Stainless steels have been widely used for elevated-temperature service, so fundamental and practical data concerning their resistance to corrosion are provided.
When stainless steels are exposed at elevated temperatures, changes can occur to the surface film. For example, at mildly elevated temperatures in an oxidising gas, a protective oxide film is formed. In an environment containing sulphur-bearing gases, the film will be in the form of sulphides which may also be protective.
In more aggressive environments, such as gas turbine ducts and stacks, with temperatures above 1600°F (871°C), the surface film may break down with sudden increase in scaling. Depending on alloy content and environment, the film may be self-healing for a period of time, followed by another breakdown.
Under extreme conditions of high temperature and corrosion, the surface film may not be protective at all. Therefore, information provided should serve only as a starting point for material selection, not as a substitute for service tests. (Table 5-11).
• Oxidation
In non-fluctuating-temperature service the oxidation resistance (or scaling resistance) of stainless steels depends primarily on the chromium content. Steels with less than 18% chromium (ferritic grades primarily) are limited to temperatures below 1500 °F (816 °C). Those containing 18-20% chromium are useful to temperatures of 1800 °F (982 °C), while adequate resistance to scaling at temperatures up to 2000°F (1093°C) requires a chromium content of about 25%, with selection of steels such as Types 309 and 310.
• Effect of Atmosphere
Developments with power generation associated with gas turbines and incineration stimulates interest in oxidation in carbon monoxide, carbon dioxide and water vapour environments. Exposure to mild conditions in these environments leads to the formation of the protective oxide film described earlier, but when conditions become too severe film breakdown can occur. The onset of this transition is unpredictable, influenced by alloy composition.
• Sulphidation
Sulphur in various forms and even in relatively small quantities accelerates corrosion in many environments. Sulphur dioxide, hydrogen sulphide and sulphur vapour are among the most corrosive forms. Sulphur vapour and hydrogen sulphide are considerably more aggressive than sulphur dioxide.
Sulphur attack, although closely related to oxidation, is more severe. Metal sulphides melt at lower temperatures than comparable oxides, and they may fuse to metal surfaces. Also, sulphides are less likely to form tenacious, continuous, protective films. Fusion and lack of adherence result in accelerated corrosion.
The resistance of stainless steels to sulphidation depends on chromium content. • Flue Gas
The corrosivity of flue gas containing sulphur dioxide or hydrogen sulphide is similar to that of most sulphur-bearing gases. Accordingly, the corrosion resistance of stainless steels in flue gas environments is improved by increased chromium content, as shown in Table 5-12. Figure 5-3 indicates the effect of chromium content on corrosion in flue gases produced by various fuels.
[Note: Corrosion rates of 1 to 2mm per year have been reported for Types 304 and 316 in the temperature range 1200- 1400 °F (649-760 °C).]
For reducing flue-gas environments, satisfactory material selection requires service tests. Details of elevated temperature properties are to be found in Section 8 of this Manual.
5.7
High Performance Grades
Increase of alloy content or addition of further alloying elements to the basic grades provides stainless steels with characteristics and properties enhanced for specific requirements or “fitness for purpose”.
The “high performance” stainless steels have superior corrosion resistance in a wide variety of aggressive environments when compared with the basic stainless steel grades such as Type 316L. Their superiority in chloride-containing environments results from increasing levels of chromium, nickel, molybdenum and nitrogen for corrosion resistance, and very low carbon contents.
There are three grades of high performance stainless steels - austenitic, ferritic and duplex (austenitic-ferritic). The higher nickel austenitic grades are generally preferred for severe acid service and for resistance to chloride pitting, crevice and stress corrosion cracking in flue gas cleaning equipment ducts and chimneys handling acid condensates. Duplex grades can be selected where higher strength is an advantage and where stress-corrosion cracking could be a problem.
Where site fabrication is an important consideration, the austenitic grades are favoured because of their relative ease of welding.
The high performance stainless steels with improved corrosion resistance are more technically demanding than Type 316L with regard to fabrication requirements.
5.7.1 Austenitic High Performance Stainless Steels
A list of wrought austenitic stainless steels is given in Table 5-12, identified by grade and UNSxix number. The grades in
Table 5-12 are arranged in the order of increasing molybdenum, chromium and nitrogen content, or increasing PRExx
number.
Type 317LMN, for example in sub-group A-2 (Table 5-12), provides improved localised corrosion resistance to Types 316L and 317L, with higher molybdenum and nitrogen contents. Nitrogen, while improving corrosion resistance, also stabilises the austenite so the nickel can be limited for optimum economy. Type 317LMN has been widely used in flue gas desulphurisation (FGD) equipment operating under moderately aggressive pitting conditions.
The 6% Mo grades, frequently called the “super” stainless steels, include AL-6XN, 1925hMo, 25-6MO and 254 SMO in sub-group A-4, were designed originally to resist localised corrosion in seawater at near-ambient temperatures by the use of relatively high levels of nitrogen, chromium and molybdenum to give a PRE number in the range of 40 to 44. They have been widely used in a variety of power plant applications.
Higher strength is obtained with high nitrogen alloys such as 4565S.
The highest level of performance combining high strength with outstanding localised corrosion resistance, good chloride stress corrosion cracking and acid resistance are provided in sub-group A-6. Significantly, they approach the nickel base alloys with respect to localised corrosion resistance, while providing much higher strength. These newer grades have an excellent potential for solving crevice corrosion problems in gasketed joints.
5.7.2 Duplex High Performance Stainless Steels
Duplex stainless steels, detailed despite limited application in FGD systems, ducts and chimneys, have a microstructure of approximately equal portions of austenite and ferrite with properties that take advantage of the better attributes of each of the two phases. These grades offer very high strength along with useful ductility and toughness. However, duplex stainless steels require careful fabrication to maintain the optimum structure, so are more demanding than the austenitic stainless steels, the probable reason why they find less application in FGD and chimneys. Table 5-13 lists wrought high performance duplex stainless steels. It should be borne in mind that duplex stainless steels can be subject to “885°F (474°C) embrittlement”.
The 25Cr duplex grades, sub-group D-3, such as Ferralium 255, use higher levels of chromium to produce better localised corrosion resistance. The chromium provides very good resistance to oxidising acids. They require higher nickel to balance the higher chromium, which improves resistance to reducing acids as well.
xviii For completeness, a wide range of materials are referenced in order to facilitate choice and availability xix Metals and alloys in the Unified Numbering System. Society of Automobile Engineers, USA.
xx The PRE was developed from “Pitting Index” value obtained from ambient temperature seawater tests. If the “Pitting
Index” based on chromium and molybdenum contents gave a value greater than 32, the alloy was resistant to pitting. If the value was greater than 36 the alloy was resistant to crevice corrosion in seawater. The inclusion of nitrogen in the calculation led to PRE and its use for FGD material selection. The PRE number (pitting resistance equivalent) is determined from PRE = %CR + 3.3 x %Mo + 16 x %N. A higher PRE number indicates a greater resistance to localised corrosion in chloride-containing environments.
Sub-group D-4 is the most highly alloyed sub-group of the duplex family. The high chromium, molybdenum, nickel and nitrogen content produces the best corrosion resistance of any of the duplex grades and higher strength than is obtainable in any high performance stainless steel. For this reason, these alloys are sometimes called “super duplex” stainless steels.
5.7.3 Mechanical Properties
The nitrogen-containing austenitic grades, together with duplex steels, have properties well above standard grades, permitting use of thinner sections to offset the increased cost of higher alloy contents.
5.7.3.1. Austenitic Stainless Steels.
The austenitic grades provide an excellent combination of strength, ductility and toughness over a broad temperature range. Most of the alloying elements used to improve corrosion resistance strengthen the steel, Figure 5-4. The effect of nitrogen on strength is shown in Figure 5-5, where a near 50% yield strength increase over Type 304 stainless steel is indicated for a nitrogen content of 0.20%.
The minimum ambient temperature mechanical property requirements for these grades as defined by the ASTM Standard Specification for plate, sheet and strip (A240) are provided in Table 5-14. Mechanical properties also improve with increased alloy and nitrogen contents, illustrated in Figure 5-6, where grades with increasing alloy content are compared with values for Type 316L. The ASME Code allowable design stress values given in Table 5-15 confirm this with values for some high performance grades more than twice those of Type 316L.
The high performance austenitic grades also retain their advantage over the standard grades at elevated temperatures. Typical short-term elevated temperature data compared with Type 316L are given in Figure 5-7. These grades can provide useful performance at considerably higher temperatures than the ferritic and duplex grades.
5.7.3.2. Duplex Stainless Steels.
Strength of the duplex grades increases and ductility decreases as the level of alloying increases, especially nitrogen content. Minimum yield strengths for sheet and plate are as high as 550 MPa (80 ksi). Table 5-16.
The elevated temperature properties, Figure 5-8, and ASME Code design stresses are given in Table 5-17.
5.7.4 Physical Properties
Ambient temperature physical properties for austenitic and duplex steels are given in Table 5-18 & Table 5-19. Elevated temperature values are detailed in Section 8, Table 8-8 & Table 8-9. Data are included for one or more standard grades to provide a basis for comparison, based primarily upon manufacturers’ data.
Young’s Modulus for the ferritic grades is about 200 GPa (29.0 x 103 ksi), and about 185 GPa (27.0 x 103 ksi), for the high-
nickel austenitic grades. Figure 5-9.
Among the austenitic grades, increased nickel contents lower thermal conductivity and expansivity. Thermal conductivity and coefficients of thermal expansion are shown as a function of temperature in Figure 5-10 & Figure 5-11.
5.7.5 Corrosion Resistance of High Performance Stainless Steels in Flue Gas Environments.
The outstanding corrosion resistance of the high performance stainless steels is due not only to their high alloy content, but also to the interaction of high chromium levels with other alloying elements. For example, molybdenum becomes more effective for chloride pitting resistance as chromium content increases.
5.7.5.1. Resistance to Inorganic Acids.
The presence of chlorides or other halides can lead to pitting when a stainless steel would otherwise be expected to display stable passive behaviour, for example when considering performance in sulphuric acid solutions.
Higher molybdenum austenitic grades can give better resistance. This is illustrated in Figure 5-12, which shows corrosion data for acid solutions containing 200 and 2,000 ppm chloride ion. The performance of the grades in sub-groups A-4 to A- 6 under these conditions makes them candidates for handling combustion product acid condensates, which often contain chloride, at moderate temperatures.
5.7.5.2. Sulphurous Acid.
Sulphurous acid is a relatively weak acid that is normally encountered as condensate in flue gases containing sulphur dioxide. It will cause pitting in Type 304, but can usually be handled with Type 316 provided it is not accompanied by sulphuric acid and chloride or fluoride ions. However, many flue gases can be very acidic and contain halide ions. In these instances, the high performance stainless steels and nickel alloys will offer substantially better corrosion resistance than Type 316L.
5.7.5.3. Chloride - and Other Halide Ion-Containing Aqueous Environments.
The pitting and crevice corrosion of stainless steels are similar, but differ in their initiation, occurring by a local breakdown of the passive chromium oxide film. There is then the localised development of an anodic corrosion site surrounded by a cathodic area that remains passive. Crevice corrosion occurs in the presence of a solid deposit, gasket, or some other crevice former to initiate corrosion. Otherwise, the two forms of corrosion are essentially identical, with resistance to crevice corrosion usually less than to pitting. Because most structures will contain crevices, crevice corrosion is more important from an engineering standpoint. Precautions that should be followed during fabrication are well documented. Post-weld removal of surface oxide (heat-tint)xxi is an essential requirement for obtaining satisfactory stainless steel weld
corrosion performance in high chloride-containing acid environments. The general environmental effects which promote crevice corrosion in stainless steels include high chloride concentrations, high acidity (low pH), high temperature, high dissolved oxygen content, and any environmental constituent which raises the corrosion potential such as oxidising metallic ions and dissolved chlorine gas. All these factors must be considered in relation to whether any grade of stainless steel will be suitable for a given situation or whether a nickel base alloy should be considered. Again, it is emphasised that materials producers should be consulted.
5.7.5.4. Ranking of Individual Grades
The evaluation of any grade of stainless steel for its “localised corrosion resistance” is often difficult because of the many variables involved. A comparative test to ASTM Standard Test Method G 48 uses ferric chloride to produce results that define a “critical pitting temperature” or a “critical crevice corrosion temperature”. Care must be taken when interpreting data as the ferric chloride test environment is very aggressive and does not provide results that translate directly to practical environments.
Data for representative grades evaluated in 10% ferric chloride are given in Figure 5-13. It shows that some critical temperatures are much lower than the temperatures to which standard stainless steels are often exposed in service, illustrating the severity of the test. For this reason it cannot be used as a basis for establishing service temperature limits, but has found use as a quality control methodxxii.
The high performance austenitic stainless steels far out-perform the standard Type 316L grade in this test as Figure 5-13 shows, approaching that of nickel base alloys. Performance of the different grades varies with chromium, molybdenum, and nitrogen alloying contents. Various formulae have been developed to relate composition to critical corrosion temperatures. The most commonly used expression gives a pitting resistance equivalent (PRE) number, for example,
PRE = %Cr + 3.3%Mo + 16%N.
Some typical correlations of the PRE number with several critical temperature indices, Figure 5-14, show the strong alloying effect of nitrogen, molybdenum and chromium, in that order.
5.7.5.5. Acidic Environments Containing Halides - Flue Gas Condensates.
Mildly acid aqueous environments containing halides (but not strong oxidants) can be handled by many of the high performance stainless steels, provided the temperature and halide concentrations remain relatively low. The likelihood of pitting and crevice corrosion increases with acidity, temperature, halide content and, especially, with reducing conditions which could lead to general corrosion, conditions frequently found in flue gas cleaning equipment, ducts and chimneys. Fuels that contain sulphur or chlorine produce the most corrosive combustion products, the most common examples being high sulphur coals, fuel oils and municipal waste.
Condensate (and scrubbing liquor) will become acidic with the SO2 and reducing with the SO3 and HCl in flue gases. The
design and method of operation of the gas-handling system will also contribute greatly to the severity of corrosive conditions that may develop. In general, raw gas condensate and gas recycling together will produce the most corrosive
xxi NiDI Publication reference 10068
xxii Quality control systems for the installation of a Hastelloy chimney lining using the wallpapering technique,
conditions. The “generic FGD system” illustrated in Figure 5-1 below defines various locations within the Flue Gas Desulphurisation system in terms of relative potential for corrosion. Zones having different degrees of corrosivity are indicated A - H. Each zone is defined in terms of both qualitative and quantitative severity in Figure 5-1. From the standpoint of metallic corrosion, the most severe zones all involve ductwork and stacks or wet/dry conditions where the pH can be <0.1 and temperatures can be as high as 182 °C (360 °F). Locations that are washed or that handle absorbent are mild or moderately corrosive.
Qualitative description of scrubber operating zones
Code Chemistry Environment Mechanical Temperature A Mild corrosive (vapour) Mild Mild
B Moderate (immersion) Mild Mild
C Moderate Moderate Mild
D Moderate Severe Mild
E Severe Mild Moderate
F Severe Mild Severe
G Severe Severe Severe
H Moderate Severe Moderate
Figure 5-1 Operating zones in a generic FGD system as defined in ASTM STP 837
The generic FGD system does not account for chloride and fluoride levels, or any operating variables such as deposit buildup, so further information is needed to assist materials selection. Stainless steels Type 316L and 317L exposed in a large number of commercial SO2 scrubbing environments, Figure 5-15 & Figure 5-16 indicated the detrimental effect of
high cloride and acidity, increasing the tendency for localised pitting or crevice corrosion. However, experience has shown that the more highly alloyed stainless steels are needed with even moderate increase of chloride plus fluoride levels and that nickel base alloys are necessary for locations handling raw gas condensate at high temperature.
The approximate behaviour of representative grades is shown in Figure 5-17 indicating that a wide range in performance and cost-effectiveness is available. The localised corrosion predictions as a function of chloride and pH in Figure 5-15 and Figure 5-16 should not be used to estimate performance for the very severe condition of raw acid condensate that may occur in ducting and stacks. When the pH begins to fall below about 1.0, the corrosion mode for most stainless steels, including the high performance grades, begins to shift toward general attack. Corrosion data for acid solutions are more applicable for these conditions. General experience has indicated that only the most highly alloyed nickel base alloys will be useful in ducting or stacks where raw acid condensate is likely to form. An exception may be the newest sub-group A-6, the austenitic high performance stainless steels which have outstanding resistance to strong acids containing chloride. The disadvantage of titanium in strong fluoride-containing acids was also confirmed by these tests.
5.8
Corrosion Acceptance Tests
The concept of using corrosion tests to verify a given particular property of a stainless steel is well established as a useful tool for the evaluation of mill products and for evaluating equipment after fabrication. In most cases, the test demonstrates the absence of a particular problem such as grain boundary carbides or intermetallic phases.
ASTM G48, which measures pitting and crevice corrosion resistance, differs from these tests because it describes only the laboratory procedures without defining the acceptance criteria. Fortunately, G48, in its focus on chloride-induced localised corrosion, is directed toward the corrosion quality criterion of most importance to the high performance stainless steels. The test method is also extremely sensitive to the effects of intermetallic phases and is applicable to all alloy types – austenitic, ferritic and duplex. Therefore, producers and users often use it as a corrosion acceptance test for these alloys when localised corrosion is a consideration. It is important to recognise that G48 does not define acceptance criteria for given alloys because any criterion will depend on factors such as the application, method of fabrication, etc., and mutual