Corrosion Coupons in Cooling Towers: Definition, How They Work, and Their Effectiveness
In the industrial world, cooling towers are the heart of cooling systems. Their efficiency directly affects equipment performance and operating costs. One of the biggest challenges in maintaining cooling towers is combating corrosion. Uncontrolled corrosion can cause structural damage, leaks, and process contamination.
So, how do you measure the actual level of corrosion occurring in your cooling water system? This is where corrosion coupons come into play. This article will discuss in depth this simple monitoring tool.
What is a Corrosion Coupon?
A corrosion coupon is a small metal sample (specimen) with standardised size, shape, and composition, designed to have the same characteristics as the construction materials of your cooling system (such as pipes, heat exchangers, or cooling tower casings).
Simply put, a corrosion coupon is an ‘early warning device’ that you place in the cooling tower water flow. By sacrificing this coupon, you can find out exactly what your metal equipment is experiencing without having to dismantle or stop operations.
How Do Corrosion Coupons Work?
The way corrosion coupons work is very straightforward and follows the scientific method. Here are the steps:
1. Preparation and Installation
Material Selection: Coupons are selected from the same material as your system, most commonly carbon steel to represent pipes and heat exchangers, or copper for heat exchanger tubes.
Pre-treatment: Before installation, the coupon is cleaned, weighed very carefully (high precision), its surface area is measured, and all initial data is recorded.
Installation: The coupons are then installed on a coupon holder or special rack designed to ensure they are exposed to a representative cooling tower water flow. These holders are usually installed on return pipes or areas where water circulates.
2. Exposure Period
The coupons are left immersed in the cooling tower water for a specific period of time, usually 30, 60, or 90 days. During this time, the coupons will experience the exact same conditions as the system equipment: water temperature, flow velocity, and chemical conditions (pH, inhibitors, microbiology).
3. Retrieval and Analysis
After the exposure period is complete, the coupons are retrieved from the system.
The coupons are then taken to a laboratory for analysis. The analysis process includes:
Cleaning: All corrosion products (rust) and other deposits are chemically cleaned to reveal the original metal surface.
Weighing: The coupons are weighed again. The difference between the initial weight and the final weight is the weight of metal lost due to corrosion.
Corrosion Rate Calculation: The weight loss, surface area, metal density, and exposure time are used to calculate the corrosion rate, usually in units of mils per year (mpy) or millimetres per year (mmpy).
Visual Inspection: The corrosion method (whether uniform or localised, such as pitting) is also recorded under a microscope.
How Effective are Corrosion Coupons?
Corrosion coupons are a highly effective tool for monitoring corrosion in cooling water systems. Here are their advantages and effectiveness:
1. Tangible and Accurate Data
Coupons provide measurements of the actual corrosion rate, not just potential indications as provided by electronic monitoring devices. The mpy data produced is a reliable figure for decision making.
2. As a Water Treatment Programme Validation Tool
This is its most important function. If your chemical treatment programme (administration of corrosion inhibitors, biocides, etc.) is effective, the corrosion rate measured on the coupon will be below the specified limit (usually <2 mpy for carbon steel). If the number is high, it is a clear signal that your treatment programme has failed and needs to be adjusted.
3. Detecting the Type of Corrosion
Not only the rate, the coupon also shows the type of corrosion that has occurred. Whether the corrosion is uniform or in the form of deep holes (pitting corrosion). Pitting corrosion is far more dangerous because it can penetrate metal quickly even though the average corrosion rate is low. Visual inspection of the coupon can reveal this problem.
4. Low Cost and Simple
Compared to the losses due to heat exchanger damage or downtime, the cost of a corrosion coupon monitoring programme is very low. The method is also easy for anyone to understand.
Limitations of Corrosion Coupons
Although highly effective, corrosion coupons have several limitations:
Historical Data, Not Real-Time: Coupons provide average data over the period of installation. They cannot detect sudden fluctuations in corrosion (e.g., due to upset conditions).
Point-in-Space: Coupons only represent the conditions at the location where they are installed. If there are differences in conditions in other parts of the system, they may not be detected.
Solution: These limitations can be overcome by combining the use of corrosion coupons with real-time monitoring tools such as corrosion probes (LPR - Linear Polarisation Resistance). This combination provides a comprehensive picture: real-time data from the probes and accurate, validated data from the coupons.
Conclusion
Corrosion coupons are an irreplaceable tool in a comprehensive cooling tower maintenance programme. They serve as a real-life ‘health report’ for your metal assets. At a relatively low cost, corrosion coupons provide critical data that can be used to:
Optimise chemical treatment programmes.
Prevent costly equipment failure.
Extending the life of cooling system assets.
Reducing downtime and repair costs.
In other words, investing time and money in a corrosion coupon monitoring programme is not an expense, but rather an investment in the reliability and operational efficiency of your plant.
Want to Start the Right Monitoring Programme for Your Cooling Tower?
We provide corrosion coupon installation, analysis, and data-driven consulting services to help you control corrosion and optimise operational costs. [Contact Us] to discuss further.