Galvanic Reversal (also known as Potential Reversal) is a phenomenon in which zinc—which is expected to protect steel from corrosion through sacrificial protection—instead accelerates steel corrosion under specific conditions.
Ideally, the zinc coating protecting the steel should act as the anode, while the steel remains the cathode. However, when Galvanic Reversal occurs, the roles are swapped: the steel becomes the anode, and the zinc becomes the cathode, causing the steel to corrode before the zinc.
While multiple conditions for Galvanic Reversal have been identified, they are not yet fully understood. Factors such as temperature, pH, and the presence of specific ions in water or soil are believed to be primarily involved.
Furthermore, it is known that while zinc effectively maintains its corrosion resistance below 50°C, its corrosion rate increases sharply around 60°C, leading to a loss of protective performance.
In short, in hot and humid environments, the formation of zinc oxide (ZnO) and zinc hydroxide (Zn(OH)2) on the galvanized layer occurs more easily. This renders the material state unstable, triggering Galvanic Reversal. As a result, the zinc in the coating layer effectively becomes a factor that causes corrosion of the steel.