The corrosion behavior of zinc coatings differs significantly between non-acidic/alkaline and acidic/alkaline atmospheres. Generally speaking, in the absence of acidic or alkaline atmospheres, the zinc coating surface reacts 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 and moisture is present, zinc sulfate (ZnSO₄), zinc chloride (ZnCl₂), zinc hydroxide [Zn(OH)₂], and zinc carbonate (ZnCO₃, note: ZnCO₈ is not a standard chemical formula for zinc carbonate) may be formed. The corrosion rate of the zinc coating varies with the pH value of the solution composed of these atmospheres and water. Figure 1-1 shows four pH value zones: below pH 6 is the strongly acidic zone with the highest dissolution rate; between pH 6 and 12.5 is the stable zone where zinc salts on the zinc coating surface form a stable protective film; pH values between 12.5 and 13.5 constitute the dilute alkaline zone; and above pH 13.5 is the strongly alkaline zone. It can be seen that the corrosion behavior of zinc coatings in non-acidic/alkaline and acidic/alkaline atmospheres is completely different.
