Asset Integrity Management

Pitting Resistance Equivalent Number

Pitting Resistance Equivalent Number (PREN): Your Guide to Understanding Material Resistance in Oil & Gas

In the harsh environments of oil and gas operations, corrosion is a constant threat. Materials used in pipelines, tanks, and other equipment are exposed to corrosive fluids, high pressures, and fluctuating temperatures, putting them at risk for pitting, a localized form of corrosion that can lead to catastrophic failures. To combat this, engineers rely on a crucial metric known as the Pitting Resistance Equivalent Number (PREN).

What is PREN?

PREN is a relative measure of a material's resistance to pitting corrosion in corrosive service. It's calculated based on the chemical composition of the material, specifically the content of chromium (Cr), molybdenum (Mo), and nitrogen (N). These elements contribute to the formation of a protective oxide layer on the material's surface, which helps resist pitting attack.

How is PREN Calculated?

The PREN is calculated using a simple formula:

PREN = %Cr + 3.3 x %Mo + 16 x %N

This formula reflects the relative effectiveness of each element in providing pitting resistance. Chromium forms the base oxide layer, while molybdenum enhances its stability and resistance to breakdown. Nitrogen, although less significant than Cr and Mo, also contributes to the protective layer.

Why is PREN Important?

PREN serves as a valuable tool for selecting suitable materials for specific applications. By knowing the PREN of different materials, engineers can choose the most appropriate option for a given corrosive environment.

Example Applications:

  • Sour Gas Environments: Materials used in sour gas production often face pitting corrosion due to the presence of hydrogen sulfide (H2S). Steels with higher PREN, such as duplex stainless steels, are preferred for these applications.
  • Downhole Applications: High-pressure and high-temperature environments downhole require materials with excellent resistance to pitting and other forms of corrosion. Austenitic stainless steels with high PREN are typically used for casing and tubing.
  • Offshore Applications: Seawater and marine environments can be highly corrosive. Stainless steels with high PREN are often used for offshore platforms, pipelines, and other structures.

Limitations of PREN:

While PREN is a helpful tool, it's important to remember its limitations:

  • It is a relative measure: PREN only provides a comparative assessment of pitting resistance. Actual performance may vary depending on the specific corrosive environment and other factors.
  • It doesn't consider all factors: Other factors, such as the presence of chloride ions, temperature, and flow rate, can influence pitting corrosion, and PREN doesn't account for these.
  • It doesn't guarantee corrosion resistance: Even materials with high PREN may not be completely resistant to pitting corrosion in extreme environments.

Conclusion:

PREN is a valuable tool for engineers to assess and select materials that can withstand pitting corrosion in oil and gas applications. By considering the specific corrosive environment and other relevant factors, engineers can effectively utilize PREN to ensure the long-term performance and safety of equipment and infrastructure.


Test Your Knowledge

PREN Quiz

Instructions: Choose the best answer for each question.

1. What does PREN stand for?

a) Pitting Resistance Equivalent Number b) Protective Resistance Evaluation Number c) Pipeline Resistance Evaluation Number d) Pitting Resistance Engineering Number

Answer

a) Pitting Resistance Equivalent Number

2. Which elements contribute to the PREN value?

a) Chromium, Manganese, and Nitrogen b) Chromium, Molybdenum, and Nickel c) Chromium, Molybdenum, and Nitrogen d) Manganese, Nickel, and Nitrogen

Answer

c) Chromium, Molybdenum, and Nitrogen

3. What is the formula for calculating PREN?

a) PREN = %Cr + 3.3 x %Mo + 16 x %N b) PREN = %Cr + 2 x %Mo + 10 x %N c) PREN = %Cr + 5 x %Mo + 20 x %N d) PREN = %Cr + 1.5 x %Mo + 8 x %N

Answer

a) PREN = %Cr + 3.3 x %Mo + 16 x %N

4. In which of the following environments would materials with a higher PREN be preferred?

a) Freshwater pipeline b) Sour gas production c) Low-pressure natural gas storage d) Air-filled storage tanks

Answer

b) Sour gas production

5. Which of the following is NOT a limitation of the PREN value?

a) PREN is a relative measure and does not guarantee complete corrosion resistance. b) PREN only considers the chemical composition of the material. c) PREN can accurately predict the exact lifespan of a material in a specific environment. d) PREN does not account for all factors influencing pitting corrosion, such as temperature and flow rate.

Answer

c) PREN can accurately predict the exact lifespan of a material in a specific environment.

PREN Exercise

Task: You are an engineer working on a project to build an offshore oil platform. The platform will be exposed to seawater and marine organisms, which can be highly corrosive. You are tasked with selecting a suitable material for the platform's structural components.

Given:

  • Two potential materials:
    • Material A: Stainless steel with %Cr = 18, %Mo = 2, %N = 0.1
    • Material B: Carbon steel with %Cr = 0, %Mo = 0, %N = 0
  • Expected corrosive environment: High chloride ion content, fluctuating temperatures, and potential for marine organism attachment.

Instructions:

  1. Calculate the PREN for both materials.
  2. Based on the PREN values and the corrosive environment, determine which material is more suitable for the offshore platform structure.
  3. Briefly explain your reasoning.

Exercice Correction

1. PREN Calculation: * **Material A:** PREN = 18 + (3.3 x 2) + (16 x 0.1) = 25.6 * **Material B:** PREN = 0 + (3.3 x 0) + (16 x 0) = 0 2. Suitable Material: Material A (stainless steel) is more suitable for the offshore platform structure. 3. Reasoning: Material A has a significantly higher PREN value compared to Material B. This indicates that stainless steel offers significantly better resistance to pitting corrosion in the harsh marine environment. The high chloride content, fluctuating temperatures, and potential for marine organism attachment all contribute to a high risk of pitting corrosion. Choosing a material with a high PREN value like Material A is crucial to ensure the long-term durability and safety of the offshore platform.


Books

  • Corrosion Engineering by Donald H. Uhlig and Ralph H. Winston
  • Materials Selection for the Oil and Gas Industry by T.R. Shives
  • Corrosion Resistance of Metals and Alloys by J.R. Davis
  • Corrosion in Oil and Gas Production by R.N. King and J.D. Neufeld

Articles

  • Pitting Corrosion Resistance of Stainless Steels by J.R. Davis (ASM International)
  • Selection of Materials for Sour Gas Environments by NACE International
  • Corrosion of Downhole Tubing in Oil and Gas Production by SPE Journal
  • Predicting Pitting Corrosion of Stainless Steels in Oil and Gas Applications by Corrosion Engineering, Science and Technology
  • Evaluation of PREN for the Selection of Stainless Steels for Oil and Gas Applications by Materials Performance

Online Resources

  • NACE International: https://www.nace.org/ (NACE is a leading organization for corrosion control, providing resources and information on materials selection, corrosion testing, and best practices).
  • ASM International: https://www.asminternational.org/ (ASM International is a materials science and engineering society offering resources and information on various materials, including corrosion resistant alloys).
  • Corrosionpedia: https://www.corrosionpedia.com/ (Corrosionpedia is a comprehensive online resource on corrosion science and engineering, providing definitions, articles, and calculators related to corrosion).
  • Corrosion Doctors: https://www.corrosiondoctors.org/ (Corrosion Doctors is a website dedicated to educating engineers and professionals on corrosion control, offering information on different types of corrosion, materials selection, and corrosion mitigation strategies).

Search Tips

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  • "PREN" + "stainless steel" + "sour gas"
  • "Materials selection" + "corrosion" + "downhole"
  • "Corrosion resistance" + "offshore" + "sea water"

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