Stainless steels are notable for their corrosion resistance, which increases with increasing chromium content. Additions of molybdenum increase corrosion resistance in reducing acids and against pitting attack in chloride solutions. Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure. Stainless steel’s resistance to corrosion and staining, low maintenance, and familiar lustre make it an ideal material for many applications where both the strength of steel and corrosion resistance are required.
Stainless steels are rolled into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, surgical instruments, major appliances and as construction material in large buildings, such as the Chrysler Building. As well as, industrial equipment (for example, in paper mills, chemical plants, water treatment), and storage tanks and tankers for chemicals and food products (for example, chemical tankers and road tankers). Stainless steel's corrosion resistance, the ease with which it can be steam cleaned and sterilized and no need for other surface coatings has also influenced its use in commercial kitchens and food processing plants.
Stainless steels do not suffer uniform corrosion, like carbon steel, when exposed to wet environments. Unprotected carbon steel rusts readily when exposed to the combination of air and moisture. The resulting iron oxide surface layer (the rust) is porous and fragile. Since iron oxide occupies a larger volume than the original steel this layer expands and tends to flake and fall away exposing the underlying steel to further attack. In comparison, stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in air and even the small amount of dissolved oxygen in water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal. This film is self-repairing if it is scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade.
However, stainless steels may suffer uniform corrosion when exposed to acidic or basic solutions. Whether a stainless steel corrodes depends on the kind and concentration of acid or base, and the solution temperature. Uniform corrosion is typically easy to avoid because of extensive published corrosion data or easy to perform laboratory testing.
Unfortunately, stainless steels are susceptible to localized corrosion under certain conditions, which need to be recognized and avoided. Such localized corrosion is problematic for stainless steels because it is unexpected and more difficult to predict.
Acidic solutions can be categorized into two general categories, reducing acids such as hydrochloric acid and dilute sulfuric acid, and oxidizing acids such as nitric acid and concentrated sulfuric acid. Increasing chromium and molybdenum contents provide increasing resistance to reducing acids, while increasing chromium and silicon contents provide increasing resistance to oxidizing acids.
Sulfuric acid is the largest tonnage industrial chemical manufactured. At room temperature Type 304 is only resistant up to 3% acid while Type 316 is resistant up to 50oC and up to 20% acid at room temperature. Thus Type 304 is rarely used in contact with sulfuric acid. Type 904 and Alloy 20 are resistant to sulfuric acid at even higher concentrations above room temperature.
All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentration and elevated temperature attack will occur and higher alloy stainless steels are required.  Concentrated sulfuric acid possesses oxidizing characteristics like nitric acid and thus silicon bearing stainless steels also find application.
In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid. As the molecular weight of organic acids increase their corrosivity decreases. Formic acid has the lowest molecular weight and is a strong acid. Type 304 can be used with formic acid though it will tend to discolor the solution. Acetic acid is probably the most commercially important of the organic acids and Type 316 is commonly used for storing and handling acetic acid.
Stainless steels Type 304 and 316 are 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.
Increasing chromium and nickel contents provide increasing resistance.
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.
Localized corrosion can occur in a number of ways, e.g. pitting corrosion, crevice corrosion and stress corrosion cracking. Such localized attack is most common in the presence of chloride ions. Increasing chromium, molybdenum and nitrogen contents provide increasing resistance to localized corrosion and thus increasing chloride levels require more highly alloyed stainless steels. In general, higher chromium, molybdenum and nitrogen contents provide greater resistance to localized corrosion. Design and good fabrication techniques combined with correct alloy selection can prevent such corrosion.
Localized corrosion can be difficult to predict because it is dependent on many factors including:
At elevated temperatures all metals react with hot gases. The most common high temperature gaseous mixture is air, and oxygen is the most reactive component of air. Carbon steel is limited to ~900oF in air. Chromium in stainless steel reacts with oxygen to form a chromium oxide scale which reduces oxygen diffusion into the material. The minimum 10.5% chromium in stainless steels provides resistance to ~1300oF, while 26% chromium provides resistance up to ~2200oF. Type 304, the most common grade of stainless steel with 18% chromium is resistant to ~1600oF. Other gases such as sulfur dioxide, hydrogen sulfide, carbon monoxide, chlorine, etc. also attack stainless steel. Resistance to other gases is dependent on the type of gas, the temperature and the alloying content of the stainless steel.
Like steel, stainless steel is a relatively poor conductor of electricity, with significantly lower electrical conductivity than copper. Other metals in contact with stainless steel, particularly in a damp or acidic environment, may suffer galvanic corrosion even though the stainless metal may be unaffected.
When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off, allowing the parts to weld together. When forcibly disassembled, the welded material may be torn and pitted, a destructive effect known as galling. 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, even mildly acidic environment may act as a 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 provide a film between the two parts and prevent galling. Low-temperature carburizing is another option that virtually eliminates galling and allows the use of similar materials without the risk of corrosion and the need for lubrication.
The corrosion resistance of iron-chromium alloys was first recognized in 1821 by French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in cutlery. 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 1872, the Englishmen Clark and Woods patented an alloy that would today be considered a stainless steel.
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. In 1911, Philip Monnartz reported on the relationship between chromium content and corrosion resistance. On 17 October 1912, Krupp engineers Benno Strauss and Eduard Maurer patented austenitic stainless steel as Nirosta.
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.
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. The metal was later marketed under the "Staybrite" brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in London in 1929. 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.
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". In 1929, before the Great Depression hit, over 25,000 tons of stainless steel were manufactured and sold in the US.
There are five main families, which are primarily classified by their crystalline structure:
Austenitic stainless steel is the largest family of stainless steels, making up about two-thirds of all stainless steel production. They possess an austenitic microstructure, which is a face-centered cubic crystal structure. This microstructure is achieved by alloying with sufficient nickel and/or manganese and nitrogen to maintain an austenitic microstructure at all temperatures from the cryogenic region to the melting point. Thus austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures. Though they can be strengthened by cold working, but this is limited to thin sheet and small diameter bar. Their austenitic microstructure gives them excellent formability and weldability and they are essentially non-magnetic and maintain their ductility at cryogenic temperatures.
Ferritic stainless steels possess a ferrite microstructure like carbon steel, which is a body-centered cubic crystal structure and contain between 10.5% and 27% chromium with very little or no nickel. This microstructure is present at all temperatures, due to the chromium addition, and like austenitic stainless steels are not hardenable by heat treatment. They cannot be strengthened by cold work to the same degree as austenitic stainless steels. They are magnetic like carbon steel. They are problematic to weld due to grain growth in the heat affected zone which reduces ductility and may result in cracks. Increasing chromium and molybdenum contents increase corrosion resistance as it does for austenitic stainless steels, however, this high alloying results in the precipitation of embrittling intermetallic phase upon welding. These microstructural problems due to welding restrict the use of ferritic stainless steels to very thin thicknesses and thus are not used in the construction of large heavy walled vessels and tanks, and structures like austenitic stainless steels.
Martensitic stainless steels are hardened by heat treatment, which is developed after quenching from an elevated temperature. They possess a body-centered tetragonal crystal structure. They are magnetic. Martensitic stainless steels contain chromium (12–16%), molybdenum (0.2–1%), nickel (less than 2%), and carbon (about 0.1–1%). They are not as corrosion resistant as the common ferritic and austenitic stainless steels due to their low chromium content. Their high carbon content enables them to be significantly hardened and thus they are used for knives, razor blades, cutlery and tools. Common grades are Types 410, 420 and 440C.
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. They are characterized by high chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the strength compared to austenitic stainless steels. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steels Types 304 and 316.
Precipitation hardening 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.
The designation "CRES" is used in various industries to refer to corrosion-resistant steel. Most mentions of CRES refer to stainless steel, although the correspondence is not absolute, because there are other materials that are corrosion-resistant but not stainless steel.
Steel no. k.h.s DIN
|1.4845||X 5 CrNi 2520||310S||S31008|
|1.4408||G-X 6 CrNiMo 18-10||316||S31600|
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.
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 still retain their original appearance. Stainless steel is used today in building construction because of its durability and because it is a weldable building metal that can be made into aesthetically pleasing shapes. An example of a building in which these properties are exploited is the Art Gallery of Alberta in Edmonton, which is wrapped in stainless steel.
The Parliament House of Australia in Canberra has a stainless steel flagpole weighing over 220 metric tons (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.
The Helix Bridge is a pedestrian bridge linking Marina Centre with Marina South in the Marina Bay area in Singapore.
Stainless steel is a modern trend for roofing material for airports due to its low glare reflectance to keep pilots from being blinded, also for its properties that allow thermal reflectance in order to keep the surface of the roof close to ambient temperature. The Hamad International Airport in Qatar was built with all stainless steel roofing for these reasons, as well as the Sacramento International Airport in California.
Stainless steels have a long history of application in contact with water due to their excellent corrosion resistance. Applications include a range of conditions from plumbing, potable and waste water treatment to desalination. Types 304 and 316 stainless steels are standard materials of construction in contact with water. However, with increasing chloride contents higher alloyed stainless steels such as Type 2205 and super austenitic and super duplex stainless steels are utilized.
Important considerations to achieve optimum corrosion performance are:
Stainless steels are used extensively in the Pulp and Paper industry for two primary reasons, to avoid iron contamination of the product and their corrosion resistance to the various chemicals used in the paper making process.
A wide range stainless steels are used throughout the paper making process. For example, duplex stainless steels are being used in digesters to convert wood chips into wood pulp. 6% Mo superaustenitics are used in the bleach plant and Type 316 is used extensively in the paper machine.
Stainless steels are used extensively in these industries for their corrosion resistance to both aqueous, gaseous and high temperature environments, their mechanical properties at all temperatures from cryogenic to the very high, and occasionally for other special physical properties.
Austenitic (300 series) stainless steel, in particular Type 304 and 316, is the material of choice for the Food & Beverage industry. Stainless steels do not affect the taste of the product, they are easily cleaned and sterilized to prevent bacterial contamination of the food, and they are durable.
Stainless steels are used extensively in:
Acidic foods with high salt additions, such as tomato sauce, and highly salted condiments, such as soya sauce may require higher alloyed stainless steels such as 6% Mo superaustenitics to prevent pitting corrosion by chloride.
The Allegheny Ludlum Corporation worked with Ford on various concept cars with stainless steel bodies from the 1930s through the 1970s to demonstrate 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 Type-304 stainless steel body panels over a glass-reinforced plastic monocoque. 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.
Some automotive manufacturers use stainless steel as decorative highlights in their vehicles.
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.
Budd also built two airplanes, the Budd BB-1 Pioneer and the Budd RB-1 Conestoga, of stainless steel tube and sheet. The first, which had fabric wing coverings, is on display at the Franklin Institute, being the longest continuous display of an aircraft ever, since 1934. The RB-2 Was almost all stainless steel, save for the control surfaces. One survives at the Pima Air & Space Museum, adjacent to Davis–Monthan Air Force Base.
Due to its thermal stability, the Bristol Aeroplane Company built the all-stainless steel Bristol 188 high-speed research aircraft, which first flew in 1963. However, the practical problems encountered meant that Concorde employed aluminium alloys.
The use of stainless steel in mainstream aircraft is hindered by its excessive weight compared to other materials, such as aluminium.
Surgical tools and medical equipment are usually made of stainless steel, because of its durability and ability to be sterilized in an autoclave. In addition, surgical implants such as bone reinforcements and replacements (e.g. hip sockets and cranial plates) are made with special alloys formulated to resist corrosion, mechanical wear, and biological reactions in vivo.
Stainless steel is used in a variety of applications in dentistry. It is common to use stainless steel in many instruments that need to be sterilized, such as needles, endodontic files in root canal therapy, metal posts in root canal–treated teeth, temporary crowns and crowns for deciduous teeth, and arch wires and brackets in orthodontics. The surgical stainless steel alloys (e.g., 316 low-carbon steel) have also been used in some of the early dental implants.
Stainless steel is often preferred for kitchen sinks because of its ruggedness, durability, heat resistance, and ease of cleaning. In better models, acoustic noise is controlled by applying resilient undercoating to dampen vibrations. The material is also used for cladding of surfaces such as appliances and backsplashes.
Cookware and bakeware may be clad in stainless steels, to enhance their cleanability and durability, and to permit their use in induction cooking (this requires a magnetic grade of stainless steel, such as 432). Because stainless steel is a poor conductor of heat, it is often used as a thin surface cladding over a core of copper or aluminium, which conduct heat more readily.
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.
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-plated to give it a gold tone. The gold tone variety is known as Sun-lite Valadium. Other "Valadium" types of alloy are trade-named differently, with such names as "Siladium" and "White Lazon".
Some firearms incorporate stainless steel components as an alternative to blued or parkerized steel. Some handgun 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 3D printing providers have developed proprietary stainless steel sintering blends for use in rapid prototyping. One of the more popular stainless steel grades used in 3D printing is 316L stainless steel. Due to the high temperature gradient and fast rate of solidification, stainless steel products manufactured via 3D printing tend to have a more refined microstructure; this in turn results in better mechanical properties. However, stainless steel is not used as much as materials like Ti6Al4V in the 3D printing industry; this is because manufacturing stainless steel products via traditional methods is currently much more economically competitive.
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. According to the International 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.
Stainless steel nanoparticles have been produced in the laboratory. This synthesis uses oxidative Kirkendall diffusion to build a thin protective barrier which prevent further oxidation. These may have applications as additives for high performance applications. For examples, sulfurization, phosphorization and nitridation treatments to produce nanoscale stainless steel based catalysts could enhance the electrocatalytic performance of stainless steel for water splitting.
Stainless steel is generally considered to be biologically inert, but some sensitive individuals develop a skin irritation due to a nickel allergy caused by certain alloys.
Stainless steel leaches small amounts of nickel and chromium during cooking.