Corrosion coupons and how to use them

Introduction

A common method of determining corrosion rates is by using corrosion coupons, which are uniform-sized, pre-weighed strips of metal. Corrosion coupons, representative of system metals, are inserted into a coupon rack installed on the system to be checked. Normally, system water is allowed to circulate over the corrosion coupons for about 30 - 90 days. The coupons are then removed and returned to a lab where they are cleaned and re-weighed. From this weight loss and the dimensions of the coupon, a corrosion rate in mils/year (mpy) is determined. 1.0 mil/year translates into 1/1,000th of an inch of metal loss per year. To convert corrosion rates expressed in millimeters/year (mm/y), a common metric measurement, to mpy, multiply mm/y times 39.4.

Interpretation of Results

Whether a corrosion rate is good or bad is relative to the water used and the operating conditions. No absolute interpretation is practical. However, Table One gives guidelines that have been published for assessing corrosion in cooling tower systems and closed loops using fresh water make up. Keep in mind these rates and comments assume general system corrosion. Pitting corrosion can cause rapid metal failure even if the overall corrosion rate is low.

The following describes different forms of attack that can be observed on coupons.

Generalized Attack - Uniform corrosion over entire surface of the coupon. Not usually a concern unless the mpy is high.

Pitting Attack - A general term given to any depression on the metal surface caused by corrosion. Pits can vary considerably in size and depth as well as density. Less than 10 pits per side is sometimes termed isolated pitting. Low inhibitor levels, high chlorides, pH excursions, under-deposit attack, or copper plating can all cause pitting.

Localized Areas of Attack - Usually the result of under-deposit corrosion. May indicate the need for better deposit control and/or low flow rates through the coupon rack. If the depression shows concentric rings with the deepest penetration in the center, it may be due to corrosive bacteria attack, such as can be caused by microbiologically induced corrosion (MIC).

Copper Plating - Results from the deposition of soluble copper on mild steel or other non-copper alloys. Copper plating can cause severe galvanic corrosion and metal failure due to pitting attack.

Edge Attack - Since the edges of coupons are highly stressed during fabrication, they tend to be preferential sites for corrosion. Edge attack does not generally indicate a major problem unless severe.

Attack Under the Coupon Holder - If metal loss is localized to the area under the coupon holder, this may merely represent the influence of the coupon holder to stimulate under-deposit or crevice attack and not reflect the characteristics of the recirculating water. Although these effects cannot be eliminated from corrosion rate calculations, they should be noted when interpreting the results. Insuring the coupon holder and bolts are fastened tightly helps minimize these effects.

Installation Notes

1. The coupons have been cleaned and accurately weighed prior to shipping. They should not be handled any more than necessary when installed. Avoid fingerprints, oil, or grease contact. Use a paper towel or similar covering while handling the coupon prior to nut. Metallic bolts and nuts will increase the probability of galvanic or contact corrosion at the secured end of the coupon.
2. Be sure the numbered corrosion coupons are recorded according to position and system for proper correlation. The dates of installation and removal from the system are critical.
3. Table Two gives the preferred order of installation for common metals.
4. To ensure that a representative sample of the bulk system water is directed over the coupons, the coupon rack should be installed on a line that is completely separate from any chemical injection points. The piping to a coupon rack should not be constructed of copper or copper alloys to reduce the potential for false high corrosion rates due to galvanic corrosion.
5. The flow should be adjusted so that there is no turbulence or air mixture. To avoid erosion of copper, a flow rate of three to five feet per second is desired. This translates into 4 to 7 gallons per minute in a three quarter inch (3⁄4”) coupon rack and 8 to 12 gallons per minute in a one inch (1”) coupon rack. A flow meter or some other type of flow control device is recommended. Keep in mind that high flow rates can contribute to erosion while low flow rates (less than two feet per second) can accelerate corrosion and biological fouling.
6. Test periods are generally 90 days. Coupons may be observed at 30 days intervals for reference and signs of corrosive conditions. In observing the coupons, do not disturb the surface by scraping or removing material.
7. Typically coupons show some rapid corrosion indications initially, usually in the form of corrosion products forming on the surface, particularly at points where numbered stamps are made, edges of coupons, and around bolt heads. The rapid corrosion tapers off with time. This is more prevalent with steel than copper. Unless corrosion is significant, there should be no cause for alarm.
8. When the coupon is removed for lab evaluation, physical handling of it should be kept to a minimum. Unless microbial induced corrosion is suspected, removal of corrosion products should be minimized and the coupon thoroughly dried before returning to the lab for analysis in the paper envelope in which it was originally sent.
9. If microbial induced corrosion is suspected, the corrosion products should be scraped off with a dry plastic utensil into a bio-sample bottle containing system water. The coupon can then be dried and returned to the lab for analysis along with the bio-sample bottle.

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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