Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Mils Penetration Per Year (MPY)

Last updated: August 25, 2017

What Does Mils Penetration Per Year (MPY) Mean?

Mils penetration per year (MPY) is a unit of measurement equal to one thousandth of an inch. Mils penetration per year is commonly known as mil in the U.S. measurement system.

Mils penetration per year is used to gauge a coupon’s corrosion rate. This unit may also be used to measure a coating’s thickness or tolerance.


Corrosionpedia Explains Mils Penetration Per Year (MPY)

The corrosion rate is the speed at which metals undergo deterioration within a particular environment. This rate depends on environmental conditions and the condition or type of metal. In the United States, the corrosion rate is typically computed using mils per year. To calculate the corrosion rate, the following must be determined:

  • Weight loss (reduction in weight during reference time)
  • Area (initial surface area)
  • Time (length of reference time)
  • Density

This measurement can also be used to measure the thickness of items such as:

  • Paint
  • Paper
  • Wires
  • Foil
  • Plastic sheets

For instance, the ideal measurement of an average plastic ID card is around 30 mils.

Mils can be very useful in terms of manufacturing tolerances and dimensions, such as in the production of vehicle engines’ head gaskets or the amount of material to remove from the automobile head in order to make adjustments to the cylinder’s compression ratio. It is also very useful in servicing auto engines, where it is used to measure ignition points and the gap in spark plugs. Additionally, this unit is used in the tolerance specification of hydraulic cylinders and the manufacturing of printed circuit boards (PCB).


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