6Mo Austenitic Stainless vs Duplex 2205 for Chloride Service: PRE, Strength, Fabricability, and Cost Trade-offs

What Is a 6Mo Austenitic Stainless? The 254 SMO Example

The "6Mo" or superaustenitic stainless steels are fully austenitic grades with around 6% molybdenum, high nickel and a deliberate nitrogen addition. The best-known is 254 SMO (EN 1.4547, X1CrNiMoCuN20-18-7, UNS S31254): nominally 20Cr-18Ni-6Mo-0.7Cu-0.2N. The 6% molybdenum — compared with about 2–2.5% in 316L and about 4–5% in 904L — is what lifts its pitting and crevice corrosion resistance into the same class as super duplex 2507. The high nitrogen (0.18% minimum) adds solid-solution strength and contributes to the elevated PRE. 254 SMO is ASME P-No. 8, Group 4. AL-6XN (UNS N08367) is the other common 6Mo grade, with higher nickel; in Europe and Asia 254 SMO is the more widely stocked of the two for pipe fittings.

Pitting Resistance Equivalent (PRE)

PRE is the standard index for comparing pitting resistance in chloride-containing media: PRE = %Cr + 3.3×%Mo + 16×%N. For 316L: PRE ≈ 24–26. For 254 SMO (20Cr-18Ni-6Mo-0.2N): PRE ≈ 43–45. For AL-6XN (21Cr-24Ni-6.3Mo-0.22N): PRE ≈ 45–47. For duplex 2205 (22Cr-5Ni-3Mo-0.17N): PRE ≈ 34–36. For super duplex 2507 (25Cr-7Ni-4Mo-0.28N): PRE ≈ 42–43, with a specified minimum of 40 in most project specifications. The 6Mo austenitic grades (254 SMO, AL-6XN) thus have PRE approximately equivalent to super duplex 2507, but significantly higher than standard duplex 2205. Crevice corrosion resistance follows a similar ranking: ferric chloride tests to ASTM G48 and seawater crevice tests to ASTM G78 consistently show 6Mo grades outperforming 2205 by a wide margin.

Critical Pitting Temperature in Seawater

PRE is only a ranking index; what matters in service is the critical pitting temperature (CPT) and, more importantly, the critical crevice temperature — the temperatures above which localised corrosion starts in a given chloride environment. In natural seawater, 254 SMO and super duplex 2507 have broadly comparable resistance, and both are a large step above 2205 and 316L. Crevice corrosion under gaskets and deposits, not open-surface pitting, is the practical limit for both, and chlorination lowers the safe temperature considerably. Typical 6Mo seawater applications are offshore cooling water and firewater systems, seawater lift and injection piping, desalination intake and concentrate streams, and seawater scrubbers on ships and offshore platforms. Where seawater temperature or chlorination exceeds what 6Mo or super duplex can tolerate, the next steps are nickel alloys such as Alloy 625 or C-276, or titanium. For background on CPT testing see pitting corrosion and critical pitting temperature.

Chloride SCC Susceptibility

Austenitic stainless steels — including 304, 316L, and the 6Mo grades — are susceptible to chloride stress corrosion cracking (Cl-SCC) above a threshold temperature and chloride concentration. For 316L, SCC initiates above ~60°C in concentrated chloride. For 254 SMO and AL-6XN, the threshold is considerably higher because of the high nickel and molybdenum content, but it is not unlimited. Duplex 2205, by contrast, is highly resistant to Cl-SCC due to its two-phase microstructure: the ferrite phase blocks propagating cracks, so much higher temperatures and chloride levels are needed before cracking occurs. Duplex is not immune — concentrated chlorides at high temperature, or chlorides combined with H₂S, can still crack it — but its margin over 316L is large. This is a critical distinction for hot chloride service: if operating temperature is high and chlorides are concentrated (evaporating, wet/dry or under insulation), duplex is generally preferred, and the 6Mo grade should only be used after its SCC margin has been checked for the actual conditions.

Mechanical Strength

Duplex 2205 has a minimum specified 0.2% proof stress of 450 MPa (ASTM A815) compared to 300 MPa for 254 SMO (ASTM A403 / EN 10253-4), and super duplex 2507 is higher still at around 550 MPa. This allows thinner wall pipe fittings in duplex for the same design pressure, reducing material and weight — an advantage that matters most in high-pressure and offshore piping. For flanges, the duplex grades also carry higher pressure-temperature ratings than 316L; always check the ASME B16.5 material group for the specific grade and product form. For low-temperature service (LNG, sub-zero process), however, the advantage reverses — duplex toughness degrades below approximately -50°C due to the ductile-to-brittle transition of the ferrite phase, while 6Mo austenitic remains fully ductile down to cryogenic temperatures (around -196°C).

254 SMO vs Super Duplex 2507 — When to Choose Which

When 2205 is not enough, the real choice is usually between 254 SMO and super duplex 2507, which have similar PRE but very different metallurgy:

Weldability and Fabrication

6Mo austenitic grades (254 SMO, AL-6XN) weld similarly to standard austenitic stainless, but not with matching filler. Weld metal made with a 6Mo-composition filler shows lower local pitting resistance than the base metal because molybdenum segregates during solidification, leaving Mo-depleted dendrite cores. The standard solution is an overmatched nickel-base filler — ERNiCrMo-3 (Alloy 625 type) is the most common, with ERNiCrMo-4 or ERNiCrMo-13 used for the most aggressive media — so that the weld deposit is not the weakest link in corrosion resistance. Heat input and interpass temperature must be kept low, as set by the WPS, to avoid precipitation of sigma and other intermetallic phases in the HAZ, but the thermal cycle sensitivity is lower than for duplex. No PWHT is required and preheat is not normally used. Post-weld pickling and passivation to remove heat tint are strongly recommended for seawater and FGD service.

Duplex 2205 welding requires careful heat input control (0.5–2.0 kJ/mm, or as specified by the WPS) and use of a duplex filler with higher nickel than the base metal (ER2209) to restore the austenite fraction in the weld metal. PWHT is not done for duplex (it would precipitate sigma phase), but maintaining the correct ferrite-austenite balance in the weld metal and HAZ, within the range set by the WPS or project specification, is critical; ASTM A923 testing is used to confirm the absence of detrimental intermetallic phases. A full solution anneal is needed only where the fitting itself has been hot-formed or otherwise heated after its original solution treatment. See duplex and super duplex welding and ferrite control for detail.

Temperature Limits and Intermetallic Phase Formation

Both 6Mo austenitic and duplex 2205 form intermetallic phases (sigma, chi) if held in the range of roughly 700–1000°C, which is why both are supplied solution annealed and welded with controlled heat input. Duplex has a second, lower-temperature limitation: the ferrite phase is susceptible to 475°C embrittlement, which can develop with prolonged exposure from around 300°C upward. In practice duplex 2205 is usually not specified above about 250°C process temperature. 6Mo austenitic grades are not affected by 475°C embrittlement and can be used at higher temperatures, subject to the allowable stresses in the applicable code; long-term service in the intermetallic precipitation range must still be avoided.

FGD Scrubber Service

254 SMO is a workhorse material for flue gas desulphurisation (FGD) scrubbers in coal and oil-fired power plants. FGD absorbers combine sulphuric acid, HCl, HF and chloride-saturated scrubbing liquor — an environment that destroys 316L and 904L. The 6Mo grade provides the PRE and crevice corrosion resistance needed for spray headers, absorber outlet ducting, mist eliminator wash piping and slurry recirculation piping, and it is often selected in preference to Hastelloy C-276 where its corrosion resistance is sufficient, because it is considerably cheaper. In the most acidic, chloride-concentrated zones, nickel alloys remain necessary. For a zone-by-zone comparison see 254 SMO vs super duplex 2507 in FGD scrubbers.

Cost and Supply Availability

Duplex 2205 buttweld fittings carry a moderate premium over 316L, while 254 SMO and AL-6XN fittings cost substantially more — in the same bracket as 904L and close to super duplex 2507. 254 SMO is more widely available than AL-6XN in most markets; AL-6XN is stronger in North American supply chains. Both 6Mo grades generally have longer lead times than duplex 2205 in larger sizes. For most chloride service applications where operating temperature is moderate and PRE in the mid-30s is sufficient, duplex 2205 offers the best combination of corrosion resistance, strength, and cost. For low-temperature service, or where pitting resistance at the super duplex level is needed without the phase-balance control that duplex welding demands, 6Mo grades are specified.

Decision Framework for Pipe Fitting Material Selection

Select duplex 2205 when: operating temperature is between 0 and about 250°C, chloride SCC risk is high, high strength is needed to reduce wall thickness, or cost must be minimised while exceeding 316L corrosion performance. Select 6Mo austenitic (254 SMO or AL-6XN) when: operating temperature is below -50°C, the system must have a single austenitic microstructure for complex or orbital welding, or the PRE must exceed 40 to resist pitting and crevice corrosion in seawater or near-seawater chloride concentrations, including FGD service. Use super duplex 2507 when maximum PRE and the highest strength are both required simultaneously. Above the capability of all three — hot chlorinated seawater, or acidic chloride media beyond 6Mo limits — move to nickel alloys such as Alloy 625 or C-276.