Corrosion Testing Using Corrosion Coupons

Corrosion coupons are one of the simplest and most common tools in the corrosion engineer’s arsenal. They provide a simple, realistic, and inexpensive method to track corrosion in all sorts of gas and liquid systems.

What are Corrosion Coupons?

Corrosion coupons are small metal samples that are inserted into a system being monitored to measure the rate and extent of corrosion that is occurring. They are widely used in industries such as oil and gas, chemical processing, water treatment, and others to monitor the corrosion of pipelines, tanks, and other equipment.

The coupons are typically made of the same material as the system being monitored, and are exposed to the same environmental conditions as the system components. They are designed to simulate the corrosion that occurs in the system, allowing operators to monitor the corrosion rates and identify potential issues before they become more severe.

Corrosion coupons can come in a variety of shapes and sizes, depending on the specific application. They can be flat, disc-shaped, or rod-shaped, and can range in size from a few millimeters to several inches. The coupons are inserted into the system being monitored and allowed to remain in place for a predetermined period, usually between 7 and 90 days. During this time, they are exposed to the corrosive environment and experience the same corrosive effects as the system components.

After the coupons have been in place for the designated period, they are retrieved and analyzed to determine the extent and severity of the corrosion that has occurred. The analysis can involve visual inspection, weight loss analysis, or electrochemical methods, depending on the specific needs of the application.

Overall, corrosion coupons are an important tool for monitoring corrosion rates and identifying potential issues before they become more severe. They provide valuable data that can be used to optimize maintenance and corrosion prevention programs, and to extend the lifespan of equipment and infrastructure.

In the images below, a corroded coupon is shown on the left. In the center is a set of three coupons which have been inserted in a flow loop. On the right is an example of immersion corrosion testing of a coupon which is halfway immersed in the liquid:

Corrosion coupon analysisImmersion Corrosion TestingCorrosion Coupons in Various Stages of Analysis

Metal Corrosion Coupons for Corrosion Testing

Metal corrosion coupons are widely used in corrosion testing to monitor the rate and extent of corrosion that is occurring in a system. They are small metal samples that are designed to simulate the corrosion that occurs in the system being monitored, allowing operators to track the rate of corrosion and identify potential issues before they become more severe.

Corrosion coupons can be made from a variety of materials, including carbon steel, stainless steel, copper, brass, and aluminum. The choice of material for the coupon is typically based on the specific application, the type of corrosion being monitored, and the environment in which the system is operating.

A common question for corrosion coupon testing is this: will any material will work or should you use the actual material from the same heat that you plan to fabricate with? Our suggestion is to always use the closest material you can have, and ideally from the same heat.

Where to Get Corrosion Coupons and Analysis?

US Corrosion is a reputable provider of corrosion coupons and analysis services that can help you detect, monitor, and manage corrosion in your assets. Here are some reasons why you should consider partnering with US Corrosion for your corrosion monitoring needs.

Coupons from US Corrosion provides early detection of corrosion. Corrosion coupons are small metal samples that are exposed to the same conditions as the equipment or structure being monitored. By regularly measuring the weight loss of these coupons, US Corrosion can detect corrosion early, before it becomes a significant problem. This allows for proactive maintenance instead of reactive repairs, which can save you time and money.

US Corrosion uses standard and advanced analytical techniques to accurately measure the weight loss of the corrosion coupons. This information is used to calculate the corrosion rate of the equipment or structure being monitored. Knowing the corrosion rate allows for more accurate predictions of when maintenance or replacement will be necessary.

We also offer customized corrosion monitoring programs to meet your specific needs. They can help you design a corrosion monitoring program that is tailored to your assets and operating conditions. This ensures that you get the most accurate and relevant data to make informed decisions about your maintenance and repair strategies. Partnering with US Corrosion for your corrosion monitoring needs can help you detect, monitor, and manage corrosion in your assets.

Corrosion Coupons Analysis

Equally important is ensuring that you have accurate analysis after testing. Our corrosion laboratory can help. US Corrosion offers many different corrosion analysis services, ranging from simple weight loss testing to thorough SEM analysis with EDS chemical element mapping. We follow many of the common ASTM corrosion testing standards, including ASTM G31, G1, G28, G48, A262 and many other common industry standards, as well as customized tests for your specific application.

Ready to get started? Contact us now!

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When metal meets moisture, you get rust. Everyone knows that, but rust is only the surface issue in this highly corrosive relationship. Simply put, water eats metal. That’s why your industrial water needs to be treated with a corrosion inhibitor. Even then, if you don’t have the right chemical feed rates or the right formulation, you can eat away at costly metal components and end up having to replace them before their time.

So how do you know if your treatment is actually working? A visual inspection would tell you, but critical heat-transfer surfaces are often hidden away inside your heating, cooling, and process systems, and can’t be seen without a borescope or some major dismantling. That’s where corrosion “coupons” come in.

What are Corrosion Coupons?

No, these coupons won’t give you a discount on your next purchase. Corrosion coupons are small strips of metal that represent the metal components contained in your building’s recirculating water systems. Each coupon strip is measured and pre-weighed. They are then placed in your treated water loop, typically in a coupon rack, to expose them to the conditions of the system for a period of 90 days. After 90 days, you can remove the coupons, visually check them for corrosion and send them to a lab to be cleaned, weighed, and analyzed. The weight loss in the coupon and exposure time will be translated to a corrosion rate in the system for that metal type. The results are a good indication of how the actual metal surfaces inside your systems are faring.

Corrosion coupon materials include stainless steel, copper/copper alloys, low carbon steel, galvanized steel, and aluminum. It’s important to properly identify the metals used in your heating and cooling systems. For instance, if your open cooling loop flows across steel and copper, testing an aluminum coupon won’t tell you anything useful.

How Do You Conduct the Test?

Coupons are held in a pipe system called a coupon rack, which is installed into your recirculating loop at a point after the water has cycled through your cooling or heating system. Within the rack, the most active metal is placed in the first position in the downstream flow of water. This is to prevent noble metals from depositing onto active metals and producing a false coupon reading.

Additionally, the broad face of each metal strip should be in a vertical orientation to reduce the accumulation of debris, which can likewise throw off test results.

Corrosion rates are measured in mils per year (MPY). To get the most accurate rating, coupons are exposed to treated water for 90 days. At that point, the coupons are removed, cleaned, and weighed to determine metal loss and corrosion rating.

Are Corrosion Coupons Worth the Effort?

Corrosion coupons are an easy and inexpensive way to protect your costly HVAC or process components. There is an upfront cost to install the rack, and the lab analyses may or may not be covered in your service contract (contact your water treatment provider if you’re unsure) – but these are minor expenses when compared to the cost of having to replace corroded equipment, or even to perform a cleaning.

Beyond that, a corrosion coupon rack puts one more set of “eyes” on your water treatment program, letting you see and measure the results you’re getting from the dollars you already spend. It also allows your treatment professional to make necessary adjustments and stay ahead of the corrosion curve. Source water chemistry can change over time and require a different dosage of inhibitor. Adjusting other treatment chemicals can also impact corrosion. For instance, if a spike in microbial activity forces you to increase your oxidizing biocide feed, this may adversely affect your corrosion protection. Coupons are a tangible way to monitor the impact of these variables.

Understanding the Test Results

A coupon’s corrosion rate is based on metal loss over a designated time period. The coupons are carefully weighed and logged before and after to determine metal loss. Corrosive “pitting” is also quantified by measuring pitted surface area.

There are several factors that affect corrosion rate, including location, exposure time, pH, water temperature, coupon design/placement, flow velocity, and current water treatments.

Once evaluated, the technician will calculate the rate in mils per year. Each metal or alloy used has its own acceptable corrosion rates.

Stainless Steel

Evaporative Water Systems Closed Water Systems Acceptable <0.1 MPY <0.1 MPY Unacceptable >0.1 MPY >0.1 MPY

Copper and Copper Alloys

Evaporative Water Systems Closed Water Systems Excellent <0.1 MPY ≤0.1 MPY Very Good 0.1-0.25 MPY 0.1-0.15 MPY Good 0.25-0.35 MPY 0.15-0.20 MPY Fair 0.35-0.5 MPY 0.2-0.3 MPY Poor 0.5-1.0 MPY 0.3-0.5 MPY Unacceptable >1.0 MPY ≥0.5 MPY

Low Carbon Steel

Evaporative Water Systems Closed Water Systems Excellent ≤1.0 MPY ≤0.2 MPY Very Good 1.0-3.0 MPY 0.2-0.3 MPY Good 3.0-5.0 MPY 0.3-0.5 MPY Fair 5.0-8.0 MPY 0.5-0.8 MPY Poor 8.0-10.0 MPY 0.8-1.0 MPY Unacceptable >10 MPY ≥1.0 MPY

Aluminum

Evaporative Water Systems Closed Water Systems Acceptable <0.2 MPY <0.2 MPY Marginal 0.2-0.25 MPY 0.2-0.25 MPY Unacceptable >0.25 MPY >0.25 MPY

Acceptable corrosion rates from the Association of Water Technologies.

I Know My Corrosion Rate. Now What?

If your corrosion rate comes back within an acceptable range, no action is needed. But don’t forget to place new coupon strips in your rack. This is an ongoing tool that will help you monitor the health of your system components. A number of different variables can change over time, affecting the protection level of your treated water.

If your corrosion rate comes back above the acceptable range, you can work with your water treatment team to create or modify your plan of action. This may require some additional diagnostics to get to the root cause of the problem, but a knowledgeable water treatment professional should be able to get you back on course in short order.

If you would like more information about corrosion coupons or corrosion protection, don’t hesitate to contact us. An experienced HOH Water Quality Engineer will be happy to answer your questions. And while you’re at it, you can ask him or her about the latest automated technology that will give you an instantaneous corrosion reading.