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How zinc protects steel from corrosion?

Addtime:2012-6-16 0:52:52

The reason for the extensive use of hot-dip galvanizing is the two-fold protective nature of the coating. As a barrier coating, it provides a tough, metallurgically bonded zinc coating that completely covers the steel surface and seals the steel from the corrosive action of the environment. Additionally, zinc’s sacrificial action protects the steel even where damage or a minor discon-tinuity in the coating occurs.

Barrier Protection

    Barrier protection is perhaps the oldest and most widely used method of corrosion protection. It acts by isolating the metal from the electrolytes in the environment. Two important properties of barrier protection are adhesion to the base metal and abrasion resistance. Paint is one example of a barrier protection system.

Cathodic Protection

    Cathodic protection is an equally important method for preventing corrosion. Cathodic protection requires changing an element of the corrosion circuit by introducing a new corrosion element, thus ensuring that the base metal becomes the cathodic element of the circuit.

    There are two major variations of the cathodic method of corrosion protection. The first is called “the impressed current method”. In this method, an external current source is used to impress a cathodic charge on all the iron or steel to be protected. While such systems generally do not use a great deal of electricity, they often are very expensive to install.

    The other form of cathodic protection is called“the sacrificial anode method”. In this method a metal or alloy that is anodic to the metal to be protected is placed in the circuit and becomes the anode. The protected metal becomes the cathode and does not corrode. The anode corrodes, thereby providing the desired sacrificial protection. In nearly all electrolytes encountered in everyday use, zinc is anodic to iron and steel. Thus, the galvanized coating provides cathodic corrosion protection as well as barrier protection.