Lean duplex: a good alternative to 304/L & 316/L?
Dear Materials & Corrosion Engineers,
This column for Stainless Steel World India & Middle East is, I believe, very relevant in the current global economy where cost reduction is paramount. After all, corrosion simply does not care about recession.
^ Weight and thickness reduction comparison example by critical strength of elastic deformation when locking both ends and central concentrated loaded. Taken from https://nssc.nssmc.com/en/campaigns/duplex/orientation/pricing.php, modified for clarity.
Article by Andry Soetiawan
The topic I would like to raise is the wider use of lean duplex stainless steel. This group of alloys is currently less well established in the oil and gas industry despite strong advantages when compared to its austenitic stainless steel counterparts such as 304/L and 316/L.
Lean duplex is a member of the duplex family. These alloys typically have a PRE number below 30, with a reduced amount of higher priced and more volatilely priced alloying elements such as nickel and molybdenum, aiming to substitute 304/L & 316/L in more applications.
Major chemical composition range of several duplex stainless steels with addition of PRE Number and yield strength for comparison. Taken from ASTM A240, modified for clarity.
To maintain desirable corrosion properties and phase balance, other alloying elements are increased in the chemical composition, such as chromium, manganese and nitrogen.
Per my knowledge, lean duplex is already quite popular for structural applications such as tanks, buildings, and bridges. However, it is my sincere wish to see more usage in the future in both the upstream and downstream oil and gas sectors, as well as in chemical, petrochemical, and food industries, etc.
Critical Pitting Temperature (CPT) comparison from two test standard (left) and SCC susceptibility related to stress level with drop evaporation test (right). Taken from Outokumpu Acom, issue 1 – 2011, modified for clarity
Advantages of lean duplex
Below are some advantages that I can see from using lean duplex:
1. The main reason for using lean duplex is its significant resistance to chloride stress corrosion cracking because of its dual phases. As a general rule of thumb, the corrosion resistance of lean duplex is likely to be similar to 304L and 316L with corresponding PRE numbers.
Iso-corrosion diagram comparison in some common acids. Taken from Outokumpu Duplex Stainless Steel Brochure, modified for clarity
2. The yield strength of duplex is more than twice that of 304L and 316L hence the wall thickness of equipment or piping can be reduced. Less thickness also means lower weight which can significantly help as regards transportation and installation, especially in remote or offshore locations.
3. Lean duplex benefits from a lower CAPEX. Alloying elements such as nickel and molybdenum carry a high price, which drives up the relative cost of 304L and 316L. The reduction of both these elements keeps lean duplex economical. And with an almost similar price per unit weight, designing lean duplex equipment with lower thicknesses can significantly reduce the total cost.
Erosion-corrosion comparison with some corrosive fluids and abrasives. Taken from Erosion-Corrosion by Solid Particles in Suspension in an Aqueous Corrosive Media, Usinor Industeel, modified for clarity.
However, there’s no such thing as a perfect material and hence there are some imitations to be aware of when considering lean duplex:
1. The design temperature range depends on the lean duplex grade but is normally between minus 50°C to 300°C. Higher temperatures may risk intermetallic formation and lower temperatures may risk brittle fracture.
Comparison CPT test on welds per ASTM G150. Taken from Welding of Lean Duplex Stainless Steel grades: microstructure, corrosion resistance and mechanical properties, NACE Corrosion Conference & Expo 2012, modified for clarity.
2. Be prepared for additional delivery time because less material is kept in stock and not all product forms may be available. This is a question of supply and demand but it is hoped that, with the further acceptance and usage of lean duplex by more companies, this problem can be solved in the future.
3. With a PRE < 30 and Mo < 1.5%, the use of most lean duplexes in sour upstream environments is in practice not compliant with NACE MR0175. Fortunately, this is not a problem in downstream usage where NACE MR0103 has no issue, as all duplexes are treated equally.
Impact of Ni & Mo alloying element cost to total cost of material. Taken from Duplex stainless steels: success story and growth perspectives, Stainless Steel World America Conference & Expo 2008, modified for clarity.
1. The duplex family achieve their phase balance and pitting corrosion resistance from nitrogen. During welding, some nitrogen is released from the metal so the use of nitrogen-containing gas is recommended.
2. The corrosion and mechanical properties of duplex are achieved with a ferrite/austenite phase balance of 40 – 60%.
3. Although the yield strength of lean duplex is more than double compared with austenitic, the allowable stress for design calculation is not.
Meet the Author
Currently a freelance material & corrosion engineer, Andry Soetiawan has twelve years of experience working across a broad range of the industry value chain including materials manufacturing, consulting, end users, etc.
A regular author, Andry writes columns to share his own observations and experiences with other engineers. However, recognizing that discussing differences is the best way to developing real understanding, Andry welcomes comments and feedback at all times. He can be reached on: https://www.linkedin.com/in/andrysoetiawan/