CORROSION COUPONS | And Coupon Rack

Made of metals and materials that closely resemble those of the system components, scaling and corrosion coupons are installed within hydraulic systems, including chilled water and condenser water systems. This allows for the assessment of the overall corrosion rate, providing insight into the current condition of the system while also enabling predictions about its future lifespan, which in turn supports preventative maintenance strategies.

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What is a corrosion coupon?

A corrosion coupon is essentially a metal strip crafted to stringent tolerances, emphasizing its weight and dimensions to ensure accurate testing after being installed in a system for a predetermined period.

These coupons are ideally made from a metal similar to that of the system materials, or from a more reactive metal such as zinc or aluminum. This choice serves to amplify the corrosive conditions, providing early warnings about corrosion damage.

Each coupon is usually marked with identification numbers, facilitating proper tracking during on-site usage and when recording testing results.

How do they work and how do you test them?

Corrosion coupons are placed into either open or closed loop systems where they must be electrically isolated to ensure that any corrosion observed is only due to the fluid in the system.

In the weight-loss testing method, each coupon must be accurately weighed on a calibrated scale prior to installation, and this information should be documented for subsequent reference.

After installation, as the system water flows over and around the coupons, they will begin to corrode. It is recommended to allow a period of 90 days for acclimatization. Using them for a shorter duration, such as less than 30 days, can result in misleadingly high corrosion rates.

Upon completion of the testing interval, the coupons are removed and sent to a laboratory for cleaning and weighing, which helps to determine the rate of corrosion.

All testing and analysis should follow established standards as described in the AWT guidelines for corrosion rates for open and closed recirculating cooling water systems:

Open Pipework System [Condenser]

Description [Open System] Carbon Steel [mils-per-year] Copper Alloy [mils-per-year] Negligible or Excellent Less than or equal to 1 Less than or equal to 0.1 Mild or Very Good 1 to 3 0.1 to 0.25 Good 3 to 5 0.25 to 0.35 Moderate to Fair 5 to 8 0.35 to 0.5 Poor 8 to 10 0.5 to 1 Very Poor to Severe >10 >1

Closed Pipework System [Chilled Water/Heating]

Description [Closed System] Carbon Steel [mils-per-year] Copper Alloy [mils-per-year] Excellent Less than or equal to 0.2 Less than or equal to 0.1 Good 0.2 to 0.5 0.1 to 0.25 Moderate 0.5 to 0.8 0.25 to 0.35 Poor 0.8 to 1 0.35 to 0.5 Very Poor to Severe Greater than or equal to 1 Greater than or equal to 0.5

What are the benefits and disadvantages of using Coupons?

Advantages

• Cost-effective.
• Easy to implement with no risk of equipment failure.
• Different metals can be simultaneously assessed.
• Corrosion and bacteria samples can be collected for further analysis if necessary.

Disadvantages

• Requires time to complete testing.
• Can only provide average readings and may not reveal anomalies over time.
• Corrosion rates are not typically constant, and coupons may not capture this variability.

What materials are coupons made from?

The materials utilized for coupons will depend on the system's materials. Common options include:

• Mild Steel
• Copper
• Stainless Steel [303, 304, 316]
• Nickel
• Brass
• Bronze
• Aluminum
• Galvanized Steel

What are the common sizes?

Most coupon strips adhere to ASTM standards, generally measuring ½ inch (12.5 mm) in width, 3 inches (75 mm) in length, and 1/16 inch (1.6 mm) in thickness.

How to install coupons?

Coupons are typically installed in a coupon rack, which features holders that facilitate the installation and securing of the metals being monitored.

Installation should ensure that the water flows from behind the coupon, positioning the broad face vertically to minimize debris accumulation.

Coupon strips must be easily installable and removable as needed.

Galvanic Series

When using different metal types for the installation, it is essential to consider the galvanic series, positioning the most active metal first to avoid unwanted metal deposits that could interfere with results.

• Aluminum [Least Noble]
• Galvanized Steel
• Mild Steel
• Brass
• Bronze
• Copper
• Nickel
• Stainless Steel [Most Noble]

What is a Corrosion Coupon Rack?

A corrosion coupon rack is typically constructed using 1" / 25mm plastic PVC, CPVC, carbon steel, or stainless steel pipework designed in accordance with ASTM requirements and is wall-mountable for easy installation.

The rack should be situated between 10 to 30 feet (3m to 9m) from the main system piping.

A water flow must move through the rack from the bottom to the top, mitigating any risks of airlocks. The desired flow rate ranges from 0.4 m/s to 1.8 m/s, which can be controlled with a rotameter.

It's common for racks to come equipped with isolation and solenoid valves for controlling water on/off, a paddlewheel water meter, coupon holders, and a Rotameter flow meter to ensure proper flow rates.

How do you monitor the Coupons?

To inspect coupons, simply remove them from the rack by unscrewing the holder and sliding them out. Ensure the system is isolated, and avoid handling the coupon with bare hands to prevent grease contaminants from affecting the results.

What are the differences between a Coupon and a Probe?

A corrosion coupon is typically a metal strip situated in a rack and subject to manual cleaning, evaluation, and weighing after a specified timeframe to gauge the corrosion level within a chilled water or condenser water system.

This method is affordable but takes time to yield results that only indicate an average corrosion level. Remedial response can also be delayed.

Conversely, probes are electronic monitoring devices integrated into the system, providing instantaneous results at a higher cost, allowing for real-time trend monitoring of corrosion fluctuations, facilitating immediate investigations, and remedial measures.

Many experts recommend utilizing both systems: an electronic probe for real-time alerts and metal coupons to visualize corrosion trends over an extended period.

How do you test and commission a Coupon Rack?

Functional testing of a coupon rack involves general checks, including:

• Ensuring the rack is installed according to the manufacturer’s specifications.
• Confirming the coupon strips are made from appropriate materials.
• Verifying coupon dimensions and weights are correct.
• Checking that each coupon is stamped and numbered.
• Reviewing the flow rate across the coupons to ensure compliance with stipulated requirements, mitigating risks of excessive wear.

Maintenance of the Coupon Rack

Routine maintenance is straightforward; during each month of operation, manually inspect all fasteners for security and observe for any leaks or unusual wear on the coupons.

If leaks are detected, there may be an issue with the O-rings at the joint displaying the leakage, which could be cut, scored, debris-laden, or even missing.

Should any abnormal wear on a coupon be noted, it may be the result of high flow rates, which should be checked and adjusted as necessary.

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