Corrosion Monitoring, Corrosion Coupons, and Corrosion Coupon Racks
Corrosion monitoring is intended to provide an accurate representation of how well metallurgies of a system are protected against corrosion and to alert facility operators and service companies if a change is necessary before too much damage takes place to the system.
- Corrosion can be expressed in mils per year (MPY) of metals degraded
- Corrosion produces by-products (oxides) that hurt systems through erosion, clogging, and fouling
- Corrosion oxides reduce system efficiency and increase energy costs by accumulating on heat transfer surfaces
- Corrosion can be reduced through proper system design, maintenance, and water treatment
- Dissolved oxygen
- Electrolysis (stray current)
- Differential metal (dielectric)
- Differential cell
- Flow, temperature, and pressure can all influence corrosion
- Offers valuable system information to building/plant operators
- Provide the only internal view of system conditions and corrosion status
- Results are useful to compare with ultrasonic thickness testing or metallurgical analysis
- Can show whether corrosion rates are increasing or decreasing over time
- Shows if a chemical inhibitor is present or effective for the particular alloy
- Provides corrosion rate data necessary to evaluate chemical program
Whether a corrosion rate is acceptable or not is dependent on water conditions and operating conditions. There is no definite standard, however; there are some general guidelines developed by water treatment firms and water treatment trade associations.
Corrosion rates should be viewed in relative, not absolute, measurements. Many variables affect corrosion, but the type of application will ultimately determine what an acceptable corrosion rate is. A system that is subjected to severe use, such as a process loop in a nuclear cooling system, will have higher corrosion rate expectations than a less abused system, such as a commercial HVAC system. Use corrosion data to compare stability of the loop over time. Improvements or regressions can be determined through monitoring.
What are corrosion coupons?
Corrosion coupons are the simplest and most commonly used form of corrosion measurement today. Corrosion coupons are small bars of various metals or alloys that are introduced into the system through a side stream coupon rack. The coupon alloy selection should be representative of system metals.
Coupons are pre-weighed and measured by a manufacturer and left in place for a fixed amount of time. After exposure, coupons are usually returned to the supplier for analysis by an independent laboratory.
Corrosion coupons vary by type of metal alloy and physical configurations:
- Mild Steel
- Stainless Steel
- Galvanized Steel
- Flush Discs
Typically, coupons are photographed upon reception, cleaned of system contaminants, inspected, measured, dried, re-weighed, and re-photographed.
A corrosion rate is determined in mills per year (MPY) from the weight of material lost over its time in place.
Typically system water must circulate over the corrosion coupons for 30-90 days. We recommend a consistent testing interval of 90 days per round of coupon tests. Interval time may also depend on the process.
Evaluating a 30 day coupon interval versus a 90 day will be of less use than a 90 to 90 where you can see if your system is stable or not. A coupon will have the most metal loss in the first 30 days and typically a lower loss rate in the last 60 days of a test. This is due to 30 days being too soon for a passivating layer to develop on the coupon.
- It’s a simple test. No sophisticated instrumentation is required for results.
- Coupons are low in cost compared to alternatives.
- Allow the evaluation of numerous materials simultaneously.
- Coupons are easily adapted to evaluate specific types of corrosion. Such as crevice vs. galvanic corrosion.
- A direct measurement is obtained
- Do not handle coupons with bare hands. Finger prints, oil, and grease contact could affect results.
- Attach to the Teflon rod with a nylon screw and nut, or rod/screw provided with rack.
- Coupons should be securely held and should be electrically isolated from contact with all other metals. Insulating materials should be used to be resistant to the environment. Failure will lead to inconclusive or erroneous data.
- When installing multiple alloys starting from the direction of flow install (1st) Aluminum, (2nd) Galvanized, (3rd) Mild Steel, (4th) Brass, (5th) Copper, (6th) Copper/Nickel, (7th) Stainless Steel.
General: Uniform corrosion over the entire coupon surface.
Pitting: Depressions on the metal surface from corrosion. Pits vary in size, depth, and density. Pitting is typically caused by one of the following: low inhibitor levels, high chlorides, copper plating, pH variations, and under-deposit corrosion.
Localized Corrosion: Caused from under-deposit corrosion. Could be caused by a low flow rate through the rack or necessity for better deposit control.
Copper Plating: Soluble copper attaches to non-copper alloy samples. Can cause severe galvanic corrosion and metal failure due to pitting.
- Leaving coupons in for too long or too short of a testing time
- Removing coupons at different times between successive tests
- Water temperature variations
- Operational interference
- Differing coupon manufacturers between successive tests
- Use of a different coupon alloy than system metal
- Texture of the coupon’s surface
- Incorrect or differing lab analysis and procedures
- Corrosion coupons do not always yield corrosion rate values relative to actual pipe wall loss. They only offer an estimate, rather than true metal lost.
- As pipe surface deposits increase, the correlation between actual corrosion rate and that measured by the coupon significantly decreases.
- Coupons need flow to work. Some no flow areas such as by-pass lines, future lines, lead and lag equipment, or out of service equipment are the most susceptible to corrosion.
- Coupons are brand new samples of metal alloy. The surfaces are smooth and polished. These surfaces may differ from the surfaces of older piping systems which might be worn or pitted. This difference could create a source of error.
- The corrosion coupon rack is installed externally to the piping system, side stream.
- For an accurate representative sample of the system water, the rack should be installed on a line that is completely separate from any chemical injection points or at least up stream.
- There should be no air mixture or turbulence from flow and a typical flow rate of 3 to 5 feet per second is standard to prevent erosion of some coupon alloys from too high of a flow. Most coupon racks come with a flow meter or flow device. A low flow rate could lead to accelerated corrosion and biological fouling.
- Without flow, corrosion coupons cannot be used to measure corrosion rates. During system drain downs or winter lay-ups, corrosion rates have been documented to spike almost 10 times the full system rate.
Operating Experience – Information is generated by the actual operation of equipment. The equipment itself is used as the corrosion coupon or corrosion test. Samples of pipe are taken for analysis of corrosion.
Model Equipment – Model equipment can be installed in parallel with actual equipment or in a pilot plant. All variables must be identical to the actual plant.
Instrumental Test Methods – Electrical resistance and polarization scans are valuable, and can generate a continuous record of corrosion rate.