How to Calculate the Stainless Steels and Nickel Alloys PREN Number?
Pitting corrosion is one of the most insidious forms of corrosion, especially in chloride-containing environments. It manifests as localized pits on the material’s surface, leading to structural failures if not properly managed. In industries where materials are exposed to such aggressive environments—like oil and gas, chemical processing, and marine applications—the selection of materials with high pitting resistance is crucial. This is where the Pitting Resistance Equivalent Number (PREN) comes into play.
Understanding PREN: The Basics
The Pitting Resistance Equivalent Number (PREN) is a theoretical calculation used to predict the pitting corrosion resistance of stainless steels and nickel alloys. It correlates the chemical composition of the material with its ability to resist the initiation of pits, especially in environments containing chlorides.
The formula for calculating the PREN is based on the concentrations of specific alloying elements, primarily chromium (Cr), molybdenum (Mo), and nitrogen (N). These elements significantly enhance the material’s resistance to pitting by forming protective passive layers and reducing the corrosion rate.
The PREN Formula
The most widely accepted formula for calculating PREN is as follows:
PREN=%Cr+3.3×%Mo+16×%N
Where:
- Cr is the chromium content in the alloy.
- Mo is the molybdenum content in the alloy.
- N is the nitrogen content in the alloy.
In some advanced alloys, the formula can be adjusted to account for tungsten (W), which also contributes to pitting resistance:
PREN=%Cr+3.3×(%Mo+0.5×%W)+16×%N
In this formula, higher levels of chromium, molybdenum, and nitrogen result in higher PREN values, which indicate greater resistance to pitting corrosion. For example, Penflex lists the following PREN ranges for different alloys:
304, 304L, 309, 310, 321: 18.0–20.0
316, 316L: 22.6–27.9
317, 317L: 27.9–33.2
AL-6XN: 39.8–45.1
Inconel® alloy 625: 46.4–56.0
Hastelloy® alloy C-276: 64.0–73.8
The Role of Alloying Elements in PREN
- Chromium (Cr): Chromium is the backbone of corrosion resistance in stainless steels and nickel alloys. It forms a stable, passive oxide layer on the material’s surface, protecting it from further oxidation. The higher the chromium content, the more robust this protective layer becomes, directly enhancing the PREN.
- Molybdenum (Mo): Molybdenum is particularly effective in improving pitting and crevice corrosion resistance. It enhances the stability of the passive film in chloride-rich environments. Every 1% increase in molybdenum content has a significant impact on the PREN, as reflected by the multiplication factor of 3.3 in the formula.
- Nitrogen (N): Nitrogen is a potent alloying element that not only strengthens the material through solid solution hardening but also significantly boosts pitting resistance. Its impact is so pronounced that the PREN formula multiplies the nitrogen content by 16.
- Tungsten (W): Tungsten is used in some super duplex stainless steels and nickel alloys to further enhance pitting resistance. Although its effect is less than that of molybdenum, it still contributes to the material’s overall resistance, especially in highly aggressive environments.
Calculating PREN: Practical Examples
To illustrate how PREN is calculated, let’s consider the following examples:
1. 304 Stainless Steel
- Composition: 18% Cr, 0% Mo, 0.1% N
- PREN Calculation:
PREN=18+3.3×0+16×0.1=18+0+1.6=19.6
Interpretation: A PREN of 19.6 indicates that 304 stainless steel has a moderate resistance to pitting, making it suitable for less aggressive environments.
2. 316 Stainless Steel
- Composition: 16.5% Cr, 2.1% Mo, 0.05% N
- PREN Calculation:
PREN=16.5+3.3×2.1+16×0.05=16.5+6.93+0.8=24.23
Interpretation: With a PREN of 24.23, 316 stainless steel offers enhanced pitting resistance, making it a preferred choice for marine and chemical processing applications.
3. Duplex Stainless Steel 2205
- Composition: 22% Cr, 3% Mo, 0.18% N
- PREN Calculation:
PREN=22+3.3×3+16×0.18=22+9.9+2.88=34.78
Interpretation: A PREN of 34.78 makes Duplex 2205 highly resistant to pitting, and suitable for highly corrosive environments such as offshore platforms and chemical plants.
4. Super Duplex Stainless Steel 2507
- Composition: 25% Cr, 4% Mo, 0.3% N, 0.5% W
- PREN Calculation:
PREN=25+3.3×(4+0.5×0.5)+16×0.3=25+13.365+4.8=43.165
Interpretation: A PREN exceeding 40 indicates exceptional pitting resistance, making Super Duplex 2507 ideal for the most demanding environments, including desalination plants and subsea pipelines.
| 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 |
Importance of PREN in Material Selection
The PREN is a vital parameter in material selection for environments prone to pitting corrosion. It allows engineers and material scientists to compare different stainless steels and nickel alloys based on their theoretical resistance to pitting. However, while PREN is a valuable guide, it should not be the sole criterion for material selection. Real-world factors such as temperature, stress, and the presence of other corrosive agents can also influence the material’s performance.
Limitations of PREN
While PREN is a useful indicator, it has limitations:
- Ignores Nickel Content: PREN does not account for nickel, which plays a significant role in general corrosion resistance and the stabilization of austenitic structures.
- Only Applicable to Pitting: PREN focuses solely on pitting resistance and does not consider other forms of corrosion such as crevice corrosion, stress corrosion cracking, or general corrosion.
- Assumes Uniformity: The formula assumes that the alloying elements are uniformly distributed, which might not always be the case in real-world materials, especially in welded areas.
Conclusion
The PREN is an essential tool in predicting the pitting resistance of stainless steels and nickel alloys, providing valuable guidance in selecting materials for corrosive environments. By understanding the role of chromium, molybdenum, nitrogen, and sometimes tungsten, engineers can make informed decisions that enhance the longevity and reliability of their structures.
When designing for corrosive environments, it’s crucial to consider not just the PREN but also the specific conditions the material will face, including temperature, pH, and exposure to other chemicals. With a comprehensive approach, you can ensure that your material selection process is robust, leading to safer and more durable applications.
The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.
