Stainless steel

Although stainless steel does rust, this only affects the outer few layers of atoms, its chromium content shielding deeper layers from oxidation.

The addition of nitrogen also improves resistance to pitting corrosion and increases mechanical strength.[6] Thus, there are numerous grades of stainless steel with varying chromium and molybdenum contents to suit the environment the alloy must endure.[7] Corrosion resistance can be increased further by the following means:

• increasing chromium content to more than 11%[6]
• adding nickel to at least 8%[6]
• adding molybdenum (which also improves resistance to pitting corrosion)[6]

Stainless steel is strong, about 3 times stronger than ordinary steel. For type AISI 302 cold-rolled, its yield strength (i.e. its elastic limit) is 520 MPa, and its ultimate strength is 860 MPa.

It is also quite hard; its hardness on the Brinell scale is 200, versus 120 for ordinary steel and 15 for pure aluminium. On the Mohs scale its hardness is 8, comparable to that of pure chromium at 8.5 HB, or quartz at 7 HB.

Stainless steel is a steel, and as such its melting point is near that of ordinary steel, and much higher that the melting points of e.g. aluminium or copper.

As with most alloys, the melting point of stainless steel is expressed in the form of a range of temperatures, and not a singular temperature.[8] This temperature range goes from 1,400 to 1,530 °C (2,550 to 2,790 °F; 1,670 to 1,800 K; 3,010 to 3,250 °R)[9] depending on the specific consistency of the alloy in question.

Like steel, stainless steels are relatively poor conductors of electricity, with significantly lower electrical conductivities than copper. In particular, the non-electrical contact resistance (ECR) of stainless steel arises as a result of the dense protective oxide layer and limits its functionality in applications as electrical connectors.[10] Copper alloys and nickel-coated connectors tend to exhibit lower ECR values and are preferred materials for such applications. Nevertheless, stainless steel connectors are employed in situations where ECR poses a lower design criteria and corrosion resistance is required, for example in high temperatures and oxidizing environments.[11]

Martensitic, duplex and ferritic stainless steels are magnetic, while austenitic stainless steel is usually non-magnetic.[12] Ferritic steel owes its magnetism to its body-centered cubic crystal structure, in which iron atoms are arranged in cubes (with one iron atom at each corner) and an additional iron atom in the center. This central iron atom is responsible for ferritic steel's magnetic properties. This arrangement also limits the amount of carbon the steel can absorb to around 0.025%.[13] Grades with low coercive field have been developed for electro-valves used in household appliances and for injection systems in internal combustion engines. Some applications require non-magnetic materials, such as magnetic resonance imaging. Austenitic stainless steels, which are usually non-magnetic, can be made slightly magnetic through work hardening. Sometimes, if austenitic steel is bent or cut, magnetism occurs along the edge of the stainless steel because the crystal structure rearranges itself.[14]

Galling, sometimes called cold welding, is a form of severe adhesive wear, which can occur when two metal surfaces are in relative motion to each other and under heavy pressure. Austenitic stainless steel fasteners are particularly susceptible to thread galling, though other alloys that self-generate a protective oxide surface film, such as aluminum and titanium, are also susceptible. Under high contact-force sliding, this oxide can be deformed, broken, and removed from parts of the component, exposing the bare reactive metal. When the two surfaces are of the same material, these exposed surfaces can easily fuse. Separation of the two surfaces can result in surface tearing and even complete seizure of metal components or fasteners.[16][17] Galling can be mitigated by the use of dissimilar materials (bronze against stainless steel) or using different stainless steels (martensitic against austenitic). Additionally, threaded joints may be lubricated to provide a film between the two parts and prevent galling. Nitronic 60, made by selective alloying with manganese, silicon, and nitrogen, has demonstrated a reduced tendency to gall.[17]

The density of stainless steel ranges from 7,500kg/m³ to 8,000kg/m³ depending on the alloy.[18]

Monument to Harry Brearley at the former Brown Firth Research Laboratory in Sheffield, England

While seeking a corrosion-resistant alloy for gun barrels in 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, discovered and subsequently industrialized a martensitic stainless steel alloy, today known as AISI type 420.[31] The discovery was announced two years later in a January 1915 newspaper article in The New York Times.[19]

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.[34] Brearley applied for a US patent during 1915 only to find that Haynes had already registered one. Brearley and Haynes pooled their funding and, with a group of investors, formed the American Stainless Steel Corporation, with headquarters in Pittsburgh, Pennsylvania.[23]^: 360

Brearley initially called his new alloy "rustless steel". The alloy was sold in the US under different brand names like "Allegheny metal" and "Nirosta steel". Even within the metallurgy industry, the name remained unsettled; in 1921, one trade journal called it "unstainable steel".[35] Brearley worked with a local cutlery manufacturer, who gave it the name "stainless steel".[36] As late as 1932, Ford Motor Company continued calling the alloy "rustless steel" in automobile promotional materials.[37]

In 1929, before the Great Depression, over 25,000 tons of stainless steel were manufactured and sold in the US annually.[38]

Major technological advances in the 1950s and 1960s allowed the production of large tonnages at an affordable cost:

• AOD process (argon oxygen decarburization), for the removal of carbon and sulfur
• Continuous casting and hot strip rolling[39]
• The Z-Mill, or Sendzimir cold rolling mill[40][41]
• The Creusot-Loire Uddeholm (CLU) and related processes which use steam instead of some or all of the argon[42]

Austenitic stainless steel[43][44] is the largest family of stainless steels, making up about two-thirds of all stainless steel production.[45] They possess an austenitic microstructure, which is a face-centered cubic crystal structure.[46] This microstructure is achieved by alloying steel with sufficient nickel and/or manganese and nitrogen to maintain an austenitic microstructure at all temperatures, ranging from the cryogenic region to the melting point.[46] Thus, austenitic stainless steels are not hardenable by heat treatment since they possess the same microstructure at all temperatures.[46]

However, "forming temperature is an essential factor for metastable austenitic stainless steel (M-ASS) products to accommodate microstructures and cryogenic mechanical performance. ... Metastable austenitic stainless steels (M-ASSs) are widely used in manufacturing cryogenic pressure vessels (CPVs), owing to their high cryogenic toughness, ductility, strength, corrosion-resistance, and economy."[47]

Cryogenic cold-forming of austenitic stainless steel is an extension of the heating-quenching-tempering cycle, where the final temperature of the material before full-load use is taken down to a cryogenic temperature range. This can remove residual stresses and improve wear resistance.[48]

Austenitic stainless steel sub-groups, 200 series and 300 series:

• 200 series[49] are chromium-manganese-nickel alloys that maximize the use of manganese and nitrogen to minimize the use of nickel. Due to their nitrogen addition, they possess approximately 50% higher yield strength than 300-series stainless sheets of steel.
  • Type 201 is hardenable through cold working.[50]
  • Type 202 is general-purpose stainless steel. Decreasing nickel content and increasing manganese results in weak corrosion resistance.
• 300 series are chromium-nickel alloys that achieve their austenitic microstructure almost exclusively by nickel alloying; some very highly alloyed grades include some nitrogen to reduce nickel requirements. 300 series is the largest group and the most widely used.
  • Type 304: The most common is type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8% or 10% nickel, respectively.
  • Type 316: The second most common austenitic stainless steel is type 316. The addition of 2% molybdenum provides greater resistance to acids and localized corrosion caused by chloride ions. Low-carbon versions, such as 316L or 304L, have carbon contents below 0.03% and are used to avoid corrosion problems caused by welding.[51]

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, so they are not capable of being hardened by heat treatment. They cannot be strengthened by cold work to the same degree as austenitic stainless steels. They are magnetic. Additions of niobium (Nb), titanium (Ti), and zirconium (Zr) to type 430 allow good weldability. Due to the near-absence of nickel, they are less expensive than austenitic steels and are present in many products, which include:

• Automobile exhaust pipes (type 409 and 409 Cb[lower-alpha 2] are used in North America; stabilized grades type 439 and 441 are used in Europe)[52]
• Architectural and structural applications (type 430, which contains 17% Cr)[53]
• Building components, such as slate hooks, roofing, and chimney ducts
• Power plates in solid oxide fuel cells operating at temperatures around 700 °C (1,300 °F) (high-chromium ferritics containing 22% Cr)[54]

Martensitic stainless steels have a body-centered cubic crystal structure, and offer a wide range of properties and are used as stainless engineering steels, stainless tool steels, and creep-resistant steels. They are magnetic, and not as corrosion-resistant as ferritic and austenitic stainless steels due to their low chromium content. They fall into four categories (with some overlap):[55]

• Fe-Cr-C grades. These were the first grades used and are still widely used in engineering and wear-resistant applications.
• Fe-Cr-Ni-C grades. Some carbon is replaced by nickel. They offer higher toughness and higher corrosion resistance. Grade EN 1.4303 (Casting grade CA6NM) with 13% Cr and 4% Ni is used for most Pelton, Kaplan, and Francis turbines in hydroelectric power plants[56] because it has good casting properties, good weldability and good resistance to cavitation erosion.
• Precipitation hardening grades. Grade EN 1.4542 (also known as 17-4 PH), the best-known grade, combines martensitic hardening and precipitation hardening. It achieves high strength and good toughness and is used in aerospace among other applications.
• Creep-resisting grades. Small additions of niobium, vanadium, boron, and cobalt increase the strength and creep resistance up to about 650 °C (1,200 °F).

Martensitic stainless steels can be heat treated to provide better mechanical properties. The heat treatment typically involves three steps:[57]

1. Austenitizing, in which the steel is heated to a temperature in the range 980–1,050 °C (1,800–1,920 °F), depending on grade. The resulting austenite has a face-centered cubic crystal structure.
2. Quenching. The austenite is transformed into martensite, a hard body-centered tetragonal crystal structure. The quenched martensite is very hard and too brittle for most applications. Some residual austenite may remain.
3. Tempering. Martensite is heated to around 500 °C (930 °F), held at temperature, then air-cooled. Higher tempering temperatures decrease yield strength and ultimate tensile strength but increase the elongation and impact resistance.

Replacing some carbon in martensitic stainless steels by nitrogen is a recent development. The limited solubility of nitrogen is increased by the pressure electroslag refining (PESR) process, in which melting is carried out under high nitrogen pressure. Steel containing up to 0.4% nitrogen has been achieved, leading to higher hardness and strength and higher corrosion resistance. As PESR is expensive, lower but significant nitrogen contents have been achieved using the standard AOD process.[58][59][60][61][62]

Duplex stainless steels have a mixed microstructure of austenite and ferrite, the ideal ratio being a 50:50 mix, though commercial alloys may have ratios of 40:60. They are characterized by higher chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. Duplex stainless steels have roughly twice the yield strength of austenitic stainless steel. Their mixed microstructure provides improved resistance to chloride stress corrosion cracking in comparison to austenitic stainless steel types 304 and 316. Duplex grades are usually divided into three sub-groups based on their corrosion resistance: lean duplex, standard duplex, and super duplex. 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. The pulp and paper industry was one of the first to extensively use duplex stainless steel. Today, the oil and gas industry is the largest user and has pushed for more corrosion resistant grades, leading to the development of super duplex and hyper duplex grades. More recently, the less expensive (and slightly less corrosion-resistant) lean duplex has been developed, chiefly for structural applications in building and construction (concrete reinforcing bars, plates for bridges, coastal works) and in the water industry.

Precipitation hardening stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than other martensitic grades. There are three types of precipitation hardening stainless steels:[63]

• Martensitic 17-4 PH (AISI 630 EN 1.4542) contains about 17% Cr, 4% Ni, 4% Cu, and 0.3% Nb.

Solution treatment at about 1,040 °C (1,900 °F) followed by quenching results in a relatively ductile martensitic structure. Subsequent aging treatment at 475 °C (887 °F) precipitates Nb and Cu-rich phases that increase the strength up to above 1000 MPa yield strength. This outstanding strength level is used in high-tech applications such as aerospace (usually after remelting to eliminate non-metallic inclusions, which increases fatigue life). Another major advantage of this steel is that aging, unlike tempering treatments, is carried out at a temperature that can be applied to (nearly) finished parts without distortion and discoloration.

• Semi-austenitic 17-7 PH (AISI 631 EN 1.4568) contains about 17% Cr, 7.2% Ni, and 1.2% Al.

Typical heat treatment involves solution treatment and quenching. At this point, the structure remains austenitic. Martensitic transformation is then obtained either by a cryogenic treatment at −75 °C (−103 °F) or by severe cold work (over 70% deformation, usually by cold rolling or wire drawing). Aging at 510 °C (950 °F) — which precipitates the Ni₃Al intermetallic phase—is carried out as above on nearly finished parts. Yield stress levels above 1400 MPa are then reached.

• Austenitic A286[64](ASTM 660 EN 1.4980) contains about Cr 15%, Ni 25%, Ti 2.1%, Mo 1.2%, V 1.3%, and B 0.005%.

The structure remains austenitic at all temperatures.

Typical heat treatment involves solution treatment and quenching, followed by aging at 715 °C (1,319 °F). Aging forms Ni₃Ti precipitates and increases the yield strength to about 650 MPa (94 ksi) at room temperature. Unlike the above grades, the mechanical properties and creep resistance of this steel remain very good at temperatures up to 700 °C (1,300 °F). As a result, A286 is classified as an Fe-based superalloy, used in jet engines, gas turbines, and turbo parts.

Uniform corrosion takes place in very aggressive environments, typically where chemicals are produced or heavily used, such as in the pulp and paper industries. The entire surface of the steel is attacked, and the corrosion is expressed as corrosion rate in mm/year (usually less than 0.1 mm/year is acceptable for such cases). Corrosion tables provide guidelines.[68]

This is typically the case when stainless steels are exposed to acidic or basic solutions. Whether 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 easily performed laboratory corrosion testing.

Stainless steel is not completely immune to corrosion, as shown in this desalination equipment.

Acidic solutions can be put 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 content provides increased resistance to reducing acids while increasing chromium and silicon content provides increased resistance to oxidizing acids. Sulfuric acid is one of the most-produced industrial chemicals. At room temperature, type 304 stainless steel is only resistant to 3% acid, while type 316 is resistant to 3% acid up to 50 °C (120 °F) and 20% acid at room temperature. Thus type 304 SS is rarely used in contact with sulfuric acid. Type 904L and Alloy 20 are resistant to sulfuric acid at even higher concentrations above room temperature.[69][70] Concentrated sulfuric acid possesses oxidizing characteristics like nitric acid, and thus silicon-bearing stainless steels are also useful. Hydrochloric acid damages any kind of stainless steel and should be avoided.[1]^: 118[71] All types of stainless steel resist attack from phosphoric acid and nitric acid at room temperature. At high concentrations and elevated temperatures, attack will occur, and higher-alloy stainless steels are required.[72][73] In general, organic acids are less corrosive than mineral acids such as hydrochloric and sulfuric acid.

Type 304 and type 316 stainless steels are unaffected by weak bases such as ammonium hydroxide, even in high concentrations and at high temperatures. The same grades exposed to stronger bases such as sodium hydroxide at high concentrations and high temperatures will likely experience some etching and cracking.[74] Increasing chromium and nickel contents provide increased resistance.

All grades resist damage from aldehydes and amines, though in the latter case type 316 is preferable to type 304; cellulose acetate damages type 304 unless the temperature is kept low. Fats and fatty acids only affect type 304 at temperatures above 150 °C (300 °F) and type 316 SS above 260 °C (500 °F), while type 317 SS is unaffected at all temperatures. Type 316L is required for the processing of urea.[1]

Localized corrosion can occur in several ways, e.g. pitting corrosion and crevice corrosion. These localized attacks are most common in the presence of chloride ions. Higher chloride levels require more highly alloyed stainless steels.

Localized corrosion can be difficult to predict because it is dependent on many factors, including:

• Chloride ion concentration. Even when chloride solution concentration is known, it is still possible for localized corrosion to occur unexpectedly. Chloride ions can become unevenly concentrated in certain areas, such as in crevices (e.g. under gaskets) or on surfaces in vapor spaces due to evaporation and condensation.
• Temperature: increasing temperature increases susceptibility.
• Acidity: increasing acidity increases susceptibility.
• Stagnation: stagnant conditions increase susceptibility.
• Oxidizing species: the presence of oxidizing species, such as ferric and cupric ions, increases susceptibility.

Pitting corrosion is considered the most common form of localized corrosion. The corrosion resistance of stainless steels to pitting corrosion is often expressed by the PREN, obtained through the formula:

${\displaystyle {\text{PREN}}=\%{\text{Cr}}+3.3\cdot \%{\text{Mo}}+16\cdot \%{\text{N}}}$,

where the terms correspond to the proportion of the contents by mass of chromium, molybdenum, and nitrogen in the steel. For example, if the steel consisted of 15% chromium %Cr would be equal to 15.

The higher the PREN, the higher the pitting corrosion resistance. Thus, increasing chromium, molybdenum, and nitrogen contents provide better resistance to pitting corrosion.

Though the PREN of certain steel may be theoretically sufficient to resist pitting corrosion, crevice corrosion can still occur when the poor design has created confined areas (overlapping plates, washer-plate interfaces, etc.) or when deposits form on the material. In these select areas, the PREN may not be high enough for the service conditions. Good design, fabrication techniques, alloy selection, proper operating conditions based on the concentration of active compounds present in the solution causing corrosion, pH, etc. can prevent such corrosion.[75]

Stress corrosion cracking (SCC) is a sudden cracking and failure of a component without deformation. It may occur when three conditions are met:

• The part is stressed (by an applied load or by residual stress).
• The environment is aggressive (high chloride level, temperature above 50 °C (120 °F), presence of H₂S).
• The stainless steel is not sufficiently SCC-resistant.

The SCC mechanism results from the following sequence of events:

1. Pitting occurs.
2. Cracks start from a pit initiation site.
3. Cracks then propagate through the metal in a transgranular or intergranular mode.
4. Failure occurs.

Whereas pitting usually leads to unsightly surfaces and, at worst, to perforation of the stainless sheet, failure by SCC can have severe consequences. It is therefore considered as a special form of corrosion.

As SCC requires several conditions to be met, it can be counteracted with relatively easy measures, including:

• Reducing the stress level (the oil and gas specifications provide requirements for maximal stress level in H₂S-containing environments).
• Assessing the aggressiveness of the environment (high chloride content, temperature above 50 °C (120 °F), etc.).
• Selecting the right type of stainless steel: super austenitic such as grade 904L or super-duplex (ferritic stainless steels and duplex stainless steels are very resistant to SCC).

The nut on the left is not stainless steel and is rusty, unlike the nut on the right.

Galvanic corrosion[76] (also called "dissimilar-metal corrosion") refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. The most common electrolyte is water, ranging from freshwater to seawater. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would alone, while the other becomes the cathode and corrodes slower than it would alone. Stainless steel, due to having a more positive electrode potential than for example carbon steel and aluminium, becomes the cathode, accelerating the corrosion of the anodic metal. An example is the corrosion of aluminium rivets fastening stainless steel sheets in contact with water.[77] The relative surface areas of the anode and the cathode are important in determining the rate of corrosion. In the above example, the surface area of the rivets is small compared to that of the stainless steel sheet, resulting in rapid corrosion.[77] However, if stainless steel fasteners are used to assemble aluminium sheets, galvanic corrosion will be much slower because the galvanic current density on the aluminium surface will be many orders of magnitude smaller.[77] A frequent mistake is to assemble stainless steel plates with carbon steel fasteners; whereas using stainless steel to fasten carbon-steel plates is usually acceptable, the reverse is not. Providing electrical insulation between the dissimilar metals, where possible, is effective at preventing this type of corrosion.[77]

At elevated temperatures, all metals react with hot gases. The most common high-temperature gaseous mixture is air, of which oxygen is the most reactive component. To avoid corrosion in air, carbon steel is limited to approximately 480 °C (900 °F). Oxidation resistance in stainless steels increases with additions of chromium, silicon, and aluminium. Small additions of cerium and yttrium increase the adhesion of the oxide layer on the surface.[78] The addition of chromium remains the most common method to increase high-temperature corrosion resistance in stainless steels; chromium 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 approximately 700 °C (1,300 °F), while 16% chromium provides resistance up to approximately 1,200 °C (2,200 °F). Type 304, the most common grade of stainless steel with 18% chromium, is resistant to approximately 870 °C (1,600 °F). Other gases, such as sulfur dioxide, hydrogen sulfide, carbon monoxide, chlorine, 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.[79][80] With the addition of up to 5% aluminium, ferritic grades Fe-Cr-Al are designed for electrical resistance and oxidation resistance at elevated temperatures. Such alloys include Kanthal, produced in the form of wire or ribbons.[81]