Mils Per Year (MPY)

Definition

Mils per year is the standard industry unit for corrosion rate, expressed as thickness lost per year, where one mil equals one thousandth of an inch. It is calculated from coupon weight loss.

MPY equals 534 times W divided by (D times A times T), where W is weight loss in mg, D is metal density in g/cm3, A is coupon surface area in in2, T is exposure time in hours, and 534 is the unit conversion factor. Carbon steel density is 7.87 g/cm3. The recommended coupon exposure is about 90 days so that passivation can occur and the sample is representative; shorter timeframes may significantly overestimate corrosion activity.

Related concepts

Corrosion Rate Interpretation · Corrosion coupons

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Part of The Blueprint of Water · 📏 measurement