1. Dry Corrosion
Dry corrosion of galvanized steel sheet occurs in ambient air with relative humidity below 30%. Under these conditions, no continuous electrolytic water film forms on the steel surface. As a result, the corrosion rate of steel is relatively slow. However, such extremely dry natural environments are quite rare.
2. Wet-Cold Corrosion
Wet-cold corrosion refers to the corrosion that occurs when the ambient air humidity is between 30% and 100%. In this environment, dust accumulates on the steel surface, and micropores and gaps in the rust layer easily absorb moisture, forming invisible condensation water (shrinkage water). This creates the conditions for micro-galvanic cells, thereby accelerating the corrosion process.
3. Wet Corrosion
Wet corrosion happens when the air humidity exceeds 100% and visible condensed water forms on the surface of the galvanized steel sheet. In such high-humidity conditions, the water film on the steel surface becomes thicker, which slows down the diffusion rate of oxygen. Nevertheless, the corrosion of steel is still closely related to the presence of this water film.
Carbon dioxide (CO₂) in the air dissolves in the water film, forming a weak electrolyte that triggers electrochemical reactions. As the reaction proceeds, the main corrosion product appears bright red. This material is commonly known as "red rust". Red rust is loose and porous, making it easy to fall off and unable to protect the underlying steel. Instead, it promotes further corrosion until the steel is completely eroded. This is a typical characteristic of ordinary carbon steel.
Conclusion
Because steel is prone to corrosion in natural environments, galvanized steel sheets must be protected by coating with other metals (such as zinc) or increasing the thickness of the protective layer. This is the fundamental reason why galvanized and color-coated steel products are widely used in construction and manufacturing industries.
