Corrosion Coupons for Corrosion Monitoring
The simplest, and longest-established, method of estimating corrosion losses in plant and equipment is weight loss analysis. A weighed sample (coupon) of the metal or alloy under consideration is introduced into the process, and later removed after a reasonable time interval. The coupon is then cleaned of all corrosion product and is reweighed. The weight loss is converted to a total thickness loss, or average corrosion rate using proper conversion equations.
Weight loss determination has a number of attractive features that account for its sustained popularity. It is:
 |
Simple: No sophisticated instrumentation is required to obtain a result. |
|
Direct: A direct measurement is obtained, with no theoretical assumptions or approximations. |
|
Versatile: It is applicable to all corrosive environments, and gives information on all forms of corrosion. |
The choice of technique for initial preparation of the coupon surface, and for cleaning the coupon after use, is critical in obtaining useful data. Both the relevance and reproducibility of weight loss data are highly sensitive to the inherent suitability of these techniques, and to the care with which they are executed.
Surface finishing methods vary across a broad range for specific applications. Blasting with glass bead, sand, or other aggregate can provide an acceptable finish for some applications. Sanding with abrasive belts, or surface or double disc grinding with abrasive stones also provides an excellent surface for evaluation. Cleaning of specimens before weighing and exposure is critical to remove any contaminants that could affect test results.
Reference should therefore be made to NACE Recommended Practice RP-0775 and ASTM G-1 G-4 for further detail on surface finishing and cleaning of weight-loss coupons.
|
See also:
|
