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Galvanization


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

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 those of 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.

Note that exposed steel is painted (much in the same way as exposed aluminium flashings) to colour match the proposed scheme colour

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". The hot-dip process slightly reduces the strength of the base metal, which is a consideration for the manufacture of wire rope and other highly-stressed products. The protection provided by this process is insufficient for products that will be constantly exposed to corrosive materials such as salt water. For these applications, more expensive stainless steel is preferred. 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.


 
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