Galvannealed

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

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Galvannealed steel is a matte uniform grey color, which can be easily painted. In comparison to galvanized steel the coating is harder, and more brittle.

Production and properties

Production of galvannealed sheet steel begins with hot dip galvanization of sheet steel. After passing through the galvanizing zinc bath the sheet steel passes through air knives to remove excess zinc, and is then heated in an annealing furnace for several seconds causing iron and zinc layers to diffuse into one another causing the formation of zinc-iron alloy layers at the surface. The annealing step is performed with the strip still hot after the galvanizing step, with the zinc still liquid. [1] The galvanising bath contains slightly over 0.1% aluminium, added to form a layer bonding between the iron and coated zinc. [2] [3] Annealing temperatures are around 500 to 565 °C. [2] Pre-1990 annealing lines used gas-fired heating; post-1990s the use of induction furnaces became common. [1]

Three distinct alloys are identified in the galvannealed surface. From the steel boundary these are named the Gamma (Γ), Zeta (ζ), and Delta (δ) layers, of compositions Fe3Zn10, FeZn10, FeZn13 respectively; resulting in an overall bulk iron content of 9-12%. The layers also contain around 1-4% aluminium. Composition depends primarily on heating time and temperature, limited by the diffusion of the two metals. [2] [3] [1]

The resulting coating has a matte appearance, and is hard and brittle - under further working such as pressing or bending powder is produced from degradation of the coating, together with cracks on the surface. [3] In comparison to a zinc (galvanized) coating galvannealed has better spot weldability, and is paintable, [4] Due to iron present in the surface alloy phase galvanneal develops a reddish patina in moist environments - it is generally used painted. [5] Zinc phosphate coating is a common pre-painting surface treatment. [4]

Galvannealed sheet can also be produced from electroplated zinc steel sheet. [6]

History

Patents relating to Galvannealed wire were obtained by the Keystone Steel and Wire Company (Peoria, Illinois, USA) c. 1923. The company used the name "Galvannealed" as a brand name. [7] The key early patent was US patent No. 1430648 (J.L. Herman, 1922, Peoria, Illinois, USA) "Process of coating and treating materials having an iron base". The patent described the galvannealing process with specific reference to iron wires. [8]

Uses

A major market for galvannealed steel is the automobile industry. [9] In the mid 1980s, the Chrysler Corporation pioneered the use of Galvannealed sheet steels in the manufacture of their vehicles. In the 1990s galvannealled coatings were used by Honda, Toyota and Ford, with hot dip galvanized, electrogalvanized and other coatings (e.g. Zn-Ni) being used by other manufacturers, with variations depending on the part within the car frame, as well as due to local price differences. [10]

Galvannealed steel is the preferred material for use in the construction of permanent debris and linen chute systems. [11]

Related Research Articles

<span class="mw-page-title-main">Galvanization</span> Process of coating steel or iron with zinc to prevent rusting

Galvanization or galvanizing 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 coated by submerging them in a bath of hot, molten zinc.

<span class="mw-page-title-main">Metallurgy</span> Field of science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.

<span class="mw-page-title-main">Rust</span> 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.

<span class="mw-page-title-main">Hot-dip galvanization</span> 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").

<span class="mw-page-title-main">Carbon steel</span> Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

<span class="mw-page-title-main">Hydrogen embrittlement</span> Reduction in ductility of a metal exposed to hydrogen

Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs most notably in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are less susceptible to hydrogen embrittlement.

Plating is a finishing process in which a metal is deposited on a 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.

Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, and rust or scale from ferrous metals, copper, precious metals and aluminum alloys. A solution called pickle liquor, which usually contains acid, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes.

PPGI is pre-painted galvanised iron, also known as pre-coated steel, coil coated steel, color coated steel etc., typically with a hot dip zinc coated steel substrate.

<span class="mw-page-title-main">Chromate conversion coating</span> Chemical treatment of metals

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.

<span class="mw-page-title-main">Electroless nickel-phosphorus plating</span> Chemical-induced nickel coating of a surface

Electroless nickel-phosphorus plating, also referred to as E-nickel, is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt. It is the most common version of electroless nickel plating and is often referred by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.

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.

Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of iron, zinc, or manganese phosphates, to achieve corrosion resistance, lubrication, or as a foundation for subsequent coatings or painting. It is one of the most common types of conversion coating. The process is also called phosphate coating, phosphatization, phosphatizing, or phosphating. It is also known by the trade name Parkerizing, especially when applied to firearms and other military equipment.

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, acronym of "Steel, Electrogalvanized, Cold-rolled, Commercial quality". Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:

<span class="mw-page-title-main">Architectural metals</span>

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.

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.

Mechanical plating, also known as peen plating, mechanical deposition, or impact plating, is a plating process that imparts the coating by cold welding fine metal particles to a workpiece. Mechanical galvanization is the same process, but applies to coatings that are thicker than 0.001 in (0.025 mm). It is commonly used to overcome hydrogen embrittlement problems. Commonly plated workpieces include nails, screws, nuts, washers, stampings, springs, clips, and sintered iron components.

Nickel electroplating is a technique of electroplating a thin layer of nickel onto a metal object. The nickel layer can be decorative, provide corrosion resistance, wear resistance, or used to build up worn or undersized parts for salvage purposes.

Bethanization is a process patented by the Bethlehem Steel Company to protect steel from corrosion by plating it with zinc, a process similar to electrogalvanization. In advertising materials, Bethlehem Steel claimed the process was more effective than hot dip galvanization, the most common means of using zinc to protect steel.

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.

References

  1. 1 2 3 GalvInfoNote 1.3 2014, Production of Galvanneal, pp.2-3.
  2. 1 2 3 GalvInfoNote 1.3 2014, Introduction, p.1.
  3. 1 2 3 GalvInfoNote 1.3 2014, Coating Composition, p.1.
  4. 1 2 GalvInfoNote 1.3 2014, Coating Weldability, Paintability, Formability and Adherence, pp.5-6.
  5. ↑ GalvInfoNote 1.3 2014, Corrosion Performance, p.6.
  6. ↑ "Propertiues of Iron-Zinc Alloy-Electroplated Galvannealed Steel Sheet" (PDF), Nippon Steel Technical Report, no. 63, Oct 1994
  7. ↑ Metal Industry, vol. 21, 1923, p. 387
  8. ↑ Process of coating and treating materials having an iron base (US Patent)
  9. ↑ Llewellyn, David; Hudd, Roger (1998), Steel: Metallurgy and Applications (3 ed.), Butterworth-Heinemann, p. 87, ISBN   0-7506-3757-9
  10. ↑ Davies, Geoffrey (2012), Materials for Automobile Bodies, pp. 280–7, 86
  11. ↑ Buchanan, James (2008), Debris Conveying Equipment, pp. 35–9, 62

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