Technical knowledge
PREN and critical pitting temperature: use both, trust neither blindly
What the pitting resistance equivalent number does and does not tell you, how critical pitting temperature is measured, and how to use both when shortlisting alloys for chloride service.
The formula and its blind spots
The common form is PREN equals percent chromium plus 3.3 times percent molybdenum plus 16 times percent nitrogen. Some variants include tungsten. The calculation takes no account of nickel content, of the phase balance in duplex alloys, of surface finish, of weld condition or of the oxidising potential of the actual fluid. A high PREN alloy with a poor microstructure can perform worse than a lower PREN alloy in good condition.
Typical values for common grades
| Grade | Typical PREN | Usual interpretation |
|---|---|---|
| 316L | About 26 | Low chloride, atmospheric |
| 317L | About 30 | Low chloride, improved |
| 904L | About 35 | Acid duty, moderate chloride |
| Duplex 2205 | 35 - 38 | Seawater, brine |
| Super duplex 2507 | Above 40 | Hot brine, subsea |
| 254SMO | Above 43 | Warm chloride, heat exchangers |
| Incoloy 825 | 30 - 33 | Acid and sour water |
| Inconel 625 | 45 - 51 | Seawater, sour, FGD |
| Hastelloy C-276 | About 68 | Aggressive chemical |
| Titanium | Not applicable | Does not pit by the same mechanism |
Critical pitting temperature and how it is measured
Critical pitting temperature is determined by exposing samples to a standard ferric chloride solution, typically to ASTM G48, at increasing temperatures until pitting occurs. Because the test solution is far more aggressive than most process fluids, the result is used for ranking rather than as a service limit. It does, however, capture the effect of alloying and of microstructure far better than a calculated index.
A practical screening sequence
- Define the credible worst case chemistry, temperature and stagnation.
- Use PREN to build a shortlist, not to make the final choice.
- Compare critical pitting temperature data for the shortlisted alloys where available.
- Adjust for crevices: a crevice corrosion temperature is typically well below the pitting temperature.
- Confirm with service experience or with testing in the actual fluid before committing to a critical application.
Questions about this topic
Is a higher PREN always better?
Generally yes within the same alloy family, but the index is a screening tool. Microstructure, weld condition, surface finish and the oxidising potential of the fluid all influence real performance.
Why does titanium have no PREN?
Titanium resists chloride by a stable oxide film rather than by the chromium molybdenum mechanism the index describes. It is compared using service experience and corrosion data rather than PREN.
How much margin should I allow for crevices?
Crevice corrosion starts at a noticeably lower temperature than pitting in the same environment. When gasket faces, deposits or tube joints create crevices, evaluate crevice data rather than pitting data.
Questions about our products
What products does Hangbo Alloy supply?
Four product families, all available in nickel-based alloys, titanium, duplex and special stainless steel: flanges, butt weld pipe fittings, elbows, and seamless tube and pipe. Flanges cover weld neck, long weld neck, slip-on, blind, socket weld, threaded and lap joint patterns. Fittings and elbows cover concentric and eccentric reducers, equal and reducing tees, caps, stub ends, and 45°, 90° and 180° returns in both long and short radius, plus 3D and 5D bends for piggable lines. Seamless tube and pipe is supplied cold drawn or hot finished from 6 mm to 630 mm outside diameter.
Which alloy grades are available for these products?
More than 90 grades are published in our grade library. The most requested are Inconel 625, 718, 600 and X-750, Hastelloy C-276, C-22 and B-2, Monel 400 and K-500, Incoloy 800H, 825 and 925, duplex 2205 and super duplex 2507, 316L, 904L and 254SMO stainless steel, and titanium Grade 2 and Grade 5. Nickel-based grades can also be supplied with VIM plus ESR double melting where cleanliness or fatigue life is critical.
What sizes, wall thicknesses and pressure classes can you supply?
Flanges run from NPS ½ to NPS 24 to ASME B16.5 and from NPS 26 to NPS 60 to ASME B16.47 Series A or B, in pressure classes 150 through 2500. Butt weld fittings and elbows run from NPS ½ to NPS 48 (DN 15 to DN 1200) in wall thickness Schedule 5S through XXS. Seamless tube and pipe covers 6 mm to 630 mm outside diameter, in random lengths of about 5 to 7 metres or fixed lengths up to 12 metres. Non-standard and intermediate sizes are quoted on request.
Which material and dimensional standards do you work to?
Forged flanges to ASTM B564, B381 and A182 with dimensions to ASME B16.5 and ASME B16.47. Butt weld fittings and elbows to ASTM B366 and A403 with dimensions to ASME B16.9 and MSS SP-43. Seamless tube and pipe to ASTM B444, B423, B622, B163, B167, B861, A312, A213 and A790. Marking follows MSS SP-25 and the applicable product standard. If your project is governed by EN, DIN, JIS or a company specification, send the document and we will confirm equivalence before quoting.
Is there a minimum order quantity?
There is no minimum for standard sizes and grades held in stock, and prototypes, spares and small trial batches are accepted. Made-to-order melts and special forgings carry a minimum weight that depends on the alloy and the route, typically from about 500 kg for special grades.
What inspection and certification is issued with the goods?
Every shipment carries an EN 10204 3.1 mill test certificate quoting the actual chemical and mechanical results for the specific heat lot, together with dimensional records and heat treatment data where applicable. Testing is carried out in house by optical emission spectrometry, tensile, impact and hardness testing, plus ultrasonic and eddy current examination, with hydrostatic pressure and intergranular corrosion testing to ASTM G28 or A262 Practice E where the specification requires it. Third party inspection by SGS or another recognised agency can be arranged, and their report is issued alongside the shipment.
What are typical lead times?
Standard stock items ship within days. Common production grades such as Inconel 625 or duplex 2205 are typically a few weeks. Special chemistries, larger forgings and items that require third party inspection take longer and are quoted individually against the requirement.
How are the products packed for export?
Flanges and fittings receive a rust preventive coating, are individually wrapped and are packed in plywood cases secured with steel banding. Tube ends are capped and bundles are strapped. Packing lists quote heat numbers so every case can be cross-checked against the mill certificate on arrival. We quote EXW, FOB, CFR, CIF and DAP, and mixed-alloy orders can be consolidated into one shipment with a single documentation package.
How do I request a quotation or a sample?
Send the grade or UNS number, the dimensions including schedule or wall thickness, the governing material and dimensional standards, quantity, any heat treatment or NDT scope and the certificate type you need; a drawing is ideal for special flanges and fittings. Email demi@nickel-alloy.com or message us on WhatsApp +86 15618279078 and we normally issue a formal quotation within 24 hours on working days. Samples and previous test reports can be arranged before the first order.
Talk to an engineer
Share your duty conditions and we will shortlist candidate alloys within one working day.