Corrosion Rate Calculator

The ASTM G1/G31 standard weight loss method calculates corrosion rate using the formula: mpy = (534 x W) / (D x A x T), where W is weight loss in milligrams, D is metal density in g/cm^3, A is exposed area in square inches, and T is exposure time in hours. The result in mpy (mils per year) represents the material thickness loss per year at the measured corrosion rate.

Mils per year (mpy) is a unit of corrosion rate where 1 mil = 0.001 inch = 0.0254 mm. A corrosion rate of 1 mpy means the metal loses 0.001 inch of thickness per year. Corrosion severity ratings: less than 1 mpy = outstanding, 1-5 mpy = excellent, 5-20 mpy = good, 20-50 mpy = fair, 50-200 mpy = poor, greater than 200 mpy = unacceptable for most applications.

ASTM G31 is the Standard Guide for Laboratory Immersion Corrosion Testing of Metals. It specifies coupon preparation (cleaning, weighing), exposure conditions (solution chemistry, temperature, aeration, duration), and calculation methods for corrosion rate from weight loss. Coupons are cleaned with the appropriate inhibited acid to remove corrosion products before the final weighing per ASTM G1.

To convert mils per year (mpy) to millimeters per year (mm/y): mm/y = mpy x 0.0254. To convert mm/y to mpy: mpy = mm/y / 0.0254 = mm/y x 39.37. For example, 10 mpy = 10 x 0.0254 = 0.254 mm/y. International corrosion literature commonly uses mm/y while US industry typically uses mpy.

Structural steel exposed to corrosive environments is typically designed with a corrosion allowance (extra wall thickness) to account for expected metal loss over the service life. NACE International and pipeline standards (ASME B31.3, B31.4, B31.8) specify corrosion allowances based on the expected corrosion rate and design life. Typical allowances are 1-3 mm (40-120 mpy x 25 years) for mild environments and 3-6 mm for moderately corrosive environments.