Mils per year or MPY is used to give the corrosion rate in a pipe, a pipe system or other metallic surfaces. It is used 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.
In an open water system a corrosion rate of around 1 MPY is normal. Having corrosion rate of around 10, you should take action. Corrosion rates of 20 MPY and above, you should be concerned, as the corrosion is „eating“ the metal rather fast.
As MPY is a rather dry figure. Here an example, if you have a 24″ pipeline and a corrosion rate of 10 MPY, you will have almost half a cubic meter material loss per kilometer pipe length. Such amount of rust gives big problems on the receiver side at the end of the pipe line.
This figure is most important to calculate the life time of a pipe. E.g in a gas pipe where the pressure is rather high, this is very critical to know how long a pipe can be used.