Stainless steels are iron (Fe) base alloys containing at least 10.5 % chromium (Cr). Few stainless steels contain more than 30 % Cr or less than 50 % Fe. These steels achieve their stainless characteristics through the formation of an invisible, self healing, and adherent Cr rich oxide surface film. This oxide forms and heals itself in the presence of oxygen (O2). Other elements added to improve particular characteristics include nickel (Ni), molybdenum (Mo), copper (Cu), titanium (Ti), aluminum (Al), silicon (Si), niobium (Nb), nitrogen (N2), sulphur (S), and selenium (Se). Carbon (C) is normally present in amounts ranging from less than 0.03 % to over 1 % in certain martensitic grades.
Stainless steel is available in the form of (i) plate, (ii) sheet, strip, and foil, (iii) hot and cold finished bars, (iv) different kind of wire products, (v) structural shapes, (vi) pipes and tubes, and (vii) semi-finished products. Stainless steel can also be available as castings. The cast stainless steels, in general, are similar to the equivalent wrought steels. Most of the cast steels are direct derivatives of one of the wrought grades.
Stainless steels are used in a wide variety of applications. Most of the structural applications occur in the chemical and power engineering industries. These applications include an extremely diversified range of uses, including nuclear reactor vessels, heat exchangers, oil industry pipes, components for chemical processing and metallurgical, pulp and paper industries, furnace parts, and boilers used in fossil fuel electric power plants.
Stainless steels are characterized by corrosion resistance, aesthetic appeal, heat resistance, low life cycle cost, full recyclability, biological neutrality, ease of fabrication, cleanability and good strength to weight ratio. The selection of stainless steels is based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific temperature ranges and product cost. However, corrosion resistance and mechanical properties are normally the most important factors in selecting a grade for a given application.
The stainless steel was discovered between 1900 and 1915. In 1904, Leon Guillet discovered alloys with composition similar to steel grades 410, 420, 442, 446 and 440-C. In 1906 he also discovered an Fe-Ni-Cr alloy which was similar to the 300 series of stainless steel. In 1909 Giessen researched on the Cr-Ni (austenitic 300 series) stainless steels. In Germany, in 1908, Monnartz & Borchers found that a relationship exists between a minimum level of Cr (10.5 %) on corrosion resistance as well as the importance of low C content and the role of Mo in increasing corrosion resistance to chlorides.
Designations for wrought stainless steels
Wrought grades of stainless steels are normally designated by the American Iron and Steel Institute (AISI) numbering system, the Unified Numbering System (UNS), and the proprietary name of the stainless steel. In addition, designation systems have been established by most of the major industrial nations. Of the two institutional numbering systems, AISI is the older and more widely used. Most of the grades have a three-digit designation. The 200 and 300 series are normally austenitic stainless steels, whereas the 400 series are either ferritic or martensitic. Some of the grades have a one-letter or two-letter suffix which indicates a particular modification of the composition.
The UNS system includes a considerably greater number of stainless steels than AISI since it incorporates all of the more recently developed stainless steels. The UNS designation for a stainless steel consists of the letter S, followed by a five-digit number. For those stainless steels which have an AISI designation, the first three digits of the UNS designation normally correspond to an AISI number. When the last two digits are 00, the number designates a basic AISI grade. Modifications of the basic grades use two digits other than zeroes. For stainless steels which contain less than 50 % Fe, the UNS designation consists of the letter N (for Ni-base alloys), followed by a five-digit number.
Classification of stainless steels
Stainless steels are normally divided into five groups (Fig 1) which are (i) austenitic stainless steels, (ii) martensitic stainless steels, (iii) ferritic stainless steels,(iv) duplex (ferritic-austenitic) stainless steels, and (v) precipitation-hardening stainless steels. In each of the three original groups of stainless steels which are austenitic, martensitic, and ferritic, there is one composition which represents the basic, general-purpose steel. All other compositions derive from this basic steel, with specific variations in composition being made to impart very specific properties.
Fig 1 Types of stainless steels
Austenitic stainless steels – These steels have a face-centered cubic (fcc) structure. This structure is attained through the liberal use of austenitizing elements such as Ni, Mn (manganese), and N2. These steels are essentially non-magnetic in the annealed condition and can be hardened only by cold working. They normally possess good cryogenic properties and high temperature strength. Cr content in these steels normally varies from 16 % to 26 %, Ni upto around 35 % and Mn upto 15 %. The 200 series steels contain N2, 4 % to 15.5% Mn, and upto 7 % Ni. The 300 series steels contain larger amounts of Ni and upto 2 % Mn. Mo, Cu, Si, Al, Ti, and Nb can be added to confer certain characteristics such as halide pitting resistance or oxidation resistance. S or Se can be added to certain grades to improve machinability.
Martensitic stainless steels – These are essentially alloys of Cr and C which possess a distorted body-centered cubic (bcc) crystal structure (martensitic) in the hardened condition. They are ferro-magnetic, hardenable by heat treatments, and are normally resistant to corrosion only to relatively mild environments. Cr content is normally in the range of 10.5 % to 18 %, and C content can exceed 1.2 %. The Cr and C contents are balanced to ensure a martensitic structure after hardening. Excess carbides can be present to increase wear resistance or to maintain cutting edges, as in the case of knife blades. Elements such as Nb, Si, W (tungsten), and V (vanadium) can be added to modify the tempering response after hardening. Small amounts of Ni can be added to improve corrosion resistance in some media and to improve toughness. S or Se is added to some grades to improve machinability.
Ferritic stainless steels – These steels are essentially Cr containing steels with bcc crystal structures. Cr content is normally in the range of 10.5 % to 30 %. Some grades can contain Mo, Si, Al, Ti and Nb for introducing particular characteristics in the steel. S or Se can be added, as in the case of the austenitic grades, to improve machinability. The ferritic stainless steels are ferro-magnetic. They can have good ductility and formability, but high-temperature strengths are relatively poor compared to the austenitic grades. Toughness can be somewhat limited at low temperatures and in heavy sections.
Duplex stainless steels – These steels have a mixed structure of bcc ferrite and fcc austenite. The amount of each phase is a function of composition and heat treatment. Most stainless steels are designed to contain about equal amounts of each phase in the annealed condition. The principal alloying elements are Cr and Ni, but N2, Mo, Cu, Si, and W can be added to control structural balance and to impart certain corrosion-resistance characteristics. The corrosion resistance of duplex stainless steels is like that of austenitic stainless steels with similar alloying contents. However, duplex stainless steels possess higher tensile and yield strengths and improved resistance to stress-corrosion cracking than their austenitic counterparts. The toughness of duplex stainless steels is between that of austenitic and ferritic stainless steels.
Precipitation hardening stainless steels – These steels are Cr-Ni stainless steels containing precipitation-hardening elements such as Cu, Al, or Ti. Precipitation-hardening stainless steels can be either austenitic or martensitic in the annealed condition. Those which are austenitic in the annealed condition are frequently transformable to martensite through conditioning heat treatments, sometimes with a sub-zero treatment. In most cases, these stainless steels attain high strength by precipitation hardening of the martensitic structure.
Production of stainless steels
With specific restrictions in certain types, the stainless steels can be shaped and fabricated in conventional ways. They are produced in cast, powder metallurgy (P/M), and wrought forms.
Fig 2 shows the most commonly employed processes for making various wrought stainless steel products.
Production of stainless steels is a two-stage process involving the melting of scrap and ferro-alloys in a primary steelmaking furnace, followed by refining by argon-oxygen-decarburization (AOD) process to adjust the C content and remove impurities. Alternative, melting and refining steps include vacuum induction melting (VIM), vacuum arc remelting (VAR), electro-slag remelting (ESR), and electron beam melting (EBM). Melting and refining of stainless steels is, however, most frequently accomplished by the primary steelmaking / AOD processing route.
During the final stages of producing basic mill forms such as sheet, strip, plate, bar, and structurals etc. and bringing these forms to specific size and tolerances, the semi-finished materials are subjected to hot reduction with or without subsequent cold rolling operations, annealing, and cleaning. Further steps are needed to produce other mill forms, such as wire, pipe, and tube.
Factors for selection
The first and most important step toward successful use of a stainless steel is selection of a type which is appropriate for the application. There are a large number of standard types which differ from one another in composition, corrosion resistance, physical properties, and mechanical properties. Selection of the optimum type for a specific application is the key to satisfactory performance at minimum total cost.
A checklist of characteristics to be considered in selecting the proper type of stainless steel for a specific application includes (i) corrosion resistance, (ii) resistance to oxidation and sulphidation, (iii) strength and ductility at ambient and service temperatures, (iv) suitability for intended fabrication techniques, (v) suitability for intended cleaning procedures, (vi) stability of properties in service, (vii) toughness, (viii) resistance to abrasion and erosion, (ix) resistance to galling and seizing, (x) surface finish and / or reflectivity, (xi) magnetic properties, (xii) thermal conductivity, (xiii) electrical resistivity, (xiv) sharpness (retention of cutting edge), and (xv) rigidity.
Corrosion resistance – It is often the most important characteristic of a stainless steel, but frequently is also the most difficult to assess for a specific application. Normal corrosion resistance to pure chemical solutions is comparatively easy to determine, but actual environments are normally much more complex. Normal corrosion is frequently much less serious than localized forms such as stress-corrosion cracking, crevice corrosion in tight spaces or under deposits, pitting attack, and inter granular attack in sensitized material such as weld heat-affected zones (HAZ). Such localized corrosion can cause unexpected and sometimes catastrophic failure while most of the structure remains unaffected, and hence is to be considered carefully in the design and selection of the proper grade of stainless steel.
Corrosive attack can also be increased dramatically by seemingly minor impurities in the medium which can be difficult to anticipate but which can have major effects, even when present in only parts-per-million concentrations, by heat transfer through the steel to or from the corrosive medium; by contact with dissimilar metallic materials, by stray electrical currents, and by many other subtle factors. At high temperatures, attack can be accelerated significantly by seemingly minor changes in atmosphere which affect scaling, sulphidation, or carburization.
Despite these complications, suitable stainless steel can be selected for most applications on the basis of experience, perhaps with assistance from the steel producer. Laboratory corrosion data can be misleading in predicting service performance. Even actual service data have limitations, because similar corrosive media can differ substantially because of slight variations in some of the corrosion factors listed above. For difficult applications, the extensive study of comparative data can be necessary, sometimes followed by pilot plant or in-service testing..
Mechanical properties – These properties at the service temperature are clearly important, but satisfactory performance at other temperatures is to be considered also. Thus, a product for arctic service is to have suitable properties at sub-zero temperatures even though steady-state operating temperature can be much higher, room-temperature properties after extended service at high temperature can be important for applications such as boilers and jet engines, which are intermittently shut down.
Fabrication and cleaning -Often a particular stainless steel is chosen for a fabrication characteristic such as formability or weldability. Even a needed or desired cleaning procedure can dictate the selection of a specific type. For example, a weldment which is to be cleaned in a medium such as nitric-hydrofluoric acid, which attacks sensitized stainless steel, is to be produced from stabilized or low C stainless steel even though sensitization cannot affect the performance under service conditions.
Experience in the use of stainless steels indicates that many factors can affect the corrosion resistance. Some of the more prominent factors are (i) chemical composition of the corrosive medium, including impurities (ii) physical state of the medium such as liquid, gaseous, solid, or combinations of these, (iii) temperature, (iv) temperature variations, (v) aeration of the medium, (vi) O2 content of the medium, (vii) bacteria content of the medium, (viii) ionization of the medium, (ix) repeated formation and collapse of bubbles in the medium, (ix) relative motion of the medium with respect to the steel, (x) chemical composition of the steel, (xi) nature and distribution of micro-structural constituents, (xii) continuity of exposure of the steel to the medium, (xiii) surface condition of the steel, (xiv) stresses in the steel during exposure to the medium, (xv) contact of the steel with one or more dissimilar metallic materials, (xvi) stray electric currents, (xvii) differences in electric potential, (xviii) marine growths such as barnacles, (xix) sludge deposits on the steel, (xx) C deposits from heated organic compounds, (xxi) dust on exposed surfaces, and (xxii) effects of welding, brazing, and soldering.
Surface finish – Other characteristics in the stainless steel selection checklist are vital for some specialized applications but of little concern for many applications. Among these characteristics, surface finish is important more frequently than any other except corrosion resistance. Stainless steels are sometimes selected since they are available in a variety of attractive finishes. Surface finish selection can be made on the basis of appearance, frictional characteristics, or sanitation. The effect of finish on sanitation sometimes is thought to be simpler than it actually is, and tests of several candidate finishes can be advisable. The selection of finish can in turn influence the selection of the stainless steel because of differences in availability or durability of the various finishes for different types. For example, a more corrosion-resistant stainless steel maintains a bright finish in a corrosive environment which can dull a lower-alloy type.
Properties of stainless steels
Typical values of different properties of stainless steels are shown in Tab 1.
|Tab 1 Typical properties of stainless steel|
|5||Melting Point||Deg C||1371-1454|
|12||Hardness (Brinell 3000 kg)||BHN||137-595|
Tensile properties – Mechanical properties of most of the stainless steels, especially ductility and toughness, are higher than the same properties of C steels. Strength and hardness can be raised by cold work for ferritic and austenitic types, and by heat treatment for precipitation-hardening and martensitic types. Certain ferritic stainless steels can also be hardened slightly by heat treatment.
Certain austenitic stainless steels (the so-called metastable types) can develop higher strengths and hardnesses than other stable types for a given amount of cold work. In metastable austenitic stainless steels, deformation triggers and transformation of austenite to martensite takes place. In stable austenitic steels (type 304) strain hardening occurs throughout the duration of the application of stress, but that the amount of strain hardening for a given increment of stress decreases as stress increases. On the other hand, metastable steels (type 301) continues to strain harden well into the plastic range. The extended strain hardening is the result of the deformation-induced transformation of austenite to martensite.
Ferritic types of stainless steel are defined as those which contain at least 10.5 % Cr and which have microstructures of ferrite plus carbides. These steels are lower in toughness than the austenitic types. Strength is enhanced only moderately by cold working.
Duplex stainless steels normally contain around 50 % of austenite and 50 % of ferrite because of the balancing of elements which stabilize austenite (C, N2, Ni, Cu, and Mn) and ferrite (Cr, Mo, and Si). Low C is maintained in most grades to minimize inter-granular carbide precipitation. The austenite-ferrite balance provides wrought material with the optimum levels of mechanical properties and corrosion resistance. Since typically less austenite is present as-cast, welding consumables with enriched Ni are normally used to maintain austenite in the weld metal at levels normally similar to those in the base steel. Yield strengths around twice that of type 316 can be achieved with annealed duplex stainless steels. Strength levels of duplex stainless steels can be improved by cold working. Lower transverse ductility and impact strength can be expected because of the directional nature of the wrought micro-structure (typically elongated austenite islands in a ferrite matrix).
Martensitic type stainless steels are Fe-Cr steels with or without small additions of other alloying elements. These steels are ferritic in the annealed condition, but are martensitic after rapid cooling in air or a liquid medium from above the critical temperature. Steels in this group normally contain 14 % Cr or less except types 440A, 440B, and 440C, which contain 16 % to 18 % Cr, and an amount of C sufficient to permit hardening. If other elements are present, the total concentration is normally no more than 2 % to 3 %. Martensitic stainless steels can be hardened and tempered in the same manner as alloy steels. They have good strength and are magnetic.
Martensitic stainless steels harden when cooled off in the mill after hot processing, and hence, they are frequently given a process anneal at 650 deg C to 760 deg C for around 4 hours. Occasionally, martensitic types are available in the tempered condition. This condition is achieved by cooling directly off the mill to harden the steel and then reheating to a tempering temperature of 540 deg C to 650 deg C, or by reheating the steel to a hardening temperature of 1,010 deg C to 1,065 deg C, cooling it, and then tempering it. In heat treating martensitic stainless steels, temperatures upto around 480 deg C are referred to as stress-relieving temperatures since little change in tensile properties occurs upon heating hardened material to these temperatures. Temperatures of 540 deg C to 650 deg C are referred to as tempering temperatures, and temperatures of 650 deg C to 760 deg C are called annealing temperatures.
Precipitation-hardening types stainless steels are normally heat treated to final properties by the fabricator. These stainless steels are of two general classes namely (i) single-treatment steels, and (ii) double-treatment steels. Single-treatment steels are solution annealed at around 1,040 deg C to dissolve the hardening agent. Upon cooling to room temperature, the structure transforms to martensite which is super-saturated with respect to the hardening agent. A single tempering treatment at around 480 deg C to 620 deg C is all which is needed to precipitate a secondary phase to strengthen the steel. Different tempering temperatures within this range produce different properties.
Double-treatment steels are solution treated at around 1,040 deg C and then water quenched to retain the hardening agent in solution in an austenitic structure. The austenite is conditioned by heating to 760 deg C to precipitate carbides and thereby unbalance the austenite so that it transforms to martensite upon cooling to a temperature below 15 deg C. Alternatively, the austenite can be conditioned at a higher temperature, 925 deg C, at which fewer carbides precipitate, and then can be transformed to martensite by cooling to room temperature, followed by refrigerating to -75 deg C, Transformation can also be effected by severe cold work (around 60 % to 70 % reduction). Once the structure has been transformed to martensite by one of these three processes, tempering at 480 deg C to 620 deg C induces precipitation of a secondary metallic phase, which strengthens the steel.
Fracture toughness – Fracture toughness data for stainless steels are limited since these steels which are suitable for use at cryogenic temperatures have very high toughness. Fracture toughness of base steels are relatively high even at -269 deg C while the fracture toughness of fusion zones of welds can be lower or higher than that of the base steel.
Fracture crack growth rates – The fatigue crack growth rates of the base steels are normally higher at room temperature than at sub-zero temperatures, or around equal at room temperature and at sub-zero temperatures. Fatigue crack growth rates in the fusion zones of welds tend to be higher than in the base steel.
Fatigue strength – Fatigue strength increases as exposure temperature is decreased. Notched samples have substantially lower fatigue strengths than corresponding un-notched samples at all testing temperatures. Reducing the surface roughness of the un-notched samples improves fatigue strength.
Physical properties – There are relatively few applications for stainless steels in which physical properties are the determining factors in selection. However, there are several applications in which physical properties are important in product design. For example, stainless steels are used for many high temperature applications, frequently in conjunction with steels of lesser alloy content. Since austenitic stainless steels have higher coefficients of thermal expansion and lower thermal conductivities than C and alloy steels, these characteristics are to be taken into account in the design of stainless steel to C steel or stainless steel-to-alloy steel products such as heat exchangers. In such products, differential thermal expansion imposes stresses on the unit which are not present if the unit is made entirely of C or alloy steel. Also, if the heat-transfer surface is made of stainless steel, it is to be larger than if it is made of C or alloy steel. Physical properties can vary slightly with product form and size, but such variations are normally not of critical importance to the application.
Corrosion properties – Stainless steels are susceptible to several forms of localized corrosive attack. The avoidance of such localized corrosion is the focus of much of the effort involved in selecting stainless steel. Also, the corrosion performance of stainless steels can be strongly affected by practices of design, fabrication, surface conditioning, and maintenance. The selection of a grade of stainless steel for a particular application involves the consideration of many factors, but always begins with corrosion resistance.
It is first necessary to characterize the probable service environment. It is not enough to consider only the design conditions. It is also necessary to consider the reasonably anticipated excursions or upsets in service conditions. Once the grades with adequate corrosion resistance have been identified, it is then appropriate to consider mechanical properties, ease of fabrication, the types and degree of risk present in the application, the availability of the necessary product forms, and cost.
Mechanism of corrosion resistance – The mechanism of corrosion protection for stainless steels differs from that for C steels and alloy steels. In these other cases, the formation of a barrier of true oxide separates the steel from the surrounding atmosphere. The degree of protection afforded by such an oxide is a function of the thickness of the oxide layer, its continuity, its coherence and adhesion to the steel, and the diffusivities of O2 and steel in the oxide layer.
In high-temperature oxidation, stainless steels use a generally similar model for corrosion protection. However, at low temperatures, stainless steels do not form a layer of true oxide.Instead, a passive film is formed. One mechanism which has been suggested is the formation of a film of hydrated oxide, but there is not total agreement on the nature of the oxide complex on the steel surface. However, the oxide film is to be continuous, non-porous, insoluble, and self-healing if broken in the presence of O2.
Passivity exists under certain conditions for particular environments. The range of conditions over which passivity can be maintained depends on the precise environment and on the family and composition of the stainless steel. When conditions are favourable for maintaining passivity, stainless steels show extremely low corrosion rates. If passivity is destroyed under conditions which do not permit the restoration of the passive film, stainless steel corrodes much like a C or low-alloy steel.
The presence of O2 is essential to the corrosion resistance of a stainless steel. The corrosion resistance of stainless steel is at its maximum when the steel is boldly exposed and the surface is maintained free of deposits by a flowing bulk environment. Covering a portion of the surface, for example, by bio-fouling, painting, or installing a gasket, produces an O2 depleted region under the covered region. The O2 depleted region is anodic relative to the well aerated boldly exposed surface, and a higher level of alloy content in the stainless steel is needed to prevent corrosion.
With appropriate grade selection, stainless steel performs for very long times with minimal corrosion, but an inadequate grade can corrode and perforate more rapidly than a plain C steel fail by uniform corrosion. The selection of the appropriate grade of stainless steel, then, is a balancing of the desire to minimize cost and the risk of corrosion damage by excursions of environmental conditions during operation or downtime.
Effects of composition
Cr is the one element essential in forming the passive film. Other elements can influence the effectiveness of Cr in forming or maintaining the film, but no other element can, by itself, create this property of stainless steel.
Chromium – The film is first observed at around 10.5 % Cr, but it is rather weak at this composition and affords only mild atmospheric protection. Increasing the Cr content to 17 % to 20 %, typical of the austenitic stainless steels, or to 26 % to 29%, as possible in the newer ferritic stainless steels greatly increases the stability of the passive film. However, higher Cr can adversely affect the mechanical properties, fabricability, weldability, or suitability for applications involving certain thermal exposures. Hence, it is frequently more efficient to improve corrosion resistance by altering other elements, with or without some increase in Cr.
Nickel – Ni in sufficient quantities stabilizes the austenitic structure. This greatly improves the mechanical properties and fabrication characteristics. Ni is effective in promoting re-passivation, especially in reducing environments. Also, it is particularly useful in resisting corrosion in mineral acids. Increasing Ni content to around 8 % to 10 % decreases resistance to stress-corrosion cracking (SCC), but further increases begin to restore SCC resistance. Resistance to SCC in most service environments is achieved at around 30 % Ni.
In the newer ferritic grades, in which the Ni addition is less than that needed to destabilize the ferritic phase, there are still substantial effects. In this range, Ni increases yield strength, toughness, and resistance to reducing acids, but makes the ferritic grades susceptible to SCC in concentrated magnesium chloride (MgCl2) solutions. It is noteworthy, however, that the higher Ni types 310 and 314 are appreciably more resistant than the others. Although this solution causes rapid cracking, it does not necessarily simulate the cracking observed in field applications.
Manganese – Mn in moderate quantities and in association with Ni additions performs many of the functions attributed to Ni. However, total replacement of Ni by Mn is not practical. Very high Mn steels have some unusual and useful mechanical properties, such as resistance to galling. Mn interacts with S in stainless steels to form Mn sulphides. The morphology and composition of these sulphides can have substantial effects on corrosion resistance, especially pitting resistance.
Molybdenum – Mo in combination with Cr is very effective in terms of stabilizing the passive film in the presence of chlorides. Mo is especially effective in increasing resistance to the initiation of pitting and crevice corrosion.
Carbon – C is useful to the extent that it permits hardenability by heat treatment, which is the basis of the martensitic grades, and provides strength in the high temperature applications of stainless steels. In all other applications, C is detrimental to corrosion resistance through its reaction with Cr. In the ferritic grades, C is also extremely detrimental to toughness.
Nitrogen – N2 is beneficial to austenitic stainless steels in that it improves pitting resistance, retards the formation of the Cr-Mo sigma phase, and strengthens the steel. N2 is essential in the newer duplex grades for increasing the austenite content, diminishing Cr and Mo segregation, and raising the corrosion resistance of the austenitic phase. N2 is highly detrimental to the mechanical properties of the ferritic grades and is to be treated as comparable to C when a stabilizing element is added to the steel.
Forms of corrosion of stainless steels
General (uniform) corrosion of a stainless steel suggests an environment capable of stripping the passive film from the surface and preventing re-passivation. Such an occurrence can indicate an error in grade selection. An example is the exposure of a lower Cr ferritic stainless steel to moderate concentration of hot sulphuric (H2SO4) acid.
Galvanic corrosion – It results when two dissimilar metals are in electrical contact in a corrosive medium. As a highly corrosion-resistant metal, stainless steel can act as a cathode when in contact with a less noble metal, such as steel. The corrosion of steel parts, for example, steel bolts in a stainless steel construction can be a significant problem. However, the effect can be used in a beneficial way for protecting critical stainless steel components within larger steel construction.
In the case of stainless steel connected to a more noble metal, the active-passive condition of the stainless steel is to be considered. If the stainless steel is passive in the environment, galvanic interaction with a more noble metal is unlikely to produce significant corrosion. If the stainless steel is active or only marginally passive, galvanic interaction with a more noble metal probably produces sustained rapid corrosion of the stainless steel without re-passivation. The most important aspect of galvanic interaction for stainless steels is the need to select fasteners and weldments of adequate corrosion resistance relative to the bulk material, which is likely to have a much larger exposed area.
Pitting – It is a localized attack which can produce the penetration of a stainless steel with almost negligible weight loss to the total structure. Pitting is associated with a local discontinuity of the passive film. It can be a mechanical imperfection, such as an inclusion or surface damage, or it can be a local chemical breakdown of the film. Chloride is the most common agent for the initiation of pitting.
Once a pit is formed, it in effect becomes a crevice. The local chemical environment is considerably more aggressive than the bulk environment. Hence, very high flow rates over a stainless steel surface tend to reduce pitting corrosion. A high flow rate prevents the concentration of corrosive agents in the pit. The stability of the passive film with respect to resistance to pitting initiation is controlled primarily by Cr and Mo. Minor alloying elements can also have an important effect by influencing the amount and type of inclusions (for example, sulphides) in the steel which can act as pitting sites.
Pitting initiation can also be influenced by surface condition, including the presence of deposits, and by temperature. For a particular environment, a grade of stainless steel can be characterized by a single temperature, or a very narrow range of temperatures, above which pitting initiates and below which pitting does not initiate. It is hence possible to select a grade which is not subject to pitting attack if the chemical environment and temperature do not exceed the critical levels.
The formation of deposits in service can reduce the pitting temperature. Although chloride is known to be the primary agent of pitting attack, it is not possible to establish a single critical chloride limit for each grade. The corrosivity of a particular concentration of chloride solution can be profoundly affected by the presence or absence of various other chemical species which can accelerate or inhibit corrosion. Chloride concentration can increase where evaporation or deposits occur. Because of the nature of pitting attack, rapid penetration with little total weight loss, it is rare for any significant amount of pitting to be acceptable in practical application.
Crevice corrosion – It can be considered a severe form of pitting. Any crevice, whether the result of a metal-to-metal joints, a gasket, fouling, or deposits, tends to restrict O2 access, resulting in attack. In practice, it is extremely difficult to prevent all crevices, but every effort is to be made to do so. Higher Cr and especially higher Mo grades are more resistant to crevice attack. Just as there is a critical pitting temperature for a particular environment, there is a critical crevice temperature. This temperature is specific to the geometry and nature of the crevice and to the precise corrosion environment for each grade. The critical crevice temperature can be useful in selecting an adequately resistant grade for a particular application.
Inter-granular corrosion – It is a preferential attack at the grain boundaries of a stainless steel. It is normally the result of sensitization. This condition occurs when a thermal cycle leads to grain-boundary precipitation of a carbide, nitride, or inter-metallic phase without providing sufficient time for Cr diffusion to fill the locally depleted region. A grain boundary precipitate is not the point of attack; instead, the low Cr region adjacent to the precipitate is susceptible.
Sensitization is not necessarily detrimental unless the grade is to be used in an environment capable of attacking the region. For example, high temperature applications for stainless steel can operate with sensitized steel, but concern for inter-granular attack is to be given to possible corrosion during downtime when condensation can provide a corrosive medium. Since Cr provides corrosion resistance, sensitization also increases the susceptibility of Cr depleted regions to other forms of corrosion, such as pitting, crevice corrosion, and SCC. The thermal exposures needed to sensitize steel can be relatively brief, as in high-temperature service.
Stress-corrosion cracking – it is a corrosion mechanism in which the combination of a susceptible steel, sustained tensile stress, and a particular environment leads to cracking of the steel. Stainless steels are particularly susceptible to SCC in chloride environments; temperature and the presence of O2 tend to aggravate chloride SCC of stainless steels.
Majority of ferritic and duplex stainless steels are either immune or highly resistant to SCC. All austenitic grades, especially types 304 and 316, are susceptible to some degree. The highly alloyed austenitic grades are resistant to sodium chloride (NaCl) solutions, but crack readily in MgCl2 solutions. Although some localized pitting or crevice corrosion probably precedes SCC, the amount of pitting or crevice attack can be so small that it is undetectable. Stress corrosion is difficult to detect while in progress, even when pervasive, and can lead to rapid catastrophic failures of pressurized equipment.
It is difficult to alleviate the environmental conditions which lead to SCC. The level of chlorides needed to produce stress-corrosion cracking is very low. In operation, there can be evaporative concentration or a concentration in the surface film on a heat-rejecting surface. Temperature is frequently a process parameter, as in the case of a heat exchanger. Tensile stress is one parameter which can be controlled. However, the residual stresses associated with fabrication, welding or thermal cycling rather than design stresses, are frequently responsible for SCC, and even stress-relieving heat treatments do not completely eliminate these residual stresses.
Erosion-corrosion – Corrosion of steel can be accelerated when there is an abrasive removal of the protective oxide layer. This form of attack is especially significant when the thickness of the oxide layer is an important factor in determining corrosion resistance. In the case of a stainless steel, erosion of the passive film can lead to some acceleration of attack.
Oxidation – Because of their high Cr contents, stainless steels tend to be very resistant to oxidation. Important factors to be considered in the selection of stainless steel grades for high-temperature service are the stability of the composition and micro-structure upon thermal exposure and the adherence of the oxide scale upon thermal cycling. Because many of the stainless steels used for high temperatures are austenitic grades with relatively high Ni contents. It is also necessary to be alert to the possibility of sulphidation attack.
Corrosion in specific environments
The selection of a suitable stainless steel for a specific environment needs consideration of several criteria. The first is corrosion resistance. Steels are available which provide resistance to mild atmospheres (for example, type 430) or to the food-processing environments (for example, type 304 stainless). Chemicals and more severe corrodents need type 316 or a more highly alloyed material.
Factors which affect the corrosivity of an environment include the concentration of chemical species, pH, aeration, flow rate (velocity), impurities (such as chlorides), and temperature, including effects from heat transfer. The second criterion is mechanical properties, or strength. High-strength materials often sacrifice resistance to some form of corrosion, particularly SCC. Third, fabrication is to be considered, including such factors as the ability of the steel to be machined, welded, or formed. Resistance of the fabricated article to the environment is to be considered, for example, the ability of the material to resist attack in crevices which cannot be avoided in the design. Fourth, total cost must be estimated, including initial price, installed cost, and the effective life expectancy of the finished product. Finally, consideration is to be given to product availability.
Many applications for stainless steels, particularly those involving heat exchangers, can be analyzed in terms of a process side and a water side. The process side is usually a specific chemical combination which has its own requirements for a stainless steel grade. The water side is common in many applications.
Atmospheric corrosion – The atmospheric pollutants most often responsible for the rusting of structural stainless steels are chlorides and metallic iron dust. Chloride contamination can originate from the calcium chloride (CaCl2) used to make concrete or from exposure in marine or industrial locations. Fe contamination can occur during fabrication or erection of the structure. Pollutants need to be minimized, if possible. The corrosivity of different atmospheric exposures can vary widely and can dictate application of different grades of stainless steel. Rural atmospheres, uncontaminated by industrial fumes or coastal salt, are extremely mild in terms of corrosivity for stainless steel, even in areas of high humidity. Industrial or marine environments can be considerably more severe.
Materials containing Mo show only slight rust stain and all grades are easily cleaned to reveal a bright surface. Type 304 stainless steel can provide satisfactory resistance in many marine applications, but more highly alloyed grades are frequently selected when the stainless is sheltered from washing by the weather and is not cleaned regularly. Types 302 and 304 stainless steels have many successful architectural applications. Type 430 stainless steel has been used in many locations, but there have been problems. For example, this stainless steel is rusted in sheltered areas after only a few months exposure in an industrial environment. It is replaced by type 302, which provides satisfactory service. In more aggressive environments, such as marine or severely contaminated atmospheres, stainless steel type 316 of is especially useful.
SCC is normally not a concern when austenitic or ferritic stainless steels are used in atmospheric exposures. Annealed and quarter-hard types 201, 301, 302, 304, and 316 stainless steels are not susceptible to SCC. In the as welded condition, only type 301 stainless steel experienced failure. Following sensitization at 650 deg C for 1.5 hours and furnace cooling, failures are obtained only for materials with C contents of 0.043 % or more.
SCC is to be considered when quench-hardened martensitic stainless steels precipitation hardening grades are used in marine environments or in industrial locations in which chlorides are present. Precipitation-hardenable grades are expected to show improved corrosion resistance when higher aging temperatures (lower strengths) are used.
Cracking of high-strength fasteners is possible and frequently results from H2 (hydrogen) generation due to corrosion or contact with a less noble material, such as Al. Resistance to SCC can be improved by optimizing the heat treatment. Fasteners for atmospheric exposure have been fabricated from a wide variety of steels. Type 430 and unhardened type 410 stainless steels have been used when moderate corrosion resistance is needed in a lower-strength material. Corrosion resistance (better than average) has been obtained by using type 305 stainless steels, when lower strength is acceptable.
Corrosion in water – Water can vary from extremely pure to chemically treated water to highly concentrated chloride solutions, such as brackish water or seawater, which can be further concentrated by recycling. This chloride content poses the danger of pitting or crevice attack of stainless steels. When the application involves moderately increased temperatures, even as low as 45 deg C, and particularly where there is heat transfer into the chloride-containing medium, there is the possibility of SCC. It is useful to consider water with two general levels of chloride content namely (i) fresh water, which can have chloride levels upto around 600 ppm, and (ii) seawater, which encompasses brackish and severely contaminated waters.
The corrosivity of a particular level of chloride can be strongly affected by the other chemical constituents present, making the water either more or less corrosive. Permanganate ion (4MnO-), which is associated with the dumping of chemicals, has been related to the pitting of stainless steel of type 304. The presence of S compounds and O2 or other oxidizing agents does not have very significant effects on stainless steels at ambient or slightly high temperatures (upto around 260 deg C).
In fresh water, type 304 stainless steel has provided excellent service for such items as valve parts, weirs, fasteners, and pump shafts in water and wastewater treatment plants. A precipitation-hardenable stainless steel has been used as shafts for large butterfly valves in potable water. The higher strength of a precipitation-hardenable stainless steel permits reduced shaft diameter and increased flow. Stainless steel of type 201 has seen service in revetment mats to reduce shoreline erosion in fresh water. Stainless steel of type 316 has been used as wire for micro-strainers in tertiary sewage treatment and is suggested for waters containing minor amounts of chloride.
Seawater is a very corrosive environment for many materials. Stainless steels are more likely to be attacked in low velocity seawater or at crevices resulting from equipment design or at attachments of barnacles. Types 304 and 316 stainless steels suffer deep pitting if the seawater flow rate decreases to below around 1.5 m/s because of the crevices produced by fouling organisms. However, in one study, type 316 stainless steel provided satisfactory service as pipes in the heat recovery section of a desalination test plant with relatively high flow rates.
The choice of stainless steel for seawater service can depend on whether stagnant conditions can be minimized or eliminated. For example, boat shafting of 17Cr-4Ni stainless steel has been used for trawlers where stagnant exposure and the associated pitting are not expected to be a problem. When seagoing vessels are expected to lie idle for extended periods of time, more resistant boat shaft materials, such as 22Cr-13Ni-5Mn stainless steel, are considered. Boat shafts with intermediate corrosion resistance are provided by 18Cr-2Ni-12Mn and high- N2 type 304 (type 304HN) stainless steels.
The possibility of galvanic corrosion is to be considered if stainless steel is to be used in contact with other metals in seawater. Preferably, only those materials which show closely related electrode potentials are to be coupled to avoid attack of the less noble material. Galvanic differences have been used to advantage in the cathodic protection of stainless steel in seawater. Crevice corrosion and pitting of austenitic types 302 and 316 stainless steels have been prevented by cathodic protection, but types 410 and 430 stainless steels develop H2 blisters at current densities below those needed for complete protection.
Other factors which are to be noted for applications of stainless steels in seawater include the effects of high velocity, aeration, and temperature. Stainless steels normally show good resistance to high velocities, impingement attack, and cavitation in seawater. Also, stainless steels provide optimum service in aerated seawater because a lack of aeration at a specific site frequently leads to crevice attack. Very little O2 is needed to maintain the passive film on a clean stainless surface. Increasing the temperature from ambient to around 50 deg C frequently reduces the attack of stainless steels, possibly because of differences in the amount of dissolved O2, changes in the surface film, or changes in the resistance of the boldly exposed sample area. Further temperature increases can result in increased corrosion, such as SCC.
Corrosion in chemical environments – The selection of stainless steels for service in chemical environments needs consideration of all forms of corrosion, as well as impurity levels and degree of aeration. When steel with sufficient general corrosion resistance has been selected, care is to be taken to ensure that the material does not fail by pitting or SCC due to chloride contamination. Aeration can be an important factor in corrosion, particularly in cases of border-line passivity. If dissimilar-metal contact or stray currents occur, the possibility of galvanic attack or H2 embrittlement is to be considered.
Stainless steel selection also depends on fabrication and operation details. If steel is to be used in the as-welded or stress relieved condition, it is to resist inter-granular attack in service after these thermal treatments. In chloride environments, the possibility of crevice corrosion is to be considered when crevices are present because of equipment design or the formation of adherent deposits. Higher flow rates can prevent the formation of deposits, but in extreme cases can also cause accelerated attack because of erosion or cavitation. Increased operating temperatures normally increase corrosion.
In heat-transfer applications, higher steel wall temperatures result in higher rates than expected from the lower temperature of the bulk solution. These and other items can need consideration in the selection of stainless steels, yet suitable materials continue to be chosen for a wide variety of chemical plant applications.
Some generalizations can be made regarding the performance of various categories of stainless steels in certain types of chemical environments. These observations relate to the compositions of the grades. For example, the presence of Ni and Cu in some austenitic grades greatly improves the resistance to H2SO4 acid compared to the resistance of the ferritic grades. However, combinations of chemicals which are encountered in practice can be either more or less corrosive than can be expected from the corrosivity of the individual components.
Mineral acids – The resistance of stainless steel to acids depends on the H2 ion (H+) concentration and the oxidizing capacity of the acid, along with such material variables as Cr, Ni, and C contents and heat treatment. For example, annealed stainless steel resists strong nitric (HNO3) acid in spite of the low pH of the acid since HNO3 acid is highly oxidizing and forms a passive film because of the Cr content of the steel. On the other hand, stainless steels are rapidly attacked by strong hydrochloric (HCl) acid since a passive film is not easily attained. Even in strong HNO3 acid, stainless steels can be rapidly attacked if they contain sufficient amounts of C and are sensitized. Oxidizing materials, such as ferric salts, result in reduced general corrosion in some acids, but can cause accelerated pitting attack if chloride ions (Cl-) are present.
In HNO3 acid, stainless steels have broad applicability, primarily because of their Cr content. Majority of 300-series stainless steels show good resistance in the annealed condition in concentrations from 0 % to 65 % upto the boiling point. More severe environments at high temperatures need steels with higher Cr.
In H2SO4 acid, stainless steels can approach the borderline between activity and passivity. Conventional ferritic grades, such as type 430, have limited use in H2SO4 acid, but the newer ferritic grades containing higher Cr and Mo(for example, 28 % Cr and 4 % Mo) with additions of at least 0.25 % Ni have shown good resistance in boiling 10 % H2SO4 acid, but corrode rapidly when acid concentration is increased. The conventional austenitic grades show good resistance in very dilute or highly concentrated H2SO4 acid at slightly high temperatures. Acid of intermediate concentration is more aggressive, and conventional grades have very limited use.
Aeration or the addition of oxidizing agents can significantly reduce the attack of stainless steels in H2SO4 acid. This occurs because the more oxidizing environment is better able to maintain the Cr-rich passive oxide film. Improved resistance to H2SO4 acid has been obtained by using austenitic grades containing high levels of Ni and Cu. In addition to reducing general corrosion, the increased Ni provides resistance to SCC.
In phosphoric (H3PO4) acid, conventional straight Cr stainless steels have very limited general corrosion resistance and show lower rates only in very dilute or more highly concentrated solutions. Conventional austenitic stainless steels provide useful general corrosion resistance over the full range of concentrations upto around 65 deg C. Use at temperatures upto the boiling point is possible for acid concentrations upto about 40 %. In practical applications, however, wet process H3PO4 acid environments include impurities derived from the phosphate rock, such as chlorides, fluorides, and H2SO4 acid. These three impurities accelerate corrosion, particularly pitting or crevice corrosion in the presence of the halogens. Higher-alloyed materials other than the conventional austenitic stainless steels are needed to resist wet-process H3PO4 acid.
HCl acid service is normally not an application for stainless steels, except perhaps for very dilute solutions at room temperature. Stainless steels can be susceptible to accelerated general corrosion, SCC, and pitting in HCl acid environments.
Sulfurous (H2SO3) acid is a reducing agent and several stainless steels have provided satisfactory service in H2SO3 acid environments. Conventional austenitic stainless steels have been used in sulphite digesters, and type 316, type 317, and other higher stainless steel grades have seen service in wet sulphur dioxide (SO2) and H2SO3 environments. Service life is improved by eliminating crevices, including those formed from the settling of suspended solids, or by using Mo containing grades. In some environments, SCC is also a possibility.
Organic acids and compounds – These are generally less aggressive than mineral acids because they do not ionize as completely, but they can be corrosive to stainless steels, especially when impurities are present. The presence of oxidizing agents in the absence of chlorides can reduce corrosion rates. For pure acetic acid, corrosion resistance has been obtained by using types 316 and 316L stainless steels over all concentrations upto the boiling point. Stainless steel of type 304 can be considered in all concentrations below around 90 % at temperatures upto the boiling point. Impurities present in the manufacture of acetic acid, such as acetaldehyde, formic acid, chlorides, and propionic acid, are expected to increase the attack of stainless steels. Chlorides can cause pitting or SCC.
Formic acid is one of the more aggressive organic acids, and corrosion rates can be higher in the condensing vapour than in the liquid. Type 304 stainless steel has been used at moderate temperatures. However, type 316 stainless steel or higher stainless steel grades are frequently preferred and high-alloy ferritic stainless steels containing 26 % Cr and 1 % Mo or 29 % Cr and 4 % Mo also show some promise.
The corrosivity of propionic and acrylic acids at a given temperature is normally similar to that of acetic acid. Impurities are important and can strongly affect the corrosion rate. In citric and tartaric acids, type 304 stainless steel has been used for moderate temperatures, and type 316 has been suggested for all concentrations up to the boiling point. Most dry organic halides do not attack stainless steels, but the presence of water allows halide acids to form and can cause pitting or SCC. Hence, care is to be exercised when using stainless steels in organic halides to ensure that water is excluded. Type 304 stainless steel has normally been satisfactory in aldehydes, in cellulose acetate at lower temperatures, and in fatty acids upto about 150 deg C. At higher temperatures, these chemicals require types 316 or 317. Stainless steel of type 316 is also used in amines, phthalic anhydride, tar, and urea service.
Stainless steels have been used in the plastics and synthetic fiber industries. Types 420 and 440C stainless steels have been used as plastic mould steels. More resistant materials have been used for extruding polyvinyl chloride (PVC) pipe.
Alkalis – All stainless steels resist general corrosion by all concentrations of sodium hydroxide (NaOH) upto around 65 deg C. Types 304 and 316 stainless steels show low rates of general corrosion in boiling NaOH upto early 20 % concentration. SCC in these grades can occur at around 100 deg C. Good resistance to general corrosion and SCC in 50 % NaOH at 135 deg C is provided by higher grades Mo stainless steels. In ammonia (NH3) and ammonium hydroxide (NH4OH), stainless steels have shown good resistance at all concentrations upto the boiling point.
Salts – Stainless steels are highly resistant to most neutral or alkaline non halide salts. In some cases, types 316 is preferred for its resistance to pitting, but even the higher Mo type 317 stainless steel is readily attacked by sodium sulphide (Na2S) solutions. Halogen salts are more corrosive to stainless steels because of the ability of the halide ions to penetrate the passive film and cause pitting. Pitting is promoted in aerated or mildly acidic oxidizing solutions. Chlorides are normally more aggressive than the other halides in their ability to cause pitting.
Gases – At lower temperatures, most austenitic stainless steels resist chlorine or fluorine gas if the gas is completely dry. The presence of even small amounts of moisture results in accelerated attack, especially pitting, and possibly SCC. At high temperatures, stainless steels resist oxidation primarily because of their Cr content. Increased Ni minimizes spalling when temperature cycling occurs. Maximum temperatures for intermittent service are lower for the austenitic stainless steels, but are higher for most of the martensitic and ferritic stainless steels.
Contamination of the air with water and carbon dioxide (CO2) frequently increases corrosion at high temperatures. Increased attack can also occur because of sulphidation as a result of SO2, H2S, or S vapour. Carburization of stainless steels can occur in carbon monoxide (CO), methane (CH4), and other hydrocarbons. Carburization can also occur when stainless steels contaminated with oil or grease is annealed without sufficient O2 to burn off the C. This can occur during vacuum or inert-gas annealing, as well as in open-air annealing of oily parts with shapes which restrict air access. Cr, Si, and Ni are useful in combating carburization.
Nitriding can occur in dissociated NH3 at high temperatures. Resistance to nitriding depends on alloy composition as well as NH3 concentration, temperature, and pressure. Stainless steels are readily attacked in pure NH3 at around 540 deg C.
Liquid metals – The 18-8 stainless steels are highly resistant to liquid sodium or sodium-potassium alloys. Mass transfer is not expected upto 540 deg C and remains at moderately low levels upto 870 deg C. The accelerated attack of stainless steels in liquid sodium occurs with O2 contamination, with a noticeable effect occurring at around 0.02 % O2. Exposure to molten lead under dynamic conditions frequently results in mass transfer in common stainless alloy systems. Particularly severe corrosion can occur in strongly oxidizing conditions. Stainless steels are normally attacked by molten aluminum, zinc, antimony, bismuth, cadmium, and tin.