Galvanization

Galvanization is the process of coating a thin layer of zinc on the surface of Iron (Fe) or steel so as to protect it from rusting or corrosion. Galvanization refers to any of several electrochemical processes named after the Italian scientist Luigi Galvani. Now the term generally refers to an electro deposition 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.

History
79 AD
Record of zinc usage in construction began in 79 AD, which could be considered the origination of galvanizing. However, the first recorded history of galvanizing dates back to 1742 when a French chemist named P.J. Malouin, in a presentation to the French Royal Academy, described a method of coating iron with molten zinc.
1772
In 1772, Luigi Galvani, galvanizing’s namesake, discovered the electrochemical process that takes place between metals during an experiment with frog legs. And in 1801, Alessandro Volta furthered the research on galvanizing when he discovered the electro potential between two metals, creating a corrosion cell.
1829
In 1829, Michael Faraday discovered zinc’s sacrificial action during an experiment involving zinc, salt water, and nails. Shortly after, in 1837, French engineer Stanislaus Tranquille Modeste Sorel took out a patent for the early galvanizing process. By 1850, the British galvanizing industry was consuming 10,000 tons of zinc annually for the production of galvanized steel.
1870
The United States, slightly behind, had its first galvanizing plant open in 1870. At the time, the steel was hand dipped in the zinc bath. Today, over 600,000 tons of zinc is consumed annually in North America to produce hot-dip galvanized steel.
Today
Galvanizing is found in almost every major application and industry where iron or steel is used. The utilities, chemical process, pulp and paper, automotive, and transportation industries, to name just a few, historically have made extensive use of galvanizing for corrosion control. They continue to do so today. For over 150 years, hot-dip galvanizing has had a proven history of commercial success as a method of corrosion protection in myriad applications worldwide.

Metal protection
Galvanization protects the material from corrosion by two ways:
a) By the formation of protective covering
Galvanization protect the corrosion of Iron or Steel by forming a barrier between Iron surface and moist air. In absence of moist air Iron is not corroded.
b) Sacrificial protection or cathodic protection
If some part of galvanized Iron is Scratched or exposed to air then it is likely to be corroded but galvanization protect from the corrosion. Here zinc act as anode and exposed part of Iron as cathode and this part form an electrochemical due to electrode reaction . The exposed part of iron is covered by Zinc layer. In this way galvanization protect article from rusting or corrosion.
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 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.
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 or smaller, the thick hot-dipped coating uses up 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. 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.
Zinc coating
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 gray 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. 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.
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, a form of Sherardizing, provides a zinc coating metallurgically on iron or copper based materials similar to hot dip galvanizing. The final surface is different than hot-dip Galvanizing; all of its zinc is alloyed.[3] Zinc is applied in a powder form with "accelerator chemicals" (generally sand[4], 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 . The dull-gray crystal structure formed by the process bonds stronger with paint, powder coating, and rubber overmolding processes than other methods. It is a preferred method for coating small, complex-shaped metals and smoothing in rough surfaces on items formed with powder metal.
Eventual corrosion
Although galvanizing will inhibit attack of the underlying steel, rusting will be inevitable, especially due to 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 ofsea 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.
Process of galvanization
Surface Preparation
Degreasing/Caustic Cleaning
A hot alkaline solution removes dirt, oil, grease, shop oil, and soluble markings.
Pickling
Dilute solutions of either hydrochloric or sulfuric acid removes surface rust and mill scale to provide a chemically clean metallic surface.
Fluxing
Steel is immersed in liquid flux (usually a zinc ammonium chloride solution) to remove oxides and to prevent oxidation prior to dipping into the bath of molten zinc. In the dry galvanizing process, the item is separately dipped in a liquid flux bath, removed, allowed to dry, and then galvanized. In the wet galvanizing process, the flux floats atop the molten zinc and the item passes through the flux immediately prior to galvanizing.
Galvanizing
The article is immersed in a bath of molten zinc between 815-850 F (435-455 C). During galvanizing, the zinc metallurgically bonds to the steel, creating a series of highly abrasion-resistant zinc-iron alloy layers, commonly topped by a layer of impact-resistant pure zinc.
Finishing
After the steel is withdrawn from the galvanizing bath, excess zinc is removed by draining, vibrating or—for small items—centrifuging. The galvanized item is then air-cooled or quenched in liquid.
Inspection
Coating-thickness and surface-condition inspections complete the process. The galvanizing process has existed for more than 250 years and has been a mainstay of North American industry since the 1890s. Galvanizing is used throughout various markets to provide steel with unmatched protection from the ravages of corrosion. A wide range of steel products from nails to highway guardrail to the Brooklyn Bridge’s suspension wires to NASA’s launch pad sound suppression system benefit from galvanizing’s superior corrosion protection properties.

Characteristics of Zinc
Galvanizing’s primary component is zinc. This vital metal is silvery, blue-gray in color, makes up an estimated 0.004% of the Earth’s crust, and ranks 27th in order of abundance. It is essential for the growth and development of almost all life: Between 1.4 and 2.3 grams of zinc are found in the average adult, and the World Health Organization has recommended a daily intake of 15 milligrams. Numerous consumer products including cold remedies, sunscreens, diaper creams, and nutritional supplements contain beneficial amounts of zinc, primarily in the form of zinc oxide.
To the eye, galvanized steel is blue-gray, but it is also “green.” The zinc and galvanizing industries work to promote sustainable development by enhancing zinc’s contribution to society and ensuring its production and use are in harmony with the natural environment and the needs of society, now and in the future. Zinc, as it is used in galvanizing, is a healthy metal, completely recyclable. The energy used to melt zinc is inversely related to the amount of zinc recycled. Galvanizing delivers incredible value in terms of protecting our infrastructure. Less steel is consumed and fewer raw materials are needed because galvanizing makes bridges, roads, buildings, etc., last longer. Over time, galvanizing helps maintain steel fabrications’ structural integrity: galvanized structures are safer. Additionally, because galvanized steel requires no maintenance for decades, its use in public construction is an efficient use of our taxes. Selecting galvanized steel for private projects makes a significant contribution to a company’s profitability.

Benefits of Galvanizing Metal Parts
Galvanization helps to extend the life of steel parts by providing a barrier between the steel and the atmosphere, preventing iron oxide from forming on the surface of the steel. Galvanization also provides superior corrosion resistance to parts exposed to the environment.
Galvanization provides a cost-effective solution for coating steel parts, Iron parts specifically those that will receive significant environmental exposure over their lifetime.
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