Wednesday, September 11, 2013

Chemical Properties Of Zinc In Galvanized Steel

Galvanized steel is used for many outdoor applications.








Galvanized steel is the common name for steel treated with an outer coating of zinc as a protection against corrosion. While true galvanizing is an electrochemical process that forms a metallic bond between the zinc and steel, galvanizing commonly refers to other methods of coating including zinc paints. Although the Romans used primitive zinc coatings, Luigi Galvani first discovered the electroplating process used for applying zinc to steel in 1772. Galvanized steel has been a common building material since the middle of the 19th century. Zinc is generally considered a low-toxicity metal, and small amounts are required in the human diet.


Physical Barrier


After galvanizing, zinc will react with oxygen in the air to form a zinc oxide, a white powdery coating on the surface. The zinc oxide will react with water molecules in the air to form zinc-hydroxide, a common and non-toxic form of zinc. The zinc oxide will then react with carbon dioxide, a common atmospheric gas, and form zinc carbonate. Zinc carbonate will tightly bond itself to the remaining zinc, and form a hard, durable and waterproof coating that protects the zinc from mechanical damage.


Impermeability to Moisture


Zinc and its associated oxides provide a high resistance to the flow of water and water vapor. This is a very desirable characteristic for the protection of steel, since corrosion cannot take place without the presence of water. Since the water is unable to penetrate the zinc coatings, the zinc coating protects the steel from corrosion.


Self-Repair








Abrasion or other physical contact may remove or damage the zinc oxide and zinc carbonate coatings. When this happens, the atmospheric moisture and gasses can attach the zinc. The reaction will reform the protective oxide and carbonate coatings, again protecting the underlying zinc from further damage. As a result, galvanized coatings are self-healing.


Cathodic Protection


If the underlying steel comes in contact to moisture, zinc forms a sacrificial anode to protect it. Similar to the function of a battery and the sacrificial zinc anodes on boats, the zinc will form a small electrical current flow that will prevent the corrosion of the iron oxide on the steel. This property allows the atmospheric exposure of a small area of the underlying steel without any rust or corrosion forming.

Tags: zinc oxide, form zinc, Galvanized steel, react with, carbonate coatings