Stainless steelThe 630 foot (192 m) high, stainless-clad (type 304) Gateway Arch defines St. Louis, Missouri's skyline.In metallurgy, stainless steel is defined as an iron-carbon alloy with a minimum of 10.5% chromium content. The name originates from the fact that stainless steel does not stain, corrode or rust as easily as ordinary steel. This material is also called corrosion resistant steel when it is not detailed exactly to its alloy type and grade, particularly in the aviation industry. As such, there are now different and easily accessible grades and surface finishes of stainless steel, to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are everyday cutlery and watch straps.
Stainless steels have higher resistance to oxidation (rust) and corrosion in many natural and man made environments; however, it is important to select the correct type and grade of stainless steel for the particular application.
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. The chromium forms a passivation layer of chromium(III) oxide (Cr2O3) when exposed to oxygen. The layer is too thin to be visible, which means that the metal remains lustrous. It is, however, impervious to water and air, protecting the metal beneath. Also, this layer quickly reforms when the surface is scratched. This phenomenon is called passivation and is seen in other metals, such as aluminium and titanium. When stainless steel parts such as nuts and bolts are forced together, the oxide layer can be scraped off causing the parts to weld together. When disassembled, the welded material may be torn and pitted, an effect that is known as galling.
Nickel also contributes to passivation, as do other less commonly used ingredients such as molybdenum and vanadium.
Commercial value of stainless steel
The pinnacle of New York's Chrysler Building is clad with type 302 stainless steel.
An art deco sculpture on the Niagara-Mohawk Power building in Syracuse, New York
Pipes and fittings made of stainless steel
Stainless steel's resistance to corrosion and staining, low maintenance, relative inexpense, and familiar luster make it an ideal base material for a host of commercial applications. There are over 150 grades of stainless steel, of which fifteen are most common. The alloy is milled into sheets, plates, bars, wire, and tubing to be used in cookware, cutlery, hardware, surgical instruments, major appliances, industrial equipment, a structural alloy in automotive and aerospace assembly and building material in skyscrapers and other large buildings.
Stainless steel is also used for jewelry and watches. The most common stainless steel alloy used for jewelry is 316L. It can be re-finished by any jeweler and unlike silver will not oxidize and turn black.
Stainless steel is 100% recyclable. In fact, an average stainless steel object is composed of about 60% recycled material, 25% originating from end-of-life products and 35% coming from manufacturing processes.
Even a high-quality alloy can corrode under certain conditions. Because these modes of corrosion are more exotic and their immediate results are less visible than rust, they often escape notice and cause problems among those who are not familiar with them.
Passivation relies upon the tough layer of oxide described above. When deprived of oxygen (or when a salt such as chloride competes as an ion), stainless steel lacks the ability to re-form a passivating film. In the worst case, almost all of the surface will be protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. While the corrosion pits only nucleate under fairly extreme circumstances, they can continue to grow even when conditions return to normal, since the interior of a pit is naturally deprived of oxygen. In extreme cases, the sharp tips of extremely long and narrow pits can cause stress concentration to the point that otherwise tough alloys can shatter, or a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in stainless alloys, but it can be prevented by ensuring that the material is exposed to oxygen (for example, by eliminating crevices) and protected from chlorides wherever possible.
Pitting corrosion can occur when stainless steel is subjected to high concentration of Chloride ions (for example, sea water) and moderately high temperatures. A textbook example for this was a replica of the Jet d'Eau fountain in Geneva, ordered by an Arab Sheikh for installation in the Red Sea – King Fahd's Fountain. The difference between the freshwater of Lake Geneva and the saltwater of the sea called for much greater specialisation of the engineering processes and materials involved, as a straight duplicate of the Geneva fountain would not have survived long in the saltwater environment.
Rouging is a very peculiar phenomenon, which occurs only on polished stainless steel surfaces with very low surface roughness in a pure water environment. This effect is most common in pharmaceutical industries. It is caused by the simple fact that pure water is lacking any ions and pulls the metal ions of the passive stainless steel surface into solution. Iron ions do not dissolve at neutral pH and will precipitate as an iron hydroxide film, which has a reddish colour, hence the name rouging.
Some compositions of stainless steel are prone to intergranular corrosion when exposed to certain environments. When heated to around 700 °C, chromium carbide forms at the intergranular boundaries, depleting the grain edges of chromium, impairing their corrosion resistance. Steel in such condition is called sensitized. Steels with carbon content 0.06% undergo sensitization in about 2 minutes, while steels with carbon content under 0.02% are not sensitive to it.
Intergranular corrosionA special case of intergranular corrosion is called "weld decay" or "knifeline attack" (KLA). Due to the elevated temperatures of welding the stainless steel can be sensitized very locally along the weld. The chromium depletion creates a galvanic couple with the well-protected alloy nearby in highly corrosive environments. As the name "knifeline attack" implies, this is limited to a small zone, often only a few micrometres across, which causes it to proceed more rapidly. This zone is very near the weld, making it even less noticeable.
It is possible to reclaim sensitized steel by heating it to above 1000 °C and holding at this temperature for a given period of time dependent on the mass of the piece, followed by quenching it in water. This process dissolves the carbide particles, then keeps them in solution.
It is also possible to stabilize the steel to avoid this effect and make it welding-friendly. Addition of titanium, niobium and/or tantalum serves this purpose; titanium carbide, niobium carbide and tantalum carbide form preferentially to chromium carbide, protecting the grains from chromium depletion. Use of extra-low carbon steels is another method and modern steel production usually ensures a carbon content of <0.03% at which level intergranular corrosion is not a problem. Light-gauge steel also does not tend to display this behavior, as the cooling after welding is too fast to cause effective carbide formation.
In the presence of reducing acids or exposure to reducing atmosphere, the passivation layer protecting steel from corrosion can break down. This wear can also depend on the mechanical construction of the parts, e.g. under gaskets, in sharp corners, or in incomplete welds. Such crevices may promote corrosion, if their size allows penetration of the corroding agent but not its free movement. The mechanism of crevice corrosion is similar to pitting corrosion, though it happens at lower temperatures.
Stress corrosion cracking
Stress corrosion cracking can be a severe form of stainless steel corrosion. It forms when the material is subjected to tensile stress and some corrosive environments, especially chloride-rich environments (sea water) at higher temperatures. The stresses can be a result of service loads, or can be caused by the type of assembly or residual stresses from fabrication (e.g. cold working); residual stresses can be relieved by annealing.
Stress Corrosion Cracking (SCC) is the result of anodic dissolution which is locally increased by deformation mechanism.
Indeed, materials submitted to high stresses (either residual or applied) suffer from local deformation which causes dislocations in the microstructure. Such dislocations are responsible for slip planes to occur. As soon as slip planes are able to emerge at the outer surface of the steel, the passive film is altered or destroyed, allowing corrosion to develop locally. The combination of galvanic coupling between depassivated zones and the rest of the surface which remain passive account for cracking to develop. The resistance of stainless steels to SCC depends on a lot of factors.
As in all aggressive media responsible for SCC, corrosion potentials situated near the active/passive transition domain are the most dangerous. This may be explained by the fact that the passive film is easily destroyed in area where slip planes arise at the surface as a result of deformation process under the effect of applied or residual stresses. So, when electrochemical conditions drive the rest potential in the active/passive transition domain, the SCC risk is high.
This limits the usefulness of stainless steels of the 300 series (304, 316) for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. In more aggressive conditions, higher alloyed austenitic stainless steels (6% Mo grades) or Mo-containing duplex stainless steels may be selected.
High Ni austenitics are much more resistant to Stress Corrosion Cracking (SCC); indeed, their deformation mechanism is different since they exhibit numerous but small dislocations.
Ferritic stainless steels, particularly high Chromium grades are very resistant to SCC if they do not contain Nickel.
Chlorine catalyzes the formation of hydrogen which hardens and embrittles the metal locally, causing concentration of the stress and a microscopic crack. The chlorine moves into the crack, continuing the process.
Sulphide stress cracking
Sulphide stress cracking is an important failure mode in the oil industry, where the steel comes into contact with liquids or gases with considerable hydrogen sulfide content, e.g., sour gas. It is influenced by the tensile stress and is worsened in the presence of chloride ions. Very high levels of hydrogen sulfide apparently inhibit the corrosion. Rising temperature increases the influence of chloride ions, but decreases the effect of sulfide, due to its increased mobility through the lattice; the most critical temperature range for sulphide stress cracking is between 60–100 °C (140–212 °F).
Galvanic corrosion occurs when a galvanic cell is formed between two dissimilar metals. The resulting electrochemical potential then leads to formation of an electric current that leads to electrolytic dissolving of the less noble material. This effect can be prevented by electrical insulation of the materials, e.g. by using rubber or plastic sleeves or washers, keeping the parts dry so there is no electrolyte to form the cell, or keeping the size of the less-noble material significantly larger than the more noble ones (e.g. stainless-steel bolts in an aluminum block won't cause corrosion, but aluminum rivets on stainless steel sheet would rapidly corrode).
If these options are not available to protect from galvanic corrosion, a sacrificial anode can be used to protect the less noble metal. For example, if a system is composed of 316 SS, a very noble alloy with a low galvanic potential, and a mild steel, a very active metal with high galvanic potential, the mild steel will corrode in the presence of an electrolyte such as salt water. If a sacrificial anode is used such as a Mil-Spec A-18001K zinc alloy, Mil-Spec A-24779(SH) aluminum alloy, or magnesium, these anodes will corrode instead, protecting the other metals in the system. The anode must be electrically connected to the protected metal(s) in order to be able to preserve them. This is common practice in the marine industry to protect ship equipment. Boats and vessels that are in salt water use either zinc alloy or aluminum alloy. If the boats are only in fresh water, a magnesium alloy is used. Magnesium has one of the highest galvanic potential of any metal. If it is used in a saltwater application on a steel or aluminum hull boat, hydrogen bubbles will form under the paint, causing blistering and peeling.
Contact corrosion is a combination of galvanic corrosion and crevice corrosion, occurring where small particles of suitable foreign material are embedded to the stainless steel. Carbon steel is a very common contaminant here, coming from nearby grinding of carbon steel or use of tools contaminated with carbon steel particles. The particle forms a galvanic cell, and quickly corrodes away, but may leave a pit in the stainless steel from which pitting corrosion may rapidly progress. Some workshops therefore have separate areas and separate sets of tools for handling carbon steel and stainless steel, and care has to be exercised to prevent direct contact between stainless steel parts and carbon steel storage racks.
Particles of carbon steel can be removed from a contaminated part by passivation with dilute nitric acid, or by pickling with a mixture of hydrofluoric acid and nitric acid.
Types 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 non-magnetic and less brittle at low temperatures. For higher hardness and strength, carbon is added. When subjected to adequate heat treatment these steels are used as razor blades, cutlery, tools etc.
Significant quantities of manganese have been used in many stainless steel compositions. Manganese preserves an austenitic structure in the steel as does nickel, but at a lower cost.
Stainless steels are also classified by their crystalline structure:
Austenitic, or 300 series, stainless steels comprise 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. A typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless is often used in flatware. Similarly 18/0 and 18/8 is also available. “Superaustenitic” stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion due to high Molybdenum contents (>6%) and nitrogen additions and the higher nickel content ensures better resistance to stress-corrosion cracking over 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.
Ferritic stainless steels are highly corrosion resistant, but less durable than austenitic grades. They contain between 10.5% and 27% chromium and very little nickel, if any. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.
Martensitic stainless steels are not as corrosion resistant as the other two classes, but are extremely strong and tough as well as highly machineable, and can be hardened by heat treatment. Martensitic stainless steel contains chromium (12–14%), molybdenum (0.2–1%), zero to less than 2% nickel, and about 0.1–1% carbon (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic. It is also known as "series-00" steel.
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. There is a rising trend in defence budgets to opt for an ultra-high-strength stainless steel if possible in new projects as it is estimated that 2% of the U.S. GDP is spent dealing with corrosion. The Lockheed-Martin JSF is the first aircraft to use a precipitation hardenable stainless steel – Carpenter Custom 465 – in its airframe.
Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50:50 mix although in commercial alloys the mix may be 40:60 respectively. Duplex steel have improved strength over austenitic stainless steels and also improved resistance to localised corrosion particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.