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.
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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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Once the coupons have been produced, they are marked with a unique serial number for identification and traceability. Coupons must have a consistent finish, free of defects to supply reliable data. This finish can be applied through blasting, grinding, machining and many forms of polishing. After finishing coupons should only be handled with nitrile or latex gloves and be protected from the environment to prevent premature corrosion or oxidation.
The final and most critical step in the production process is to weigh the coupon on an analytical balance to 4 decimal places. The balance should be calibrated and validated frequently to ensure accurate weights.
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