Most people would agree that a traditional coupon is useful! Who doesn’t want to save $3 on box of laundry detergent? But when it comes to water treatment, there is a different kind of coupon to appreciate: the corrosion coupon.
Corrosion coupons are used to measure the effectiveness of inhibitor chemistry on corrosion rates in recirculated cooling and heating process water. These coupons are pre-weighed metal strips that are placed directly in a process stream. After 60, 90 or 120 days of exposure, the coupons are removed from the system, cleaned and re-weighed. If there is a loss in weight on the coupon, this will help to indicate what the metal loss potential would be on the cooling or heating system for the year. Primary system metallurgy, along with other secondary metals found in the system, will help dictate the type of coupons that are needed. Additionally, each coupon may have alloys that will need to be matched with the system metal. Mild steel and copper are the most common testing coupons followed by stainless, aluminum and brass.
Galvanized steel coupons are available but are not recommended for monitoring corrosion in open recirculation systems. The cooling tower galvanizing process does not form consistent and uniform layers of zinc on the sheet metal. Interpreting the metal loss of a galvanized coupon and how this relates to overall galvanizing zinc loss is complex and could be misread. Additionally, there are currently no specifications or recommended acceptable corrosion rates for galvanized steel coupons. The use of 100% zinc coupons versus galvanized sheet metal coupons may provide a more complete indication of galvanizing corrosion.
While corrosion monitoring using coupons can be an inexpensive and effective means of interpreting overall system corrosion rates, this method is not without some drawbacks. Water temperature and flow can have a significant impact on corrosion and pitting potential and are not easy to duplicate the total system conditions.
Do you have coupons you want tested? Kurita America’s analytical services laboratory has the ability to report quantitative metal loss and qualitative and microscopic pitting interpretation for all metal coupons. Contact us to discuss corrosion testing and corrosion control treatment programs for all water process systems.
About the Author
Paul Sharpe is an industry consultant at Kurita America, specializing in boiler and cooling water. Paul has over 30 years of water treatment experience including chemical treatment formulations, sales, marketing, and technical product applications.