Corrosion Coupons - An Introduction
Water Treatment Corrosion Coupons
Rod (Cylindrical) Corrosion Coupons
Flush Disc Corrosion Coupons
Standard Strip and Scale Corrosion Coupons
Corrosion Coupons Before Cleaning
Post Cleaning Corrosion Coupons
Heavy Corrosion and Pitting
Heavy Corrosion on a Disc Coupon
Magnified Image of Corrosion on a Strip Coupon
Corrosion coupons are the simplest and most cost-effective way to monitor internal corrosion in pipelines, plants and process equipment. The purpose of corrosion coupons is to give the operator of the equipment an estimate of the rate at which the pipe or equipment is corroding from the inside out. In other words, how corrosive is the fluid or vapor inside the system? Knowing the rate of corrosion will allow the operator to gage the effectiveness of the corrosion mitigation program and determine if any changes are needed. A proper corrosion monitoring and mitigation program is critical to preventing corrosion-related failures. Failures in production equipment can result in death or serious injury, environmental disasters, litigation and destruction of property, plant, and equipment. The largest consumers of corrosion coupons are the oil & gas, water treatment, chemical, and laboratory industries.
Corrosion coupons are small metal samples made from metal alloys which are (to the extent economically feasible) identical to the alloys in the system. For example, a carbon steel corrosion coupon will be used in a carbon steel pipeline, a copper corrosion coupon will be used in a copper heat exchanger, and a galvanized steel coupon will be used in a galvanized cooling tower. If identical material is not available or is cost prohibitive the closest available substitute should be used. For example, 1018 carbon steel is often used to monitor pipeline corrosion though the pipe itself maybe a slightly different grade of steel.
Emphasis should be placed on the trend data generated by corrosion coupons in any case. It is practically and economically impossible to mimic the exact condition of the material inside process equipment. Equipment has that has been in service for years may have passivation layers, oxide layers, existing corrosion, scale build up and many other variables that will cause the system itself to corrode at a different rate than indicated by the corrosion coupon. Nonetheless the coupon will give the operator trend data that can be acted upon and will help provide answers to questions such as: did the corrosion rate go up or down over the previous period? Is the corrosion rate higher or lower than last year or 5 years ago? Did the change in water chemistry increase or decrease the corrosion rate? What is the corrosion rate in similar systems? All of this is extremely helpful in determining the effectiveness of a corrosion mitigation program and can potentially save millions of dollars in chemical corrosion inhibitors and ultimately prevent equipment failure.
Corrosion coupons come in a wide variety of shapes and sizes such as rods, discs and flat strips. Regardless of the form factor, users should consider the following when deploying corrosion coupons in their monitoring program:
1. The coupon is made from a known alloy which can be proven through positive material identification techniques or through traceability back to the mill test report which is the “birth certificate” of the material.
2. The corrosion coupon is manufactured in a stable, consistent, and reproducible manner such that a coupon manufactured today, 10 years ago, and 10 years in the future is equivalent and will result in reproducible data.
3. In particular, the coupons should have a consistent and reproducible finish. Generally, this is achieved through media blasting, grinding, or polishing. It should be noted that the finish type will have a significant impact on the corrosion rate, a rougher finish is expected to be more reactive due to the higher surface area and result in higher corrosion rates. For this reason, corrosion rates across different finish types should not be considered comparable. It is also worth noting that none of the standard finish techniques are likely to exactly mimic the internal finish of the process equipment. For this and many other reasons corrosion rate as measure by corrosion coupons will not be identical to the corrosion rate experienced within the process environment and the data should be interpreted accordingly. Emphasis should again be placed on generating a consistent specimen that will produce accurate trend data. The Cooling Technology Institute in their standard Corrosion Testing Procedures published in 2000 states “Corrosion coupons, when prepared similarly, can provide a near standardized method to compare relative performance of various water treatment conditions within a cooling water system. This standard will provide uniformity in the evaluation process. Coupon corrosion rates are not necessarily an indication of the actual system corrosion, but rather are a relative performance indicator.”
4. The corrosion coupon should be weighed to “4 decimal places” or to the nearest 0.1 milligram. Accurate and precise weights are critical to generating accurate corrosion rate data. Incorrect pre- or post-exposure weights will result in incorrect corrosion rates. For this reason, corrosion coupons should we weighed on a high-quality analytical balance. The balance must be calibrated by a third party a minimum of once per year and should be spot checked daily prior to and during use with NIST-traceable calibration weights.
5. Corrosion coupons must be stored in protective packaging before and after exposure to prevent premature or additional corrosion from taking place.
6. Corrosion coupons should not be re-used after initial exposure even in the same system. A used coupon cannot ensure accurate corrosion rates due to the irreversible changes in surface area and composition.
Corrosion coupons are exposed to the process environment by various methods including mounting in a coupon bypass rack, mounting on a holder directly in the production stream, or by hanging in a tank or test cell. While installing and handling coupons gloves should be worn at all times to prevent skin oils from transferring to the coupon, which adds mass and can potentially block fluid from interacting with the surface of the coupon. These and other unforeseen effects can occur, causing unknown variations in corrosion rates. Typical test periods are 30, 60 or 90 days but can vary based on the corrosivity of the environment. Relatively speaking, shorter exposure periods are expected to produce higher corrosion rates. The corrosion rate can decrease over time as films formed on the surface of the coupon can form a protective layer.
After exposure, the coupon is removed, again the coupon must only be handled with gloves. The coupon must be dried, placed back in its original protective packing and sent to a laboratory for analysis. The lab will clean the coupon to remove any deposits, then inspect and if necessary, photograph the coupon. If any pitting is present the pit depth can be measured and recorded. Finally, the coupon is weighed, and the weight loss calculated. Using the weight loss data and the information about the dimensions and makeup of the corrosion coupon, a corrosion rate can be calculated. Corrosion rates are typically reported in “mils per year”, which is the rate that the material thickness is reduced, in thousandths of an inch, per year.
The corrosion rate is calculated using the following equation:
CR = (22270 x W)/(ATD)
where:
CR = average corrosion rate, mils per year (mpy)
W = mass loss, grams (g)
A = initial exposed surface area of coupon, square inches
T = exposure time, days
D = density of coupon metal, grams per cubic centimeter
For additional information on corrosion coupons please visit our website www.pacificsensor.com, contact me, or review the references below.
Will Ritter
Pacific Sensor
972-242-5750
2429 Dickerson Pkwy
Carrollton, TX 75006
References
1. NACE SP0775 (latest revision), “Preparation, Installation, Analysis, and Interpretation of Corrosion Coupons in Oilfield Applications” (Houston, TX: NACE).
2. ASTM G1 (latest revision), “Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens” (West Conshohocken, PA: ASTM International).
3. CTI Code STD-149 (latest revision), “Corrosion Testing Procedures Corrosion Coupon Testing and Test Devices” (Houston, TX: Cooling Technology Institute)