Pitting Resistance in Stainless Steel: Understanding PREN for Optimal Material Selection
Exploring the determination of pitting resistance equivalent numbers (PREN) and their significance in understanding material corrosion resistance.
Pitting corrosion resistance is a critical factor in selecting components and materials for piping systems. However, gauging resistance levels solely by name or grade is challenging due to variations in metallurgical composition. Pitting resistance equivalent numbers (PRENs), also known as PRE-values, offer a simple means of comparing alloys regarding their ability to withstand pitting corrosion. Although not absolute, PRENs provide a valuable reference for comparing different alloys. The British Stainless Steel Association (BSSA) advises that while PRENs are useful for ranking grades, they cannot predict a specific grade's suitability for an application involving pitting corrosion. It's always recommended to consult with an engineer to ensure materials meet all requirements.
WHAT IS PITTING CORROSION?
Understanding PREN values requires familiarity with pitting corrosion, which affects metals with protective films, such as stainless steel. Initiated by poor aeration or chemical exposure, pitting corrosion leads to localized attacks spreading quickly from the protective surface layer to the metal itself.
Common causes of pitting corrosion include exposure to bromides, chlorides, fluorides, hypochlorites, iodides, sulfides, and water.
HOW IS THE PREN OF AN ALLOY DETERMINED?
The PREN of an alloy is determined using widely accepted formulas that evaluate resistance to localized pitting corrosion by chlorides. The formulas take into account chromium (%Cr), molybdenum(%Mo), nitrogen(%N), and tungsten (%W) present in the alloy.
For super duplex stainless alloys with tungsten:
PREN=%Cr+3.3×(%Mo+0.5%W)+16×%N
Results typically range from 16 to over 40, with numbers greater than 32 often required for components in oil and gas industries. For environments with hydrogen sulphide (H2S) or seawater, a PREN of 40 or higher is typical. However, these numbers can vary based on environmental factors such as corrosive media, service temperatures, component surface finishes, system design, and acidity.
| STEEL TYPE | CHROMIUM % | MOLYBDENUM % | NITROGEN % | PREN |
|---|---|---|---|---|
| Ferritic Steels | ||||
| 430 | 16.0-18.0 |
NS | NS | 16.0-18.0 |
| 434 | 16.0-18.0 |
0.9-1.4 | NS | 19.0-22.6 |
| 441 | 17.5-18.5 | NS | NS | 17.5-18.5 |
| 444 | 17.0-20.0 | 1.8-2.5 | 0.030 MAX | 23.0-28.7 |
| Austenitic Steels | ||||
| 304 | 17.5-19.5 | NS | 0.11 MAX | 17.5-20.8 |
| 304LN | 17.5-19.5 | NS | 0.12-0.22 | 19.4-23.0 |
| 316/316L | 16.5-18.5 | 2.0-2.5 | 0.11 MAX | 23.1-28.5 |
| 316L (2.5% min Mo) | 17.0-19.0 | 2.5-3.2 | 0.11 MAX | 25.3-30.7 |
| 316LN | 16.5-18.5 | 2.0-2.5 | 0.12-0.22 | 25.0-30.3 |
| 904L | 19.0-21.0 | 4.0-5.0 | 0.15 MAX | 32.2-39.9 |
| Sanicro 28 | 24.0-26.0 | 3.0-4.0 | 0.11 MAX | 35.9-43.0 |
| 254SMO | 19.5-20.5 | 6.0-7.0 | 0.18-0.25 | 42.2-47.6 |
| 1925hMo | 19.0-21.0 | 6.0-7.0 | 0.15-0.25 | 41.2-48.1 |
| 4565S | 24.0-26.0 | 4.0-5.0 | 0.30-0.60 | 42.0-52.1 |
| Duplex Steels | ||||
| 2202 | 22.0 | 0.4 | 0.20 | 26.5 |
| 2101LDX | 21.0-22.0 | 0.1-0.8 | 0.20-0.25 | 24.5-28.6 |
| SAF 2304 | 22.0-24.0 | 0.1-0.6 | 0.05-0.20 | 23.1-29.2 |
| SAF 2205 | 21.0-23.0 | 2.5-3.5 | 0.10-0.22 | 30.8-38.1 |
| SAF 2507 | 24.0-26.0 | 3.0-4.0 | 0.24-0.35 | > 40 |
| Zeron 100 | 24.0-26.0 | 3.0-4.0 | 0.20-0.30 | > 40 |
| Ferrinox 255 | 24.0-26.0 | 3.0-4.0 | 0.20-0.30 | > 40 |
WHAT CAN PREN VALUES TELL YOU ABOUT A MATERIAL CHOICE?
While PREN values are not absolute, they provide a relative understanding of pitting corrosion resistance within the same stainless steel family (austenitic, ferritic, duplex, etc.). For oil and gas industries, a PREN greater than 32 is often required. However, material choice should consider various factors, including availability, cost, fabricability, and physical traits.
SUMMARY
This guide serves as informative material and not a direct recommendation. Consulting with an engineer is crucial before making material choices to ensure regulatory compliance and efficiency in your materials investment. Despite their limitations, PREN values offer a relative understanding of pitting corrosion resistance when combined with data on costs, availability, and fabricability.
For over decades, INOX-TEK has been a leading provider of high-quality stainless steel products in the world. Specializing in the oil, gas, and maritime industries, we understand the challenges these environments pose to piping systems. Contact us today for expert advice on achieving your next system design or operation goals.
REFERENCES
- British Stainless Steel Association: Calculation of pitting resistance equivalent numbers (PREN)
- Corrosionpedia: Pitting Resistance Equivalent Number (PREN)
- Wikipedia: Pitting resistance equivalent number
- ASTM International: ASTM G48 - 11(2015) Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution