The electrodeposition of zinc-manganese
Electroplated zinc alloys can provide increased corrosion protection for automotive body panels compared with zinc coatings of similar thickness. As a result, their use is becoming widespread. In particular, outstanding results have been reported in the literature for the corrosion resistance of zinc-manganese, and the alloys are said to offer good paintability, weldability and formability. However, the production of zinc-manganese coatings is restricted by the instability of the sulphate-citrate bath from which the alloy is usually deposited. The solution deteriorates rapidly and precipitates appear in the bath. The aim of this research was to investigate both the process of alloy deposition from the sulphate-citrate bath and the solution instability. The electrodeposition of zinc-manganese was assessed in terms of the polarisation behaviour of the system, in order to determine the role of the bath components and the operating parameters in the deposition process. The coatings were analysed using scanning electron microscopy, and the efficiency of metal deposition was established. The sodium citrate complexant was not found to significantly move the deposition potentials of zinc and manganese together. Instead, its action as a buffer can be used to explain metal deposition and the polarisation behaviour of the system. The sodium citrate suppresses the hydrogen evolution reaction, such that manganese can be deposited from the electrolyte. The bath deterioration with time and the resulting precipitate were examined using a variety of chemical techniques including spectroscopic methods, namely ultraviolet and visible, atomic absorption and infrared. Bath discolouration and precipitation were found to be two separate phenomena. The precipitate was determined to be a citrate of known composition with a ratio of Mn2+ ions to Zn2+ ions of 2:1. The formation took several days, and it was precipitated once the solubility limit in the bath was exceeded. Solution discolouration resulted from reactions occurring after the oxidation of Mn2+ ions and could be delayed by the use of antioxidants.