White rust forms when a layer of condensed water adhering to the surface of the galvanized coating interacts with oxygen, carbon dioxide, hydrogen sulfide, sulfur dioxide, chemical compounds, soot particles, dust, and other atmospheric gases. This interaction produces a corrosive aqueous solution that adheres to the zinc surface, creating an electrolyte. The electrochemical corrosion reaction then occurs between this electrolyte and the zinc coating, which exhibits relatively poor chemical stability. This process generates a powdery corrosion product commonly known as "white rust."
For example, when the surface of a steel pipe is exposed to humid air, moisture condenses on the surface and absorbs carbon dioxide from the atmosphere. This creates a thin electrolyte film that forms micro-galvanic cells, triggering electrochemical corrosion. The dissolution of the zinc coating then produces the characteristic white rust deposits.
