What is a metal coupon?

Updated September 28, 2022

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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