Oxidation of galvanized surfaces
Like all metals, zinc oxidizes when exposed to air and moisture. However, this element does not rust like most other metals.
Iron, for example, reacts with water and oxygen in the atmosphere to form hydrated iron oxide (III) on the metal surface. This by-product, called "rust", is known for its reddish brown color and its harmful effects. Hydrated iron oxides are brittle and flake easily, thus exposing the underlying metal to the atmosphere for further corrosion. This cycle is repeated until the iron is completely consumed by the corrosion process.
Zinc, on the other hand, reacts with oxygen to form a thin oxide layer. This layer then reacts with moisture to produce zinc hydroxide (white rust), which further reacts with carbon dioxide to create zinc carbonate. Unlike iron oxides, which flake easily, zinc carbonate is resistant, chemically stable and adheres firmly to the metal surface. This layer acts as a protective barrier that prevents air and moisture from coming into contact with the underlying substrate and prevents further corrosion and deterioration.
Zinc hydroxide is superficial and has a white and dusty appearance, while the carbonate layer has protective properties and allows the galvanized surfaces to corrode very slowly over time. This is why the galvanizing process exists, to prevent corrosion and increase the physical characteristics of the treated surfaces. Galvanizing ensures barrier protection to the treated surface (iron or steel), making it impermeable to air and humidity, it also guarantees the so-called galvanic (or cathodic) protection. Since zinc is a highly reactive and electronegative element, it will corrode in place of the underlying surface, until it is totally consumed.
Types of galvanizing process:
Hot dip galvanizing
Hot dip galvanizing is one of the most common forms of galvanizing. This process involves coating an iron or steel object by immersing it in a molten zinc bath at temperatures of about 450 ° C. Once removed from the bath, the zinc coating on the outside of the iron or steel reacts with oxygen in the atmosphere to form zinc oxide (ZnO).
Zinc oxide reacts further with carbon dioxide to form the protective layer known as zinc carbonate (ZnCO3). This opaque grayish film is relatively stable and adheres perfectly to the surface of iron or steel. In hot dip galvanizing, zinc binds chemically and becomes part of the steel to be protected.