Effect of oxide species and bath temperature on reactions in a galvanizing bath of Si-containing steel

Abstract

Reaction behavior was investigated in a galvanizing bath between Si-containing steel and molten Zn, in
order to understand the influence of Si oxides and solute Si on galvanizing reactions. For the 0.2Si steel, only
Mn2SiO4 formed on the surface of the substrate after reduction annealing. After galvanizing, the Fe amount
in the coatings slightly increased with the rise of the bath temperature. In contrast, the Al amount at the
substrate/coating interface (interfacial Al) decreased. As such behavior is similar to 0.01Si steel, it was
suggested that the influence of Mn2SiO4 on a galvanizing reaction is small. For the 1.2Si steel, on the other hand,
SiO2 formed on the surface of the substrate in addition to Mn2SiO4 after reduction annealing, and the galvanizing
reaction was quite different from other kinds of steel. Although both the Fe in the coatings and interfacial Al were
much lower below 450ºC, the Fe in the coatings increased sharply with a rise in bath temperature to more than
460ºC. As SiO2 was detected on the coating/substrate interface, even after galvanizing at 470ºC for the 1.2Si steel,
it was considered that SiO2 exhibits a barrier effect on the reaction in the bath.
Furthermore, for the 1.2Si steel, the Fe-Zn compounds seemed to form easily compared to other kinds of
steel. It was implied that the balance of stability between the Fe-Zn compounds and Fe-Al compounds was changed
by the solute Si in the substrate in addition to Si oxides.