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What are the benefits and downfalls of corrosion coupons?
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2 point PVC coupon rack.
 
Water and moisture actively eat away at metal components within your water treatment system. That’s why treating your industrial water with a corrosion inhibitor is essential. However, if your chemical feed rates or formulation aren’t correct, you could still be damaging expensive metal components, leading to premature replacements.
 
But how can you be sure your treatment is effective? While a visual inspection might seem like the answer, heat-transfer surfaces are often hidden deep within systems, making them inaccessible. This is where corrosion coupons become invaluable.
 
What are corrosion coupons?
Corrosion coupons are small strips of metal that are pre-weighed and installed in your recirculating water system to detect corrosion over time.  They are normally exposed to the system for 60, 90 or 120 days to get a representation of corrosion of the metal surfaces.  After the set time, the coupons are removed and sent to a lab to be cleaned, weighed, and analyzed. The results are reported in a corrosion rate of mils per year, MPY.  Coupons are a predictive maintenance tool that offer an estimate of the corrosivity of the water and not the actual loss of pipe wall.
 
Examples of new corrosion coupons in aluminum, steel, brass and copper.
 
 
Examples of coupons from a system with substantial corrosion.  
 
What factors should be considered when working with corrosion coupons?
Many factors will affect the accuracy of the test. The proper design, installation, flow, and types of metal surfaces is important for accurate results.
  • Typical coupons are installed in a coupon rack in a closed-loop installation after the water has cycled through the system.  Installing the coupon rack after the heat source will increase the corrosion rates where installing it after the tower has cooled the water will result in lower corrosion rates.
  • The order of installing the coupons in the rack is important as less noble metals (aluminum, carbon steel) should be mounted first in the direction of the flow into the rack.  More noble metals (copper, stainless steel) should be installed last in the rack.  This prevents more noble metals from plating on the less noble metals and giving you false readings.  
Installation Chart
Aluminum: Least noble
Galvanized: Second
Mild Steel: Third
Brass: Fourth
Copper: Fifth
Nickel: Sixth
Stainless Steel: Most noble  
 
Examples of coupon racks for a cold and hot water system.

  • Proper and constant flow rates are a must. Flow rates below 3 feet per second allow particles to settle, increasing the corrosion. While flow rates over 5 feet per second can erode softer metals.  The optimal range is about 4 to 7 gallons per minute in a ¾ inch pipe.  If your system has bypass lines, lead and lag equipment, equalization lines, or out of service equipment, you create no flow or low flow conditions and the coupons results will not properly reflect actual corrosion rates. No flow or low flow conditions cause significant corrosion in most systems regardless of water quality.
  • Contamination of process leaks will affect the results and coupons installation should be delayed until contamination is removed.
  • Debris may collect on the coupon causing under deposit corrosion which results in a false higher rate. 
Results of the coupons analysis will be in a laboratory report with mils per year (MPY) value and any visual observations (ex. Pitting, general etch, etc.)  One mil is a thousandth of an inch (.001), so the corrosion rate measurement is very small. Below are guidelines for corrosion rates in both open and closed systems.
 
 
What steps should be taken after receiving corrosion results?
If your corrosion rate falls within an acceptable range, no further action is required. Just remember to replace the coupon strips in your rack. This ongoing monitoring is essential for continuously tracking the health of your system components.
 
If the corrosion rate exceeds the acceptable range, it’s time to collaborate with your water treatment team to revise your plan and get things back on track.
 
Are corrosion coupons worth the investment?
Corrosion coupons are a valuable predictive tool to help you evaluate the proper chemical inhibitor treatments, biocides, pH, alkalinity and hardness in your systems.  
 
However, using it to predict the corrosion rate for a entire recirculation system has some downfalls. The flows, temperatures, and velocities will never be the same as the corrosion through the entire system. It doesn’t take into account the galvanic influence of different metals to metals in water. Corrosion is an electro-chemical reaction and most coupons are installed on a plastic holder in PVC tubing so they are unaffected by the anode/cathode reactions. 
 
Even with these considerations in mind, a corrosion coupon rack can still offer an affordable way to predict water conditions in your system. While there’s an initial cost to installing the rack, this expense is minor compared to the cost of replacing corroded equipment or even conducting a cleaning. A corrosion coupon rack serves as an additional set of eyes on your water treatment program, enabling your treatment professional to make necessary adjustments to stay ahead of corrosion issues.
 
For more information about corrosion coupons or corrosion protection, reach out to Watertech and one of our technical engineers will be in touch.  
   
 
Rob Pierick
Senior Territory Manager
Rob Pierick has worked for Watertech of America for 19 years and has over 25 years’ experience providing industrial water treatment solutions for boiler, cooling, wastewater applications and healthcare water management, including water testing, Legionella testing and remediation. Rob is a Certified Water Technologist (CWT) through the Association of Water Technologies. The CWT designation involves rigorous testing, peer review, and an ethics declaration. It must be renewed every five years to maintain this designation. Rob holds a BS from the University of Wisconsin Platteville.

 
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