Mils per year: the standard US unit of corrosion rate, where one mil is 0.001 inch and 1 mpy equals 0.0254 mm per year. It is calculated from coupon weight loss, ER probe trends or repeat thickness readings. As a rough guide for carbon steel, under 1 mpy is low, 1 to 5 moderate and over 10 severe, but a low average can hide much faster local pitting, which is reported separately.
Coupons off the water injection header averaged 3 mpy, but the pitting rate was over 20.
Also heard as
- mils per year
Related terms
Corrosion coupon
Weighed metal strip exposed inside a system for a set time, then reweighed to calculate corrosion rate.
ER probe
Electrical resistance probe that tracks corrosion by measuring the rising resistance of a thinning metal element.
Corrosion allowance
Extra wall thickness added beyond what pressure design needs, meant to be used up by corrosion over the service life.
Uniform corrosion
Corrosion spread evenly over a whole surface, thinning the metal at a fairly predictable rate.