Galvanizing
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Wikipedia-Article "Galvanizing"
- Galvanize redirects here. Galvanize is also the name of a song by the Chemical Brothers.
Galvanization refers to any of several electrochemical processes named after the Italian scientist Luigi Galvani.
- 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 galvanized when meaning 'stirred to sudden action'. Its claims to health benefits have largely been disproven, except for some limited uses in psychiatry. See also: Galvanism, Violet wand
- 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.
- In current use, it typically means hot-dip galvanizing, a chemical process that is used to coat steel or iron with zinc. This is done to reduce corrosion (specifically rusting) of the ferrous item; while it is accomplished by non-electrochemical means, it serves an electrochemial purpose.
The remainder of the article is about zinc anti-corrosion coatings.
Zinc coatings prevent oxidation of the protected metal by forming a barrier, and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not cause a breakdown of the substrate's surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, 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 "electro-galvanized". Where the metal is not to be painted or is to be used in critical exposure conditions such as near salt water, "hot-dip galvanized" is preferred for its long term durability. Galvanized nails are now usually electro-galvanized but these are greatly inferior to the hot-dipped kind, particularly when used outdoors.
Galvanic protection (also known as sacrificial-anode or cathodic protection) can be achieved by connecting zinc both electronically (often by direct bonding to the protected metal) and ionically (by submerging both into the same body of elecrolyte, such as a drop of rain). In such a configuration the zinc is absorbed into the electrolyte in preference to the metal that it protects, and maintains that metal's structure by inducing an electric current. In the usual example, ingots of zinc are used to protect a boat's hull and propellers, with the ocean as the common electrolyte.
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.
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