Corrosion coupons are small specimens of known metallurgy exposed to the process water for a defined time. After exposure, deposits and corrosion products are removed using an appropriate cleaning procedure, and the metal weight loss is converted to a corrosion rate.
Information required
- Initial and final coupon weight.
- Exposed surface area.
- Exposure time.
- Metal density.
- Correct cleaning method and coupon metallurgy.
Common metric formula
One commonly used form for uniform corrosion rate is:
Corrosion rate (mm/year) = 87.6 × W ÷ (D × A × T)
Where W = weight loss in mg, D = metal density in g/cm³, A = exposed area in cm², and T = exposure time in hours.
Example
If a carbon-steel coupon loses 120 mg over 720 hours, with density 7.86 g/cm³ and exposed area 28 cm²:
Rate = 87.6 × 120 ÷ (7.86 × 28 × 720) ≈ 0.066 mm/year.
Why calculation is only part of interpretation
A coupon can show a low average weight-loss rate while still having serious localized pitting. Always inspect the coupon visually and document pits, deposits, discoloration and uneven attack.
Installation matters
Coupons should be installed in representative flow, without being electrically shorted to the rack. Orientation, velocity and exposure period should be consistent if results will be compared over time.
Cleaning matters too
Over-cleaning can remove base metal and create an artificially high corrosion result; under-cleaning leaves corrosion products and creates an artificially low weight loss. Follow a recognized coupon-cleaning method suitable for the alloy.
Use coupon results as part of a broader corrosion-control picture that includes water chemistry, treatment residuals, inspection and operating history.