CORROSION COUPONS | And Coupon Rack

Made of similar metals and materials to the system components; scaling and or corrosion coupons are installed within a hydraulic system [chilled water/condenser water] to allow the examination of the overall &#;corrosion rate&#;, allowing the system&#;s current condition to be understood and the ability to also predict its future life expectancy allowing for preventative maintenance measures.

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What is a corrosion coupon

A coupon is a &#;strip&#; of metal that is manufactured to precise tolerances focusing upon its weight and dimensions to allow accurate testing after a period of being installed within a system.

Coupons should be manufactured from a metal similar to the system materials, or from a one that is more reactive, similar to zinc or aluminum, to accelerate the corrosive conditions, providing earlier warnings.

They are usually stenciled with numbers which will allow proper identification when being used on-site and recording the testing.

How do they work and how do you test them?

They are installed into a closed or open loop system where they should be electrically isolated to ensure any corrosive effect is only from the fluid within the system.

If using the &#;weight-loss method&#;, prior to installation, each coupon should be weighed, with an accurate calibrated scale and the information recorded for future reference.

During the time they are installed and the system water is flowing over and around them, they will start to corrode &#; the recommended installation time for the coupons would be 90 days. This is to allow acclimatization to the system. It has been noticed if only install for a short period of time, under 30 days, the corrosion rate is usually higher.

Once the testing has been completed, they are removed and sent to a laboratory, where they will be cleaned and weighed to understand the rate of corrosion.

All testing and analysis should be completed in line with the following standards:

As can be seen within the AWT Standard for corrosion rates the classification of corrosion rates for open and closed recirculating cooling water systems are:

Open Pipework System [Condenser]

Description [Open System]Carbon Steel [mils-per-year]Copper Alloy [mils-per-year]Negligible or ExcellentLess than or equal to 1Less than or equal to 0.1Mild or Very Good1 to 30.1 to 0.25Good3 to 50.25 to 0.35Moderate to Fair5 to 80.35 to 0.5Poor8 to 100.5 to 1Very Poor to Severe>10>1

Closed Pipework System [Chilled Water/Heating]

Description [Closed System]Carbon Steel [mils-per-year]Copper Alloy [mils-per-year]ExcellentLess than or equal to 0.2Less than or equal to 0.1Good0.2 to 0.50.1 to 0.25Moderate0.5 to 0.80.25 to 0.35Poor0.8 to 10.35 to 0.5Very Poor to SevereGreater than or equal to 1Greater than or equal to 0.5

What are the benefits and disadvantages of using Coupons?

Advantages

• Inexpensive.
• Simple to use, no risk of &#;instrument&#; failure.
• Different metals can be tested at the same time.
• Corrosion and bacteria, if required, can be obtained from the surface for further anaysis.

Disadvantages:

• Time to complete the testing.
• Can only provide average readings and won&#;t be able to show annomolies across a time period.
• Corrosion is not normally constant, the coupons will not be able to show this.

What materials are coupons made from?

The coupon material used for monitoring will depend upon the materials in the system, below is a list of materials that are available:

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

What are the common sizes?

Coupon strips are generally ½ inch (12.5 mm) wide, 3 inch (75 mm) in length, and 1/16 inch (1.6 mm) thick as per the ASTM standards.

How to install coupons?

Coupons would usually be installed in a &#;coupon rack&#;, see below for additional information. Within the rack will be &#;coupon holders&#;, usually one to six numbers, to allow installation and fixing of the metals being used.

They should be installed so that the flow of the water is from the back of the coupon and that its broad face is in the vertical position, this reduces the risk of accumulation of debris on its face.

The coupon strips should be easily installed and removed when needed.

Galvanic Series

If using multiple types of metals for the installation, the galvanic series should be considered.

The most active metal should be installed in the first position then leading through the metals in the next positions until the last being the most noble of the metals being tested.

For example:

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

What is a Corrosion Coupon Rack?

A rack will usually consist of a range of 1&#; / 25mm plastic PVC, CPVC, Carbon Steel or Stainless Steel pipework designed to the ASTM requirements, and wall-mountable for easy installation.

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The rack should be installed at a distance of between 10 to 30 feet / 3m to 9m from the main system pipework.

A flow of water should pass through the rack from the bottom to the top, reducing any risks of airlocks, and at a flow rate that is between 0.4 m/s to 1.8 m/s. The flow can be controlled by a &#;rotameter&#;

They can be supplied with isolation valves, and solenoid valves for controlling the water [on/off], a paddlewheel water meter, coupon holders, and a Rotameter flow meter to ensure the required flow rates are met.

How do you monitor the Coupons

If the coupons are needing to be inspected then they can easily be removed from the &#;Rack&#;. This is usually achieved by unscrewing the co

upon holder and sliding out. Make sure the system is isolated and do not touch the &#;coupon&#; as grease from the hands may interfere with the results.

What are the differences between a Coupon and a Probe?

As noted in this article, a coupon is usually a metal strip that is installed into a &#;rack&#; and is manually cleaned, evaluated and weighed after a period of time has passed to understand the corrosion rate within a chilled water/condenser water system.

The installation is quite inexpensive but takes time to obtain any results which only show an average of corrosion, and sometimes the reaction to the problem can be quite a while.

Probes on the other hand are electronic monitoring devices that are installed into the system and are a lot more expensive.

They can provide instantaneous results, allowing trend monitoring to see all peaks and troughs of the corrosion being measured and immediate investigations and remedial actions can be taken.

Some people suggest having both types, the electronic probe for instantaneous detection and remedial actions, and the metal coupon strips for providing a visual picture of the corrosion in the system over a longer period of time.

How do you test and commissioning a Coupon Rack?

There is not much to be done to functionally test a coupon rack, general checks to complete would be:

• Ensure installed inline with manufacturers literature and requirements.
• Ensure coupon strips are of the correct material.
• Coupons are of the correct dimensions and weight.
• Coupons are stamped and numbered.
• Review the flow rate across the coupons to ensure it meets the requirements and that there is no risk of wear happening.

Maintenance of the Coupon Rack

Maintenance is quite simple, each month that the system is in operation, manually check all fasteners and visibly check for leaks and if there is any abnormal wear on the coupons.

If leaks are detected then there could be an issue with the &#;O&#; Rings at the joint the leakage is seen [cut, scored, debris, or missing].

If notice abnormal wear on the coupon, this could be due to high flow rates, so check those and reduce where necessary.

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Your water system is vulnerable to a wide array of potential issues, from Legionella bacterial overgrowth to rust and sediment. Creating a proper water treatment plan that addresses these risks comprehensively requires more than just guesswork; you will need to conduct testing to understand what vulnerabilities your system is facing. One such option is the use of corrosion coupons. If this term is unfamiliar to you, or if you are just looking into using coupons to better understand the health of your water system, continue reading to learn more about how this diagnostic approach works.

What Is A Corrosion Coupon?

A corrosion coupon is a material that is used to determine how corrosive your water is&#;and by extension, how much damage your pipes may be suffering as a result. Typically, this coupon takes the form of a small metal piece (often in the shape of a coupon, hence the name) that is affixed within your water system so that the water can flow freely past it for a set period of time.

After that time, you can remove the coupon and see how it has reacted to your system&#;s water; this will give you an indication of what your pipes are experiencing and how to remedy the situation. If it is corroded, you can expect some corrosion damage on your pipes, too.

Benefits Of Corrosion Coupon Monitoring In Your System

Corrosion coupon monitoring can provide valuable insight into the acidity and other elements of your water without having to completely remove a segment of piping in order to check its durability and condition. Identifying corrosion as soon as possible is vital so that you can avoid premature repair and replacement of critical equipment; corrosion can cause not only leaks that cost you money in water damage and higher water bills, but it can also contribute to inadequate performance of your machinery.

If you operate cooling towers, for example, corrosion may impact how heat is exchanged. Corrosion coupon monitoring is a simple and effective way to gather important data about your system so that you can make a plan.

How To Properly Set Up A Coupon

Setting up a corrosion coupon is not as simple as just placing it into a pipe. Instead, you must be strategic about where it is located so that you can gather the most relevant data about your system. A professional can help you to identify the most appropriate location for your coupon if you need assistance.

Duration

One of the most important aspects to decide when you are installing a corrosion coupon is how long you will test. While it is possible to test for only 30 days, this may produce results that are more difficult to learn from. Instead, the recommended time frame for corrosion coupon monitoring is at least 90 days, though you may continue for longer if needed.

Placement

Coupons should be placed so that water flows from the holder (rather than toward it), and they should be located differently depending on the material you are testing. Follow the galvanic series; the most active metal should be placed first so that unwanted metal deposits do not cloud the results.

Flow And Environment

In order to procure accurate results, water flow past the corrosion coupon should not dip below three feet per second, up to a maximum of five. Any faster could result in erosion of the material, which will cloud the results of the test. Place your coupon in the area with the warmest water, as cool water can artificially lower the rate of corrosion.

Things To Avoid

Never couple your corrosion rack to valves made of galvanized steel or brass. These materials will react with the corrosion coupon setup through the release of copper or zinc ions that will skew the results. Copper ions will accelerate corrosion, and zinc ions may synergize to artificially create a layer of protection against corrosion that is not present in the entire system, leading to the false belief that your system is safe from corrosion.

Trust The Experts To Help You Manage Your Water System Effectively

Whether you are just hearing of corrosion coupons for the first time or you are ready to install them but are not sure how to do so by yourself, trust the experts. The professionals at Tower Water set the standard in water treatment and would be happy to help you identify the proper location and usage for your corrosion coupons so that you can gather relevant data. Reach out to learn more or to schedule an appointment to achieve your water treatment goals.

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