by Gary Coates, Technical Director, Nickel Institute and Dr. David Jenkinson, Director Nickel Institute Australasia.
Stainless steel is the name given to a group of corrosion resistant and high temperature steels. Their remarkable resistance to corrosion is due to a chromium-rich oxide film which forms on the surface. When ordinary carbon steel is exposed to rain water, for example, it corrodes forming a brown iron oxide, commonly called rust, on the surface. This is not protective and eventually the entire piece of steel will corrode and be converted to rust. But when enough chromium (more than about 10%) is added to ordinary steel, the oxide on the surface is transformed - it is very thin, virtually invisible and protective in a wide range of corrosive media. This is what we call stainless steel and there are several different types, and many different grades.
Types of Stainless Steel
The basic composition of stainless steel is iron (Fe) and chromium (Cr). This is the simplest form of stainless steel, with this family known as the ferritic stainless steels because their crystal structure is called ferrite. (This is also the structure of mild steel.) The ferritic stainless steels are magnetic like ordinary steel. A commonly used grade is Type 430 (S43000) which is used for automotive trim and inside dishwashers and clothes dryers. They are often the least expensive stainless steels but can be more difficult to form and weld.
If we wish to make carbon steel strong and hard, such as for a drive shaft or wear plate, we might increase the carbon content, and then heat treat the steel by quenching and tempering it. We can do the same with stainless steel - if we increase the carbon content of ferritic stainless steels we produce the family of martensitic stainless steels, used for items such as knives, razor blades and corrosion resistant bearings. Martensitic grades are strong and hard, but are brittle and difficult to form and weld. Type 420 (S42000) is a typical example. Like ferritic stainless steels, martensitic stainless steels are magnetic.
The majority of stainless steels contain nickel (Ni), which is added for a number of reasons but particularly to change the crystal structure from ferrite to austenite. Austenitic stainless steels are ductile, tough and, most importantly, easy to form and weld. These steels are not magnetic in the annealed condition. The most common example is Type 304 (S30400) or "18/8" - the most widely used stainless steel in the world. The lower carbon version, Type 304L (S30403) is always preferred in more corrosive environments where welding is involved. There are numerous applications for this grade, ranging from domestic kitchen sinks and building facades to commercial food processing equipment and chemical plant piping.
Molybdenum (Mo) is added to some stainless steels to increase their corrosion resistance, particularly in marine and acidic environments. It increases an alloy's pitting and crevice corrosion resistance. These corrosion forms are caused by the common and highly aggressive chloride ion (Cl¯), which is present in salts, such as sea salt and table salt. When 2-3% molybdenum is added to Type 304 or 304L, we create Type 316 (S31600) or 316L (S31603) stainless steel. They are sometimes referred to as the marine grades of stainless steel, since they are widely used for items such as boat fittings. They are also known as the acid resistant grades, since they have better corrosion resistance in some acids such as sulphuric acid. But their range of applications is wide, from building facades in aggressive atmospheres to piping onboard chemical tankers.
Halfway between the ferritic and austenitic stainless steels is a family called the duplex stainless steels, which are about 50% ferrite and 50% austenite. Because of this duplex structure, they are resistant to stress corrosion cracking, which can affect the austenitic stainless steels in hot waters containing chlorides. The most common duplex stainless steel is 2205 (including both S31803 and S32205) and it is used in many applications such as hot water tanks.
Nitrogen (N) is added to some stainless steels, but is very important in duplex grades. It has several beneficial effects. Like nickel, nitrogen promotes austenite (especially important for welding) and, like molybdenum, it improves resistance to pitting and crevice corrosion. It also increases strength. Duplex stainless steels are inherently stronger, but a grade such as 2205, which contains about 0.15% nitrogen, has over twice the yield strength of Type 316L. Thus, 2205 is commonly used in tanks for seagoing chemical tankers where both strength and corrosion resistance are required, and for components such as rods and connectors for glass curtain walls in public buildings where the high strength means that the components can be small and so make the structure seem lighter and more transparent.
There is one more family - the precipitation hardening stainless steels. This is a specialised family which has very high strength achieved by adding elements such as copper, which form very fine particles during heat treatment. They generally have slightly higher corrosion resistance than the martensitic stainless steels but, at best, they have slightly less resistance than Type 304. They are commonly used in the aerospace and defence industries, but also find use in items such as pump shafts. 17-4PH (S17400) is a typical example.
In addition to the common grades mentioned above, there are many more specialised grades of stainless steel for applications which require greater corrosion resistance or higher strength. Three which are listed in the accompanying table are Alloy 904L (N08904), which was originally developed for sulphuric acid service, the super-austenitic grade Alloy 254 (S31254), representing a group of 6% Mo stainless steels; and the grade Alloy 2507 (S32750), representing a group of super-duplex alloys. The last two are 'seawater resistant' - they will not suffer pitting or crevice corrosion when immersed in ambient temperature seawater. There are also grades developed for such special needs as improved machinability. Cast versions of most wrought grades are also available, usually slightly modified to improve castability.
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Nickel-containing stainless steels and nickel alloys play an important role in providing corrosion resistant, and hence leak resistant, materials of construction for projects internationally. Some of these materials also play a critical role in handling gas production, particularly in liquefied form, thus helping to develop difficult-to-access gas reserves.
Mechanical and Physical Properties
For most corrosion resistant applications, strength is not a key issue. There are exceptions, such as pressure vessels, and here the high strength of duplex grades can make them attractive. A characteristic of the austenitic stainless steels is that they work harden easily - that is, their strength increases rapidly when they are formed at ambient temperatures, such as in rolling or wire drawing operations. The accompanying table (below) shows the large increase in strength of Type 304 in the 1/2 hard condition. This characteristic is valuable in items like bolts and springs.
Two important physical properties are thermal conductivity and thermal expansion rate. The common austenitic stainless steels, such as Type 304, have lower thermal conductivity than carbon steels and this is useful in applications such as cappuccino cups and thermos flasks. Their rate of thermal expansion is also greater than ordinary steel (but less than materials such as aluminium) so care must be taken during welding to ensure that the recommended jigging and tacking procedures and welding sequences are followed.
Corrosion Resistance
When carbon steel rusts, it does so by uniform corrosion - the entire surface of the steel corrodes more or less uniformly. Except in special environments, such as strong acids, stainless steels do not corrode in this way. If corrosion does occur, it is normally by localised corrosion and the most common forms of this are as follows:
Pitting is localised corrosion at individual sites on the surface of stainless steel. Pitting starts at points of weakness in the protective oxide film, such as at manganese sulphide inclusions on the steel surface.
Crevice corrosion takes place where physical crevices are present, such as at the joint between two overlapping sheets of stainless steel, in the crevice between a stainless steel flange and a non-metallic gasket or under surface deposits.
The mechanisms of pitting and crevice corrosion are similar and both most commonly occur in chloride environments. But crevice corrosion occurs more readily since it is assisted by the existence of a physical crevice, whereas pitting has to initiate on a surface which is effectively flat. It is often not possible to control environmental factors such as the amount of chloride or the temperature, so it is usually necessary to choose a sufficiently corrosion resistant grade for the service. An indication of pitting and crevice corrosion resistance is given by the 'Pitting Resistance Equivalent' (PRE) number:
PRE = %Cr + 3.3%Mo + 16%N
This formula shows the beneficial effect of chromium, molybdenum and nitrogen and illustrates why Type 316, with 2-3% Mo, has better resistance than Type 304 to marine environments. However, for resistance to corrosion when immersed in seawater on a longterm basis, it is necessary to move up to a grade with a relatively high PRE number, such as a super-austenitic 6% Mo grade or a super-duplex such as 2507.
Chloride stress corrosion cracking (SCC) can occur in chloride-containing solutions at elevated temperature, normally above 50 degrees C, when tensile stress is present. It particularly affects austenitic stainless steels, and a common failure observed in the field is cracking from the outside of tanks or pipes carrying hot fluids. For example, if a water leak occurs into insulation on the outside, chlorides can concentrate through evapouration, and SCC can take place because of the tensile stresses present in the outside surface of pipes and tanks.
Chloride SCC is most comonly overcome by using a duplex stainless steel, such as 2205, or a grade with a higher nickel content, such as a 6% Mo material or high-nickel alloys like Alloy 825 (N08825). Ferritic stainless steels are very resistant to SCC but grades with equivalent pitting resistance to the austenitic grades have other major drawbacks.
Intergranular corrosion (IGC) is preferential attack at the grain boundaries of a stainless steel and is commonly associated with welding.
If stainless steel is heated into a sensitising temperature range, such as can occur in the heat affected zone of a weld, then chromium can combine with carbon in the steel to form chromium carbides in the grain boundaries. Such a stainless steel is said to be 'sensitised'. Around each chromium carbide particle is an area low in chromium so that, when the material is placed in a corrosive environment, attack of these low-chromium regions can occur. This is called IGC.
The most common way today to avoid IGC is to specify a low carbon 'L' grade of stainless steel when welding, such as Type 304L . In the past, when it was difficult for mills to achieve low carbon levels, titanium (Ti) or niobium (Nb) were added since these elements preferentially combine with carbon and so leave the chromium unaffected. Grades containing these additions include Type 321 (S32100) containing Ti and Type 347 (S34700) containing Nb.
Galvanic corrosion can occur when different metals are in contact in an electrically conductive liquid. Stainless steel is not normally corroded in such a galvanic couple, since it is usually the more corrosion resistant of the two metals and acts as the cathode. When the other metal which is in contact with the stainless steel is less corrosion resistant, it acts as the anode and corrodes preferentially. The rate of corrosion of the second metal can be rapid if its surface area is small relative to the area of the stainless steel cathode with which it is in contact. An example of this would be the use of galvanised steel fasteners to hold stainless steel sheets, a poor design unless the system is always dry. Galvanic couples are not necessarily a problem and can, in fact, be used to benefit in some designs.
Applications
Stainless steels find use in a very wide variety of applications. Some typical examples are:
Consumer goods: Applications here include domestic kitchenware and tableware, kitchen sinks, laundry equipment and electrical and electronic appliances.
Architecture, building & construction: Stainless steel has been used in numerous famous buildings. The Chrysler Building in New York, the world's tallest building when it was built in 1929, used Type 302 (similar to Type 304) for the roof and upper structure. Type 316 is used to clad the outside of Petronas Twin Towers in Kuala Lumpur, currently the world's tallest buildings, and Jin Mao Tower in Shanghai, the third tallest. More common applications are balustrades, column wraps, roofing and guttering, signage, curtain wall supports, light poles, elevator doors and public seating. Stainless steel rebar is used in bridges, barrier walls and decking to extend the life of critical areas of roadways and marine structures.
Food and beverage industry: Type 304 and, in more aggressive situations, Type 316 are widely used in this industry for food and beverage production (milk silos, cheese vats, beer and wine fermenters, fruit juice tanks and piping), storage (wine tanks, beer kegs), cooking (large commercial kitchens) and serving (display cabinets, bench tops). Stainless steel equipment is easy to clean (sanitize) and also preserves the purity of the food.
Transportation: A wide range of both decorative and functional components are fabricated from stainless steel, such as automotive exhaust systems, fasteners, trim, wheel covers and windscreen wiper arms; passenger railcars, coal wagons, bus frames and milk tankers; and seagoing chemical tankers.
Chemical, petrochemical, oil and gas, pulp and paper industries, and power generation: This field represents a very diversified market for stainless steel with many specialised applications such as heat exchangers, vessels for various types of chemicals, pipe, fittings, valves, pumps, mixers, high temperature furnace equipment, components for nuclear reactors, and gas and water turbines.
End of Life
When a system which uses stainless steel components reaches the end of its useful life, and if the equipment cannot be reused, it should always be recycled. Stainless steel, especially the nickel-containing grades, has a high scrap value and can be recycled to produce new stainless steel without any loss of quality. Most stainless steel today is made using a significant percentage of scrap stainless steel.
Footnotes
1 High temperature grades and applications are not covered here.
2 The traditional AISI (American Iron and Steel Institute) numbering system groups stainless steels into 300, 400 etc series, such as Type 304, Type 430 etc. The newer UNS (Unified Numbering System) uses numbers of the form S30400 and S43000 for these same grades.
