PRE

Understanding PRE: Pitting Resistance Equivalent Number in Oil & Gas

In the oil and gas industry, corrosion is a significant concern, leading to costly repairs, production downtime, and environmental hazards. To combat this, materials used in the industry must possess exceptional resistance to various forms of corrosion, particularly pitting corrosion. One key metric used to assess this resistance is the Pitting Resistance Equivalent Number (PRE).

What is Pitting Resistance Equivalent Number (PRE)?

PRE is a numerical value that quantifies the resistance of a material to pitting corrosion, a localized form of corrosion that attacks specific areas on a metal surface. It is calculated based on the material's chemical composition, primarily focusing on the concentration of chromium (Cr) and molybdenum (Mo), which are known to enhance pitting resistance.

Why is PRE Important?

Predicting Material Performance: PRE provides a standardized method for comparing the pitting resistance of various materials. This allows engineers to select materials best suited for specific environments and applications.
Ensuring Safety and Reliability: By selecting materials with adequate PRE values, companies can minimize the risk of pitting corrosion, preventing leaks, failures, and potential environmental disasters.
Cost Optimization: Knowing the PRE of materials allows engineers to select the most cost-effective options without compromising on safety or performance.

Calculating PRE:

The most commonly used formula for calculating PRE is:

PRE = %Cr + 3.3 x %Mo

Where:

%Cr represents the percentage of chromium in the material
%Mo represents the percentage of molybdenum in the material

Higher PRE values generally indicate greater resistance to pitting corrosion. However, it's important to consider other factors like the specific environment, stress levels, and the presence of other corrosive elements when evaluating material suitability.

Example of PRE Application:

Stainless Steels: 316L stainless steel, with a PRE of approximately 35, offers excellent resistance to pitting corrosion in many oil and gas applications.
Nickel Alloys: Alloys like Hastelloy C-276, with PRE values exceeding 50, are highly resistant to pitting corrosion in aggressive environments like sour gas production.

Conclusion:

PRE is an essential tool for engineers and professionals involved in the oil and gas industry. By understanding and utilizing this metric, they can ensure the selection of materials that provide optimal resistance to pitting corrosion, ultimately improving safety, reliability, and cost-effectiveness throughout the project lifecycle.

Test Your Knowledge

Quiz: Pitting Resistance Equivalent Number (PRE)

Instructions: Choose the best answer for each question.

1. What does PRE stand for?

a) Pitting Resistance Efficiency b) Pitting Resistance Equivalent Number c) Protective Resistance Equivalent Number d) Percentage Resistance Evaluation

Answer

b) Pitting Resistance Equivalent Number

2. Which elements primarily contribute to a material's PRE value?

a) Iron and Nickel b) Chromium and Molybdenum c) Copper and Zinc d) Aluminum and Magnesium

Answer

b) Chromium and Molybdenum

3. A higher PRE value generally indicates:

a) Greater resistance to pitting corrosion b) Lower resistance to pitting corrosion c) No relation to pitting corrosion d) Increased susceptibility to other forms of corrosion

Answer

a) Greater resistance to pitting corrosion

4. Which of the following is NOT a benefit of utilizing PRE in material selection?

a) Predicting material performance b) Ensuring safety and reliability c) Cost optimization d) Determining the exact lifespan of a material

Answer

d) Determining the exact lifespan of a material

5. What is the PRE value of a material containing 12% Chromium and 4% Molybdenum?

a) 12 b) 24 c) 26.2 d) 36

Answer

c) 26.2 (PRE = %Cr + 3.3 x %Mo = 12 + 3.3 x 4 = 26.2)

Exercise: Material Selection

Scenario: You are designing a pipeline for transporting sour gas. Sour gas contains hydrogen sulfide (H2S), which is highly corrosive. You need to select a suitable material for the pipeline.

Materials:

Material A: Stainless Steel 304 (PRE = 19)
Material B: Stainless Steel 316L (PRE = 35)
Material C: Nickel Alloy Hastelloy C-276 (PRE = 55)

Instructions:

Based on the given PRE values, rank the materials from least to most resistant to pitting corrosion in the sour gas environment.
Explain your reasoning for choosing the most resistant material for the sour gas pipeline.

Exercice Correction

1. **Material Ranking (Least to Most Resistant):** * Material A: Stainless Steel 304 (PRE = 19) * Material B: Stainless Steel 316L (PRE = 35) * Material C: Nickel Alloy Hastelloy C-276 (PRE = 55)

2. **Reasoning for Choosing Material C:** * Sour gas environments are highly corrosive due to the presence of H2S. * Material C (Hastelloy C-276) has the highest PRE value (55), indicating exceptional resistance to pitting corrosion. * Although Material B (316L) offers good resistance, Material C's superior PRE value makes it the most suitable choice for this aggressive application, ensuring better long-term performance and safety.

Books

Corrosion Engineering by Dennis R. Uhlig and Reginald Winston Revie
Corrosion and its Control in the Oil and Gas Industry by A.K.T. Lau and D.P. La
Corrosion Basics: An Introduction by ASM International
Materials Selection for Oil and Gas Production by A.K.T. Lau

Articles

Pitting Corrosion: A Review by M.G. Fontana and N.D. Greene (Journal of Electrochemical Society, 1958)
Pitting Resistance Equivalent Number (PRE) for Stainless Steels by R.D. McCright (Corrosion, 1978)
The Use of Pitting Resistance Equivalent Number (PRE) to Select Corrosion Resistant Alloys for Oil and Gas Applications by A.K.T. Lau (Corrosion, 2003)
Pitting Corrosion: A Critical Review of Current Knowledge by S.P. Nunes et al. (Corrosion Reviews, 2015)