Corrosion Rate from Coupon Weight Loss

Hang a pre-weighed steel coupon in a bypass rack, leave it 60 to 90 days, clean it per ASTM G1 and weigh it again: the mass it lost, spread evenly over its area and divided by the metal's density, is a thickness — and scaled up to a year it becomes the corrosion rate. The solver reports that as annual penetration, so you can read it in mils (0.001 in) or micrometres directly. The famous shortcut MPY = 534W/(D·A·T), with W in milligrams, D in g/cm³, A in in² and T in hours, is nothing but this same equation with the unit conversions and a 365-day year folded into the number 534.

Typical open recirculating cooling water targets are under 3 mpy (76 μm/yr) on mild steel and under 0.2 mpy on copper; closed loops should be under 1 mpy. A worked example: a coupon of 8.0 g/cm³ alloy with 10 cm² of area losing 1000 mg while penetrating 250 μm/yr must have been in service 182.5 days. The trap is that a coupon measures uniform corrosion only — a coupon showing a beautiful 1 mpy can still be pitted straight through, and pitting is what actually fails a tube. Always inspect the cleaned coupon under a loupe and note pit density and depth alongside the number, and never trust an exposure shorter than 30 days, where initial film formation dominates the reading.