Effect of silicon on the reaction between solid iron and liquid Zn-22.3 wt.% Al bath

• The τ5, and τ4 phase will form in the reaction zone first, when proper amount of Si (1.1 and 3.8 wt.%) added in 600 °C Zn-Al bath.
• The thicknesses of the reaction layer which dipped in 1.8 wt.% Si bath, is the minimum value.
• The growth of the reaction layer is diffusion controlled for various content Si baths in this study.

The coating microstructures and thicknesses of the iron panels galvanized in liquid Zn-22.3 wt.% Al baths containing 0.0, 0.7, 1.1, 1.8 and 3.8 wt.% Si for 10 s, 30 s, 60 s, 180 s, 300 s and 600 s have been studied detailedly. With increasing of the Si content in various Zn-Al bath, the phase sequence in the reaction zone of the panel dipped for 600 s was iron substrate/Fe2Al5/Fe2Al5 + τ1/FeAl3/FeAl3 + τ5C/overlay, iron substrate/Fe2Al5/Fe2Al5 + τ1/FeAl3/FeAl3 + τ1/τ5C + τ5H/overlay, iron substrate /Fe2Al5/Fe2Al5 + τ1/FeAl3/FeAl3 + τ1/τ5H/overlay, iron substrate/Fe2Al5/Fe2Al5 + τ1/FeAl3/FeAl3 + τ2/τ5H/overlay and iron substrate/Fe2Al5/Fe2Al5 + τ1/FeAl3 + τ1/τ2/τ4/overlay, respectively. Seven phases were found in these reaction Zones, i.e. Fe2Al5, FeAl3, τ5C, τ5H, τ1, τ2 and τ4. The growth kinetics of the reaction zone were also studied. The thicknesses of the reaction layer which dipped in 1.8 wt.% Si bath, is the minimum value. The growth of the reaction layer is diffusion controlled for various content Si baths in this study. With the absence of Si in the bath, the liquid phase erodes the iron substrate directly, and the growth of the reaction layer is interface controlled.