Concept
- Coupon test is used to gauge a material’s corrosion rate in a corrosive environment.
- It is also used to test the efficiency of corrosion inhibitors.
Parameters
- Material – The coupons are made from the same or similar material as that of the actual component
- Electrolyte – The electrolyte for exposure is the same or similar to the actual environment. Sometimes, coupons are placed in the actual environment with the component, for example in the soil with submerged pipelines.
Steps
- Clean the coupon of any oil/grease/rust
- Measure the weight of original coupon – W
- Measure the Area of coupon to be exposed – A.
- Take photographs and/or optical micrographs of the coupon
- Immerse the coupon in the electrolyte without inhibitor
- Note the time of measurement – t. This is decided by the concerned parties or as per the standards followed.
- Take out the coupon after the time.
- Take photographs and/or optical micrographs of the coupon
- Clean the corrosion products off the coupon. DO NOT USE ROUGH POLISHING as it can remove the underlying pits and surface as well.
- Take photographs and/or optical micrographs of the coupon.
- Measure the final weight of the coupon again – Wf
To test the efficiency of an inhibitor, add the inhibitor to the electrolyte and then use a DIFFERENT COUPON. Follow the above steps again.
Calculations
Weight loss Wo = Original weight W โ final Weight Wf
corrosion rate C.R = weight loss W / Area A* time of exposure t
degree of surface coverage ฮธ = (corrosion rate C.Ro without inhibitorโ corrosion rate with inhibitor C.Rinhi) / corrosion rate without inhibitor C.Ro
inhibition efficiency (Einhi%) = (corrosion rate C.Ro without inhibitorโ corrosion rate with inhibitor C.Rinhi) * 100 / corrosion rate without inhibitor C.Ro
For standard samples with the same Area A and time of exposure t, the C.R may be changed to weight loss W.
