Corrosion Coupon Analysis in Cooling Towers

Last Updated: May 17, 2019

There are many factors that affect corrosion rates in a cooling tower system. High concentrations of dissolved oxygen, low alkalinity, low hardness (soft water), low pH, are some of the factors that can cause corrosion. With proper chemical treatment and monitoring, corrosion levels can be maintained at appropriate levels.  

Duration of coupon:  A short duration (30 days or less) will results in higher corrosion rates than those in the system for 90 days. Short term exposed coupons do not have the appropriate amount of time to acclimate to the system for which they are in or create a passive oxide layer, which is why we typically see a higher rate of corrosion.

Flow through coupon rack:  It is recommended to control the flow through the rack to be 3-5 feet per second. This can be done with a dole valve or rotameter which can be purchased with a coupon rack.  

Coupon Placement & Orientation:  Placement should follow the galvanic series, with the most active metal or alloy placed in the number one position downstream of the water.  Additionally, coupons should be placed in the rack such that water flows from the back of the coupon to the front. Coupons should be orientated with the broad face in a vertical position, reducing the accumulation of debris onto the face of the coupon which can accelerate corrosion.

Obtaining corrosion rates can be quite simple via digital or mechanical devices. The most simple device is a corrosion coupon rack (link). These racks can be made with 1, 2 or 3 ports, or places to put a corrosion coupon. ASTM standard is to leave the coupon in the system for 90 days before sending it to be analyzed. US Chemical Supply will analyze corrosion coupons for free with the purchase of cooling tower chemical. Digital corrosion monitoring systems or Linear Polarization instruments may also be used if desired. These instruments give a real-time look into the exact corrosion rates at a given time. They are extremely beneficial for customers who have extreme corrosion potential, or who want to closely monitor corrosion rates. These systems are becoming more and more economical and my be a good solution.

Below is a chart showing acceptable corrosion rates for open and closed recirculating systems: