What are corrosion coupons?
A corrosion coupon is an in-line intrusive corrosion monitoring device used widely in oil and gas assets. Corrosion coupons are typically constructed from carbon steel. These can be generic carbon steel or may be selected to match the material used for a specific purpose; for example, X65 pipeline steel or weld consumables.
Any corrosion coupon should be certified by recording its serial number, weight in grams, dimensions, material, and surface finish.
Why are corrosion coupons used?
Corrosion coupons are used to monitor the corrosivity of a system. Corrosion coupons are installed to duplicate the corrosion rate experienced in piping, pipelines, or vessels. They can also be used to estimate the effectiveness of the mitigation program in place or evaluate the suitability of a specific material for a specific system.
What are the advantages and disadvantages of using corrosion coupons?
Advantages:
Simplicity – No electronic monitoring equipment is required
Versatility – The technique is extremely versatile because weight loss coupons can be fabricated from any commercially available alloy. Also, using appropriate geometric designs, a wide variety of corrosive environments may be studied
Applicability – The technique is applicable to all environments and both corrosive and erosive species- gases, liquids, solids/particulates, stagnant or flowing conditions
Reliability – Coupons are not subject to the various forms of interference that can lead to false corrosion readings or probe failures
Visual Assessment – Visual and microscopic assessment can be undertaken
Surface Analysis – Surface deposits can be observed and analysed
Disadvantages:
Delayed Data – In a typical monitoring program, coupons are exposed for a three-month to one-year duration before being removed for laboratory analysis. As such, an indication of a corrosion event may only be obtained significantly after the event may have occurred
Lack of Time Determination – If a specific corrosion event occurs during the period of exposure, the coupon will not be able to identify the time of occurrence
Time Averaging – Depending upon the peak corrosion rate of a corrosion event and its duration, the coupon will not indicate the peak corrosion rate of the event, and short-term events may not even register a statistically significant increased weight loss
Galvanic Corrosion – Corrosion coupons are typically isolated from any metal-metal contact. Thus, they are unaffected by the galvanic interactions present in the system
What data does a corrosion coupon provide?
Corrosion coupons should be located where corrosion is expected. Corrosion coupons provide the general and pitting corrosion rates. The general corrosion rate is assessed through metal loss from the coupons over a certain time period. The worst pitting corrosion rate is assessed using the depth of the deepest pit over a period of time.
Several factors should be taken into consideration during the interpretation of corrosion coupon data; such as:
To ensure effective monitoring in a given system, coupons should be placed throughout the different areas in the system where the worst-case corrosion rates are predicted. This could include low points, stagnant areas, and high velocity/turbulent areas. Additionally, both the 6 o’clock and the 12 o’clock positions should be monitored for piping and pipelines.
Long exposure to the service fluid (3-6 months) is required to evaluate corrosion rates. It should be noted that some conditions such as bacteria fouling take time to develop on the coupon surface.
How should the coupons be oriented and positioned?
Corrosion coupons orientation should be consistent in order to assess and compare different data sets. Typically, a corrosion coupon should be oriented in a direction parallel to the process flow. Both of the 6 O’clock and the 12 O’clock positions should be monitored and this is especially important for systems with multiphase/stratified flow or systems where bottom of line deposits could occur.
The corrosion coupon should be positioned in the plane of the vessel or pipe wall to provide a good representation of the flow regime of the vessel/pipe. The best representation of flow in a given system will be achieved by using flush mounted coupons rather than intrusive coupons.
How many coupons should be installed?
Typically, at any coupon monitoring point, more than one coupon is installed for two reasons:
Many uncontrollable factors may exist and reduce the accuracy of a single coupon. The use of paired coupons, with an average value taken, substantially improves data reliability and accuracy.
Corrosion coupons give an indication of the corrosion that has already occurred. If it is required to verify whether the corrosion rate was uniform or varying over a certain time, then multiple corrosion coupons could be installed at the same location and retrieved at different times.
Types of corrosion coupons
There are many different coupon types. The most common coupon types are described below and presented in Figure 1:
Corrosion coupon holders
A coupon holder is used to fix two or more coupons at a certain location. Depending on the operating conditions, number and type of coupons, configuration and system entry method, the coupon holder is selected.
The common type of holders used are:
Additionally, ladder coupon holders may be used instead of single or paired coupon holders to facilitate monitoring across the diameter of piping or pipelines allowing simultaneous monitoring of the 6 o’clock position, 12 o’clock position and the center line.
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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.
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.
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.
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.
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 MPYCopper 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 MPYLow 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 MPYAluminum
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 MPYAcceptable corrosion rates from the Association of Water Technologies.
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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