S32750 Corrosion Resistance: A Field Engineer's Guide to Pitting, Crevice, SCC and Sour Service

Supporting article for the Ultimate Guide to Super Duplex S32750 · Audience: piping engineers, materials specialists, procurement · Reading time: 12 min

Super duplex S32750 (UNS S32750, EN 1.4410, 2507, F53) is the grade you reach for when 316L and duplex 2205 have already failed in chloride service. This article unpacks the corrosion behaviour in the order you actually meet it on a project: pitting, crevice, chloride stress corrosion cracking, sour service, acid resistance and the embrittlement events that quietly strip the corrosion resistance you paid for.

S32750 resists pitting to roughly 70–85 °C in 6% FeCl₃ (ASTM G48 Method A), resists crevice corrosion to roughly 35–50 °C (ASTM G48 Method B), is essentially immune to chloride stress corrosion cracking in neutral chloride up to its 250 °C application ceiling, and is qualified for sour service under NACE MR0175 / ISO 15156 at hardness ≤ 28 HRC. Its single biggest reliability risk is not the alloy itself but the heat-affected zone after welding or any exposure between 300 °C and 1 000 °C — that is where sigma, chi and Cr₂N precipitation quietly reset the corrosion resistance back to 2205 levels or worse.

Why Does S32750 Resist Chloride Corrosion Better Than 316L and 2205?

Because its chemistry is engineered to a Pitting Resistance Equivalent (PREN) above 40, and its 50/50 austenite–ferrite microstructure gives each phase a backstop the other phase cannot provide.

Three levers move corrosion resistance in stainless grades, and S32750 pushes all three at once. Chromium thickens and stabilises the passive oxide film; molybdenum widens the passive range in chloride and acid; nitrogen raises both the bulk pitting resistance and the repassivation speed when the film is breached. S32750 sits at 25% Cr, 3.5% Mo and 0.25% N — the high end of the duplex band on every one of those levers.

The Pitting Resistance Equivalent Number (PREN) compresses those three levers into one figure most engineers compare on: PREN = %Cr + 3.3·(%Mo + 0.25·%N). S32750 lands at PREN 41–43 in production chemistries — above the PREN ≥ 40 line that defines a super duplex and well above 2205 (PREN 34–38) and 316L (PREN 23–27). PREN is a useful ranking tool but it is not a service limit; the real service limits are the temperature thresholds in the next sections.

PREN is blind to microstructure. A 25Cr ferritic with PREN 43 would still pit aggressively in seawater because it has no austenite to repassivate the surface. S32750 keeps its 40–50% austenite phase to provide the ductile, corrosion-tolerant second network, and the producer controls the phase balance through the anneal so that neither phase falls below roughly 40%.

For the chemistry that produces this PREN and the mechanical strength that comes with it, see our S32750 chemical composition and S32750 mechanical properties articles.

How Does S32750 Resist Pitting Corrosion?

S32750 resists pitting by combining a Cr–Mo–N film that self-repairs, a microstructure where austenite heals the ferrite pits, and a critical pitting temperature (CPT) of 70–85 °C in ASTM G48 Method A — about three times higher than 316L and almost double that of 2205.

Pitting is localised breakdown of the passive film at sites where chlorides concentrate — under deposits, at scratches, in dead legs. Once a pit breaks through, the metal inside the pit goes acidic and the pit propagates by its own chemistry. The trick is to either stop initiation or stop propagation, and S32750 does both.

Initiation: chromium above roughly 22% thickens the passive Cr₂O₃ film; molybdenum extends the passive window into acidic and chloride-bearing solutions; nitrogen raises the charge the surface can absorb before the film collapses. Together they push the CPT in 6% FeCl₃ from the 5–15 °C band of 316L up to the 70–85 °C band of S32750.

Propagation: in duplex grades the austenite phase is more noble than the ferrite, so when a pit does start in the ferrite, it hits the austenite boundary and effectively stops. This is why a super duplex can show a high CPT even though one of its phases would pit on its own — the structure, not the chemistry alone, is doing the work.

What this means in practice: In clean, oxygenated seawater S32750 is in its comfort zone and will not pit at ambient temperature. In stagnant, deposit-covered, or warm chloride service (≥ 40 °C), CPT becomes the real design number, and you should keep the surface temperature at least 10 °C below the certified CPT for the actual heat — not the catalogue CPT.

Real seawater service and desalination case data are collected in our S32750 seawater desalination applications and S32750 offshore applications articles.

How Does S32750 Resist Crevice Corrosion?

S32750 resists crevice corrosion to roughly 35–50 °C in ASTM G48 Method B — about 25 °C lower than its own CPT, because a crevice is a pre-built pit, but still well above the temperature where 316L and 2205 begin to fail at flanges, gaskets and under-deposit zones.

A crevice is a geometric feature — a gasket face, a lap joint, a deposit, a thread root — where chemistry can drift more aggressive than the bulk fluid. Because the electrolyte inside the crevice is already depleted, the alloy has to fight from a worse starting position than it does on an open surface, so the critical crevice temperature (CCT) is always lower than the CPT for the same grade.

For S32750 that gap is roughly 25 °C. Catalogue CPT is 70–85 °C; catalogue CCT is 35–50 °C. For 316L the CCT is below ambient, which is why you see under-deposit pitting and gasket-face attack on 316L flanges in relatively mild service where the pipe body is fine.

Design takeaway: Crevices are the leading cause of super duplex failures that get reported as “pitting”. If you can, eliminate the geometry before you blame the alloy: use weld-neck flanges instead of slip-on, avoid gasket materials that wick, specify full-penetration welds, and require pickling and passivation after fabrication so the as-welded oxide scale — a near-perfect crevice — is removed before commissioning.

The fabrication-side controls are covered in detail in S32750 welding guide and S32750 heat treatment.

Is S32750 Immune to Chloride Stress Corrosion Cracking (SCC)?

In practical terms, yes. In neutral chloride up to its 250 °C application ceiling, S32750 does not chloride-stress-corrosion-crack under realistic design stresses, where 316L will crack in a few hundred hours and 2205 starts to crack above 60–80 °C.

Chloride SCC in stainless is a transgranular brittle crack driven by the combination of tensile stress, temperature and chloride concentration. The classical screen test is ASTM G36 (boiling 45% MgCl₂), where 316L cracks in hours and S32750 survives the 500-hour exposure used for screening. Real service does not look like boiling MgCl₂, but the ranking is correct.

The mechanism: austenitic stainless grades slip by planar dislocation glide, which exposes bare metal to the chloride and starts a crack. Duplex grades slip by wavy glide in the ferrite phase, and the crack has to switch direction every time it crosses an austenite island. Under realistic neutral-pH chloride, that path-blocking effect is enough to essentially stop the crack.

The honest caveat: “Immune” only applies to neutral chloride. If the environment goes acidic and stressed, or sour and cathodically charged, you move from chloride SCC to hydrogen-induced stress cracking (HISC) or sulphide stress cracking (SSC), and S32750 is no longer the right tool. See the sections on sour service and on what destroys corrosion resistance.

Is S32750 Suitable for Sour Service Under NACE MR0175 / ISO 15156?

Yes. S32750 is listed in NACE MR0175 / ISO 15156-3 for sour service when supplied in the solution-annealed condition with hardness ≤ 28 HRC, and this is the single most common reason it is chosen over 316L for upstream oil and gas piping.

Sour service means an environment containing H₂S, where the risk is sulphide stress cracking (SSC) — a form of hydrogen embrittlement. The 28 HRC hardness ceiling is the key acceptance criterion in MR0175; S32750 in the solution-annealed condition lands at roughly 22–28 HRC, so the headroom is thin. Any cold work, any weld without a full solution anneal, any sigma precipitation, pushes the hardness above 28 HRC and takes the material out of compliance.

The recurring mistake: Specifying “S32750 to NACE MR0175” on the purchase order, then welding it in the field without re-anneal. The pipe body stays compliant; the weld and HAZ do not. For sour service you should either weld with a qualified procedure that holds the HAZ under 28 HRC (rare without a full solution anneal), or specify that the assembly is solution annealed after welding before going into sour service.

Sour service case data, selection thresholds and the comparison against S32760, 2205 and 316L are collected in our S32750 oil & gas applications article.

How Well Does S32750 Resist Acids?

S32750 resists oxidising acids (nitric, sulphuric with oxidising contaminants), dilute organic acids and phosphoric acid very well. It does not resist reducing acids (concentrated HCl, hot concentrated H₂SO₄ without oxidiser, HF) — for those, switch to a nickel-based grade.

The same Mo and Cu that make S32750 great in chloride also help it in many acid environments, but the boundary is whether the acid can maintain the passive film. Oxidising acids keep the film alive and S32750 performs well; reducing acids dissolve it and no amount of alloying saves the surface.

For reducing-acid service the correct family is nickel-based; see our Hastelloy C-276 and Inconel 625 product pages, or our Inconel 625 vs Hastelloy C-276 selection guide.

What Quietly Destroys S32750 Corrosion Resistance?

Sigma phase, chi phase, chromium carbides and chromium nitrides — all of which precipitate in the 600–1 000 °C band — and 475 °C embrittlement in the 300–525 °C band. Each strips chromium and molybdenum out of the matrix and resets the local PREN to 2205 or lower, sometimes invisibly.

The single most under-diagnosed cause of “S32750 corrosion failures” is not the alloy, the seawater, or the chemistry — it is a heat excursion that the project team never logged. Welding without a qualified procedure, slow cooling through the sigma range, stress relief in the forbidden temperature band, or even a sustained service temperature above 280 °C will precipitate intermetallics that locally deplete the matrix.

What to put on the mill certificate and on the WPS: Ask for an ASTM A923 Method A + C test on every plate and pipe, an ASTM G48 Method A CPT ≥ 70 °C, and a ferrite number 35–55 FN. On the welding side, hold interpass below 150 °C, use a nitrogen-backed purge, qualify with a Charpy and a G48 on the weldment, and — if the assembly is for sour service — specify solution anneal after welding.

The welding-side controls and the heat-treatment window that avoids all of the above are covered in our S32750 welding guide and S32750 heat treatment articles, with the standard-driven acceptance tests in S32750 ASTM standards.

How Do You Specify S32750 Corrosion Performance on a Purchase Order?

Ask for chemistry (ASTM A240 / A790), phase balance (40–55% ferrite, FN 35–55 per ASTM E562 / AWS A4.2), absence of intermetallics (ASTM A923 A + C), and a corrosion acceptance test (ASTM G48 Method A, CPT ≥ 70 °C). These four blocks cover 90% of the corrosion failures seen in service.

Procurement shortcut: If the supplier cannot quote all four blocks (chemistry + phase + A923 + G48) on the mill certificate, treat the material as unverified super duplex. The cost of the tests is small; the cost of putting sigma-bearing S32750 into a seawater heat exchanger is not.

For the standards map and the equivalent grade cross-references, see S32750 ASTM standards and S32750 equivalent grades.

Where Is S32750 the Right Corrosion Answer?

S32750 is the right answer in warm chloride service (seawater, brine, desalination, swimming pool atmospheres), in upstream sour oil and gas, and in chemical process equipment handling oxidising or organic acids — wherever 316L and 2205 stop being safe but a nickel-based grade is overkill.

Application-specific selection data is in our S32750 seawater desalination, offshore, oil & gas and heat exchanger articles; for product selection, see S32750 pipe selection guide and S32750 plate selection guide.

When Should You Not Use S32750?

Do not use S32750 in concentrated reducing acids (HCl, hot concentrated H₂SO₄ without oxidiser, HF), in service above 250 °C for long durations (sigma and 475 °C embrittlement), or where a heavily cold-formed zone has to stay in sour service without re-anneal.

The mistakes that get super duplex a bad name are almost always selection mistakes — the grade was right for the chemistry of the fluid but wrong for the temperature window, or right for the parent metal but wrong for the as-welded condition.

• Do not use S32750 where the service temperature stays above 280 °C for long durations — you are inside the 475 °C band.

• Do not use S32750 in concentrated HCl, HF, or hot concentrated H₂SO₄ without oxidiser — reducing acids dissolve the passive film; switch to nickel-based grades.

• Do not use S32750 in heavily cold-formed sour service without re-anneal — the cold work takes the HAZ over 28 HRC.

• Do not use S32750 with cathodic protection more negative than −0.85 V vs Ag/AgCl in dynamic loading — hydrogen charging triggers HISC.

• Do not use S32750 in service where the surface is allowed to stay as-welded, unpickled and unpassivated — the heat tint is a perfect crevice and pits first.

For the broader decision framework, see Why choose S32750, and for the failure-mode roll call, see S32750 common problems and solutions.

Frequently Asked Questions

What is the PREN of S32750?

S32750 has a Pitting Resistance Equivalent Number (PREN) of 41–43, calculated as %Cr + 3.3·(%Mo + 0.25·%N). This is above the PREN ≥ 40 line that defines a super duplex and well above 2205 (PREN 34–38) and 316L (PREN 23–27).

What is the critical pitting temperature (CPT) of S32750?

In 6% FeCl₃ to ASTM G48 Method A, the CPT of solution-annealed S32750 is 70–85 °C depending on the heat, roughly three times higher than 316L (5–15 °C) and almost double that of duplex 2205 (40–55 °C).

What is the critical crevice temperature (CCT) of S32750?

In 6% FeCl₃ to ASTM G48 Method B, the CCT of solution-annealed S32750 is roughly 35–50 °C — about 25 °C lower than its own CPT, because a crevice is a pre-built pit, but still well above 316L and 2205.

Is S32750 suitable for seawater?

Yes. In clean, oxygenated seawater at ambient temperature, S32750 is in its comfort zone and will not pit. The design threshold is to keep the surface temperature at least 10 °C below the certified CPT for the actual heat.

Is S32750 suitable for sour service?

Yes. S32750 is listed in NACE MR0175 / ISO 15156-3 for sour service when supplied in the solution-annealed condition with hardness ≤ 28 HRC. The HAZ of as-welded S32750 may exceed 28 HRC; specify re-anneal after welding for sour service.

What is the hardness limit for S32750 in sour service?

28 HRC. S32750 in the solution-annealed condition lands at roughly 22–28 HRC; any cold work, sigma precipitation, or unqualified weld can push the local hardness above 28 HRC and disqualify the material from MR0175 compliance.

Is S32750 immune to chloride stress corrosion cracking?

In practical terms, yes. In neutral chloride up to its 250 °C application ceiling, S32750 does not chloride-stress-corrosion-crack under realistic design stresses. In acidic, sour or cathodically charged service, the failure mode shifts to HISC or SSC and S32750 is no longer the right tool.

Does S32750 resist HCl?

Only at very dilute concentrations (< 1%) and ambient temperature. In concentrated HCl, S32750 loses passivity and is not recommended; switch to a nickel-based grade such as Hastelloy C-276 or Inconel 625.

Does S32750 resist sulphuric acid?

In dilute (< 10%), cool (< 50 °C) sulphuric acid, S32750 is suitable. In concentrated sulphuric acid, S32750 is marginal; carbon steel or cast iron is often the better choice. In reducing hot sulphuric, switch to a nickel-based grade.

What is sigma phase and why does it matter for corrosion?

Sigma phase is an intermetallic that precipitates in S32750 between roughly 700 and 1 000 °C (peak rate around 850–925 °C). It consumes chromium and molybdenum from the matrix, locally dropping the PREN below 30 and resetting the corrosion resistance to 2205 levels or worse. Detect it with ASTM A923 Method A and Method C.

What is 475 °C embrittlement?

It is hardening and embrittlement caused by spinodal decomposition of the ferrite in the 300–525 °C band (peak around 475 °C). It also reduces local corrosion resistance. The rule of thumb is to keep service temperature below 280 °C for long durations to stay clear of the band.

How do you test S32750 for intermetallic phases before putting it into service?

Run ASTM A923 Method A (metallography) for visible sigma and Cr₂N, and ASTM A923 Method C (Charpy V-notch at −40 °C) for the toughness loss that sigma causes. Typical acceptance: no sigma in Method A; ≥ 54 J average and ≥ 40 J single in Method C.

How do you keep the weld HAZ from destroying the corrosion resistance?

Use a nitrogen-backed purge, hold interpass below 150 °C, qualify the WPS with an ASTM G48 test on the weldment, and — for sour service — solution anneal after welding. Without these, the HAZ can lose 20–30 °C of CPT versus the parent metal.

What should be on the mill certificate for S32750 in corrosive service?

Chemistry per ASTM A240 / A790, ferrite 40–55% (or FN 35–55), ASTM A923 Method A pass and Method C ≥ 54 J average, ASTM G48 Method A CPT ≥ 70 °C, and — for sour service — hardness ≤ 28 HRC and an MR0175 compliance statement.

Why does S32750 pit after a hydrostatic test?

Almost always because the test water was left to stagnate in the pipe. Even clean potable water concentrates chlorides under deposits as it evaporates, and the resulting pits are reported as “S32750 pitting” when the real cause was test-water management. Drain, dry and passivate after every hydrotest.

Is S32750 better than 2205 for corrosion?

Yes, measurably. S32750 has a higher PREN (41–43 vs 34–38), a higher CPT (70–85 °C vs 40–55 °C) and a higher CCT. Choose S32750 when 2205’s thresholds are too close to the service temperature; choose 2205 when the service is clearly below 2205’s limits and cost dominates.

When should you choose S32760 over S32750?

When the service needs slightly higher tungsten-assisted crevice resistance, or when the producer listing you need (e.g. Norsok M-630) specifies S32760. Otherwise S32750 is the default and S32760 is a near-equivalent. See our S32750 vs S32760 comparison for the full table.

Related reading: Start at the Ultimate Guide to Super Duplex S32750 for the full chemistry, mechanical properties, welding, heat treatment and standards map. The corrosion-specific companion articles below extend each topic covered here: S32750 welding guide, S32750 heat treatment, S32750 common problems and solutions, and the S32750 FAQ.