Encyclopedia
In metallurgy,
stainless steel is defined as a
ferrous alloy with a minimum of 10%
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.
Stainless
steels have higher resistance to
oxidation 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%
chromium, and up to 26% is used for harsh environments. The chromium forms a passivation layer of chromium oxide when exposed to
oxygen. The layer is too thin to be visible, meaning the metal stays shiny. It is, however, impervious to
water and air, protecting the metal beneath. Also, when the surface is scratched this layer quickly reforms. This phenomenon is called passivation by
materials scientists, and is seen in other metals, such as
aluminium. 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.
Commercial value 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, and building material in
skyscrapers and large buildings. See
"Use in sculpture and building facades", below, for more.
Stainless steel is 100%
recyclable. In fact, over 50% of new stainless steel is made from remelted scrap metal, rendering it a somewhat eco-friendly material.
Corrosion
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.
Pitting corrosion
Passivation relies upon the tough layer of oxide described above. When deprived of oxygen , 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 and protected from chlorides wherever possible.
Pitting corrosion can occur when stainless steel is subjected to high concentration of chloride ions 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. The replica did not last long, because the engineers responsible failed to take into account the difference between the freshwater of
Lake Geneva and the saltwater of the sea.
Weld decay and knifeline attack
Due to the elevated temperatures of
welding or during improper heat treatment, chromium
carbides can form in the
grain boundaries of stainless steel. This chemical reaction robs the alloy of chromium in the zone near the grain boundary, making those areas much less resistant to corrosion. This creates a
galvanic couple with the well-protected alloy nearby, which leads to
weld decay in highly corrosive environments. Special alloys, either with low carbon content or with added carbon "getters" such as
titanium and
niobium , can prevent this effect, but the latter require special heat treatment after welding to prevent the similar phenomenon of
knifeline attack. As its name 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. Modern steel-making technologies largely avoid these problems by controlling the carbon content of stainless steels to <0.03% and historically such grades were referred to as "L" grades such as 316L; in practice most stainless steels are now produced at these low carbon contents.
Rouging
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 mostly common in the pharmaceutical industries. The whole effect is caused by the simple fact, pure water is lacking any ions and pulls the metal ions of the passive stainless steel surface into the 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.
Intergranular corrosion
This is a largely historical problem related to the high carbon contents of steels from the past, for modern steels it is very rarely an issue.
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.
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.
Crevice corrosion
In the presence of reducing acids or exposition 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, eg. 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 is a rapid and severe form of stainless steel corrosion. It forms when the material is subjected to tensile stress and some kinds of corrosive environments, especially chloride-rich environments at higher temperatures. The stresses can be a result of the service loads, or can be caused by the type of assembly or residual stresses from fabrication ; the residual stresses can be relieved by annealing. This limits the usefulness of stainless steels of the 300 series for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. In more agressive conditions, higher alloyed austenitic stainless steels or Mo containing duplex stainless steels may be selected.
Stress corrosion cracking depends on the
nickel content.
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.
Galvanic corrosion
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
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 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 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
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.
See also and
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 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 is 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 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 means they are fearsomely expensive and similar performance can usually be achieved using duplex steels at much lower cost.
- Ferritic stainless steels are highly corrosion resistant, but far less durable than austenitic grades and cannot be hardened by heat treatment. 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 , molybdenum , no nickel, and about 0.1-1% carbon . 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.
- 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 and molybdenum and lower nickel contents than austenitic stainless steels.
Comparison of standardized steels
EN-standard
Steel no. | EN-standard
Steel name | ASTM/AISI
Steel type | UNS |
|---|
1.4016 | X6Cr17 | 430 | | 1.4512 | X6CrTi12 | 409 | | 1.4310 | X10CrNi18-8 | 301 | | 1.4318 | X2CrNiN18-7 | 301LN | | 1.4307 | X2CrNi18-9 | 304L | S30403 | 1.4306 | X2CrNi19-11 | 304L | S30403 | 1.4311 | X2CrNiN18-10 | 304LN | S30453 | 1.4301 | X5CrNi18-10 | 304 | S30400 | 1.4948 | X6CrNi18-11 | 304H | S30409 | 1.4303 | X5CrNi18 12 | 305 | 1.4541 | X6CrNiTi18-10 | 321 | 1.4878 | X12CrNiTi18-9 | 321H | S32109 | 1.4404 | X2CrNiMo17-12-2 | 316L | S31603 | 1.4401 | X5CrNiMo17-12-2 | 316 | S31600 | 1.4406 | X2CrNiMoN17-12-2 | 316LN | S31653 | 1.4432 | X2CrNiMo17-12-3 | 316L | S31603 | 1.4435 | X2CrNiMo18-14-3 | 316L | S31603 | 1.4436 | X3CrNiMo17-13-3 | 316 | S31600 | 1.4571 | X6CrNiMoTi17-12-2 | 316Ti | S31635 | 1.4429 | X2CrNiMoN17-13-3 | 316LN | S31653 | 1.4438 | X2CrNiMo18-15-4 | 317L | S31703 | 1.4539 | X1NiCrMoCu25-20-5 | 904L | N08904 | 1.4547 | X1CrNiMoCuN20-18-7 | | S31254 |
Stainless steel finishes
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 is removed by
pickling, and the 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 polished rollers
- No, 2BA - Bright Anealed same as above with highly polished rollers
- No. 3 - coarse abrasive finish applied mechanically
- No. 4 - fine abrasive finish
- No. 6 - matte finish
- No. 7 - reflective finish
- No. 8 - mirror finish
History
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 the year AD 400. However, unlike stainless steel, 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 layer of
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 the
French metallurgist Pierre Berthier, who noted their resistance against attack by some acids and suggested their use in
cutlery. However, the 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 of practical interest.
This situation changed in the late 1890s, when Hans Goldschmidt of
Germany developed an aluminothermic process for producing carbon-free chromium. In the years 1904–1911, several researchers, particularly Leon Guillet of France, prepared alloys that would today be considered stainless steel. In 1911, Philip Monnartz of Germany reported on the relationship between the chromium content and corrosion resistance of these alloys.
Harry Brearley of the Brown-Firth research laboratory in
Sheffield, England is most commonly credited as the "inventor" of stainless steel. In 1913, while seeking an erosion-resistant alloy for gun barrels, he discovered and subsequently industrialized a martensitic stainless steel alloy. However, similar industrial developments were taking place contemporaneously at the
Krupp Iron Works in Germany, where Eduard Maurer and Benno Strauss were developing an austenitic alloy , and in the United States, where Christian Dantsizen and Frederick Becket were industrializing ferritic stainless.
Already in the year 1908 Krupp had built a famous sailing-yacht featuring a chrome-nickel steel hull, or so it seems - its wreck being currently investigated by the Bureau of Archaeological Research of the State of Florida.
Use in sculpture and building facades
- Stainless steel was particularly in vogue during the art deco period. The most famous example of this is the upper portion of the Chrysler Building . Diners and fast food restaurants feature large ornamental panels, stainless fixtures and furniture. Owing to the durability of the material, many of these buildings still retain their original and spectacular appearance.
- In recent years the forging of stainless steel has given rise to a fresh approach to architectural blacksmithing. The work of illustrates this well.
- Also pictured above, the Gateway Arch is clad entirely in stainless steel: 886 Tons of 1/4" plate, #3 Finish, Type 304.
- 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.
- Stainless Steel is the fourth common material used in metal wall tiles, and is used for its corrosion resistance properties in kitchens and bathrooms.
See also
- AISI steel grades
- Budd Company Historically notable user of stainless steel
References
External links
- by International Stainless Steel Forum
- by The Stainless Steel Information Center
- by Cambridge University
