Protecting Steel Structures with Zinc
Mild steel is the one of the most commonly used material in building structures on land and at sea. Steel, however, has a high corrosion rate - especially when exposed to fresh water and sea water. There are various methods in practice to protect steel structures from corroding away. The following article describes an economic and effective way of protecting steel structures by electroplating them with Zinc.
Zinc (Zn), is a metal that is found in abundance in nature all over the world. In addition to its importance in pharmaceutical industries, it is used for galvanizing iron and steel against corrosion. Zinc itself forms a protective coating of its oxide when exposed to the atmosphere, making it more resistant to corrosion than iron and steel.
Electroplating Steel Structures with Zinc
This is used for structures that are built for marine environments like SeaPods, and oil rigs. This is known as the barrier method which prevents oxygen from reaching the surface of the steel structure. Steel structures are hot-dip galvanized to get a thin layer of zinc enveloping the complete structure.
When a hot-dip galvanized steel structure us removed from the zinc bath, its surface reacts with Oxygen in the air. The resultant oxide layer has very little ability to protect against corrosion. When this zinc-plated steel structure is dipped in water, the oxide layer is converted to zinc carbonate, resulting in an additional, sealed layer of protection. Zinc carbonate has very low solubility in water and it takes very long for sea water to cause corrosion to the steel structure.
Corrosion rate of zinc is low and stable in liquids that have a pH range of 5.55 to 12.5. The pH value of sea water is usually in this range with an average of 8.1 to 8.2.
The longevity of the steel structure in sea water depends on the thickness of zinc coating. Tests have proved that steel structures that have a coating of 100 μm (micrometers) can protect steel for up to 5 years and a coating of 250 μm can protect the steel structure over 15 years.
Zinc-coated anodes have been used in the manufacturing sector for decades because of their cathodic protection properties, longevity, low cost and strength. This is ideally suited for marine environments where corrosion rates are extremely high.