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ancient history

Ancient history is the aggregate of past events^[1] from the beginning of recorded human history to the Early Middle Ages or the Postclassical Era. The span of recorded history is roughly 5,000 years, beginning with Sumerian Cuneiform script, the oldest discovered form of coherent writing from the protoliterate period around the 30th century BC.^[2]

The term classical antiquity is often used to refer to history in the Old World from the beginning of recorded Greek history in 776 BC (First Olympiad). This roughly coincides with the traditional date of the founding of Rome in 753 BC, the beginning of the history ofancient Rome, and the beginning of the Archaic period in Ancient Greece. Although the ending date of ancient history is disputed, some Western scholars use the fall of the Western Roman Empire in 476 AD,^[3][4] the closure of the Platonic Academy in 529 AD,^[5]the death of the emperor Justinian I,^[6] the coming of Islam^[7] or the rise of Charlemagne^[8]as the end of ancient and Classical European history.

In India, the period includes the early period of the Middle Kingdoms,^[9][10][11] and, in China, the time up to the Qin Dynasty is included.^[12][13]

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stainless steel

A few corrosion-resistant iron artifacts survive from antiquity. A famous example is the Iron Pillar of Delhi, erected by order of Kumara Gupta I around AD 400. Unlike stainless steel, however, these artifacts owe their durability not to chromium but to their high phosphorus content, which, together with favorable local weather conditions, promotes the formation of a solid protective passivation layerof iron oxides and phosphates, rather than the non-protective cracked rust layer that develops on most ironwork.

The corrosion resistance of iron-chromium alloys was first recognized in 1821 by French metallurgistPierre Berthier, who noted their resistance against attack by some acids and suggested their use incutlery. Metallurgists of the 19th century were unable to produce the combination of low carbon and high chromium found in most modern stainless steels, and the high-chromium alloys they could produce were too brittle to be practical.

In the late 1890s Hans Goldschmidt of Germany developed an aluminothermic (thermite) process for producing carbon-free chromium. Between 1904 and 1911 several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel.

Friedrich Krupp Germaniawerft built the 366-ton sailing yacht Germania featuring a chrome-nickel steel hull in Germany in 1908.^[5] In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On 17 October 1912, Kruppengineers Benno Strauss and Eduard Maurer patented austenitic stainless steel as Nirosta.^[6]

Similar developments were taking place contemporaneously in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless steel. In 1912, Elwood Haynes applied for a US patent on a martensitic stainless steel alloy, which was not granted until 1919.^[7]

Also in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times.^[4] The metal was later marketed under the 'Staybrite' brand byFirth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929.^[8] Brearley applied for a US patent during 1915 only to find that Haynes had already registered a patent. Brearley and Haynes pooled their funding and with a group of investors formed the American Stainless Steel Corporation, with headquarters in Pittsburgh, Pennsylvania.^[9]

In the beginning stainless steel was sold in the US under different brand names like 'Allegheny metal' and 'Nirosta steel'. Even within the metallurgy industry the eventual name remained unsettled; in 1921 one trade journal was calling it "unstainable steel".^[10] In 1929, before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.^[11]

Properties[edit]

Oxidation[edit]

High oxidation resistance in air at ambient temperature is normally achieved with additions of a minimum of 13% (by weight)chromium, and up to 26% is used for harsh environments.^[12] The chromium forms a passivation layer of chromium(III) oxide (Cr₂O₃) when exposed to oxygen. The layer is too thin to be visible, and the metal remains lustrous and smooth. The layer is impervious towater and air, protecting the metal beneath, and this layer quickly reforms when the surface is scratched. This phenomenon is calledpassivation and is seen in other metals, such as aluminium and titanium. Corrosion resistance can be adversely affected if the component is used in a non-oxygenated environment, a typical example being underwater keel bolts buried in timber.

When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off, allowing the parts to weldtogether. When forcibly disassembled, the welded material may be torn and pitted, an effect known as galling. This destructive galling can be avoided by the use of dissimilar materials for the parts forced together, for example bronze and stainless steel, or even different types of stainless steels (martensitic against austenitic). However, two different alloys electrically connected in a humid environment may act as Voltaic pile and corrode faster. Nitronic alloys made by selective alloying with manganese and nitrogen may have a reduced tendency to gall. Additionally, threaded joints may be lubricated to prevent galling.

Acids[edit]

Stainless steel is generally highly resistant to attack from acids, but this quality depends on the kind and concentration of the acid, the surrounding temperature, and the type of steel. Type 304 is resistant to sulfuric acid at room temperature, even in high concentrations, but type 316 and 317 are only resistant at low concentrations. All types of stainless steel resist attack from phosphoric acid, 316 and 317 more so than 304; and Types 304L and 430 have been successfully used with nitric acid. Hydrochloric acid will damage any kind of stainless steel, and should be avoided.^[13]

Bases[edit]

The 300 series of stainless steel grades is unaffected by any of the weak bases such as ammonium hydroxide, even in high concentrations and at high temperatures. The same grades of stainless exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some etching and cracking, especially with solutions containingchlorides.^[13]

Organics[edit]

Types 316 and 317 are both useful for storing and handling acetic acid, especially in solutions where it is combined with formic acidand when aeration is not present (oxygen helps protect stainless steel under such conditions), though 317 provides the greatest level of resistance to corrosion. Type 304 is also commonly used with formic acid though it will tend to discolor the solution. All grades resist damage from aldehydes and amines, though in the latter case grade 316 is preferable to 304; cellulose acetate will damage 304 unless the temperature is kept low. Fats and fatty acids only affect grade 304 at temperatures above 150 °C (302 °F), and grade 316 above 260 °C (500 °F), while 317 is unaffected at all temperatures. Type 316L is required for processing of urea.^[13]

Electricity and magnetism[edit]

Similarly to steel, stainless steel is a relatively poor conductor of electricity, with a lower electrical conductivity than that of copper.

Ferritic and martensitic stainless steels are magnetic. Austenitic stainless steels are non-magnetic.

Applications[edit]

The 630-foot-high (190 m), stainless-clad (type 304) Gateway Archdefines St. Louis'sskyline The pinnacle of New York's Chrysler Building is clad with Nirostastainless steel, a form of Type 302[14][15] An art deco sculpture on the Niagara-Mohawk Power building inSyracuse, New York

Stainless steel’s resistance to corrosion and staining, low maintenance and familiar lustre make it an ideal material for many applications. There are over 150 grades of stainless steel, of which fifteen are most commonly used. The alloy is milled into coils, sheets, plates, bars, wire, and tubing to be used in cookware,cutlery, household hardware, surgical instruments, major appliances, industrial equipment (for example, insugar refineries) and as an automotive and aerospace structural alloy and construction material in large buildings. Storage tanks and tankers used to transport orange juice and other food are often made of stainless steel, because of its corrosion resistance. This also influences its use in commercial kitchens and food processing plants, as it can be steam-cleaned and sterilized and does not need paint or other surface finishes.

Stainless steel is used for jewelry and watches with 316L being the type commonly used for such applications. It can be re-finished by any jeweler and will not oxidize or turn black.

Some firearms incorporate stainless steel components as an alternative to blued or parkerized steel. Somehandgun models, such as the Smith & Wesson Model 60 and the Colt M1911 pistol, can be made entirely from stainless steel. This gives a high-luster finish similar in appearance to nickel plating. Unlike plating, the finish is not subject to flaking, peeling, wear-off from rubbing (as when repeatedly removed from a holster), or rust when scratched.

Some automotive manufacturers use stainless steel as decorative highlights in their vehicles.

Architecture[edit]

Stainless steel is used for buildings for both practical and aesthetic reasons. Stainless steel was in vogue during the art deco period. The most famous example of this is the upper portion of the Chrysler Building(pictured). Some diners and fast-food restaurants use large ornamental panels and stainless fixtures and furniture. Because of the durability of the material, many of these buildings retain their original appearance.

Type 316 stainless is used on the exterior of both the Petronas Twin Towers and the Jin Mao Building, two of the world's tallest skyscrapers.^[15]

The Parliament House of Australia in Canberra has a stainless steel flagpole weighing over 220 tonnes (240 short tons).

The aeration building in the Edmonton Composting Facility, the size of 14 hockey rinks, is the largest stainless steel building in North America.

Bridges[edit]

• Cala Galdana Bridge in Minorca (Spain) was the first stainless steel road bridge.
• Sant Fruitos Pedestrian Bridge (Catalonia, Spain), arch pedestrian bridge.
• Padre Arrupe Bridge (Bilbao, Spain) links the Guggenheim museum to the University of Deusto.^[16]

Monuments and sculptures[edit]

• The Unisphere, constructed as the theme symbol of the 1964-5 World's Fair in New York City, is constructed of Type 304L stainless steel as a sphere with a diameter of 120 feet, or 36.57 meters.
• The Gateway Arch (pictured) is clad entirely in stainless steel: 886 tons (804 metric tonnes) of 0.25 in (6.4 mm) plate, #3 finish, type 304 stainless steel.^[17]
• The United States Air Force Memorial has an austenitic stainless steel structural skin.
• The Atomium in Brussels, Belgium was renovated with stainless-steel cladding in a renovation completed in 2006; previously the spheres and tubes of the structure were clad in aluminium.
• The Cloud Gate sculpture by Anish Kapoor, in Chicago US.
• The Sibelius monument in Helsinki, Finland, is made entirely of stainless steel tubes.

Other[edit]

Automotive bodies

The Allegheny Ludlum Corporation worked with Ford on various concept cars with stainless steel bodies from the 1930s through the 1970s, as demonstrations of the material's potential. The 1957 and 1958 Cadillac Eldorado Brougham had a stainless steel roof. In 1981 and 1982, the DeLorean DMC-12 production automobile used stainless steel body panels over a glass-reinforced plasticmonocoque. Intercity buses made by Motor Coach Industries are partially made of stainless steel. The aft body panel of the Porsche Cayman model (2-door coupe hatchback) is made of stainless steel. It was discovered during early body prototyping that conventional steel could not be formed without cracking (due to the many curves and angles in that automobile). Thus, Porsche was forced to use stainless steel on the Cayman.

Passenger rail cars

Rail cars have commonly been manufactured using corrugated stainless steel panels (for additional structural strength). This was particularly popular during the 1960s and 1970s, but has since declined. One notable example was the early Pioneer Zephyr. Notable former manufacturers of stainless steel rolling stock included the Budd Company (USA), which has been licensed to Japan's Tokyu Car Corporation, and the Portuguese company Sorefame. Many railcars in the United States are still manufactured with stainless steel, unlike other countries who have shifted away.

Aircraft

Budd also built an airplane, the Budd BB-1 Pioneer, of stainless steel tube and sheet, which is on display at the Franklin Institute.

The American Fleetwings Sea Bird amphibious aircraft of 1936 was also built using a spot-welded stainless steel hull.

The Bristol Aeroplane Company built the all-stainless steel Bristol 188 high-speed research aircraft, which first flew in 1963.

The use of stainless steel in mainstream aircraft is hindered by its excessive weight compared to other materials, such as aluminum.

Jewelry

Valadium, a stainless steel and 12% nickel alloy is used to make class and military rings. Valadium is usually silver-toned, but can be electro-charged to give it a gold tone. The gold tone variety is known as Sun-lite Valadium.^[18]

Maintenance of stainless steel[edit]

If treated or stored incorrectly, any grade of stainless steel may discolor or stain. To maintain optimum appearance, the surface should be cared for regularly.

Maintenance during installation[edit]

The quality of installation affects the durability and lifespan of stainless steel.^[19] Therefore it is important to make sure stainless steel is in good condition before installation. Normally, giving it a quick clean is enough prior to installation. However, if surface contamination is present, more attention is required. In fields such as aerospace, pharmaceuticals and food handling, an extremely high standard of cleanliness may be required so extra care should be taken.

Routine maintenance[edit]

Maintenance is required to maintain the quality and appearance of steel. Depending on the environment, it is carried out between one and ten times per year. A proper maintenance routine significantly prolongs the life of stainless steel.^[20]

Tools used for maintenance[edit]

• Soft cloth and water: suitable for cosmetic issues and general cleaning
• Mild detergent: needed if stains cannot be easily lifted with water
• Glass cleaner: useful for removing fingerprints and similar stains

Recycling and reuse[edit]

Stainless steel is 100% recyclable. An average stainless steel object is composed of about 60% recycled material of which approximately 40% originates from end-of-life products and about 60% comes from manufacturing processes.^[21] According to theInternational Resource Panel's Metal Stocks in Society report, the per capita stock of stainless steel in use in society is 80–180 kg in more developed countries and 15 kg in less-developed countries.

There is a secondary market that recycles usable scrap for many stainless steel markets. The product is mostly coil, sheet and blanks. This material is purchased at a less-than-prime price and sold to commercial quality stampers and sheet metal houses. The material may have scratches, pits and dents but is made to the current specifications.

Types of stainless steel[edit]

Pipes and fittings made of stainless steel

There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels virtually non-magnetic and lessbrittle at low temperatures. For greater hardness and strength, more carbon is added. With properheat treatment, these steels are used for such products as razor blades, cutlery, and tools.

Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel, similar to nickel, but at a lower cost.

Stainless steels are also classified by their crystalline structure:

• Austenitic, or 200 and 300 series, stainless steels have an austenitic crystalline structure, which is a face-centered cubic crystal structure. Austenite steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.

• 200 Series—austenitic chromium-nickel-manganese alloys. Type 201 is hardenable through cold working; Type 202 is a general purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.^[22]

• 300 Series—The most widely used austenite steel is the 304, also known as 18/8 for its composition of 18% chromium and 8% nickel.^[23] 304 may be referred to as A2 stainless (not to be confused with A2 grade steel, also named Tool steel, a steel). The second most common austenite steel is the 316 grade, also called marine grade stainless, used primarily for its increased resistance to corrosion. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless, is often used in cutlery and high-quality cookware. 18/0 is also available.

Superaustenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion because of high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus the 300 series. The higher alloy content of superaustenitic steels makes them more expensive. Other steels can offer similar performance at lower cost and are preferred in certain applications, for example ASTM A387 is used in pressure vessels but is a low-alloy carbon steel with a chromium content of 0.5% to 9%.^[24] Low-carbon versions, for example 316L or 304L, are used to avoid corrosion problems caused by welding. Grade 316LVM is preferred wherebiocompatibility is required (such as body implants and piercings).^[25] The "L" means that the carbon content of the alloy is below 0.03%, which reduces the sensitization effect (precipitation of chromium carbides at grain boundaries) caused by the high temperatures involved in welding.

• Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, because of the lower chromium and nickel content. They are also usually less expensive. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni. These alloys can be degraded by the presence of  chromium, an intermetallic phase which can precipitate upon welding.

Swiss Army knives are made of martensitic stainless steel.

• Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machinable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12–14%), molybdenum (0.2–1%), nickel (less than 2%), and carbon (about 0.1–1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic.

• Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, uses about 17% chromium and 4% nickel.

• Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Duplex stainless steels have roughly twice the strength compared to austenitic stainless steels and also improved resistance to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterized by high chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.

The properties of duplex stainless steels are achieved with an overall lower alloy content than similar-performing super-austenitic grades, making their use cost-effective for many applications. Duplex grades are characterized into groups based on their alloy content and corrosion resistance.

• Lean duplex refers to grades such as UNS S32101 (LDX 2101), S32304, and S32003.
• Standard duplex is 22% chromium with UNS S31803/S32205 known as 2205 being the most widely used.
• Super duplex is by definition a duplex stainless steel with a Pitting Resistance Equivalent Number (PREN) > 40, where PREN = %Cr + 3.3x(%Mo + 0.5x%W) + 16x%N. Usually super duplex grades have 25% chromium or more and some common examples are S32760 (Zeron 100 via Rolled Alloys), S32750 (2507) and S32550 (Ferralium),.
• Hyper duplex refers to duplex grades with a PRE > 48 and at the moment only UNS S32707 and S33207 are available on the market.

Comparison of standardized steels[edit]

More Stainless Steel Datasheets are listed at the following reference.^[26]

Stainless steel grades[edit]

There are a number of systems for grading stainless and other steels. The article on US SAE steel grades details a large number of grades with their properties.

Stainless steel in 3D printing[edit]

Some 3D printing providers have developed proprietary stainless steel sintering^[27] blends for use in rapid prototyping. Currently available grades do not vary significantly in their properties.^[28]

Stainless steel finishes[edit]

316L stainless steel, with an unpolished, mill finish

Standard mill finishes can be applied to flat rolled stainless steel directly by the rollers and by mechanical abrasives. Steel is first rolled to size and thickness and then annealed to change the properties of the final material. Any oxidation that forms on the surface (mill scale) is removed by pickling, and a passivation layer is created on the surface. A final finish can then be applied to achieve the desired aesthetic appearance.

• No. 0: Hot rolled, annealed, thicker plates
• No. 1: Hot rolled, annealed and passivated
• No. 2D: Cold rolled, annealed, pickled and passivated
• No. 2B: Same as above with additional pass-through highly polished rollers
• No. 2BA: Bright annealed (BA or 2R) same as above then bright annealed under oxygen-free atmospheric condition
• No. 3: Coarse abrasive finish applied mechanically
• No. 4: Brushed finish
• No. 5: Satin finish
• No. 6: Matte finish (brushed but smoother than #4)
• No. 7: Reflective finish
• No. 8: Mirror finish
• No. 9: Bead blast finish
• No. 10: Heat colored finish-wide range of electropolished and heat colored surfaces

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stainless steel

n metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is a steel alloy with a minimum of 10.5%^[1] chromium content by mass.

Stainless steel does not readily corrode, rust or stain with water as ordinary steel does, but despite the name it is not fully stain-proof, most notably under low-oxygen, high-salinity, or poor-circulation environments.^[2] There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and corrosion resistance are required.

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ozone

The ozone layer was discovered in 1913 by the French physicists Charles fabry and Henri bussion. Its properties were explored in detail by the British meterologist G.M.B dobson, who developed a simple spectrophoto meter that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The Dobson unit  a  conveninent measure of the amount of ozone over head is named in his houour.

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acid

An acid (from the Latin acidus/ac?re meaning sour^[1]) is a chemical substance whose aqueous solutions are characterized by a sour taste, the ability to turn blue litmus red, and the ability to react with bases and certain metals (like calcium) to form salts. Aqueous solutions of acids have a pH of less than 7. A lower pH means a higher acidity, and thus a higher concentration of positive hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic.

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dehydration property of sulphuric acid

eaction with water and dehydrating property[edit]

Drops of concentrated sulfuric acid rapidly dehydrate a piece of cotton towel.

Because the hydration reaction of sulfuric acid is highly exothermic, dilution should always be performed by adding the acid to the water rather than the water to the acid.^[14] Because the reaction is in an equilibrium that favors the rapid protonation of water, addition of acid to the water ensures that the acid is the limiting reagent. This reaction is best thought of as the formation of hydroniumions:

H
2SO
4 + H
2O ? H
3O+
 + HSO₄^?    K₁ = 2.4×10⁶   (strong acid)

HSO?
4 + H
2O ? H
3O+
 + SO2?
4     K
2 = 1.0×10^?2 [15]

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sulphuric acid

Although nearly 99% sulfuric acid can be made, the subsequent loss of SO
3 at the boiling point brings the concentration to 98.3% acid. The 98% grade is more stable in storage, and is the usual form of what is described as "concentrated sulfuric acid." Other concentrations are used for different purposes. Some common concentrations are:^[9][10]

"Chamber acid" and "tower acid" were the two concentrations of sulfuric acid produced by the lead chamber process, chamber acid being the acid produced in lead chamber itself (<70% to avoid contamination with nitrosylsulfuric acid) and tower acid being the acid recovered from the bottom of the Glover tower.^[9][10] They are now obsolete as commercial concentrations of sulfuric acid, although they may be prepared in the laboratory from concentrated sulfuric acid if needed. In particular, "10M" sulfuric acid (the modern equivalent of chamber acid, used in many titrations) is prepared by slowly adding 98% sulfuric acid to an equal volume of water, with good stirring: the temperature of the mixture can rise to 80 °C (176 °F) or higher.^[10]

Sulfuric acid reacts with its anhydride, SO
3, to form H
2S
2O
7, called pyrosulfuric acid, fuming sulfuric acid, Disulfuric acid or oleumor, less commonly, Nordhausen acid. Concentrations of oleum are either expressed in terms of % SO
3 (called % oleum) or as %H
2SO
4 (the amount made if H
2O were added); common concentrations are 40% oleum (109% H
2SO
4) and 65% oleum (114.6%H
2SO
4). Pure H
2S
2O
7 is a solid with melting point 36 °C.

Pure sulfuric acid has a vapor pressure of <0.001 torr at 25 °C and 1 torr at 145.8 °C,^[11] and 98% sulfuric acid has a <1 mmHg vapor pressure at 40 °C.^[12]

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sulphuric acid

Sulfuric acid is a diprotic acid and shows different properties depending upon its concentration. Its corrosiveness on other materials, like metals, living tissues (e.g. skinand flesh) or even stones, can be mainly ascribed to its strong acidic nature and, if concentrated, strong dehydrating and oxidizing property. Sulfuric acid at a highconcentration can cause very serious damage upon contact, as it not only causeschemical burns via hydrolysis, but also secondary thermal burns via dehydration. It burns the cornea and can lead to permanent blindness if splashed onto eyes. Accordingly, safety precautions should be strictly observed when handling it.

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sulphuric acid

Sulfuric acid (alternative spelling sulphuric acid) is a highly corrosive strong mineral acid with the molecular formula H₂SO₄. It is a pungent-ethereal, colorless to slightly yellow viscous liquid which is soluble in water at all concentrations.^[4] Sometimes, it is dyed dark brown during production to alert people to its hazards.^[5] The historical name of this acid is oil of vitriol.^[6]

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atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by acloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other by chemical bonds based on the same force, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it is positively or negatively charged and is known as an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines thechemical element, and the number of neutrons determines the isotope of the element.^[1]

Chemical atoms, which in science now carry the simple name of "atom," are minuscule objects with diameters of a few tenths of a nanometer and tiny masses proportional to the volume implied by these dimensions. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.94% of an atom's mass is concentrated in the nucleus,^{[note 1]} with protons and neutrons having roughly equal mass. Each element has at least one isotope with an unstable nucleus that can undergoradioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus.^[2] Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emittingphotons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties. 

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chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard and brittle metal^[2] which takes a high polish, resists tarnishing, and has a high melting point. The name of the element is derived from the Greek word ?????, chr?ma, meaning colour,^[3] because many of its compounds are intensely coloured.

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aluminium

Aluminium is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminium serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa.^[7]Aluminium has about one-third the density and stiffness of steel. It is easily machined,cast, drawn and extruded.

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