What Does Galvanic Coating Mean?
Galvanic coatings are rich in zinc content, which provides a sacrificial coat, and cathodic protection to ferrous substrates. It is a primer coating and must be applied directly on the substrate.
This coating is used for preventing the damaging effects of corrosion. For iron and steel products, galvanic coatings are normally chosen for their low cost, low maintenance, long life, sustainability and aesthetics.
Zinc and zinc alloys are employed as a metal coating, which protects the base material, steel, against corrosion. Zinc in contact with the substrate acts as anode—it dissolves completely and covers with a layer of oxides, which inhibits further corrosion.
Corrosionpedia Explains Galvanic Coating
Galvanic coatings are metal particles that are non-noble relative to the metal substrate. On exposure, these particles (zinc dust on steel) corrode preferentially, while at the original metal surface only a cathodic reaction takes place.
For example, zinc coatings prevent corrosion of the protected metal by forming a physical barrier and by acting as a sacrificial anode even if this barrier is damaged. When exposed to oxygen, zinc reacts to form zinc oxide, which further reacts with water molecules in the air to form zinc hydroxide. In turn, zinc hydroxide reacts with carbon dioxide in the atmosphere to produce a thin, impermeable, long-lasting and insoluble dull gray layer of zinc carbonate, which adheres extremely strongly to the underlying zinc, protecting it from further corrosion.
Galvanic coatings can be applied by:
- Hot-dip galvanizing - Deposits a thick, robust layer which protects of the underlying metal in applications such as automobile bodies
- Thermal diffusion galvanizing - Preferred method for coating small, complex-shaped metals, and for smoothing rough surfaces on items formed with powder metal
- Electro Galvanizing (Plating) - Suitable only for indoor applications, otherwise it may be depleted quickly.
Galvanized steel can endure longer if other means are maintained, like paint coatings and additional sacrificial anodes.