Galvanization

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Galvanization (or galvanisation) classically refers to any of several electrochemical processes named after the Italian scientist Luigi Galvani. Now the term generally refers to an electrodeposition process used to add a thin layer of another metal to an item made of steel, in order to prevent rusting. More recently, though, the term has been broadened in common usage to include applying a protective metallic coating to an underlying piece of metal, using a process called hot-dip galvanization, which produces similar results, but which does not employ electrochemical deposition.

Contents

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".[1] Its claims to health benefits have largely been disproved, 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, the term refers to the coating of steel or iron with zinc. This is done to prevent galvanic corrosion (specifically rusting) of the ferrous item. The value of galvanizing stems from the relative corrosion resistance of zinc, which, under most service conditions, is considerably less than those of iron and steel. The effect of this is that the zinc is consumed first as a sacrificial anode, so that it cathodically protects exposed steel. This means that in case of scratches through the zinc coating, the exposed steel will be cathodically protected by the surrounding zinc coating, unlike an item which is painted with no prior galvanizing, where a scratched surface would rust. 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.

The term galvanizing, while correctly referring to the application of the zinc coating by the use of a galvanic cell (also known as electroplating), sometimes is also used to refer to hot dip zinc coating (commonly incorrectly referred to as hot dip galvanizing). The practical difference is that hot dip zinc coating produces a much thicker, durable coating, whereas genuine galvanizing (electroplating) produces a very thin coating. Another difference, which makes it possible to determine visually which process has been used if an item is described as 'galvanized', is that electroplating produces a nice, shiny surface, whereas hot dip zinc coating produces a matte, grey surface. The thin coating produced by electroplating is much more quickly consumed, after which corrosion turns to the steel or iron itself. This makes electroplating unsuitable for outdoor applications, except in very dry climates. For example, nails for indoor use are electroplated (shiny), while nails for outdoor use are hot dip zinc coated (matte grey). However, electroplating and subsequent painting is a durable combination because the paint slows down the consumption of the zinc. Car bodies of some premium makes are corrosion protected using this combination.

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