Oxidation of galvanized surfaces

20/02/2020

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.
Now let's see some types of galvanizing:

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.
To allow this process, the products are previously subjected to various treatment steps: degreasing, pickling and flushing. Degreasing is used to remove dirt, oil and other organic residues; pickling further cleans the steel by removing its oxides and finally the flushing covers the steel with a saline protective layer (based on zinc and ammonium salts) which promotes the wettability of the subsequent galvanizing process.
Once the cleaning and drying process is complete, proceed with immersion in the molten zinc. It flows in and around the iron or steel object, covering it completely to protect all surfaces. The coated material is then removed from the bath and dried in the air before the inspection.

Cold galvanizing
Cold galvanizing is simply the application of a zinc-rich paint to the surface.
Zinc paints can be applied with brushes, rollers, spray guns, etc. The coatings can also be applied with electrogalvanising means. The zinc-rich paints used in cold galvanizing are different from conventional coatings due to the presence of a binding compound. These binders allow zinc to mechanically bond to steel to offer an effective level of protection.
Like hot dip galvanizing, cold galvanizing can provide barrier protection and even some degree of cathodic protection. However, the zinc powder present in the paint or coating must be sufficiently concentrated to favor electrical conductivity between steel and zinc.
The surface preparation required for applying zinc-rich coatings is less demanding than hot dipping techniques. Before starting the coating operations, the steel surface must be clean and dry. Usually, a wire brush is first used to remove rust or other corrosion products that may be present. Dirt, grease, chemicals and other organic compounds also need to be removed accordingly.
Since cold galvanizing is simply a coating, it cannot bind to the metal on a chemical level and, as such, it does not have the same duration, abrasion resistance and cathodic protection capacity as hot galvanizing, however it allows a quick application and convenient on smaller structures and components.

Electrogalvanization
It occurs by immersion in an electrolytic solution containing zinc salts.
Electrolytic cold galvanizing allows a micrometric level of deposition control, a good aesthetic result and an excellent quality and price ratio. A further advantage of the treatment is its reversibility: it can be removed and applied again without compromising any characteristics of the substrate. The procedure involves a passage of direct electric current through a zinc-saline solution, where an anode and a cathode are immersed (object to be galvanized, which acts as a conductor) .The main use is the protection of small and light elements, such as screws, nails, metal wires, bolts, small parts in general and connecting means.