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ASTM G48 Method E, Critical Pitting Temperature Test
ASTM G48 Method E is the industry-standard laboratory test for determining the Critical Pitting Temperature (CPT) of stainless steels and nickel-base alloys in chloride solution. The test uses a 6 percent ferric chloride (FeCl3) solution as an aggressive oxidising chloride electrolyte. The specimen is immersed at a starting temperature, examined after a fixed exposure period, and the temperature is raised in 5 degree Celsius steps until pitting is detected. The CPT is the lowest temperature at which pitting initiates. For super duplex 2507 (UNS S32750, EN 1.4410) the CPT is normally 50 degrees Celsius or above. Method F is the parallel test for Critical Crevice Temperature (CCT), using a PTFE crevice former; for super duplex 2507 the CCT is typically 35 degrees Celsius or above.
ASTM G48 covers six methods total (A through F). Method A is a simple weight-loss screening test at fixed temperature (the most common acceptance test on welded fabrications). Method E and Method F are the temperature-stepped tests used to establish numeric CPT and CCT values for material qualification and ranking. NORSOK M-630 invokes Method A as the standard acceptance test; Method E and F are invoked by project specification when a quantitative CPT or CCT value is required.
Method E Test Procedure (CPT)
| Step | Detail |
|---|---|
| 1. Specimen preparation | Cut to approximately 25 x 50 mm; wet ground to 120 grit; cleaned, degreased, dried; passivated optional per project |
| 2. Test solution | 6.0 percent FeCl3 by mass (60 g FeCl3 per litre, anhydrous basis), prepared from reagent-grade salt and deionised water |
| 3. Solution volume | Minimum 5 mL of solution per square centimetre of specimen surface area |
| 4. Specimen support | Glass cradle or PTFE rack; specimen suspended fully immersed, no contact with vessel wall |
| 5. Starting temperature | Typically 35 deg C for super duplex (selectable per project) |
| 6. Exposure time per step | 24 hours (standard) or 72 hours (extended) |
| 7. Inspection between steps | Visual examination at 10x to 20x magnification for pits 25 micrometres and larger |
| 8. Temperature step | 5 deg C increments; fresh solution at each step |
| 9. Endpoint | Lowest temperature at which pitting (one or more pits 25 micrometres or larger) is detected; CPT is reported as that temperature |
Typical CPT Values
CPT values below are representative ranges from published mill data and from independent laboratory testing of correctly heat-treated material. Material with sigma-phase contamination, ferrite excess, or out-of-spec chemistry will show lower CPT than the table indicates.
| Alloy | UNS | Typical CPT (Method E), deg C |
|---|---|---|
| 304L austenitic | S30403 | Below 0 |
| 316L austenitic | S31603 | 10 to 20 |
| Duplex 2205 | S31803 / S32205 | 30 to 40 |
| Super Duplex 2507 | S32750 | 50 to 70 |
| Zeron 100 (super duplex) | S32760 | 55 to 75 |
| 904L superaustenitic | N08904 | 40 to 50 |
| 254 SMO (6 percent Mo) | S31254 | 70 to 85 |
| Alloy 625 (Ni-base) | N06625 | Above 90 (no pitting in standard test range) |
Method F Test Procedure (CCT)
Method F is the crevice-corrosion analogue of Method E. The same 6 percent FeCl3 solution is used; the specimen geometry differs by addition of a PTFE crevice former clamped to one face. The crevice former creates a tight, restricted-flow region that mimics the geometry of a gasket, flange seat, or a deposit on a heat-exchanger tube, locally concentrating chlorides and depleting oxygen at the underlying metal surface. Crevice corrosion typically initiates at lower temperatures than open-surface pitting because the local environment under the crevice is more aggressive than the bulk solution.
| Step | Detail |
|---|---|
| Specimen preparation | Same as Method E (25 x 50 mm; wet ground to 120 grit; cleaned) |
| Crevice former | PTFE block with multiple flat faces (typically 8 to 12 contact points); clamped to specimen at controlled torque (typically 1.7 to 2.8 N m) |
| Test solution | 6.0 percent FeCl3, identical to Method E |
| Starting temperature | Typically 25 deg C for super duplex; selectable per project |
| Step size and exposure | 5 deg C steps; 24 hours each step; fresh solution per step |
| Endpoint | Lowest temperature at which crevice corrosion (depth 25 micrometres and larger) is detected under any contact face |
Typical CCT Values
| Alloy | UNS | Typical CCT (Method F), deg C |
|---|---|---|
| 316L austenitic | S31603 | Below 0 to 5 |
| Duplex 2205 | S31803 / S32205 | 15 to 25 |
| Super Duplex 2507 | S32750 | 35 to 50 |
| Zeron 100 | S32760 | 40 to 55 |
| 254 SMO | S31254 | 45 to 60 |
Method A (Routine Acceptance Test)
Method A is the most common ASTM G48 test on production fabrications because it is fast (24-hour exposure) and does not require temperature stepping. The specimen is immersed in 6 percent FeCl3 at a fixed temperature (35 deg C is the standard for duplex acceptance per NORSOK M-630) for 24 hours. The acceptance criterion is weight loss below 4.0 grams per square metre of exposed surface, with no pitting visible at 20x magnification. Method A is invoked on welded fabrications to confirm that the welding cycle has not degraded pitting resistance below the project threshold.
| Test | Use | Acceptance Criterion |
|---|---|---|
| Method A | Routine production acceptance on welded duplex fabrications | Below 4.0 g per square metre at 35 deg C, 24 hours, no pits 25 micrometres or larger |
| Method B | Same as A but with crevice former; less common than F | Per project specification |
| Method E | Quantitative CPT determination, qualification testing, alloy ranking | Reports lowest pit-initiation temperature |
| Method F | Quantitative CCT determination, qualification testing, alloy ranking | Reports lowest crevice-initiation temperature |
Interpretation of Low CPT or CCT
A super duplex 2507 specimen returning a CPT below 50 deg C or a CCT below 35 deg C is non-conforming and warrants root-cause investigation. The most common causes are:
- Sigma-phase contamination: chromium and molybdenum locally robbed from the matrix to feed sigma precipitates. PREN is preserved on a bulk-chemistry basis but is degraded at the grain boundaries that control pit initiation. Confirmed by ASTM A923 Method A etch test.
- Out-of-spec chemistry: bulk PREN below 40 (lower-end chromium, molybdenum, or nitrogen). Confirmed by re-checking the heat analysis on the mill test certificate.
- Inadequate solution-anneal quench: ferrite excess (above 65 percent), Cr2N nitride precipitation, or sub-microscopic sigma. Confirmed by Feritscope and Charpy testing.
- Welding heat input above 2.5 kJ per mm or interpass above 150 deg C: HAZ sigma-phase contamination. Confirmed by metallography across the weld.
- Surface contamination (iron pickup, embedded chloride salts): false-low CPT from surface effects rather than bulk metallurgy. Confirmed by re-test with additional surface preparation (re-grind, re-passivate).
Related Super Duplex 2507 References
- UNS S32750 reference page
- Corrosion resistance
- PREN explainer
- NORSOK M-630
- Sigma-phase embrittlement
- Ferrite content
- Super duplex 2507 pipe
ASTM G48 Method E FAQ
What does ASTM G48 Method E measure?
Method E measures the Critical Pitting Temperature (CPT) in 6 percent ferric chloride solution. The specimen is immersed at a starting temperature (typically 35 deg C for super duplex), examined after 24 hours, and the temperature is raised in 5 deg C increments with fresh solution at each step. The CPT is the lowest temperature at which pitting (one or more pits 25 micrometres or larger) is detected.
What is the typical CPT for super duplex 2507?
50 to 70 degrees Celsius for material that meets the chemistry and heat-treatment requirements. Mill heats from Sandvik (SAF 2507), Industeel (UR 52N+), and Outokumpu (Avesta 2507) routinely return CPT in this band. CPT below 50 deg C signals either sigma-phase contamination, out-of-spec chemistry, or inadequate solution-anneal heat treatment.
What is the difference between Method A and Method E?
Method A is a fixed-temperature, 24-hour weight-loss test for routine production acceptance (typical pass criterion: below 4.0 g per square metre weight loss at 35 deg C, no visible pits). Method E is a temperature-stepped test that returns a quantitative CPT value; it is more time-consuming but provides a numeric ranking suitable for material qualification, alloy comparison, or dispute resolution.
What is Method F and how does it differ from Method E?
Method F is the crevice-corrosion analogue of Method E. A PTFE crevice former is clamped to the specimen at controlled torque, creating tight contact zones where local environment becomes more aggressive than the bulk solution. The test returns the Critical Crevice Temperature (CCT). For super duplex 2507, typical CCT is 35 to 50 deg C, normally 10 to 20 deg C below the open-surface CPT measured on the same alloy.
Why is ferric chloride used as the test electrolyte?
FeCl3 is an aggressive oxidising chloride that accelerates pit initiation by providing both the chloride ion and the cathodic oxidant in a single solution. The combination is more aggressive than seawater (which has roughly 19,000 ppm chloride at neutral pH). FeCl3 testing therefore gives a conservative ranking: an alloy that resists pitting in 6 percent FeCl3 at a given temperature will normally also resist pitting in seawater at higher temperature.
How does CPT relate to PREN?
CPT and PREN are positively correlated. PREN is calculated from chemistry (Cr + 3.3 Mo + 16 N) and predicts pitting resistance; CPT is measured experimentally and confirms it. For super duplex 2507 with PREN around 41 to 43, the typical Method E CPT is 50 to 70 deg C. PREN is the cheap, fast indicator from the mill test certificate; CPT is the slow, quantitative laboratory measurement that validates PREN under realistic service-relevant conditions.
What does a CPT result below 50 deg C indicate?
A super duplex 2507 specimen returning CPT below 50 deg C is non-conforming. Most common causes are sigma-phase contamination from inadequate solution-anneal quench or excessive welding heat input, ferrite excess from incorrect cooling rate, out-of-spec chemistry (PREN below 40), or surface iron contamination. Diagnosis requires Charpy testing, Feritscope ferrite measurement, ASTM A923 sigma screening, and re-check of mill heat analysis.
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