The corrosion behavior of zinc coatings in non-acidic/alkaline atmospheres differs significantly from that in acidic/alkaline atmospheres. Generally speaking, in the absence of acidic or alkaline conditions, the zinc coating surface comes into contact with oxygen in the air to form zinc oxide (ZnO). In the presence of moisture, zinc hydroxide [Zn(OH)₂] can also be formed.
When the zinc coating surface comes into contact with acidic or alkaline atmospheres accompanied by moisture, zinc sulfate (ZnSO₄), zinc chloride (ZnCl₂), zinc hydroxide [Zn(OH)₂], zinc carbonate (ZnCO₃, note: ZnCO₈ is not a valid chemical formula for zinc carbonate), and other compounds may be formed. The corrosion rate of the zinc coating varies with the pH value of the solution composed of these atmospheres and water. As shown in Figure 1-1, there are four pH value regions: below pH 6 is the strongly acidic region with the highest dissolution rate; between pH 6 and 12.5 is the stable region where zinc salts on the zinc coating surface form a stable protective film; pH 12.5 to 13.5 is the dilute alkaline region; and above pH 13.5 is the strongly alkaline region. It can be seen that the corrosion behavior of zinc coatings in non-acidic/alkaline atmospheres is completely different from that in acidic/alkaline atmospheres.
