Mils Per Year (MPY): Measuring Material Degradation
In the world of materials science and engineering, understanding the rate at which materials degrade is crucial. This degradation can be caused by various factors such as corrosion, abrasion, or erosion, all of which can lead to material loss and ultimately, failure. To quantify this degradation, a common unit of measurement is Mils Per Year (MPY).
What is MPY?
MPY is a simple and widely used measure that expresses the average rate of material loss in terms of thickness, specifically in mils, over a period of one year. One mil is equal to one-thousandth of an inch (0.001"). Therefore, an MPY of 10 means that the material is losing an average of 10 thousandths of an inch of thickness every year.
How is MPY calculated?
Calculating MPY involves a few key factors:
- Material Thickness (T): This is the original thickness of the material in mils.
- Weight Loss (WL): This is the amount of material lost due to corrosion, abrasion, or erosion, measured in grams or milligrams.
- Density (D): This is the density of the material in grams per cubic centimeter (g/cm³).
- Surface Area (SA): This is the total area of the material exposed to the degrading environment in square inches (in²).
- Time (t): This is the duration of exposure in years.
The MPY is then calculated using the following formula:
MPY = (WL * 3.45 * T) / (D * SA * t)
Limitations of MPY:
While MPY provides a general indication of material loss, it has certain limitations:
- Average Representation: MPY represents an average rate of material loss across the entire surface. It does not account for localized corrosion, such as pitting, where the material loss might be significantly higher in specific areas.
- Uniform Degradation Assumption: MPY assumes uniform material loss across the surface. In reality, the rate of degradation can vary depending on factors such as temperature, stress, and the presence of corrosive agents.
- Limited Applicability: MPY is primarily used for metallic materials and might not be applicable to other materials such as polymers or ceramics.
Applications of MPY:
MPY is widely used in various industries, including:
- Oil and Gas: Assessing the rate of corrosion in pipelines and other equipment.
- Chemical Processing: Evaluating the degradation of equipment exposed to corrosive environments.
- Aerospace: Analyzing the corrosion resistance of aircraft components.
- Construction: Determining the service life of materials used in buildings and infrastructure.
Conclusion:
MPY provides a useful and straightforward method for quantifying the average rate of material loss due to corrosion, abrasion, or erosion. However, it's important to recognize its limitations and consider the potential for localized degradation when making critical decisions related to material selection, design, and maintenance.
Test Your Knowledge
MPY Quiz
Instructions: Choose the best answer for each question.
1. What does MPY stand for? (a) Miles Per Year (b) Mils Per Year (c) Meters Per Year (d) Millimeters Per Year
Answer
(b) Mils Per Year
2. What is the unit of measurement for material thickness in the MPY formula? (a) Millimeters (b) Inches (c) Mils (d) Centimeters
Answer
(c) Mils
3. Which of the following is NOT a factor considered in the MPY calculation? (a) Material Density (b) Surface Area (c) Time (d) Material Strength
Answer
(d) Material Strength
4. What is the primary limitation of MPY? (a) It only measures corrosion. (b) It is not applicable to metals. (c) It assumes uniform material loss across the surface. (d) It is too complicated to calculate.
Answer
(c) It assumes uniform material loss across the surface.
5. Which industry commonly uses MPY to assess the rate of corrosion in pipelines? (a) Aerospace (b) Construction (c) Chemical Processing (d) Oil and Gas
Answer
(d) Oil and Gas
MPY Exercise
Scenario: A steel pipe with a thickness of 0.25 inches is exposed to a corrosive environment for 5 years. The weight loss measured after 5 years is 10 grams. The density of steel is 7.85 g/cm³. The surface area of the pipe exposed to the environment is 100 square inches.
Task: Calculate the MPY for this scenario.
Exercice Correction
1. **Convert thickness to mils:** 0.25 inches * 1000 mils/inch = 250 mils
2. **Convert density to grams per cubic inch:** 7.85 g/cm³ * (1 cm/0.3937 inch)³ = 0.283 lb/in³
3. **Calculate MPY:** MPY = (10 g * 3.45 * 250 mils) / (0.283 lb/in³ * 100 in² * 5 years) ≈ **19.3 MPY**
Books
- Corrosion Engineering: By Donald H. Peck & Ralph M. Steigerwald (This comprehensive textbook covers various aspects of corrosion, including corrosion rate measurement using MPY.)
- ASM Handbook: Volume 13A, Corrosion: Edited by Robert W. Revie (This handbook offers detailed information on corrosion science and engineering, including sections on corrosion rate calculations and MPY.)
- Materials Science and Engineering: An Introduction: By William D. Callister, Jr. (This introductory textbook covers the fundamentals of materials science, including corrosion and its measurement using MPY.)
- Corrosion in Oil and Gas Production: By NACE International (This book provides insights into corrosion challenges specific to the oil and gas industry, including methods for assessing corrosion rates using MPY.)
Articles
- Corrosion Rate Measurements: A Review of Common Techniques: By R.S. P. Singh & A.K. Shukla (This article provides an overview of different corrosion rate measurement techniques, including the use of MPY.)
- A Practical Guide to Corrosion Measurement Techniques: By NACE International (This guide covers various corrosion measurement techniques, including MPY, and discusses their applications and limitations.)
- Understanding Corrosion Rates and How to Measure Them: By Corrosionpedia (This article offers a clear explanation of corrosion rates and their measurement, including the calculation of MPY.)
- Corrosion Rate Measurement Using Weight Loss Technique: By The Electrochemical Society (This article focuses on the weight loss method for corrosion rate determination and its relation to MPY.)
Online Resources
- Corrosionpedia: https://www.corrosionpedia.com/ (A comprehensive online resource for corrosion information, including articles on corrosion rate measurements and MPY.)
- NACE International: https://www.nace.org/ (A leading organization in corrosion control with various resources on corrosion measurement techniques, including MPY.)
- ASM International: https://www.asminternational.org/ (A materials science and engineering society with resources on corrosion, including information on corrosion rate measurements and MPY.)
- NIST Corrosion Data Center: https://www.nist.gov/corrosion (A website dedicated to corrosion research and data, including information on corrosion rate measurement and MPY.)
Search Tips
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- "MPY corrosion measurement methods": This search will provide information on different techniques used for corrosion rate measurement, including MPY.
- "MPY limitations": This search will highlight the limitations of MPY as a corrosion rate measurement technique.
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