What Corrosion Coupons are & Their Importance

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Updated September 28,

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 coupons. 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.

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, 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.

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            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 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 = ( 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.

 

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