How can I convert corrosion rate between mils per year and mm/y?

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.)