscopy measurements, showed that morphology of the deposits is highly influenced by deposition current density. Zn-Mn coating deposited at 30 mA cm(-2) was more compact and possessed more homogeneous structure (more uniform agglomeration size) than the coating deposited at 80 mA cm(-2). Such dependence of morphology on the current density could be explained by the high rate of hydrogen evolution reaction during the electrodeposition process.