A pre-weighed sample of metal, usually a strip or disk of the system alloy, mounted in a holder and inserted into a pipeline, vessel or cooling water loop, commonly for 30 to 90 days. After removal it is cleaned, reweighed and inspected; weight loss, area, density and time give the average rate in mpy, and pit depths give the pitting rate. Coupons are cheap, direct and show the type of attack, but only report averages over the exposure.
Pull the corrosion coupon on the produced water line this week, it's been in 90 days.
Put coupons where corrosion is worst, usually the bottom of the line at 6 o'clock in liquid service. A coupon up in the gas space can read clean while the bottom pits.
Also heard as
- weight-loss coupon
- mass-loss coupon
- coupon
Related terms
mpy
Mils per year, the US corrosion rate unit: thousandths of an inch of metal lost per year.
ER probe
Electrical resistance probe that tracks corrosion by measuring the rising resistance of a thinning metal element.
LPR
Linear polarization resistance: an electrochemical method that reads corrosion rate in minutes in conductive liquids.
Corrosion inhibitor
Chemical added in small doses to a fluid to slow corrosion of the metal it touches.