The corrosion mechanisms of zinc coatings exhibit significant differences between acidic/alkaline environments and neutral atmospheres. Generally speaking, in neutral atmospheres without acidic or alkaline constituents, the zinc surface reacts with oxygen in the air to form zinc oxide (ZnO). When moisture is present, zinc hydroxide [Zn(OH)₂] may form.
However, when the zinc surface is exposed to acidic or alkaline atmospheres containing moisture, the corrosion products become more complex, including zinc sulfate (ZnSO₄), zinc chloride (ZnCl₂), zinc hydroxide [Zn(OH)₂], and zinc carbonate (ZnCO₃). The corrosion rate of the zinc coating varies with the pH of the electrolyte solution formed by these atmospheric constituents and moisture. As illustrated in Figure 1-1, four distinct pH regions can be identified:
Strongly acidic region (pH < 6): Characterized by maximum dissolution rate
Stable region (pH 6–12.5): Formation of protective zinc salt films on the coating surface
Dilute alkaline region (pH 12.5–13.5)
Strongly alkaline region (pH > 13.5)
This demonstrates fundamentally different corrosion behaviors between neutral environments and those containing acidic/alkaline constituents. The presence of aggressive ions in acidic/alkaline atmospheres significantly alters the corrosion kinetics and product formation compared to neutral conditions, with pH being a critical determining factor in the corrosion mechanism.
