Corrosion Rate (Mass Loss & mm/y ⇄ mpy)
Turn a coupon mass-loss test into a corrosion rate, or convert straight between mm/y and mils per year.
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The engineering
The mass-loss method is the workhorse of corrosion monitoring: expose a weighed coupon, pull it after a known time, clean and reweigh it, and the metal that vanished becomes a uniform penetration rate. ASTM G102 fixes the constant so grams, square centimeters, hours, and g/cm³ come out as mm/y — the 8.76×10⁴ folds in the 8760 hours per year and the unit shuffle.
Rule of thumb: 1 mpy ≈ 0.0254 mm/y, and general-purpose steel structures are usually happy below ~5 mpy (0.13 mm/y). Two gotchas — the rate assumes uniform attack, so it wildly understates pitting or crevice damage, and you must use the density of the actual alloy (7.87 for iron, 8.0 for 304 SS, 2.70 for aluminum), not a generic 'metal' value.
Where this math comes from
Weight-loss coupons predate any standard — corrosion engineers were hanging tagged metal strips in seawater and cooling loops well before World War II, then scraping, pickling, and weighing them on analytical balances. The messy part was always the arithmetic and the cleaning: how do you back out corrosion product without eating base metal, and what constant turns milligrams into a penetration rate everyone agrees on?
NACE International (now AMPP) and ASTM codified the practice mid-century, with ASTM G1 pinning down coupon preparation and cleaning and G102 giving the electrochemical-to-rate conversions and the mass-loss constant this card uses. The mils-per-year unit is an American holdover — a mil is one thousandth of an inch — that persists in refinery and pipeline shops long after the rest of the world went metric.
- 1943NACE (founding pipeline engineers)National Association of Corrosion Engineers formed, standardizing field corrosion practice.
- 1972ASTM Committee G01ASTM G1 sets coupon preparation, cleaning, and mass-loss evaluation procedures.
- 1989ASTM Committee G01ASTM G102 defines the mass-loss and electrochemical corrosion-rate conversion constants.
- 2021AMPPNACE and SSPC merge into AMPP, carrying the corrosion standards forward.
See the full timeline of the math behind every calculator →
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