MPY - mils per year
Mils per year or MPY is used to give the corrosion rate in a pipe, a pipe system or other metallic surfaces. To calculate the material loss or weight loss of a metal surface, there is a formula using the type of metal, the size of the sample area and the time of exposure, giving the value of mils per year. The expression MPY is mostly used in the United States. One Mil is equal to one thousandth of an Inch.
In metric expression one mil equals to 0.0254 mm/Y.
In an open water system a corrosion rate of around 1 MPY is normal. Having corrosion rate of around 10 MPY, you should take action. Corrosion rates of 20 MPY and above, you should be concerned, as the corrosion is eating the metal rather fast.
See also proven cases of corrosion reduction on an offshore oilrig, a steam condensate system and a cooling loop.