What is PREN?
PREN (Pitting Resistance Equivalent Number) is an index used to evaluate the resistance of stainless steels and corrosion-resistant alloys to pitting corrosion in chloride-containing environments.
Pitting is a localized form of corrosion that typically occurs in seawater, chemical processing fluids, or other chloride-rich media.
The higher the PREN value, the greater the material’s resistance to pitting corrosion.
PREN Calculation Formula
PREN = %Cr + 3.3 × (%Mo + 0.5 × %W) + 16 × %N
Where:
- Cr = Chromium content (%)
- Mo = Molybdenum content (%)
- W = Tungsten content (%) (often included as it enhances corrosion resistance)
- N = Nitrogen content (%)
This formula accounts for the combined effect of molybdenum and tungsten on pitting corrosion resistance, with tungsten weighted as half as effective as molybdenum.
You can also check the PREN value through the following table:
PREN Value Comparison Table (Pitting Resistance Equivalent Number)
| Material Name | UNS No. | Main Composition (Cr/Mo/N, %) | PREN Range | Pitting Resistance |
| 304 | S30400 | 18Cr / ~0Mo / ~0N | 18–19 | Low |
| 304L | S30403 | Similar to 304, lower carbon | 18–19 | Low |
| 316 | S31600 | 16Cr / 2.1Mo / ~0N | 23–28 | Moderate |
| 316L | S31603 | Similar to 316, lower carbon | 23–28 | Moderate |
| 317L | S31703 | 18Cr / 3.1Mo / ~0.03N | 28–30 | Good |
| 904L | N08904 | 20Cr / 4.5Mo / 0.02N | 34–36 | High |
| 2205 Duplex | S31803 / S32205 | 22Cr / 3.0Mo / 0.14–0.2N | 34–38 | High |
| 2304 Duplex | S32304 | 23Cr / 0Mo / 0.10N | 24–26 | Moderate |
| Super Duplex | S32750 | 25Cr / 4.0Mo / 0.27N | 41–45 | Very High |
| Zeron 100 | S32760 | 25.5Cr / 3.5Mo / 0.22N + Cu | 40–44 | Very High |
| Alloy 20 | N08020 | 20Cr / 2Mo / — | ~30 | Moderate |
| Incoloy 800 | N08800 | 21Cr / — / — | ~21–22 | Low |
| Incoloy 825 | N08825 | 20Cr / 2.6Mo / — | 30–32 | Moderate |
| Inconel 625 | N06625 | 21.5Cr / 9Mo / — | 45–52 | Very High |
| Inconel 718 | N07718 | 19Cr / 3Mo / — | ~33–36 | High |
| Hastelloy C-22 | N06022 | 22Cr / 13Mo / 0.01N | 68–72 | Super |
| Hastelloy C-276 | N10276 | 15.5Cr / 16Mo / 0.02N | 67–72 | Super |
| Hastelloy C-2000 | N06200 | 23Cr / 16Mo / 0.02N | 70–74 | Super |
| Alloy 59 | N06059 | 23Cr / 16Mo / — | 66–70 | Super |
| Titanium Gr.2 | R50400 | — | ∞ (infinite) | Immune |
PREN Value Comparison Chart
Through the chart from Nickel System , you can gain a clearer picture of the PREN comparison of different materials.
Importance of PREN in Material Selection
PREN is a critical guideline when selecting materials for corrosion-prone environments such as:
- Seawater systems
- Chloride-containing cooling water systems
- Chemical processing equipment
- Oil and gas transmission pipelines
PREN Value Reference Ranges
| PREN Range | Pitting Resistance Level | Typical Materials |
| < 20 | Low resistance | Common stainless steels (e.g., 304) |
| 20–30 | Moderate resistance | 316L, 317L |
| 30–40 | Good resistance (seawater-grade) | Duplex S31803 / S32205 |
| ≥ 40 | Excellent resistance | Super Duplex S32750 / S32760, Hastelloy, Inconel 625 |
FAQ About PREN (Pitting Resistance Equivalent Number)
Materials with PREN ≥ 32 are generally considered suitable for chloride environments with effective pitting resistance.
Molybdenum and nitrogen greatly enhance the material’s resistance to pitting and crevice corrosion and are key elements in increasing PREN.
PREN mainly indicates pitting resistance; however, overall corrosion resistance also depends on factors like microstructure and environmental conditions.
Austenitic stainless steels typically have lower PREN values, while duplex and super duplex stainless steels have higher PREN values, with some high-nickel alloys being even higher.
Summary
PREN (Pitting Resistance Equivalent Number) is an important index used to evaluate the ability of stainless steels and corrosion-resistant alloys to resist pitting corrosion in chloride-containing environments. It quantifies the corrosion resistance of a material based on the content of chromium, molybdenum, and nitrogen. The higher the PREN value, the stronger the material’s resistance to pitting and crevice corrosion, making it suitable for harsh environments such as seawater, chemical processing, and oil and gas industries. Selecting materials with an appropriate PREN value is key to ensuring long-term safety and extending the service life of equipment.
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