Pitting corrosion is one of the most critical localized corrosion forms affecting stainless steels, particularly in chloride-containing environments. Because pits can propagate rapidly and lead to catastrophic failure, evaluating pitting resistance is essential for alloy selection in demanding applications such as offshore, desalination, and chemical processing industries.
Two widely adopted approaches are the measurement of the pitting potential (Epit) through electrochemical testing, and the calculation of the pitting resistance equivalent number (PREN) from alloy composition. While the former provides an experimental threshold for pit initiation, the latter serves as a predictive index based on material chemistry. Understanding the correlation between these two parameters is vital for assessing the corrosion resistance of duplex stainless steels and comparing them with conventional austenitic grades.
Pitting Potential (Epit)
The pitting potential represents the critical potential at which a stainless steel loses its passivity in a chloride solution and localized attack initiates. It is typically expressed in millivolts relative to the saturated calomel electrode (mV vs SCE). A higher Epit indicates greater stability of the passive film and hence better resistance to pitting.
It is usually expressed in mV versus a reference electrode (such as the saturated calomel electrode, SCE, or the Ag/AgCl electrode).
Significance:
- A higher Epit indicates better resistance to pitting.
- A low Epit suggests that the material is more prone to pitting in chloride-containing environments.
Examples:
- Type 304 stainless steel exhibits a relatively low Epit and is therefore susceptible to pitting in chloride environments.
- In contrast, duplex stainless steels such as 2205, or high-alloy grades such as 2507 and Inconel 625, show much higher Epit values, reflecting their superior resistance to pitting.
Pitting Resistance Equivalent Number (PREN)
PREN is an empirical index derived from the alloy composition, reflecting the beneficial effects of chromium, molybdenum, nitrogen, and sometimes tungsten on pitting resistance. The most widely used formula is:
PREN=%Cr+3.3×%Mo+16×%NPREN
An extended version includes tungsten:
PREN=%Cr+3.3×%Mo+16×%N+1.65×%W
Higher PREN values correspond to improved theoretical resistance to pitting.
Correlation Between PREN and Pitting Potential
Although determined through different methods, PREN and pitting potential generally show a positive correlation. Alloys with higher PREN values tend to exhibit higher Epit, confirming enhanced resistance to localized corrosion. Nevertheless, PREN is a compositional guideline and must be validated through electrochemical measurements, since metallurgical factors and environmental conditions can also influence pitting susceptibility.
Examples of Duplex Stainless Steels
The relationship between PREN and Epit can be illustrated using common duplex stainless steels:
- 2205 (UNS S32205): PREN ≈ 30–35; Epit ≈ +300 to +500 mV vs SCE.
- 2507 (UNS S32750): PREN ≈ 40–45; Epit ≈ +600 to +800 mV vs SCE.
- Zeron 100 (UNS S32760): PREN ≈ 40–50; Epit ≈ +700 to +900 mV vs SCE.
These examples demonstrate that super duplex grades with higher PREN consistently achieve higher pitting potentials compared with standard duplex alloys.
Comparative Data for Stainless Steels
Table 1 provides a comparative overview of PREN values and typical pitting potentials for commonly used austenitic and duplex stainless steels, along with a nickel-based alloy for reference.
| Material (Grade) | PREN (typical range) | Pitting Potential (mV vs SCE, 3.5% NaCl) | Pitting Resistance |
| 304 (UNS S30400) | 18–20 | +50 ~ +150 | Very prone to pitting |
| 316L (UNS S31603) | 23–28 | +200 ~ +350 | Moderate resistance |
| 904L (UNS N08904) | 33–35 | +350 ~ +500 | High resistance |
| 2205 Duplex (UNS S32205) | 30–35 | +300 ~ +500 | Excellent resistance |
| 2507 Super Duplex (UNS S32750) | 40–45 | +600 ~ +800 | Superior resistance |
| Zeron 100 (UNS S32760) | 40–50 | +700 ~ +900 | Superior resistance |
| Alloy 625 (UNS N06625, Ni-base) | 45–55 | +800 ~ +1000 | Outstanding resistance |
Conclusion
Both pitting potential and PREN are essential parameters for evaluating the corrosion resistance of stainless steels. Pitting potential provides an experimentally measured threshold for pit initiation, while PREN offers a compositional guideline for alloy design. The positive correlation between these two indicators confirms that alloys with higher PREN, such as super duplex grades (2507, Zeron 100) and nickel-based alloys (Alloy 625), exhibit significantly higher resistance to pitting than standard austenitic steels. Thus, the combined use of PREN calculations and electrochemical testing offers a robust framework for material selection in chloride-rich environments.
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