Corrosion Coupons: Why Relying on One Test Method Isn't Enough
Although they're simple and inexpensive, corrosion coupons often produce misleading results about corrosion rates.
An accurate assessment of corrosion rates in any surroundings is important in terms of the price of upkeep and repair as a result of material failure. Corrosion coupons provide an inexpensive way to monitor and measure the corrosivity inside a system. By watching the mils-per-year corrosion rate of an exposed coupon, valuable data may be provided relating to the material's life span.
Corrosion coupons have historically served this purpose well, and have been the typical monitoring tool for facility managers, engineers, corrosion consultants and water treatment companies. However, although this type of monitoring is simple and straightforward, mistakes will be created that may render poor quality (or even worthless) coupon corrosion information. When it comes to corrosion coupons, attention to detail can't be overemphasized, particularly when those coupons represent actual components/structures.
Furthermore, the reliance on corrosion coupons as the sole testing technique is just plain dangerous. In some cases, an absolute belief and trust in the information provided by corrosion coupon testing has allowed years of runaway corrosion activity to destroy materials or structures beyond the scope of maintenance—and at a huge financial cost. (For background reading on other techniques, see Corrosion Assessment: 8 Corrosion Tests That Help Engineers Mitigate Corrosion.)
So, when does a corrosion coupon work? When does it fail? And how can you understand the difference? Let's find out. But first, a quick review of corrosion coupons.
What Is a Corrosion Coupon?
A corrosion coupon is a piece of metal of a preset form, size and area. It is a metal of comparable chemical composition to the method instrumentation and has a consistent surface finish as well as a characteristic serial number. It is exposed to a corrosive medium that's similar to the structures that it's designed to represent, and therefore gives a visual indication of the kind of corrosion that may occur, and the rate at which it's occurring. As such, corrosion coupons are an easy and, in many cases, effective tool for providing a quantitative estimation of corrosion rates occurring in an operating system. As a testing tool, corrosion coupons greatly facilitate precise corrosion rate watching in many important surroundings. As a result, they're used to make projections about:
- Repair prices
- Measurement of material failure
How Do They Work?
Corrosion coupons are rigorously machined bars of varied metals, generally mild steel, that are inserted into an external "coupon rack" or zigzag layout of piping in a special rack. Some racks are made of steel pipe, particularly in areas of high operative pressure. Nowadays, usually wherever pressure permits, pre-fabricated coupon racks made from PVC or plastic are installed.
Each corrosion coupon is pre-weighted by the manufacturer to an accuracy of four decimal places before installation, and generally left in place for a period of between one and three months. It’s put in on a plastic or phenolic resin post that electrically insulates it from the remainder of the installation. Usually, multiple coupons are put into the rack.
After exposure, the coupons are removed and sent back to the place of purchase, to a corrosion authority or to an independent laboratory for analysis. Longer or shorter check periods may apply. Exposed coupons are generally photographed as received, cleansed of any debris and deposits, visually inspected, dried and re-weighted, and then photographed once more to indicate surface status. The coupon’s corrosion rate, in mils per year (MPY), will then be calculated primarily based on the weight of material lost over its time in service. This is done using a weight-derived calculation concerning all six surfaces instead of a real wall thickness loss measure. 1.0 mil/year is represented as 1/1000th of an inch of metal depletion per year.
In this way, corrosion coupons provide an estimate of the corrosivity of the water or liquid, and how it will affect a brand-new metal surface. What they can't do is indicate corrosion activity occurring within a pipe or another enclosed system.
Interpreting Corrosion Coupons
The appearance of the cleansed coupons can provide vital information about corrosion rates. The following are descriptions of various kinds of attacks for which corrosion coupons may provide clues.
- Generalized attack
Uniform corrosion over the entire surface of the coupon. This type of corrosion often isn't a priority unless the MPY is high.
- Pitting attack
Pitting attack is a general term given to any depression on the metal surface caused by corrosion. Pits will vary significantly in size and depth, as well as density. Up to 10 pits per side is typically called isolated corrosion. Low substance levels, high chlorides, pH scale excursions, under-deposit attacks or copper plating will all cause this type of corrosion.
- Localized attack
A localized attack in a specific area should indicate the necessity for higher deposit management and/or low flow rates through the coupon rack. If the depression shows homocentric rings with the deepest penetration within the center, it could indicate a corrosive bacterium attack, which may be caused by microbiologically induced corrosion (MIC).
- Copper plating
Occurs as a result of the deposition of soluble copper on low-carbon steel or alternative non-copper alloys. Copper plating will cause severe galvanic corrosion and metal failure.
- Edge attack
Because the sides of coupons are extremely stressed throughout fabrication, they have an inclination to be detonative sites for corrosion. However, edge attack doesn't typically indicate a serious failure unless it's severe.
- Attack beneath the coupon holder
If metal loss is localized to the space beneath the coupon holder and the remainder of the coupon surface isn't attacked, this could simply represent the influence of the coupon holder to stimulate under-deposit or crevice attacks, and not mirror the characteristics of the recirculating water. Ensuring that the coupon holder and bolts are squared and fixed tightly helps minimize these effects.
The Benefits of Coupons
If coupons are monitored over time and the results are accumulated, fluctuations in corrosion activity may be detected. Although coupons have their limits, they usually give the sole indication of how much corrosion is occurring, as well as an idea of the kinds of deposits that may exist within a piping system.
Corrosion coupons become a good additional prognostic maintenance tool once results are compared to confirmed wall loss information, like that obtained via ultrasonic thickness testing, spool piece measure, or actual pipe removal and metallurgical analysis. Wherever regular testing under strictly controlled conditions exists, corrosion coupons can give a superb indication of whether the potential for corrosion to occur is increasing or decreasing.
Corrosion coupons can quickly document whether a corrosive chemical substance exists or determine whether a particular substance is effective at protecting a selected metal by an absence of great wall loss, or equally show whether or not the suggested substance is effective for providing protection to a selected metal. Another nice benefit is that they produce short-term corrosion rate data, which can be helpful during harsh chemical cleansing or chemical program analysis. (Learn more about the benefits in How Corrosion Coupon Holders Can Preserve Your Valuable Assets.)
The Limitations of Corrosion Coupons
Corrosion coupons are a superb source of data for any building owner or plant operator, but they don't give an accurate or realistic estimate of the particular corrosion activity happening at the surface of the pipe.
As the corrosion rate of the piping system itself increases, it's common to examine a larger distinction between coupons' reportable losses and actual pipe wall loss. The presence of even a moderate quantity of interior deposits can nearly eliminate any connection between a coupon's corrosion rate and the wall loss that is truly occurring on a pipe's surface. Besides, basing a whole chemical treatment program on the results of corrosion coupons taken at just one point within the system is risky in itself, given the low chance that any one check area is representative of the whole piping system.
Corrosion coupons are electrically isolated, exist under completely different flow conditions, and infrequently experience erosion as a result of particulates. Their mirror-sleek surface limits the attachment of rust and small biological growths, and their typically short test interval of 45–90 days prevents the buildup of deposits common to older piping systems—a serious reason for high corrosion loss. As a result, the entirely different operative conditions between the coupon rack and actual pipe means that coupons may under-report actual corrosion rates by five to 10 times.
The Key Factors that Contribute to Coupon Failure
Many factors affect the accuracy of corrosion coupons and result in their failure. Below are the key factors that can contribute to this failure.
- Coupon rack location
The coupon rack itself, put in outwardly to the piping system, limits several of the influences acting against any current water system. Variations in water flow, construction materials, rack layout, pipe size or filtering of the coupon rack assembly will considerably alter corrosion rate estimates.
- System reliance
The variety of piping affects corrosion activity. Whereas closed systems show low corrosion and pitting activity, open systems show higher activity. As a result of variable wall thickness, any corrosion coupon testing performed at one space isn't representative of the system as a whole.
- Galvanic influence
Because corrosion coupons are generally isolated from any metal-to-metal contact through the use of galvanic insulators, metal losses caused by galvanic influence isn't measured.
- Lack of flow conditions
With no flow obtainable, corrosion coupon testing fails to assess the inside of a piping system, leaving the most vulnerable areas of the complete piping system unaddressed.
- Surface alterations
Mirror-sleek, polished surfaces are seldom attacked in the same manner as an aged piping system with a worn and pitted surface.
- Testing interludes
The most common check interval for corrosion coupons is between 30 and 90 days. In many cases, 30 days is simply too short to assess most corrosion, whereas 90 days may be too long.
- Surface deposits
The buildup of interior deposits or lack thereof is the greatest limitation to correct corrosion coupon testing. Because pipe surface deposits increase the correlation between the specific corrosion rate, the presence of those deposits can render information gleaned from coupon corrosion very inaccurate.
- Ancillary corrosion problems
Accumulated internal deposits usually produce a localized and severe secondary metal loss called concentration cell or oxygen cell corrosion, and should make conditions auspicious to small, biologically influenced corrosion.
- Interrupted watching
Coupons are generally replaced with new check coupons and, therefore, the entire testing method starts again from the beginning.
As a result of the reasons mentioned above, coupons typically fail to provide corrosion rate values relative to actual pipe wall loss. At best, they deliver an estimate of the corrosivity of the fluid, instead of a measure of actual metal lost from the pipe.
Overall, corrosion coupons provide some diagnostic info, although with very clear limitations. It is evident that the data they supply is just about worthless. A comparison results in additional direct and reliable testing strategies like ultrasonic testing to ensure the utmost safety and to prevent failure.
Written by Alan Kehr | Managing Consultant, Alan Kehr Anti-Corrosion, LLC
Alan Kehr has more than 40 years’ experience in the pipeline and reinforcing steel coatings industries, specializing in research and development of coatings, marketing, and technical service. Starting his career in the lab and field at 3M for several decades, Alan has since become world-recognized expert in fusion-bonded epoxy (FBE) and epoxy-coated rebar, now holding three patents for innovative FBE coating chemistries.