Domestic and international applications of duplex stainless steel

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         Domestic and international applications of duplex stainless steel 1. Oil refining industry & A& r4 Y7 e5 L$ Y. |- V W This is one of the fields where duplex stainless steel is widely used; it is primarily employed in units such as atmospheric and vacuum distillation, catalytic cracking, and hydrodesulfurization. There are successful application cases both domestically and internationally, as well as instances of failure. ( G# X/ S. B2 V ① Atmospheric and vacuum distillation units: The low-temperature light oil section at the top of the tower, as well as the condensation and cooling systems, are exposed to a corrosive environment characterized by HCl-H2S-H2O at temperatures below 120°C. Especially when processing crude oils with high salt and sulfur content, the Clˉ concentration can reach 2000–4000 mg/L, with typical values around 100 mg/L; H2S levels are generally between 32–65 mg/L. In such an environment, carbon steel suffers from severe uniform corrosion, 0Cr13 experiences pitting corrosion, while austenitic stainless steels suffer from stress corrosion cracking. When deep desalination of crude oil and comprehensive anti-corrosion measures are employed, Clˉ can be kept at 20–30 mg/L and H2S at ≤32 mg/L. However, if the oil is complex and the demulsifier is not effective enough, desalination may fail to achieve the desired level of Clˉ control, which can still lead to equipment corrosion.   As early as the 1970s, the Nanjing Refinery and Jinan Refinery—which used oil transported via the Luning Oil Pipeline—adopted 00Cr18Ni5Mo3Si2 duplex stainless steel in this system. This material is primarily used for the lining (or clad plates) at the tops of atmospheric and vacuum distillation towers, internal tower components, atmospheric tower top air coolers, vacuum tower top humidified air coolers, tube bundles of post-vacuum tower cooling water chillers, and linings of oil-water separators. It has yielded excellent performance in service; some components made from this material have been in use for up to 20 years. Typical medium conditions (taking condensate water as an example): % }/ u- D" a* |& q$ 3 k4 N3 o- w# j7 c- c: In Sweden, 10 air-cooled units have been using SAF 2205 steel since 1983; the KP value of the effluent (where the corrosion coefficient KP = mol%NH3 × mol%H2S) ranges from 0.1 to 1, with a Clˉ concentration of 20 ppm, a pressure of 12 MPa, an inlet flow rate of 6 m/s, and an inlet temperature of 130°C. After several years of operation, no corrosion or wear corrosion has occurred in these units.   At a refinery in California, USA, three heat exchanger tube bundles successively used 3RE60 steel; during an inspection in 1992, they were found to be in good condition. Several air coolers of another US oil company (with a process stream containing 65-85% H2, hydrocarbons, H2S, at 5-8 MPa and 110-205 °C) have been in use for 8-11 years, and the heat exchangers upstream and downstream of them have also not experienced any problems. However, there are also cases of failure. In another U.S. oil refinery, a heat exchanger made of 3RE60 steel was used. The inlet temperature on the tube side was 285°C, and it was maintained at 210°C. The liquid components coming from the low-pressure separator entered the shell side at a temperature of 88°C, which was kept at 220°C. The pressure on the tube side was 11.5 MPa, while the pressure on the shell side was 0.6 MPa. The equipment leaked after 3 years of use in 1994, with cracks appearing on the inner and outer walls of the tube near the tube sheet. Analysis shows that the outer wall of the tube is covered with FeS corrosion products, with a Cl˻ concentration as high as 1%; the inner wall of the tube is relatively clean, with only a few pitting spots. There are few cracks on the outer wall, which are due to chloride stress corrosion cracking; there are many cracks on the inner wall, which are due to sulfide stress corrosion cracking (SSCC). ' w3 P& j: U5 A9 R0 p" P8 P. In other refineries in the Middle East and Eastern Europe, similar problems occurred in 1992 after the 3RE60 air coolers had been in operation for several years; the cracks appeared in the heat-affected zones at the welds between the tubes and the tube sheets, where the ferrite content was as high as 100% and >75% respectively. Such microstructures inevitably make these components susceptible to hydrogen-induced cracking (HIC).   Looking at these failure cases, most are related to equipment manufacturing technology; however, it seems that despite years of experience in use, the fact that upper limits for safe and reliable use of duplex stainless steel in hydrogenation systems have not yet been established is also a contributing factor. ) {0 V8 s3 T0 j4 A2. Petrochemical and chemical industry) The corrosive environments in the petrochemical and chemical industry are characterized by high reaction temperatures, and the media often contain high or moderate concentrations of chlorides, which can easily lead to stress corrosion cracking in stainless steel. In this field, not only duplex stainless steel is used, but also super duplex stainless steel to a greater extent. : k1 d9 v7 g* P① Vinyl chloride production plant: The corrosive environment in which vinyl chloride is produced by the oxychlorination method is quite severe. Stainless steels of various grades are used to manufacture the vessels, heat exchangers, and piping systems in this plant. The most severe corrosion conditions occur when there is contact with hydrochloric acid; super duplex stainless steels are commonly used for manufacturing key equipment such as the oxychlorination reactor, HCl cooler, and vinyl chloride tower. 4 ] S, h t6 \9 }) c/ s& v& O In the oxygen chlorination reactors of Qilu Petrochemical Company in China, imported SAF 2205 steel tubes are used for the cooling coils; this company is covered in detail in articles on this website. Super duplex stainless steel is not used in China, and it remains to be seen what the results of using such materials will be. ' v' e; U' ]" l& s6 U6 {- r The oxygen chlorination reactor is of the coil type, where dichloroethane reacts with HCl in the presence of a catalyst to produce vinyl chloride. Process conditions: - y. S3 C: q. P4 E" S Pipe side: water ； 9 a2 O3 s2 X6 e7 S# | Shell side: HCl, water vapor, dichloroethane, 230°C, 0.2 MPa % p8 @7 F; U F9 z7 | The gas exiting the oxychlorination reactor is quenched and washed before entering the pre-condenser (air cooler); the tube bundles equipped with aluminum fins are typically made of 3RE60 steel or 2205 steel abroad. The vinyl chloride reboiler is the same: EDC flows inside the tubes, along with trace amounts of hydrochloric acid, at 100°C and 1.2 MPa ； Steam is passed through the shell side at 200–230°C and 1.3 MPa. The 3RE60 steel showed no damage after 5 years. ②The methanol synthesis reactor # Y$ s, n' ]* q+ r/ |2 t. Methanol is an important basic organic chemical raw material; it is a key product in carbon chemistry, serves as a starting point for the chemical processing of syngas, and can also be used as a solvent or as a fuel for vehicles. In the early 1980s, domestic low-pressure methanol synthesis pilot plants were already in operation; to this day, a large-scale methanol synthesis plant with a capacity of 200,000 tons that uses coal as raw material has been in use for several years, while dozens of other smaller and medium-sized plants are also operating. The equipment that utilizes the most duplex stainless steel tubes within methanol synthesis reactors is the small-sized reactors; their catalyst tubes are mostly made of 00Cr18Ni5Mo3Si2 steel. For medium- and large-sized reactors, 00Cr22Ni5Mo3N steel is used instead. Operating conditions: Inner tube side: CO, H2, CH3OH synthesis gas, 265°C, 5.2 MPa : y. x6 N2 e& C! r Outer tube side: Steam, deionized water, 250–265°C, 3.9–5.1 MPa : z# h% o" X7 T7 ^0 J High-pressure equipment made of duplex stainless steel shows no signs of corrosion after 10 years of use. Abroad, in addition to reactors, duplex stainless steel is also used for other components such as heat exchangers for materials and effluents, as well as piping systems. - @1 m0 i/ @' t( w8 X$ m   The application of duplex stainless steels in acidic environments primarily involves determining their safe operating limits. The use of Cr18 and Cr22-type duplex stainless steels is relatively well-established. NACE MR0175 specifies the safe operating limits for SAF 2205 steel in acidic environments: the maximum operating temperature is ≤232°C; the partial pressure of H2S must be ≤0.3 psi (2.1×10⁻³ MPa); σ0.2 should be ≤145 ksi (1015 MPa); and the hardness on the Rockwell C scale should be ≤34. Shell Corporation has further proposed guidelines for using SAF 2205 steel. In addition to the requirements outlined in MR0175, it is permitted to raise the partial pressure of H2S to ≤0.9 psi (6.3×10⁻³ MPa) when the temperature ranges between 177–232°C; however, this applies only to linings used in production pipelines. Additionally, when using duplex stainless steels, hydrochloric acid should be avoided as an acidifying agent; instead, degassed, clean brine should be utilized. Super duplex stainless steels are also employed in the oil and gas industry; for instance, UR52N+ is used in oil and gas gathering and transportation pipelines in the North Sea. SAF 2507 steel is utilized in oil and gas wells, offshore platforms, and pipelines in Alaska, the North Sea, the Gulf of Mexico, etc., especially in harsh acidic environments where no corrosion inhibitors can be used. It remains difficult to promote the use of duplex stainless steels in domestic oil and gas pipelines due to their high cost. Nevertheless, they can still be used in equipment requiring resistance to Cl⁻ and H2S, such as gas gathering pipelines and heat exchangers. Currently, they are only used in limited quantities in the South China Sea oilfields, with all such materials being imported. Regarding the major West-to-East Gas Transmission project aimed at developing the western region, the gas collection pipelines starting from the Tarim Basin might consider using large-diameter welded pipes made of duplex stainless steel, and domestic production capabilities for such materials already exist. 4. Pulp and paper industry: Duplex stainless steel has been in use in the foreign pulp and paper industry for 30 years now, and it is one of the industries that makes the most use of this type of steel. The pulp and paper industry is highly developed in Scandinavia, where almost all pressure vessels used in this industry are manufactured from duplex stainless steel. The Pori factory in Finland has produced over a hundred such pressure vessels, with the largest steaming vessels having a capacity of 400 m3. Most of these vessels are made from 2205 steel; other duplex and superduplex stainless steels such as UR47N (00Cr25Ni6.5Mo3N) and UR52N+ (00Cr25Ni6.5Mo3.5CuN) have also been utilized. These materials have proven effective in manufacturing wood chip pre-steaming units, batch and continuous pulp steaming vessels, papermaking pressure rollers, and recycling equipment. In continuous sulfuric acid steaming systems, the Clˉ concentration in wood chip pre-steaming units can reach up to 8000 ppm; 316L steel can only be used for one year under such conditions, while UR47N can remain corrosion-free for four years. Corrosion in continuous steaming units is less severe than that in batch steaming units, so 304L or 316L composite sheets are often used. In recent years, due to wear corrosion and stress corrosion, 2205 steel has been adopted, mainly for use in the lower parts of steaming units. In batch steaming units, the cooking liquid consists of Na2S, Na2SO4, Na2CO3, NaCl, and 20% NaOH. Currently, there are dozens of batch steaming units around the world that are made from UR45N (equivalent to 2205) or UR47N composite sheets or whole sheets, with a service life of over 10 years. In chlorine dioxide bleaching systems, the typical operating conditions are pH 6–7, 70°C, ≥600 ppm Clˉ, 300 ppm ClO3ˉ, and 20–200 ppm ClO2. In such conditions, although UR45N (SAF 2205) performs better than 316L steel, pitting corrosion still occurs. It is better to use superduplex stainless steels such as UR52N+ and SAF 2507, which can compete with austenitic stainless steels containing 6% Mo. Domestic use of these materials is virtually non-existent, though in recent years some papermaking equipment has been imported from abroad. Fertilizer industry: With the development of duplex stainless steel, its use in the urea and phosphate fertilizer industries has increased. In the urea industry, wrought products are commonly used, while in the phosphate fertilizer industry, cast products are preferred. In the urea industry, high-alloy austenitic stainless steels, or even titanium materials, are currently used in the CO2 stripping and NH3 stripping processes for urea production. In China, high-chromium duplex stainless steel is now being used in the pump bodies of ammonium methanate pumps in large-scale urea plants, replacing 316L (urea-grade) steel. Since the 1980s, in the ACES process for urea production developed by Toyo Engineering Corporation, three high-pressure components—the stripping tower, ammonium methanate condenser, and high-pressure decomposition tower—have been made from DP-12 (a 25Cr-7Ni-3Mo-N steel containing trace amounts of tungsten and copper) and R-5 (00Cr25Ni6.5Mo1.5N) duplex stainless steel. The former is used for making stripping tubes, condensation tubes, decomposition tubes, and delivery pipes, while the latter is used for the shells of high-pressure components. The ACES process equipment has been exported to countries such as Spain, **, and Indonesia; ** has also introduced large-scale plants with an annual urea production capacity of 520,000 tons. Since the 1980s, China has begun researching and using duplex stainless steel in urea-related applications. Initially, 00Cr18Ni5Mo3Si2 duplex stainless steel was used in place of 304L tube bundles in the three-stage coolers of the CO2 compressors at the Qixia Mountain Fertilizer Plant. The 304L steel started leaking after just one month of use, as the operating conditions for these devices are extremely harsh: the inlet temperature of the wet CO2 in the tube side is 180°C, while the outlet temperature is 45–50°C, with a pressure of 8–9 MPa. The cooling water on the shell side contains 80–100 ppm of Clˉ and 6–10 ppm of O2; its inlet temperature is 32–34°C, and the outlet temperature is 80°C, with a pressure of 0.4 MPa. Cracks in the tubes occurred at the junctions where the high-temperature end tube sheet met the tubes, as well as at the gaps in the tube sheets. These were typical cases of intergranular SCC. The duplex stainless steel was used for nearly 5 years before the second and third stage coolers were also equipped with this material; currently, 00Cr22Ni5Mo3N steel is being used. Regarding pumps and valves made of duplex stainless steel, their excellent resistance to corrosion fatigue, thanks to their duplex structure, makes them very suitable for manufacturing key equipment used in urea production, such as ammonium carbamate pumps. The selection of materials for the body of ammonium carbamate pumps depends on the production process used in urea manufacturing. Currently, many large-scale fertilizer plants in China employ the stripping process; under this process, the temperature around the pump body is approximately 75°C. Smaller and medium-sized fertilizer plants generally use the total recycle process, where the temperature is around 95°C. For both of these processes, it is feasible to use 316L steel (urea-grade). However, using duplex stainless steel with better resistance to corrosion fatigue would yield even better results. Given that the structure and operating conditions of large pump bodies are quite complex, the materials must pass the intergranular corrosion test according to the Huey method. Therefore, Cr25-type duplex stainless steels such as 329 (0Cr25Ni5Mo2), 00Cr25Ni6Mo2N, and even 00Cr25Ni7Mo3N are utilized. The latter two types are commonly used in China. For instance, since 1987, Qixiashan Fertilizer Plant has been using single-casing pumps made from 00Cr25Ni6Mo2N steel. Subsequently, Dongting Nitrogen Fertilizer Plant and Heilongjiang Fertilizer Plant began using three-casing pumps made from 00Cr25Ni7Mo3N steel. Most smaller and medium-sized fertilizer plants in China opt for 00Cr18Ni5Mo3Si2 steel; however, many manufacturers also choose 00Cr25Ni6Mo2N steel. Although Cr18-type steels fail to meet the requirements of the Huey test, they are relatively inexpensive and have a longer service life compared to previously used 0Cr18Ni12Mo2Ti or 316 steels.

Another issue relates to the materials used for check valves at the inlet and outlet of ammonium carbamate pumps, as well as high-pressure globe valves along the main process pipelines. These materials must exhibit good resistance to wear and corrosion. Some domestic plants, such as Chishui Natural Gas Fertilizer Plant and Luzhou Natural Gas Chemical Plant, utilize valve components made from 00Cr25Ni6Mo2N steel in their ammonium carbamate and liquid ammonia pumps. At the Triamine Plant of Sichuan Chemical Industry Group, valve components for check valves on ammonium carbamate pumps and various high-pressure globe valves along the main process pipelines are made from 0Cr25Ni6Mo3CuN steel; these materials perform satisfactorily. However, from the standpoint of corrosion resistance, 0Cr25Ni6Mo3CuN steel is an age-hardening duplex stainless steel with superior resistance to wear and corrosion compared to 00Cr25Ni6Mo2N steel. The latter, however, offers better resistance to corrosion fatigue, making it more suitable for fabricating pump bodies.

8. Phosphate Fertilizer Industry
Pure phosphoric acid isn’t particularly corrosive; nevertheless, impurities present in the medium—such as hydrofluoric acid, fluorosilicic acid, excess sulfuric acid, chlorides, and fluorides—can significantly increase its corrosiveness. At present, over 95% of phosphoric acid is produced by the so-called wet process, which involves leaching phosphate rock with sulfuric acid. The effect of temperature, impurities such as F⁻ and Cl⁻, as well as wear and corrosion of the slurry, all increase the corrosivity of the medium. Therefore, the composition of phosphate rocks and the process conditions have a significant impact on the corrosion of materials. In the production of wet-process phosphoric acid, the wear and corrosion of the components in slurry pumps and concentrated phosphoric acid pumps are particularly severe; these pumps are key equipment in phosphate fertilizer production. Most phosphate rocks in China contain more than 10% SiO2, and the concentrations of ions that accelerate corrosion, such as Clˉ and Fˉ, vary greatly. For example, the concentration of Cl3ˉ ranges from 100 to 1000 ppm, while the concentrations of ions that inhibit corrosion, such as Mg2+, Al3+, and Fe3+, also differ. Overall, these rocks exhibit moderate corrosivity, but due to their high silicon content, they cause severe wear. Hence, duplex stainless steel, which offers good resistance to wear and corrosion, holds promise for use in such applications. In domestic facilities for producing nitrate phosphate fertilizers, the operating conditions are not extremely harsh, and slurry pumps are made from American ACI CD-4MCu (Cr26Ni5.5Mo2Cu3) cast steel, a material that is well-established in foreign countries. In facilities for producing ammonium phosphate using the dihydrate method, the flow components of horizontal centrifugal pumps for concentrated and dilute phosphoric acid are made from domestically produced CD-4MCu castings.*