Abstract: The relationships of activities with 1% solute as standard state and mass fraction of solute, and hot-dip temperature, were given on the base of Miedema's model, Tanaka expression, some basic thermodynamic relationships; and discussion was carried out on Zn-Mn and Zn-Ti binary alloys by calculation, in which varied colors can be achieved on the hot-dip steel sheets. The results indicate that the activity of solute shows positive deviation relative to Henry's law for both Zn-Mn and Zn-Ti binary dilute solution. The degree of, deviation increases with increasing solute and decreases with increasing bath temperature. As the solution is very dilute solution (w (Mn)≤40% for Zn-Mn alloy，w (Ti)≤8% for Zn-Ti alloy), the two binary baths can all be treated as ideal dilute solutions.

Key words:  activities; colored hot dip galvanization; solution