2205 Duplex Stainless Steel for Pharmaceutical and Biotechnology Applications
Introduction
In the pharmaceutical and biotechnology industries, hygiene, corrosion resistance, and durability are paramount, and selecting suitable materials for manufacturing processes is critical. 2205 Duplex Stainless Steel is an optimal choice due to its unique combination of strength, corrosion resistance, and toughness. Known for its high resistance to stress corrosion cracking, pitting, and crevice corrosion, this material provides a reliable and cost-effective solution for equipment used in harsh environments. With its balanced microstructure of austenite and ferrite, 2205 Duplex Stainless Steel also offers superior weldability and mechanical performance, making it a preferred choice in applications ranging from pharmaceutical manufacturing to biotechnology research.
This blog explores the properties, applications, and benefits of 2205 Duplex Stainless Steel in these demanding fields.
Materials of Construction
The pharmaceutical and biotechnology industries have relatively high hygienic requirements, and the construction materials for processing vessels and piping systems must demonstrate outstanding corrosion resistance and cleanability to ensure the purity and integrity of the drug product. Materials must withstand the temperature, pressure, and corrosive nature of the production environments and all sanitizing and cleaning procedures. In addition, candidate materials must have good weldability and be capable of meeting the industry’s surface finish requirements.
The primary construction material for processing equipment in the pharmaceutical and biotechnology industries is Type 316L (UNS S31603, EN 1.4404) austenitic stainless steel. The corrosion resistance, weldability, electropolishing properties, and availability of the 316L grade make it an ideal candidate for most pharmaceutical applications. Although Type 316L performs well in many process environments, users are continually looking to enhance its properties through judicious selection of special 316L chemical compositions and enhanced production processes such as electroslag remelting (ESR).
Figure 1. Pharmaceutical R&D vessel fabricated from 10 gauge 2205 duplex stainless steel sheet and 3/16 inch (4.8 mm) thick 2205 plate. Product contact surfaces were electropolished to an ASME BPE – SF4 finish.
When process environments are too aggressive for Type 316L, users have either accepted the increased maintenance costs of a 316L system or moved to more highly alloyed 6% Mo super austenitic stainless steels such as AL-6XN® (UNS N08367) or 254 SMO® (UNS S31254, EN 1.4547). The biotechnology industry has recently recognized the benefits of constructing processing equipment from Type 2205 (UNS S32205, EN 1.4462) duplex stainless steel.
What is 2205 Duplex Stainless Steel?
316L stainless steel contains an austenite phase with a minimal amount of ferrite, which results from the addition of sufficient nickel to stabilize the austenite. The nickel content in wrought 316L stainless steel ranges from 10% to 11%. Duplex stainless steels are designed to maintain a balanced microstructure with approximately equal proportions of austenite and ferrite. Regarding 2205 duplex stainless steel, manufacturers lower the nickel content to around 5% while increasing the levels of manganese and nitrogen. This adjustment results in a composition that achieves a 40-50% ferrite phase, optimizing the material’s strength and resistance to corrosion. The chemical composition 2205 ensures that the austenite and ferrite phases exhibit comparable corrosion resistance. The term “duplex” refers to this combination of the two microstructural phases.
The increased nitrogen content and fine-grained microstructure of 2205 duplex stainless steel result in higher strength levels than common austenitic grades such as Types 304L and 316L stainless steel. In the solution-annealed condition, the 2205 grade has about double the yield strength of Type 316L. Depending on the design code used in the construction of the processing facility, this higher strength can result in much higher allowable stresses for the 2205 grade. In many applications, this allows the reduction of the wall thickness, resulting in reduced weight and cost savings.
Figure 2. (A) The microstructure of a wrought Type 316L stainless steel shows austenite grains and an occasional ferrite stringer.
Table 1. Comparison of chemical composition ranges for Type 316L and 2205 stainless steel based on ASTM A 240 requirements*.
| Grade | UNS No. | C | Mn | P | S | Si | Cr | Ni | Mo | N |
| 316L | S31603 | 0.03 | 2.00 | 0.045 | 0.030 | 0.75 | 16.0–18.0 | 10.0–14.0 | 2.0–3.0 | 0.10 |
| 2205 | S32205 | 0.03 | 2.00 | 0.030 | 0.020 | 1.00 | 22.0–23.0 | 4.5–6.5 | 3.0–3.5 | 0.14–0.20 |
*Maximum unless the range is indicated.
Table 2. Comparison of the mechanical properties of solution-annealed, dual-certified Type 316/316L and 2205 stainless steel according to ASTM A240*.
| Grade | UNS No. | Tensile Strength | Yield Strength | Elongation | Hardness, max | |||
| Ksi | MPa | Ksi | MPa | Brinell | Rockwell | |||
| 316/316L** | S31603 | 75 | 515 | 30 | 205 | 40% | 217 | 95 HRB |
| 2205 | S32205 | 95 | 655 | 65 | 450 | 25% | 293 | 31 HRC |
* Minimum unless otherwise indicated.
** Strength minimum for dual-certified 316/316L; the 316L-only certified grade would have even lower minimum strength requirements.
2205 Duplex Stainless Steel Corrosion Properties
Pitting Resistance
In pharmaceutical and biotechnology applications, pitting corrosion is the most common type of corrosion experienced with stainless steel, particularly in chloride-bearing environments. The higher levels of chromium (Cr), molybdenum (Mo), and nitrogen (N) in 2205 stainless steel significantly enhance its resistance to pitting and crevice corrosion compared to 316L stainless steel. The critical pitting temperature (CPT) is measured in a standardized solution, such as 6% ferric chloride, to determine the relative pitting resistance of stainless steel. As demonstrated in Figure 3, 2205 stainless steel exhibits a critical pitting temperature that falls between Type 316L and 6% molybdenum (Mo) super austenitic stainless steels. It’s important to note that while CPTs measured in a ferric chloride solution provide a reliable comparison of chloride pitting resistance, they should not be used to predict pitting behavior in other chloride-bearing environments.
Figure 3. Comparison of the critical pitting temperatures measured in a 6% FeCl3 test solution. (Plotted from data reported in reference 1)
Stress-Corrosion Cracking
At temperatures above 150°F (60°C), the combination of tensile stresses and chlorides can readily crack the 316L grade. This catastrophic attack mode, chloride stress-corrosion cracking (SCC), must be considered when choosing materials for hot process streams. Type 316L should be avoided for applications involving chlorides and temperatures of 150°F (60°C) and higher. As shown in Figure 4, 2205 duplex stainless steel resists SCC in simple salt solutions up to temperatures of at least 250°F (120°C).
Figure 4. Comparison of the chloride stress corrosion cracking thresholds for Type 316L and 2205 duplex stainless steel. (After reference 2, online 2205 datasheet)
Rouging
Stainless steels exposed to high-purity water environments can show a thin surface stain or deposit termed rouge or rouging (Figure 5). This staining consists predominantly of ferric oxide or hydroxide particles, which can have a variety of colors, including shades of red, gold, blue, gray, and dark brown. The cause of rouge remains poorly understood, but material variables, such as the specific grade of stainless steel and the surface finish, can significantly influence its formation.
Figure 5. Split stainless steel tubing showing a gold-colored (A) and dark gray-colored (B) rouge accumulation on the inside diameter.
In the pharmaceutical and biotechnology industries, clean steam and high-purity water environments encountered in water for injection (WFI) systems often show rouge formation. Components such as distillation units, storage tanks, process vessels, pumps, valves, and piping can all be affected.
Because of possible product contamination, heavily rouged surfaces require cleaning, which can be expensive and time-consuming. Therefore, candidate materials for pharmaceutical and biotechnology applications must be at least as resistant to rouge formation as the Type 316L grade. A systematic rouging investigation has included 316L stainless steel and 2205 duplex stainless steel (3, 4). Based on this investigation, the 2205 grade is at least as resistant to rouging as the 316L grade.
2205 Duplex Stainless Steel Fabrication Properties
Cold-forming Operations
In many respects, fabricating with 2205 duplex stainless steel mirrors the process used for 316L stainless steel. However, particular crucial distinctions must be considered. Cold-forming operations require attention to duplex stainless steel’s higher strength and more significant work-hardening tendencies. Fabricators may need to adjust forming equipment to accommodate higher load capacities. Additionally, 2205 duplex steel exhibits more spring-back during forming operations than the standard austenitic grades. Its increased strength also makes it more challenging to machine than Type 316L.
Heat Input and Interpass Temperatures Control
The same welding methods used for Type 316L are applicable for joining 2205 duplex stainless steel. However, it is essential to closely control heat input and interpass temperatures to maintain the desired austenite-ferrite balance and prevent the formation of undesirable intermetallic phases. Introducing a small amount of nitrogen into the welding gas can help prevent these issues. When qualifying a duplex stainless steel weld procedure, industry practice commonly involves assessing the austenite-ferrite ratio using a ferrite gauge or metallographic examination. The ASTM A 923 test methods are often employed to confirm the absence of undesirable intermetallic phases.
Figure 6. Automatic orbital welding of stainless steel tubing
The recommended weld filler for this process is ER2209 (UNS S39209, EN 1600). If you choose to weld without filler (autogenous welding), ensure that the weld undergoes solution annealing after the process to restore its corrosion properties. To perform solution annealing, heat the weld to a minimum of 1900°F (1040°C) and cool it rapidly.
Duplex stainless steels such as 2205 show lower penetration and fluidity than 316L, resulting in lower welding speeds. The reduced penetration of the 2205 grade can require modifying the joint geometry. The 2205 geometries may require wider joint angles, more significant root gaps, and smaller root lands than 316L to achieve full penetration.
If the welding equipment can accommodate a filler wire, you can use 2209 filler wire to make orbital welds for joining 2205 tubes. Alternatively, an appropriate over-alloyed consumable insert can serve as a substitute for the filler wire. Table 3 summarizes the insert materials used to join 2205 tubing. For further guidance on welding duplex stainless steel pipe and tube, refer to Reference (5).
Table 3. Consumable insert materials used to join 2205 tubing
| Insert Materials | Specific Alloys and Grades |
| NiCrMo Alloys | UNS – N06625, N10276, N06022 (EN – 2.4856, 2.4819, 2.4602) |
| 6% Mo Grades | UNS – N08367, S31254, N08926 (EN – NA., 1.4547, 1.4529) |
| Super Duplex Stainless Steel | UNS – S32750, S32760 (EN – 1.4410, 1.4501) |
Electropolishing of 2205 Duplex Stainless Steel
Many pharmaceutical and biotechnology applications require electropolished product contact surfaces. Achieving a high-quality electropolished finish is a crucial material characteristic. The 2205 grade can be electropolished to a smoothness of 15 micro-inches (0.38 micrometers) or smoother, meeting or exceeding the surface finish requirements for electropolished surfaces in the ASME BPE Standard. Although 2205 duplex stainless steel can readily meet the pharmaceutical and biotechnology industry’s surface finish requirements, an electropolished 2205 surface is not as bright and lustrous as an electropolished 316L surface. This difference is due to the tendency for slightly higher metal dissolution rates in the ferrite phase compared to the austenite phase during electropolishing.
Specs. and QC Standards for 2205 Duplex Stainless Steel
The 2205 grade is specified in various North American and European industry and government standards. ASTM A240 lists two variations of this grade: S31803 and S32205. The S32205 grade has slightly higher minimum levels of Cr, Mo, and N but still falls within the chemical composition range of S31803. To address potential corrosion and toughness losses in the heat-affected zone (HAZ) of certain S31803 welds, it is recommended to specify the S32205 grade. Suppose product specifications require the S31803 designation, which may be necessary due to its inclusion in the ASME code. In that case, users should ensure that all S31803 products meet the composition requirements for S32205 to maintain consistent properties.
Table 4 summarizes essential ASTM product and quality control standards for the 2205 grade, and Table 5 lists some relevant international standards for duplex stainless steel. The latest edition of the ASME BPE-2009 Standard includes 2205 duplex stainless steel, providing specific guidelines for its use in the design, materials, fabrication, and testing of process equipment. Additionally, duplex stainless steels are specified in the API 650 Standard, which outlines the design, materials, fabrication, erection, and testing requirements for vertical, cylindrical, and aboveground tanks.
Table 4. Summary of ASTM product and quality control standards for 2205 duplex stainless steel
| Product Form | Relevant ASTM Specification |
| Forged Pipe Flanges & Fittings | A182 – Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service |
| Plate, Sheet, & Strip | A240 – Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and General Applications |
| Seamless & Welded Sanitary Tubing | A270 – Seamless and Welded Austenitic and Ferritic/Austenitic Stainless Steel Sanitary Tubing |
| Bar & Shapes | A276 – Stainless Steel Bars and Shapes |
| Seamless & Welded Tubing | A789 – Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing for General Service |
| Seamless & Welded Pipe | A790 – Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe |
| Pipe Fittings | A815 – Wrought Ferritic, Ferritic/Austenitic, Martensitic Stainless Steel Piping Fittings |
| Welded Pipe | A928 – Ferritic/Austenitic (Duplex) Stainless Steel Pipe Electric Fusion Welded with Addition of Filler Metal |
| Castings | A890 – Standard Specification for Castings, Iron-Chromium-Nickel-Molybdenum Corrosion-Resistant, Duplex (Austenitic/Ferritic) for General Application
A995 – Standard Specification for Castings, Austenitic-Ferritic (Duplex) Stainless Steel, for Pressure-Containing Parts |
| Quality Control | A923 – Detecting Detrimental Intermetallic Phase in Duplex (Austenitic/Ferritic) Stainless Steels |
| Corrosion Testing | G48 – Standard Test Method for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution |
Table 5. Applicable European material standards
| EN 10028-7 | Flat products for pressure purposes – Stainless steel |
| EN 10088-2 | Stainless steels – Corrosion-resisting sheet/plate/strip for general and construction purposes |
| EN 10088-3 | Stainless steels – Corrosion-resisting semi-finished products/bars/rods/wire/sections for general and construction purposes |
| EN 10217-7 | Welded steel tubes for pressure purposes – Stainless steel tubes |
| EN 10272 | Stainless steel bars for pressure purposes |
| EN 10296-2 | Welded circular steel tubes for mechanical and general engineering purposes – Stainless steel tubes |
| VdTÜV WB 418 | Ferritisch-austenitischer Walz- und Schmiedestahl, 1.4462 |
| Norsok M-CR 630, MDS D45 | Piping material data sheets for duplex stainless steel |
Conclusion
In conclusion, 2205 Duplex Stainless Steel offers outstanding performance for pharmaceutical and biotechnology applications, where stringent corrosion resistance, strength, and durability requirements are essential. Its unique duplex structure, combining ferrite and austenite, ensures it can withstand harsh operating conditions, such as exposure to aggressive chemicals and extreme temperatures, all while maintaining excellent mechanical properties. Moreover, its high resistance to stress corrosion cracking, pitting, and crevice corrosion makes it a reliable material for ensuring the longevity and safety of pharmaceutical equipment and biotechnology facilities. By choosing 2205 Duplex Stainless Steel, industries can optimize their operations with a cost-effective, high-performance material that excels in demanding environments.
If you have a pharmaceutical or biotechnology project demand for 2205 Duplex Stainless Steel, please feel free to contact [email protected] for a quote.
