Corrosion Coupons | Metal Samples

Accurate monitoring of corrosion rates in various environments is crucial, as it directly affects maintenance and repair costs stemming from corrosion and material failure. Corrosion coupons offer a cost-effective method for real-time monitoring, enabling precise measurements of corrosivity within a system. By examining the mils-per-year corrosion rate of an exposed coupon, significant insights can be gleaned regarding the lifespan of the material used. Metal Samples can fabricate coupons in any required size, shape, or material. Each coupon can be marked with alloy and sequence numbers for easy identification. Detailed mill test reports, confirming element compositions of the utilized materials, are included with all orders.

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Note: When ordering test coupons, please be prepared to provide the sales representative with details regarding the type of material being tested, the size and style of the coupons needed, the desired surface finish, and any applicable information related to the mounting hole size and location.

Updated September 28,

What do Corrosion Coupons do?

Corrosion coupons are designed to monitor the extent of corrosion over time within water systems. Initially, they are weighed without corrosion and later, their mass is compared after being subjected to the water system for a specified duration, often 60, 90, or 120 days. Available in various materials, these corrosion coupons can assess multiple systems, and the technique employed during the testing process significantly determines effectiveness.

Benefits of Corrosion Coupons

Utilizing corrosion coupons presents several advantages for tracking corrosion in systems:

• They are an efficient tool for detecting corrosion. With proper monitoring, the changes observed in corrosion coupons over time can serve as essential indicators to confirm the effectiveness of chemical treatment programs.
• Ease of use is another benefit. While numerous indicators of corrosion can occur, the straightforward method of inserting and retrieving corrosion coupons allows for clear comparison of noticeable changes throughout time.

For assistance with corrosion coupons, reach out to the experts at Chardon Laboratories here.

Using Corrosion Coupons Effectively

Effective analysis of corrosion coupons requires careful consideration of various variables that may significantly influence the results:

• Avoid direct contact with the coupons, as oils from your skin can promote corrosion and skew the test results.
• Ensure a constant flow rate of 3-5 feet per second during the test period. Rates below 3 fps may allow particulates to settle on the coupon, affecting readings, while rates above 5 fps could erode softer metals like copper and brass. For 3/4” PVC pipe, this flow rate corresponds to 5-8 GPM.
• Avoid conducting corrosion coupon tests in comfort cooling systems during spring and fall. Consistent flow ensures more reliable results, whereas varying flow could lead to elevated corrosion rates.
• Temperature significantly influences corrosion rates. Thus, placing the coupon rack downstream of the heat source can lead to higher corrosion rates, while positioning it after the cooling tower may result in lower rates.
• Coupons should be arranged in the rack with less noble metals, such as carbon steel, situated upstream of more noble metals like copper. This arrangement helps prevent copper ions from artificially inflating corrosion rates on iron coupons.
• Coupons placed in systems contaminated with oil or hydrocarbons may also reflect exaggerated corrosion rates due to sulfide attacks. It is advisable to either wait until process leaks are resolved or avoid coupon studies in such environments entirely.
• The water flow's direction must be correct, as it should travel from the unattached end towards the attachment end of the coupon, ensuring minimal turbulence that may otherwise impact the corrosion rates.
• Do not remove the coupons for inspection prior to the conclusion of the test period, as exposure to air can increase corrosion.

Prior to the installation of coupons, complete all necessary information on the coupon packaging. Turn off the system, mount the coupons securely on the holder arms, and employ a paper or cloth towel to prevent skin oils from impacting results. Reactivate flow to the rack and adjust to maintain the 3-5 FPS flow rate. Ensure a steady flow throughout the evaluation period. Store coupon envelopes in a controlled environment near the coupon rack.

Remove the coupons after 60, 90, or 120 days, per study specifications. Carefully dry each coupon using a paper towel without disturbing any deposits, which are crucial to the evaluation process. Return the coupon to its original envelope without the plastic bag, document the removal date, and promptly send it to the Technical Support Laboratory for analysis. Results will be detailed in a Laboratory Report, featuring a written analysis, a photograph of the coupon, and a calculated corrosion rate in mills per year (MPY). Utilize the following table for evaluating your study results:

Cooling Tower Result (MPY)

Closed Loop Result (MPY)

Corrosion Rate

Material	Excellent	Good	Moderate	Poor
Carbon Steel	1-3	3-5	5-8	>8
Copper	0.1-0.3	0.25-0.35	0.35-0.5	>1

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What Corrosion Coupons DON’T do

Though installed to estimate overall system corrosion rates, the limitations of corrosion coupons are noteworthy, given that flow and temperature variances through coupon racks seldom replicate corrosion forces present elsewhere in the systems. The coupons themselves are also less affected by temperature compared to practical experiences of severe corrosivity.

Moreover, they are not subject to galvanic reactions affecting the metal due to their placement, thereby overlooking critical electrochemical reactions and possibly losing some tracking capability. Areas with low flow can accumulate corrosion that goes undetected if coupons are not strategically placed in those locations. Ultimately, they primarily measure general corrosion, neglecting specific phenomena like under-deposit corrosion or significant deposit accumulation in high-risk zones. Total iron concentration in recirculating water remains the most reliable corrosion indicator. Nonetheless, many facilities still rely on coupon studies, detailed processes for installation and monitoring shall follow.

Common Mistakes that Make Corrosion Coupons Ineffective

• Failing to account for system types. Open systems frequently exhibit more variability in corrosion compared to closed systems, which show less.
• Only testing limited system sections. For a more thorough diagnosis, monitoring various locations is advisable.
• Inconsistent monitoring or improper placement of coupons. Removing coupons prematurely or inconsistent test procedures can complicate the tracking of pitting and surface irregularities, leading to inaccurate data.

We utilize corrosion coupons at Chardon to monitor water quality effectively. For assistance in detecting corrosion or related water quality concerns, please reach out to us here.

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