Anomalous codeposition and characterization of Zn-Fe alloy

Abstract

A sulphate bath containing ZnSO4. 7H2O, FeSO 4.VH2O, ascorbic acid and hexamine is proposed for the deposition of bright iron rich Zn-Fe alloy onto mild steel substrate. Due to inherent character of Fe+2 ions to oxidize into Fe+3, the main part of the current applied during electroplating is spent on the reduction of ferric ions into ferrous ions. This results in decrease of cathode current efficiencies and will give instability to the bath. Hence the present work has been carried out with ascorbic acid as antioxidant with hexamine as brightener. Hull cell studies were used to optimize the concentrations of the all constituents in the bath. The influence of bath constituents, current density (c.d.) and pH on the appearance and composition of the deposits were examined. A stable bath has been formulated for deposition of Zn-Fe alloy having about 60% Fe for automobile application, by choosing very less zinc content (8 g/1) in the bath. The bath exhibits the anomalous codeposition with preferential deposition of zinc as characterized by any Zn-Fe group metal alloys. It was found that the due to less Zn+2 ions in the bath, the deposition of more noble iron has not been inhibited resulting in formation of Fe rich alloy. The compositions of the deposit at different conditions were related with their brightness, thickness, hardness and their corrosion behavior. Increase of the bath temperature enhanced the percentage of more readily depositable metal (zinc) in the deposit. Thickness of the deposit increased with c.d. as a characteristic of its iron content. A very thin and bright deposit obtained at about 1 A.dm-2 is found to have about 53% iron and a thick and bright deposit at 4.0 A.dm-2 contains about 80% iron. Photomicrograph observation showed that the phase structures of the deposits are influenced by the applied c.d. and pH of the bath. The proposed bath found to give a smooth and uniform deposit (64% Fe) with good performance against corrosion at about 2.0 A.dm-2. A stable acid sulphate bath has been proposed for iron rich Zn-Fe alloy plating for automobile applications.