Steel Protection By Hot Dip Galvanizing & Duplex Systems

Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA

About Corrosion and Rust Prevention

Corrosion can be defined as the destruction or deterioration of a material by reaction with its environment. Corrosion usually results in compromising the function of a metal, damage to its surroundings, or damage to the technical system in which they are both included (figures 1 to 4). Broadly speaking, all metals, with the possible exception of precious metals, are corroded and destroyed with time.

For steel to corrode - or rust - in normal environments, it must have access to both oxygen and water. in most environments both oxygen and water are available in sufficient quantities through most of the year to enable the corrosion process to take place.

The engineering properties of steel have made it the most widely used metal. However, its tendency to corrode readily means that corrosion protection is of great economic importance. The rusting process in steel can be impeded by any of the following:

• By alloying the steel with elements such as chromium, nickel, molybdenum, etc. For ordinary steel structures, however, these steels are too expensive.
  

• Changing of the corrosive environment by reducing the access of water and oxygen through techniques such as dehumidification, inert atmospheric blankets, etc. in totally immersed aqueous environments inhibitors may be added to reduce the aggresivity of the solution.
  

• Cathodic protection through the utilization of sacrificial anodes or impressed direct current. The method using sacrificial anodes can be said to be a form of controlled galvanic corrosion, since the metals are arranged so that one of them is allowed to corrode while the other is protected. Cathodic protection can only be used in the presence of an electrolyte, such as water or moist soil. The method is used for the protection of ships, small boats, quays, offshore oil platforms, tanks, pipelines, etc..

• Coating with inorganic or organic material, for the purpose of excluding water and oxygen from the steel surface. This is the most widely used method of protection against corrosion. The inorganic materials can be metals and vitreous enamels, The organic materials can be paints, bitumen products or plastics.

Metal coating of steel will provide protection against corrosion, give wear resistance, and sometimes a decorative effect.

Only a few of the metals that can be deposited on steel are cost-effective and cathodic to steel. In fact, only zinc and aluminium can really be considered, Cadmium is used to some extent but environmental concerns limit its use.

Aluminium has good durability in most environments, although it is difficult to apply. Thin sheet is aluminized on a small scale, Thermal spraying is used to a certain extent.

A more detailed analysis of the different aspects of corrosion and corrosion prevention would go beyond the scope of this publication. For those who are interested, further information about related problems can be obtained from the Hot Dip Galvanizers Association Southern Africa.