Galvanization

Galvanization or galvanisation refers to any of several electrochemical processes named after the Italian scientist Luigi Galvani.

Meanings

Electricity

Originally, galvanization was the administration of electric shocks (in the 19th century also termed "Faradism", after Michael Faraday). It stemmed from Galvani's induction of twitches in severed frogs' legs, by his accidental generation of electricity. This archaic sense is the origin of the meaning of "galvanic" when meaning "affected/affecting, as if by a shock of electricity; startled". [cite web | url=http://dictionary.reference.com/search?r=10&q=galvanic | title=Galvanic; Dictionary.com | accessdate=2006-11-30] Its claims to health benefits have largely been disproven, except for some limited uses in psychiatry in the form of electroconvulsive therapy (ECT). Later the word was used for processes of electrodeposition. This remains a useful and broadly applied technology, but the term "galvanization" has largely come to be associated with zinc coatings, to the exclusion of other metals.

Metal protection

In current use, it typically means hot-dip galvanizing, a metallurgical process that is used to coat steel or iron with zinc. This is done to prevent galvanic corrosion (specifically rusting) of the ferrous item; while it is accomplished by non-electrochemical means, it serves an electrochemical purpose.

Hot-dip galvanized steel has been effectively used for more than 150 years. The value of hot-dip galvanizing stems from the relative corrosion resistance of zinc, which, under most service conditions, is considerably better than iron and steel. In addition to forming a physical barrier against corrosion, zinc, applied as a hot-dip galvanized coating, cathodically protects exposed steel. Furthermore, galvanizing for protection of iron and steel is favored because of its low cost, the ease of application, and the extended maintenance-free service that it provides.

Zinc coatings

Zinc coatings prevent corrosion of the protected metal by forming a physical barrier, and by acting as a sacrificial anode if this barrier is damaged. When exposed to the atmosphere, zinc reacts with oxygen to form zinc oxide, which further reacts with water molecules in the air to form zinc hydroxide. Finally zinc hydroxide reacts with carbon dioxide in the atmosphere to yield a thin, impermeable, tenacious and quite insoluble dull grey layer of zinc carbonate which adheres extremely well to the underlying zinc, so protecting it from further corrosion, in a way similar to the protection afforded to aluminium and stainless steels by their oxide layers.

Hot dip galvanizing deposits a thick robust layer that may be more than is necessary for the protection of the underlying metal in some applications. This is the case in automobile bodies, where additional rust proofing paint will be applied. Here, a thinner form of galvanizing is applied by electroplating, called "electrogalvanization". However, the protection this process provides is insufficient for products that will be constantly exposed to corrosive materials such as salt water. Nevertheless, most nails made today are electro-galvanized.

As noted previously, both mechanisms are often at work in practical applications. For example, the traditional measure of a coating's effectiveness is resistance to a salt spray. Thin coatings cannot remain intact indefinitely when subject to surface abrasion, and the galvanic protection offered by zinc can be sharply contrasted to more noble metals. As an example, a scratched or incomplete coating of chromium actually exacerbates corrosion of the underlying steel, since it is less electrochemically active than the substrate.

The size of crystallites in galvanized coatings is an aesthetic feature, known as spangle. By varying the number of particles added for heterogeneous nucleation and the rate of cooling in a hot-dip process, the spangle can be adjusted from an apparently uniform surface (crystallites too small to see with the naked eye) to grains several centimeters wide. Visible crystallites are rare in other engineering materials. Protective coatings for steel constitute the largest use of zinc and rely upon the galvanic or sacrificial property of zinc relative to steel.

Thermal diffusion galvanizing is a new "green" process which creates a zinc coating metallurgically similar to hot dip galvanizing. Instead of dipping parts in molten zinc, zinc is applied in a powder form with accelerator chemicals. The parts and the zinc compound are sealed in a drum which is rotated in an oven. Due to accelerator chemicals added to the zinc powder, the zinc/iron diffusion (alloying) takes place at a lower temperature than hot dip galvanizing, and results in a more uniform and wear resistant coating. The process also eliminates the need for hazardous caustic, acid, and flux baths required to prepare parts for hot dip galvanizing. The unique crystal structure formed by the process provides a strong bond with paint, powder coating, and rubber overmolding processes.

Eventual corrosion

Although galvanizing will inhibit attack of the underlying steel, rusting will be inevitable, especially if the local rainfall is at all acidic in nature. So for example, corrugated iron sheet roofing will start to degrade within a few years despite the protective action of the zinc coating. Marine environments also lower the lifetime of galvanized iron roofs and similar products, because the high electrical conductivity of sea water encourages and increases the rate of corrosion. Corrugated steel roofs can last for many years if further protected by a paint layer. Galvanized car frames may also be at risk, especially where road salting is used extensively.

See also

* Corrugated galvanised iron
* Sherardising

References

External links

* [http://www.zeroplus.eu:http://ec.europa.eu/environment/life/ Life Program regarding liquid waste management in galvanic processes.]
* [http://www.galvanizeit.org/ Americam Galvanizers Association]