Duplex stainless steels combine higher strength and better corrosion resistance than standard austenitic grades such as Alloy 316L, which opens up applications where 316L is not sufficient but super duplex, super austenitic or nickel alloys would cost more than needed. This makes 2205 duplex a workhorse across oil and gas, marine, chemical processing, water treatment and more.
Before outlining the benefits, it helps to distinguish between duplex, super duplex and hyper duplex stainless steels. All are based on a duplex microstructure combining austenitic and ferritic grains, so all can be described as a form of duplex. Because the grades differ enough, this article focuses on the more standard duplex stainless steels based on 22% chromium.
There is a subtle difference between UNS S31803 (F51) and UNS S32205 (F60), although the terms are often used interchangeably. For simplicity we use 2205, or duplex, as shorthand.
Pitting resistance equivalent number
The pitting resistance equivalent number (PREN) is a quick reference to the corrosion resistance of stainless steels and other corrosion resistant alloys (CRA). For 2205 it is 34, a significant uplift over 316 at 25. Raising the chromium content from around 18% to around 22% improves resistance to pitting corrosion by making the passive layer that forms on exposure to oxygen more effective.
As a very crude rule of thumb: Alloy 316L can be used successfully in rural, urban and estuary environments but is limited in more exposed coastal settings; 2205 duplex can be used comfortably in estuary, marine and offshore environments; but applications with long-term immersion may need a super duplex.
Applications where 316L is not sufficient
Pulp and paper
Duplex alloys were actually invented with this application in mind, where equipment is exposed not only to water but to the various solutions and liquors used to break down, process and bleach the wood pulp.
Water treatment and desalination
This is an increasingly significant market for 2205 duplex grades. In desalination, the corrosive nature of seawater decreases through the process, allowing a range of alloys to be used along the length of the process units.
Geothermal energy
Depending on the location, and therefore the composition of the fluid being handled, duplex alloys can be used in extracting and recovering energy from the earth. Some locations where brine is present will require super duplex or super austenitic alloys due to the more aggressive conditions.
Chemical industry
For a great many solutions, particularly at lower concentrations and temperatures, 2205 duplex gives acceptable performance. It is widely used to machine the bodies of smaller valves, and occasionally as fasteners.
Marine
Propeller drive shafts are frequently manufactured from 2205 duplex, often with a ground finish specified to reduce fouling and improve straightness during that additional processing step. Other applications include rudders, shaft seals, pumps, bolts, fasteners, valves and instrumentation.
Oil and gas
Within this segment, 2205 is frequently used for pumps, valves, pipe, vessels, wellhead equipment, subsea equipment, separators and heat exchangers. Duplex grades offer excellent resistance to stress corrosion cracking (SCC), which can open up applications in more aggressive environments. This property is related to the presence of ferrite grains within the mixed microstructure.
Food production
Although Alloy 316L is the default choice for most food and drink facilities, 2205 duplex is used for applications such as brewery piping, evaporators, hot liquor tanks and presses, where the temperature is higher, the solution more aggressive, or the strength requirement greater.
Architectural
While most of the above focus on corrosion resistance, for load-bearing applications such as pedestrian footbridges the additional strength of 2205 duplex is the main benefit.
Oil and chemical storage tankers
Here the additional strength (over 50% more than Alloy 316L) helps offset the higher cost of the more highly alloyed grade. It can provide the same structural performance from a lighter section, reducing the amount of metal needed as well as the size of supporting structures.
What we stock
We stock 2205 (SAF2205, Sanmac® 2205, 1.4462, UNS S31803, UNS 32205, F51, F60) duplex stainless steel as solid bars, hollow bars, plate and pipe. Solid bars are available ex-stock from ½" to over 16" diameter, while plate is available from our USA warehouse up to 1" thick.