How Galvanizing Protects Steel

Corrosion of Steel

Differences in electrical potential are caused on surface areas of exposed steel by non-uniformity of surface composition, by surface moisture or by the electrolyte in which it is immersed. Small electrolytic cells are formed comprising anodes and cathodes. One such cell is shown diagrammatically.

As the result of differences in electrical potential within the cell, negatively charged electrons flow from anode to cathode and iron atoms in the anode area are converted to positively charged ions.

The positively charged iron ions of the anode attract and react with negatively charged hydroxyl ions in the electrolyte to form iron oxide or rust. Negatively charged electrons react at the cathode surface with positively charged hydrogen ions in the electrolyte to form hydrogen gas.

 

            

 

Under suitable conditions corrosion occurs at the rate of billions of complete reactions every second and soon results in a layer of rust appearing over the surface of the anode area.

The anode and cathode areas on a piece of steel are actually microscopic. When greatly magnified the surface might appear as the mosaic of anodes and cathodes visualised here, all electrically connected by the underlying steel. Corrosion occurs in the anode areas.

As anode areas corrode new material of different composition and structure is exposed. This results in changes in electrical potentials, causing anodes and cathodes to exchange roles, though not all at once, and areas previously uncorroded are now attacked. These processes may continue until the steel is entirely consumed.

 

           

 



Barrier Protection

Barrier protection, as its name implies, works by providing an impermeable barrier over the steel item. Galvanizing provides barrier protection in two ways: firstly, the galvanized layer provides a protective physical envelope around the steel; secondly, the galvanized layer also develops a protective patina on its surface upon exposure to the environment. This is made up of insoluble zinc oxides, hydroxides, carbonates and basic zinc salts depending on the nature of the environment. Once the patina stabilises, it reduces the exposure of the base galvanized steel to the environment, thus considerably slowing the corrosion process. This patina regenerates itself after damage by very slowly consuming the zinc outer coating.

Once the pure zinc of the outer layer has been consumed, the iron-zinc alloys are exposed to the environment and their corrosion resistance is up to 30% greater, providing even longer life.

 

 

The barrier protection qualities of galvanized steel are also enhanced by the fact that it is immune to ultraviolet radiation and thus will not degrade on exposure to Australia’s harsh environment.  Most other corrosion protection coatings will degrade on exposure to solar radiation.  This is usually one of the key limiting factors to the performance of such coatings.

The problem with conventional barrier protection such as painting is that it will not prevent corrosion if the base steel is exposed due to mechanical impact damage or abrasion. In fact, barrier protection can allow corrosion to proceed undetected.  This is known as underfilm corrosion.  In the event of severe mechanical damage and exposure of the base steel to the environment, galvanizing also provides cathodic protection. The galvanizing performs in a similar way to other sacrificial protection systems, except in this case the sacrificial anode is distributed over the article to be protected and electrical continuity is assured. The cathodic protection characteristics of galvanizing ensure that mechanical damage does not result in concealed under-film corrosion and potential catastrophic failure prevalent in some other protective coatings. For further information refer to the section on Cathodic Protection.

About Hot-Dip Galvanizing

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