Galvanized zinc-coated steel, corrosion resistance

Galvanized steel A zinc-coated steel sheet or plate with good corrosion resistance properties used for ductwork and other applications. 

Like chromium, zinc is a corrosion-resistant metal. One of its principal uses is as a coating on iron and steel surfaces to prevent rusting. In the process called galvanizing, a surface coating of zinc is applied to iron by dipping the iron into molten zinc. Zinc is also important when alloyed with other metals. 

The most promising corrosion-resistant rebars are galvanized (zinc-coated) rebars, stainless steel-clad rebars, and solid stainless steel rebars. Titanium has also been considered as a rebar metal, but its cost is prohibitive although it is highly corrosion-resistant. 

Zinc-coated steel is increasingly used in situations that otherwise would be too aggressive to steel. Five-year trials in the agricultural industry, particularly in animal waste environments, showed (Kim, 1991) the superior corrosion resistance of galvanizing after fabrication, compared to epoxy-coated steel. 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

Zinc (Zn) Zinc is a bluish silver metal. Medical applications of zinc compounds include calamine lotion and zinc oxide (sun block). Galvanized steel is resistant to corrosion and is prepared by coating iron with a thin layer of zinc. 

Galvanizing. The process where steel is coated with a layer of zinc, which provides the steel with greater corrosion resistance... 

The external layer of pure zinc is of primary importance with regard to the corrosion resistance of the bars. If galvanized steel is exposed to a neutral environment, such as the atmosphere, the duration of protection is primarily dependent on the thickness of the zinc coating, and its composition and microstructure has a negligible effect. Similarly, for galvanized-steel bars embedded in concrete, the protective properties of zinc coatings are due for the most part to the external layer of pure zinc, which can form a passive film if it has a sufficient thickness [36-39]. In fact, a loss of thickness of 5-10 pm is required prior to passivation, while, if the thickness is insufficient, the underlying layers of Zn-Fe alloy passivate with more difficulty. 

The most important change in materials over the years has been the transition from uncoated mild steel to zinc pre-coated steel and other corrosion-resistant metals. Hot-dip galvanized steels were used in the early stages in the motor vehicle industry. Hot-dipped galvanized steels were corrosion resistant but had a spangled surface that resulted in poor appearance after painting. Hot-dipped galvanized steel is still used on most body parts and interior surface of body outer panels. 

Galvanized steel is a common example of galvanic coupling where steel (Fe) with a standard electrode potential of—0.440 Vvs. SHE is cathodicaUy protected by a coating of zinc with a more active standard electrode potential of—0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects the steel from corrosion because of its sacrificial properties. Because zinc is less noble than steel, it acts as the anode. The sacrificial anode is continuously consumed by anodic dissolution and protects the more positive metal from corrosion. In practice, sacrificial anodes are... 

---