How can I convert the corrosion rate between the mils per year and the metric equivalent millimeter per year (mm/y), and why does this matter?

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How can I convert the corrosion rate between the mils per year and the metric equivalent millimeter per year (mm/y), and why does this matter?


A corrosion rate conversion is a technique to change the measurement units of a given corrosion rate to another unit for comparison purposes or a specific engineering application. When converting a corrosion rate between these units you must keep in mind that a mil is a thousandth of an inch (i.e., a thousand mils equals one inch) and that one inch equals 25.4 millimeters:

1000 mils = 1 inch = 25.4 mm

Hence, one millimeter equals 39.37 mils:

1 mm = 1000 mils/25.4 mm = 39.37 mils

So, to convert a given corrosion rate from mm/y to mils/y, you must multiply by 39.37

and, to convert a given corrosion rate from mils/y to mm/y, you must divide by 39.37

Note: mils per year (mm/y) is often known as the mils penetration per year and abbreviated as mpy.

Conversion examples:

  • A corrosion rate of 0.10 mm/y is equals to what in mpy?

0.10 mm/y * 39.37 mils/mm = 3.94 mpy

  • What is the equivalent of 5 mpy in mm/y?

5 mils/y / 39.37 mils/mm = 0.13 mm/y

Having the corrosion rate expressed in the appropriate unit of measurement is important because unit consistency facilitates calculating the remaining service life of an asset. For example, if your equipment wall thickness is measured in inches and your corrosion rate in mm/y, and you want know how long it will take corrosion to consume a certain amount of wall thickness (the wall loss), then you will need to first convert the corrosion rate to mpy, and then to inch/y, before making the calculation. (To learn more about conversions, read Corrosion Rate Conversion: Simple Ways to Convert Data Between Common Corrosion Units.)

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Written by Osvaldo Mejia
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Osvaldo Mejia currently serves as a Sr. Pipeline Integrity Engineer at Petroleum Development Oman (PDO) LLC. He has over 15 years of experience designing and implementing corrosion control programs, risk based inspection and maintenance strategies, and conducting fitness for service evaluations and integrity assessments of assets for the oil & gas, process and petrochemical industries.

He is the author and lecturer of various training courses related to corrosion control, risk management and integrity management. He has authored and lectured various technical papers at national and international conferences and technical seminars. He is an approved NACE instructor for the Pipeline Corrosion Integrity Management (PCIM), In-Line Inspection (ILI), Direct Assessment (DA), Cathodic Protection Tester (CP1) and Cathodic Protection Technician (CP2) courses.

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