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Metal protection

In current use, the term refers to the coating of steel or iron with zinc. This is done to prevent rusting of the ferrous item. The value of galvanizing stems from the corrosion resistance of zinc, which, under most service conditions, is considerably greater than that of iron and steel. The zinc serves as a sacrificial anode, so that it cathodically protects exposed steel. This means that even if the coating is scratched or abraded, the exposed steel will still be protected from corrosion by the remaining zinc - an advantage absent from paint, enamel, powder coating and other methods. Galvanizing is also favored as a means of protective coating because of its low cost, ease of application and comparatively long maintenance-free service life.

The term galvanizing, while technically referring specifically to the application of zinc coating by the use of a galvanic cell (also known as electroplating), is also generally understood to include hot-dip zinc coating. The practical difference is that hot-dip galvanization produces a thick, durable and matte gray coating - electroplated coatings tend to be thin and brightly reflective. Due to its thinness, the zinc of electroplated coatings is quickly depleted, making them unsuitable for outdoor applications (except in very dry climates). When combined with subsequent painting (which slows zinc consumption), electroplating is durable enough to be used in some premium auto body coatings.

Nonetheless, electroplating is used on its own for many outdoor applications because it is cheaper than hot dip zinc coating and looks good when new. Another reason not to use hot dip zinc coating is that for bolts and nuts size M10 (US 3/8") or smaller, the thick hot-dipped coating fills in too much of the threads, which reduces strength (because the dimension of the steel prior to coating must be reduced for the fasteners to fit together). This means that for cars, bicycles and many other 'light' mechanical products, the alternative to electroplating bolts and nuts is not hot dip zinc coating but making the bolts and nuts from stainless steel (known by the corrosion grades A4 and A2).

Electroplated steel is visually indistinguishable from stainless steel when new.^{[citation
needed]} To determine whether a part is electroplated or stainless steel, apply a magnet. The most common stainless steel alloys (including those used for bolts and nuts) are not magnetic or only very slightly attracted to a magnet.

[edit] History

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. Its claims to health benefits have largely been disproved, except for some limited uses in psychiatry in the form of electroconvulsive therapy (ECT). This archaic sense is the origin of the meaning of galvanic when meaning "affected/affecting, as if by a shock of electricity; startled".^[1] and the metaphorical "galvanize into action" referring to suddenly stimulating a complacent person or group to take action. Later the word was used for processes of electrodeposition, which 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.

Galvanic paint, a precursor to hot-dip galvanization, was patented by Stanislas Sorel, of Paris, France in December, 1837.^[2]

The earliest known example of galvanizing of iron was found on 17th century Indian armor in the Royal Armouries Museum collection.^[3]

[edit] Zinc coatings

Main articles: Hot-dip galvanizing and Sherardizing

Zinc coatings prevent corrosion of the protected metal by forming a physical barrier, and by acting as a sacrificial anode even 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. In turn, zinc hydroxide reacts with carbon dioxide in the atmosphere to yield a thin, impermeable, tenacious and quite insoluble dull gray layer of zinc carbonate which adheres extremely well to the underlying zinc, so protecting it from further corrosion. This is 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. Some 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.

Galvanized surface with visible spangle

The size of crystallites in galvanized coatings is a visible and 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 centimetres wide. Visible crystallites are rare in other engineering materials.

Thermal diffusion galvanizing, a form of Sherardizing, provides a zinc coating on iron or copper based materials partially similar to hot dip galvanizing, but the final surface that results is different from that yielded with hot-dip galvanizing in that all of the zinc is alloyed.^[4] Zinc is applied in a powder form with "accelerator chemicals" (generally sand,^[5] but other chemicals are patented). The parts and the zinc powder are tumbled in a sealed drum while it is heated to slightly below zinc's melting temperature. The drum must be heated evenly, or complications will arise. Due to the chemicals added to the zinc powder, the zinc/iron makes an alloy at a lower temperature than hot dip galvanizing. This process requires generally fewer preparatory cleanings than other methods. The dull-grey crystal structure formed by the process bonds more strongly with paint, powder coating, and rubber overmolding processes than other methods. It is a preferred method for coating small, complex-shaped metals, and for smoothing in rough surfaces on items formed with powder metal.

[edit] Eventual corrosion

Rusted corrugated steel roof

Although galvanizing will inhibit attack of the underlying steel, rusting will be inevitable, especially if exposed to the natural acidity of rain. For example, corrugated iron sheet roofing will start to degrade within a few years despite the protective action of the zinc coating. Marine and salty environments also lower the lifetime of galvanized iron because the high electrical conductivity of sea water increases the rate of corrosion. Galvanized car frames exemplify this; they corrode much quicker in cold environments due to road salt. Galvanized steel can last for many years if other means are maintained, such as paint coatings and additional sacrificial anodes.

[edit] Galvanized piping

See also: Galvanic corrosion

In the early 20th century, galvanized piping replaced cast iron and lead in cold-water plumbing. Typically, galvanized piping rusts from the inside out, building up plaques on the inside of the piping, causing both water pressure problems and eventual pipe failure. These plaques can flake off, leading to visible impurities in water and a slight metallic taste. The life expectancy of such piping is about 70 years, but it may vary by region due to impurities in the water supply and the proximity of electrical grids for which interior piping acts as a pathway (the flow of electricity can accelerate chemical corrosion). Pipe longevity also depends on the thickness of zinc in the original galvanization, which ranges on a scale from G40 to G210, and whether the pipe was galvanized on both the inside and outside, or just the outside. Since World War II, copper and plastic piping has replaced galvanized piping for interior drinking water service, but galvanized steel pipes are still used in outdoor applications where mechanical strength is required.

This lends some truth to the urban myth that water purity in outdoor water faucets is lower, but the actual impurities (iron, zinc, calcium) are harmless. This is not always the case in pre-1986 copper pipe where lead-containing solder was commonly used. In installations where copper pipe has been fitted to replace a section of corroded galvanized pipe, a special dielectric fitting called a "union" must be used to join the two types of pipes; otherwise the presence of water in contact with differing metals creates an electrical current that can cause "galvanic corrosion". In some amateur installations, the failure to use this special fitting has caused the lead in the solder to leach into the drinking water. A common location where this occurs is where a home's copper piping connects to a galvanized steel municipal supply line.

The presence of galvanized piping detracts from the appraised value of housing stock because piping can fail, increasing the risk of water damage. Galvanized piping will eventually need to be replaced if housing stock is to outlast a 50 to 70 year life expectancy, and some jurisdictions require galvanized piping to be replaced before sale. One option to extend the life expectancy of existing galvanized piping is to line it with an epoxy resin.