What Corrosion Coupons are & Their Importance

Updated September 28, 2022

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What do Corrosion Coupons do?

Corrosion coupons are utilized to detect corrosion over time in water systems. They are weighed initially without any corrosion, and then their rate is compared after a certain amount of exposure to being inside the water of the system. The days they are exposed typically are 60, 90, or 120. These corrosion coupons come in various materials to asses different systems. The technique used with corrosion coupons, will greatly impact its effectiveness.

 

Benefits of Corrosion Coupons

There are several benefits to utilizing corrosion coupons to track corrosion in your systems.

• It is an effective way to find corrosion. It can be a useful tool if tracked correctly, by monitoring changes in corrosion coupons over time. It is an important indicator to make sure that a chemical treatment program is working effectively.
• They are relatively easy to use. There are many warning signs of corrosion, but leaving these in, taking them out, and seeing noticeable changes over time is a simple method of doing so.

For corrosion coupon help contact the experts at Chardon Laboratories here.

Using Corrosion Coupons Effectively

 

Corrosion coupon analysis involves a number of variables which may significantly affect the results of the analysis.  Make sure the design of the system and the installation of the coupon rack will produce accurate test results.

• Be careful not to touch the couhttps://www.chardonlabs.com/corrosion-coupon-rack/pons. Oil from your hands will promote corrosion and bias the test results.
• Maintain constant flow at 3-5 feet per second during the test period. Flow rates below 3 fps will allow particulates to settle on the coupon and will bias the results.  Flow rates in excess of 5 fps will erode soft metals such as copper and brass.  In 3/4″ PVC pipe, 3-5 fps is equivalent to 5-8 GPM.
• Avoid corrosion coupon tests in comfort cooling systems during the spring and fall. Constant, consistent flow provides more realistic results.  Inconsistent flow will cause higher corrosion results.
• Since the temperature of the water effects corrosion rate, installing the coupon rack after the heat source will result in higher corrosion rates. Conversely, installing the coupon rack after the tower will result in lower corrosion rates.
• The order of coupons in the rack is important. Less noble metals such as carbon steel should be mounted upstream of more noble metals such as copper.  This prevents copper ions from plating onto the iron coupons and causing artificially high corrosion rates.
• Corrosion coupons installed in systems with oil or other hydrocarbon contamination will show artificially high corrosion rates due to sulfide attack. Corrosion coupon projects should be delayed until process leaks are under control or avoided altogether in contaminated systems.
• The direction of water flow is important. Water should flow from the unattached end of the coupon toward the attachment end of the coupon.  This reduces turbulence that can adversely affect the corrosion rate.
• Do not remove the coupons to look at them except at the end of the test period. Exposure to air, once the coupons are installed, can increase corrosion.

Before installing the coupons, complete the information on the coupon package.  Turn off the system and carefully mount the coupons on the holder arms.  Use a paper or cloth towel to prevent the oils on your skin from affecting the results.  Once installed, turn the flow to the rack back on and adjust the flow rate to 3-5 FPS.  Maintain constant flow throughout the study.  Store the coupon envelopes in the controller or in a dry place near the coupon rack.

Remove the coupons after 60, 90 or 120 days as specified by the study.  Carefully remove and dry each coupon on a paper towel.  Take care not to remove material deposited on the coupon because deposits are an important part of the evaluation.  Place the coupon into the original envelope without the plastic bag.  Record the date of removal and send the coupon back to the Technical Support Laboratory as soon as possible.  The results of the corrosion coupon analysis will be presented on a Laboratory Report for and include a written analysis, a photograph of the coupon, and a corrosion rate in mills per year, or MPY.  Use the table below to evaluate the results from your study.

  Cooling Tower Result (MPY)   Closed Loop Result (MPY)   Corrosion Rate Carbon Steel Copper Carbon Steel Copper Excellent 1-3 0.1-0.3 0.2-0.5 0.1-0.25 Good 3-5 0.25-0.35 0.5-0.8 0.25-0.35 Moderate 5-8 0.35-0.5 0.8-1.0 0.35-0.5 Poor >8 >1 >1 >0.5

 

What Corrosion Coupons DON’T do

 

Corrosion coupons are installed in a system with the intention of predicting the corrosion rate for the entire system.  The obvious flaw with this reasoning is that the flow and temperature through a corrosion coupon rack will never duplicate the corrosion forces elsewhere in most systems.  There is little effect on the coupon by temperature which would typically be a severe corrosive effect in actuality. 

 

An additional factor to consider is that they are not effected by galvanic reactions caused on the metal. Corrosion coupons are placed in a position that does not directly touch the metal surfaces. Various electrochemical reactions are not measured because of this, and some corrosion tracking capability is lost. It does not account for areas with low flow. Low flow areas will accumulate corrosion and corrosion coupons not placed in those areas will leave them undetectable. It only accounts for general corrosion. Other factors such as under deposit corrosion and large deposit settlement in high-risk areas may go unnoticed by corrosion coupons. The most reliable indication of corrosion in the concentration of the total iron in the recirculating water.  Even so, many facilities require coupon studies and below is the detailed procedure for installing and monitoring coupons.

 

Common Mistakes that Make Corrosion Coupons Ineffective

 

• Not accounting for system type. Open systems show more corrosion with more variability. In contrast, closed systems show less corrosion with less variability.
• Only testing part of the system. For a more effective diagnosis, it is better to test various locations in the system for corrosion.
• Inconsistent monitoring or placement of the corrosion coupons. Corrosion coupons may be taken out too early and replaced with a new one. This makes it difficult to track pitting and interior wall surface irregularities. Additionally, not following instructions or not being consistent with how you test them, will lead to flawed data.

We utilize corrosion coupons at Chardon to monitor water quality. To have us detect corrosion or other water quality issues reach out to us here.

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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.

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