Effect of Mg Addition (in Zinc Bath) on Galvanized Sheet Quality

In order to improve the corrosion resistance of the conventional hot dip galvanized sheets, a new type of Zn-Mg coating has been developed. Experiments were conducted in HDPS by varying zinc bath Mg level, bath temperature & dipping time. It has been found that steel sheet galvanized in zinc bath composition of 0.50%Mg, 0.25%Al, 0.08%Si & 0.08%Sb and processed at bath temperature of ~460oC and dipping time : 2.0 sec. results in best combination of properties in terms of corrosion rate : 4.0 mpy (~1/3rd w.r.t conventional GI sheets : ~12 mpy), formability of the composite (Ev : 10.4 mm for sheet thickness of 0.8 mm) at par with substrate material, coating adherence as per Lock Forming Quality (LFQ) standard and appearance bright and smooth. The improved corrosion resistance of newly developed coating is attributed to the presence of Mg-Zn intermetallic phases along the grain boundary inhibiting the cathodic reaction as well as formation of protective MgO & protective corrosion products of zinc. The presence of Sb in the galvanizing bath has also led to improved intergranular corrosion resistance. Superior formability, adherence and appearance of the coating is due to presence of adequate Al & Si level in the galvanizing bath, which led to suppression of brittle Fe-Zn phase formation at the coating-steel interface.