How does the application of a duplex coating system (e.g., galvanizing plus paint) enhance corrosion protection compared to galvanizing alone?

A duplex coating system, such as galvanizing plus paint, enhances corrosion protection through synergistic effects, combining the benefits of both systems. Galvanizing, which is coating steel with a layer of zinc, provides cathodic protection. Cathodic protection means the zinc corrodes preferentially to the steel, sacrificing itself to protect the underlying steel even if the coating is scratched or damaged. However, galvanizing alone has a limited lifespan, especially in harsh environments. Applying a paint coating over the galvanizing extends the life of the system significantly. The paint acts as a barrier, slowing down the rate at which the zinc corrodes. It also protects the galvanizing from abrasion and chemical attack. Furthermore, the paint layer can be easily inspected and recoated, providing ongoing protection. When the paint layer is scratched or damaged, the exposed zinc from the galvanizing layer still provides cathodic protection, preventing corrosion from spreading under the paint. This combination creates a robust and long-lasting corrosion protection system that outperforms either galvanizing or paint used alone.