Formula
Where W is weight loss (mg), D is metal density (g/cm³), A is exposed area (in²), and T is exposure time (hours). This is the standard ASTM G1 weight-loss formula.
To report in mm per year: 1 mpy = 0.0254 mm/y.
Worked Example
A carbon steel coupon (density 7.86 g/cm³) with 2.0 in² exposed area loses 25 mg over 30 days (720 hours):
Engineering Notes & Assumptions
This assumes uniform general corrosion. It won't capture localized attack like pitting or crevice corrosion, which can be far more damaging than the average weight-loss rate suggests. Always inspect the coupon visually for pitting alongside the calculated rate.
Frequently Asked Questions
How is corrosion rate calculated from a coupon?
Weigh the cleaned coupon before and after exposure, then use corrosion rate (mpy) = 534 × W / (D × A × T), with W in mg, D in g/cm³, A in in² and T in hours.
How do I convert mpy to mm/y?
Multiply by 0.0254. For example, 5 mpy is 0.127 mm/y.