Galvanization

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A lamppost in Singapore showing the characteristic spangle of hot-dip galvanization Lamppost-singapore.jpg
A lamppost in Singapore showing the characteristic spangle of hot-dip galvanization

Galvanization or galvanizing (also spelled galvanisation or galvanising) [1] is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged in a bath of molten hot zinc.

Contents

Protective action

Galvanizing protects the underlying iron or steel in the following main ways:

History and etymology

Galvanized nails NailsCloseup.jpg
Galvanized nails

The earliest known example of galvanized iron was encountered by Europeans on 17th-century Indian armour in the Royal Armouries Museum collection. [3]

The etymology of galvanisation is via French from the name of Italian scientist Luigi Galvani. However this is an obscure back-formation; Galvani had no involvement in zinc coating.

The earliest use of the term was in late 18th-century scientific research and medical practice by Galvani and meant the stimulation of a muscle by the application of an electric current. Although Galvani was the first to study this, it was Alessandro Volta who then developed a better understanding of its cause and effect. Galvani's explanation of 'animal electricity' as a cause was replaced by Volta's invention of the electric battery and its use to stimulate animal tissue. Despite the superseding of his experimental results, it was Galvani's name rather than Volta's which became associated with the field.

The term "galvanized" continues to be used metaphorically of any stimulus which results in activity by a person or group of people, [4] such as to "galvanize into action" meaning stimulating a complacent person or group to take action.

In modern usage, the term "galvanizing" has largely come to be associated with zinc coatings, to the exclusion of other metals. Galvanic paint, a precursor to hot-dip galvanizing, was patented by Stanislas Sorel, of Paris, on June 10, 1837, as an adoption of a term from a highly fashionable field of contemporary science, despite having no evident relation to it. [5] [6]

Methods

Hot-dip galvanizing deposits a thick, robust layer of zinc iron alloys on the surface of a steel item. In the case of automobile bodies, where additional decorative coatings of paint will be applied, a thinner form of galvanizing is applied by electrogalvanizing. The hot-dip process generally does not reduce strength on a measurable scale,[ citation needed ] with the exception of high-strength steels (>1100 MPa) where hydrogen embrittlement can become a problem. [7] This deficiency is a consideration affecting the manufacture of wire rope and other highly stressed products.

The protection provided by hot-dip galvanizing is insufficient for products that will be constantly exposed to corrosive materials such as acids, including acid rain in outdoor uses. For these applications, more expensive stainless steel is preferred. Some nails made today are galvanized. 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 of 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 practical alternative to electroplating bolts and nuts is not hot-dip zinc coating, but making the fasteners from stainless steel or titanium.

Galvanized surface with visible spangle Feuerverzinktes Gelander.jpg
Galvanized surface with visible spangle

The size of crystallites (grains) 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, even though they are usually present.

Thermal diffusion galvanizing, or Sherardizing, provides a zinc diffusion coating on iron- or copper-based materials. [8] [9] Parts and zinc powder are tumbled in a sealed rotating drum. Around 300 °C (572 °F), zinc will diffuse into the substrate to form a zinc alloy. The advance surface preparation of the goods can be carried out by shot blasting. The process is also known as "dry galvanizing", because no liquids are involved; this can avoid possible problems caused by hydrogen embrittlement. The dull-grey crystal structure of the zinc diffusion coating has a good adhesion to paint, powder coatings, or rubber. It is a preferred method for coating small, complex-shaped metals, and for smoothing rough surfaces on items formed with sintered metal.

Eventual corrosion

Rusted corrugated steel roof Rust 1.jpeg
Rusted corrugated steel roof

Although galvanizing will inhibit attack of the underlying steel, rusting will be inevitable after some decades' exposure to weather, especially if exposed to acidic conditions. 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, primarily through converting the solid zinc to soluble zinc chloride which simply washes away. Galvanized car frames exemplify this; they corrode much faster in cold environments due to road salt, though they will last longer than unprotected steel.

Galvanized steel can last for many decades if other supplementary measures are maintained, such as paint coatings and additional sacrificial anodes. The rate of corrosion in non-salty environments is caused mainly by levels of sulfur dioxide in the air. [10]

Galvanized construction steel

This is the most common use for galvanized metal, and hundreds of thousands of tons of steel products are galvanized annually worldwide. In developed countries most larger cities have several galvanizing factories, and many items of steel manufacture are galvanized for protection. Typically these include: street furniture, building frameworks, balconies, verandahs, staircases, ladders, walkways, and more. Hot dip galvanized steel is also used for making steel frames as a basic construction material for steel frame buildings. [11]

Galvanized piping

In the early 20th century, galvanized piping replaced previously-used cast iron and lead in cold-water plumbing. Typically, galvanized piping rusts from the inside out, building up layers of plaque 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 galvanized piping is about 40–50 years, [12] but it may vary on how well the pipes were built and installed. Pipe longevity also depends on the thickness of zinc in the original galvanizing, which ranges on a scale from G01 to G360, [13] and whether the pipe was galvanized on both the inside and outside, or just the outside.

Since World War 2, copper and plastic piping have replaced galvanized piping for interior drinking water service, but galvanized steel pipes are still used in outdoor applications requiring steel's superior mechanical strength. The use of galvanized pipes 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.[ citation needed ]

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[ which? ] 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.[ citation needed ]

See also

Related Research Articles

Rust Type of iron oxide

Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.

Corrosion Gradual destruction of materials by chemical reaction with its environment

Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, carbonate or sulfide. It is the gradual destruction of materials by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.

Galvanic anode Main component of cathodic protection

A galvanic anode, or sacrificial anode, is the main component of a galvanic cathodic protection (CP) system used to protect buried or submerged metal structures from corrosion.

Hot-dip galvanization Process of coating iron or steel with molten zinc

Hot-dip galvanization is a form of galvanization. It is the process of coating iron and steel with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around 450 °C (842 °F). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material that protects the steel underneath from further corrosion in many circumstances. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and is considered superior in terms of cost and life-cycle. It can be identified by the crystallization patterning on the surface (often called a "spangle").

Cathodic protection Corrosion prevention technique

Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrificial metal" to act as the anode. The sacrificial metal then corrodes instead of the protected metal. For structures such as long pipelines, where passive galvanic cathodic protection is not adequate, an external DC electrical power source is used to provide sufficient current.

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.

A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that comes into contact with the fluid. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime. Corrosion inhibitors are common in industry, and also found in over-the-counter products, typically in spray form in combination with a lubricant and sometimes a penetrating oil. They may be added to water to prevent leaching of lead or copper from pipes.

Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, modify electrical conductivity, remove burrs and other surface flaws, and control the surface friction. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item. An unfinished surface is often called mill finish.

Chromate conversion coating

Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives, as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack on soft metals.

Sherardising is a process of galvanization of ferrous metal surfaces, also called vapour galvanising and dry galvanizing. The process is named after British metallurgist Sherard Osborn Cowper-Coles who invented and patented the method c. 1900. This process involves heating the steel parts up to c. 500 °C in a closed rotating drum that contains metallic zinc dust and possibly an inert filler, such as sand. At temperatures above 300 °C, zinc evaporates and diffuses into the steel substrate forming diffusion bonded Zn-Fe-phases.

The salt spray test is a standardized and popular corrosion test method, used to check corrosion resistance of materials and surface coatings. Usually, the materials to be tested are metallic and finished with a surface coating which is intended to provide a degree of corrosion protection to the underlying metal.

Aluminized steel

Aluminized steel is steel that has been hot-dip coated on both sides with aluminium-silicon alloy. This process assures a tight metallurgical bond between the steel sheet and its aluminium coating, producing a material with a unique combination of properties possessed neither by steel nor by aluminium alone. Aluminized steel shows a better behavior against corrosion and keeps the properties of the base material steel for temperature lower than 800 °C (1,470 °F). For example, it is commonly used for heat exchangers in residential furnaces, commercial rooftop HVAC units, automotive mufflers, ovens, kitchen ranges, water heaters, fireplaces, barbecue burners, and baking pans. This steel is very useful for heating things up because it transfers heat faster than most other steels.

Electrogalvanizing is a process in which a layer of zinc is bonded to steel in order to protect against corrosion. The process involves electroplating, running a current of electricity through a saline/zinc solution with a zinc anode and steel conductor. Such Zinc electroplating or Zinc alloy electroplating maintains a dominant position among other electroplating process options, based upon electroplated tonnage per annum. According to the International Zinc Association, more than 5 million tons are used yearly for both hot dip galvanizing and electroplating. The plating of zinc was developed at the beginning of the 20th century. At that time, the electrolyte was cyanide based. A significant innovation occurred in the 1960s, with the introduction of the first acid chloride based electrolyte. The 1980s saw a return to alkaline electrolytes, only this time, without the use of cyanide. The most commonly used electrogalvanized cold rolled steel is SECC steel. Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:

Galvannealed or galvanneal is the result from the processes of galvanizing followed by annealing of sheet steel.

Architectural metals

Metals used for architectural purposes include lead, for water pipes, roofing, and windows; tin, formed into tinplate; zinc, copper and aluminium, in a range of applications including roofing and decoration; and iron, which has structural and other uses in the form of cast iron or wrought iron, or made into steel. Metal alloys used in building include bronze ; brass ; monel metal and nickel silver, mainly consisting of nickel and copper; and stainless steel, with important components of nickel and chromium.

Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for appearance, and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.

Corrosion engineering is an engineering specialty that applies scientific, technical, engineering skills, and knowledge of natural laws and physical resources to design and implement materials, structures, devices, systems, and procedures to manage corrosion.

Low hydrogen annealing, commonly known as "baking" is a heat treatment in metallurgy for the reduction or elimination of hydrogen in a material to prevent hydrogen embrittlement.Hydrogen embrittlement is the hydrogen-induced cracking of metals, particularly steel which results in degraded mechanical properties such as plasticity, ductility and fracture toughness at low temperature. Low hydrogen annealing is called a de-embrittlement process. Low hydrogen annealing is an effective method compared to alternatives such as electroplating the material with zinc to provide a barrier for hydrogen ingress which results in coating defects.

Zinc flake coatings are non-electrolytically applied coatings, which provide good protection against corrosion. These coatings consist of a mixture of zinc and aluminium flakes, which are bonded together by an inorganic matrix.

Galvanic corrosion Electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another

Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices.

References

  1. "Galvanize". Cambridge English Dictionary . Retrieved 10 November 2019.
  2. Magalhães, A. A. O; Margarit, I. C. P; Mattos, O. R (1999-07-31). "Electrochemical characterization of chromate coatings on galvanized steel". Electrochimica Acta. 44 (24): 4281–4287. doi:10.1016/S0013-4686(99)00143-7. ISSN   0013-4686.
  3. Summary of XRF analysis conducted on or about 30 September 1999 by the Royal Armouries Museum in Leeds and written up as part of a thesis by Helen Bowstead Stallybrass at the Department of Archaeological Sciences, Bradford University.
  4. James A. H. Murray; et al., eds. (1989). The Oxford English Dictionary . VI (2 ed.). Oxford University Press. p.  340. ISBN   0 19 861218-4.
  5. Process for protecting articles made of Iron or Steel from oxidation." Specification of patent granted to M. Sorel, of Paris, France, December, 1837. Journal of the Franklin Institute (Philadelphia, Pa.), Published by Pergamon Press, 1838, via Google Book Search.
  6. via the INPI database.
  7. "Steel Selection". American Galvanizers Association. Retrieved 3 April 2015.
  8. Porter, Frank C. (1991). Zinc Handbook. CRC Press. ISBN   978-0-8247-8340-2.
  9. Natrup, F.; Graf, W. (21 November 2014). "20 - Sherardizing: corrosion protection of steels by zinc diffusion coatings". In Mittemeijer, Eric J.; Somers, Marcel A. J. (eds.). Thermochemical Surface Engineering of Steels: Improving Materials Performance. Elsevier Science. p. 737. ISBN   978-0-85709-652-4.
  10. "Atmospheric Resistance". Galvanising Association (UK). Archived from the original on 2014-02-22.
  11. marshall (2019-10-11). "Galvanized Steel: Types, Uses, Benefits". National Material Company - Steel Processing Facilities. Retrieved 2021-02-06.
  12. Biard & Crockett (2016-05-16). "How Long Will My Galvanized Pipes Last?". Biard & Crockett. Retrieved 2021-02-06.
  13. American Galvanizers Association. "Zinc Coatings" (PDF). courtgalvanizinginc.com.