The same line item appears on two quotes: 6-inch duplex pipe, 500 metres, SCH 40S. One says 2205, the other says 2507, and the second number is 22% higher. Both suppliers are honest, both mills are certified, both weld the same. The buyer instinct is to take the cheaper one - and about half the time, that instinct is right. The other half, it is the most expensive saving in the project.
The difference between Duplex 2205 and Super Duplex 2507 pipe is not "better and worse." It is two different products with two different jobs, and the money follows three things: the crevice corrosion gap, the strength credit, and the way you are being priced. Here is how the decision actually works.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
First: The Index Everyone Quotes, and the One That Actually Decides
| Index | Duplex 2205 (S32205) | Super Duplex 2507 (S32750) | What it means |
| PREN | About 34-36 | About 41-42.5 | The number every datasheet leads with |
| CPT (ASTM G48, pitting) | About 20-25 C | About 40-50 C | Initiation of pitting on a polished surface |
| CCT (crevice) | About 15-20 C | About 35-42 C | Initiation under a gasket, bolt hole or deposit - the number that decides real service |
Here is the part the SERP never says: the crevice gap is wider than the pitting gap, and crevices are where duplex piping actually fails. Flanged joints, socket welds, marine growth, deposits under clamps - every real piping system is a crevice machine. A 2205 line in warm seawater can pass every coupon test and pit first at the flange faces. If your system runs continuous seawater above roughly 20-25 C with bolted joints, the CCT row - not the PREN row - is the one your specification should be built on, and that row points at 2507.
Conversely: splash zones, atmospheric-marine, brackish cooling, and chemical duty with moderate chlorides are all comfortably 2205 territory - and paying the super duplex premium there buys margin nobody asked for. A seawater injection package on a Gulf coast facility (Ras Laffan-class conditions are the classic case) is the boundary example where the spec-writer choice between the two rows is a real engineering decision, not a formality.
Second: The Strength Credit Nobody Prices
| Property | 2205 | 2507 | The consequence |
| Yield, 0.2% | 450 MPa min | 550 MPa min | About 20% more allowable stress in pressure design |
| Tensile | 655 MPa min | 800 MPa min | - |
| Elongation | 25% min | 15% min | 2507 is a stiffer, less forgiving product to fabricate |
For pressure pipe, wall thickness follows allowable stress. Run the same design pressure through ASME B31.3 with the higher 2507 allowable stress and the required wall drops roughly 15% on the pressure term. On a large-diameter line, that is real tonnage.
| Line of the calculation | 2205 | 2507 |
| Price per kg (FOB China, indicative) | $3.2-4.5 | $4.0-5.5 |
| Required wall for same pressure duty | t (baseline) | About 0.85 x t |
| kg/m at that wall | baseline | About 0.85 x baseline |
| Cost per metre | baseline | About 0.9-1.1 x baseline |
This is the "per-metre, not per-kg" rule, and it is the reason a 22% per-kg premium can shrink to nearly nothing on pressure-governed lines: you buy pipes by the metre, not by the kilogram. On corrosion-governed walls (where the schedule is set by corrosion allowance and availability, not pressure), the credit disappears and the premium is the premium. Ask which term governs your wall before you benchmark the two quotes - if the answer is "corrosion," the 2205 decision needs to be won on corrosion data alone.
Third: The Duties, Line by Line
| Duty | Verdict | Why |
| Atmospheric / splash marine, 316L marginal | 2205 | CCT headroom over austenitics at moderate chloride |
| Brackish / estuarine cooling water | 2205 | Typically below the CCT line |
| Continuous seawater, ambient, welded systems | 2205 usually holds; audit every crevice | Borderline - the flange faces decide |
| Continuous seawater, warm (above 20-25 C), bolted joints | 2507 | CCT row governs |
| Seawater + H2S within NACE MR0175 envelope | 2507 typically | Sour + chloride is the super duplex home ground |
| High-pressure injection lines | 2507 | Strength credit + chloride, premium pays back per metre |
| Chloride SCC risk where austenitics crack | Either - 2205 first on price | Duplex structure itself is the SCC answer |
Fabrication and Welding: Two Different Shop Temperaments
Both weld without preheat and without PWHT, with nickel-over-alloyed filler - 2209 for 2205, 2594 for 2507 - but they are not the same customer in the shop. 2205 tolerates ordinary duplex-qualified procedures. 2507 is a stricter product: tighter heat-input windows, lower interpass limits, and cold-forming rules that bite sooner (its higher strength means higher springback and faster work-hardening). A fabricator new to super duplex will burn more hours on a 2507 spool than on a 2205 one - price that in, or pick the alloy your shop already knows.
The Bottom Line
| If your project is... | Buy |
| Moderate chloride, corrosion-governed wall, budget-driven | 2205 |
| Warm seawater, crevice-heavy, or pressure-governed wall | 2507 |
| Between the two rows of every table above | Send the duty sheet to a supplier who will pick a side instead of quoting both |
Send your medium (chloride level, temperature, crevice count), design pressure and line list to market@htpipe.com - we hold both alloys and will quote the per-metre economics of each instead of selling you the one we happen to stock.
