16. What is the difference between the corrosion of zinc layer in acid-free and alkaline atmosphere and in acid and alkaline atmosphere?

The corrosion behavior of zinc coatings differs significantly between acidic and alkaline environments. Generally, in non-acidic or non-alkaline conditions, zinc oxide (ZnO) forms when the coating surface reacts with atmospheric oxygen. In the presence of moisture, zinc hydroxide (Zn(OH)₂) may also be generated.
When zinc coatings are exposed to acidic or alkaline atmospheres with moisture, they generate zinc sulfate (ZnSO₄), zinc chloride (ZnCl₂), zinc hydroxide [Zn(OH)₂], and zinc carbonate (ZnCO₈). The corrosion rate of zinc coatings varies depending on the pH of the resulting solution. Figure 1-1 shows four distinct pH zones: below pH 6 (strongly acidic) indicates the fastest dissolution rate; pH 6–12.5 (stable zone) forms a protective film of zinc salts; pH 12.5–13.5 (weakly alkaline) indicates corrosion; and pH above 13.5 (strongly alkaline) shows accelerated corrosion. This demonstrates that zinc coatings exhibit entirely different corrosion behaviors in acidic/alkaline versus neutral environments.