A metallized coating (a metallic alloy applied to a base metal or concrete) is intended, in most applications, to be a form of protection to an underlying metal substrate. The act of applying the coating is referred to as metallizing. Metallizing can be achieved in several ways such as hot-dipped galvanizing or thermal arc spraying, applied in situ or in a shop. Zinc, which was first used in construction in 79 AD, is the most used metal for this process. Half of the zinc produced today is used for corrosion protection of steel structure.
Zinc is the prominent metal of choice for metallized coatings as it corrodes slower than ferrous metals and is less noble than steel. Noble metals are resistant to chemical actions and oxidation and do not corrode. In the Galvanic Series of Metals, metals are ranked from 'noble' to 'active' based on the metals' potential. Zinc is often a good choice as an anode due to its electronegative potential (Figure 1).
Figure 1. Basic galvanic series of metals for construction.
In electropotential relationships, when two metals are joined, the more electronegative metal becomes anodic to the more noble metal. At this state, the anodic metal corrodes preferentially to the more noble metal. This is commonly referred to as sacrificial corrosion, where the anodic metal sacrifices itself to the cathodic metal.
The range of metallized zinc coatings can be seen in Figure 2. This image depicts the types and thickness of the applied metal to the substrate. These coatings are generally termed as galvanized coatings, though their application methods differ.
The focus of this article is to look at metalized coatings applied to steel and concrete where the coating is used to prevent corrosion and extend the service life of the treated component in steel structure.
Figure 2. Various zinc coating thickness comparison.