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Misleading Information

Corrosion coupons are the most widely used form of corrosion measurement and monitoring today.  They are precisely machined thin bars of various metals which are inserted into an external “rack” or layout of 1 in. piping to the main circulating loop.  A large variety of metal alloys are available in various physical configurations, although for HVAC and process cooling applications, rectangular bars of mild carbon steel and soft copper are the primary materials used.

In most cases, the pressure differential across the suction and discharge of the circulating pump is sufficient to flow a small side stream of water through the external coupon rack.  This flow rate is regulated based upon various procedural guidelines in order to ensure uniform conditions between tests.  At a few feet per second, however, it never approaches actual flow conditions within the piping system.  Where no flow exists, such as at dead legs or by-pass crossovers, corrosion coupon testing is generally not pursued.

Typically, the external rack is configured to allow the simultaneous testing of two or four corrosion coupons of similar or different metals as the water navigates the standard zig zag piping configuration.  Coupon racks are constructed of either steel or PVC – a choice often dependent upon operating pressure and convenience, and may exist immediately near or far from to the actual piping system.

Standard Rack Configuration

• How Coupons Work

Each corrosion coupon is pre-weighed by the manufacturer to an accuracy of four decimal places – 0.001g.  They are usually left in place for a duration of between one month to three months, and are then removed and sent back to their source for analysis.  Longer or shorter test intervals may apply.  Coupons are given a serial number relating back to the supplier.

Each coupon is typically photographed as received, cleaned of any attached debris and deposits, visually inspected, dried and re-weighed, and then photographed again to show surface conditions.  The corrosion rate of the coupon in mils per year can then be estimated based upon the weight of material lost over its time in service.

Weight loss against its 6 generally unequal sides is then translated into a corrosion rate acting against one side of the piping surface – an arguably questionable relationship.

The information provided by corrosion coupons is not what most individuals relying upon their information presume.  Corrosion coupons provide an estimate to the corrosivity of the water or liquid itself against a new metal surface.  They do not, however, indicate corrosion activity occurring at the pipe wall – a differentiation often first raised only after a major corrosion problem is realized.

• Comparison Improves Reliability

Corrosion coupons offer some information to any building property or plant operator – especially if monitoring is continuously maintained and a history of coupon test results accumulated.  Though limited in many respects, they will often provide the only inside look at the conditions and type of deposits existing within a piping system, and the only indication of corrosion status.  Corrosion coupons become an even more valuable predictive maintenance tool when results are compared to confirmed wall loss information such as provided through ultrasonic thickness testing, spool piece measurement, or actual pipe removal and metallurgical analysis.

Due to a wide variety of reasons, however, corrosion coupons generally fail to produce corrosion rate values relative to actual pipe wall loss and typically show a wide difference in corrosion rate estimates except where corrosion activity is inherently low.  At best, they offer an estimate of the corrosivity of the fluid, rather than a true measurement of the metal lost from the pipe itself.  Where regular testing under rigorously controlled conditions exist, corrosion coupons will provide an excellent indication of whether the potential for corrosion to occur is increasing or decreasing.

• Very Clear Limitations

Corrosion coupons can document if a chemical inhibitor is present and effective by an absence of wall loss, or similarly show whether the recommended inhibitor is effective for providing protection to a particular metal.  However, they rarely provide real world corrosion data regarding the actual wall loss at the pipe itself due a wide variety of reasons.  Under deposit corrosion, probably the greatest threat to any piping system, is never represented by the wall loss at a new corrosion coupon surface, nor ever present galvanic influences since the coupon is electrically insulated from the piping system.

Our own comparative product testing has shown strong uniformity of wall loss between actual pipe and corrosion coupons when equally exposed under a salt spray fog.  The wide variance commonly found in comparing corrosion coupon results to actual pipe loss we attribute to the following issues:

• Different Conditions

The corrosion coupon rack itself, installed externally to the piping system, limits many of the influences acting against any circulating water system.  Variations in water flow can dramatically influence corrosion estimates by as much as five to ten fold, as can materials of construction, rack layout, pipe size, or filtering of the coupon rack assembly.

Racks constructed of PVC will greatly eliminate any possible galvanic activity.  Even the physical location of the coupon rack itself can produce differences in measured corrosion rate between extremes of the system.

• Draining Down

Corrosion coupons cannot be used to measure the significantly higher corrosion activity occurring during a winter lay-up or periodic drain down – documented in many cases to reach ten times that of water filled pipe since, by definition, no flow exists.

While high concentrations of water treatment chemicals prior to drain down are suggested as one method to coat the pipe and provide protection, little actual benefit has ever been shown.

• Galvanic Influence

Since corrosion coupons are typically isolated from any metal to metal contact through the use of a center located plastic or galvanic insulator, they are totally unaffected by the many anode/cathode electrochemical reactions always present in an established piping system.  The well recognized steel pipe to brass valve or copper pipe effect is an example of galvanic forces which always exist to some degree.

As a result, a major corrosion mechanism responsible for a significant amount of material loss in most piping systems is never measured.

• No Flow Conditions

Some of the highest corrosion and pitting conditions are found at areas of no flow.  This is common at by-pass lines, future lines, lead and lag equipment, as well as at the very end of some small diameter piping distribution systems.

With no flow available, corrosion coupon testing is therefore impossible – leaving the most vulnerable areas of the entire piping system unaddressed.

• Surface Differences

Typically having a mirror smooth polished surface, which minimizes the adhesion of iron oxide, dirt and microorganisms, corrosion coupons are rarely attacked in the same manner as an aged piping system having an irregularly worn and pitted surface.

For an older piping system typically worn and pitted, the corrosion coupons bear no resemblance to the pipe surface – thereby further amplifying reporting error.  In those examples where a high corrosion or pitting condition is found after a 30 or 60 day coupon exposure, a new coupon is put in its place rather than the same worn coupon returned.

Different corrosion coupon manufacturers produce test specimens having significantly different surface texture.

• Testing Interval

The most common test interval for corrosion coupons is between 30 and 90 days.  In reality, 30 days is too soon for the coupon to develop a passivating layer of rust protection and can actually lead to the reporting of falsely high corrosion rates.

On the opposite end, 90 days is far too short a time period necessary for the smooth surface of the coupon to accumulate any microbiological or deposit buildup typically existing in an actual piping system.

Both are well recognized reasons for the under reporting or over reporting of corrosion activity using corrosion coupons as a test method, and are all too often used to explain away a questionable or high test result.

• Surface Deposits

Once a solid layer of iron oxide or scale deposits form on the pipe’s interior, an entirely new set of corrosion mechanisms typically form which simply cannot be duplicated, nor measured by any remotely located corrosion coupon.  For that reason, most authorities recognize that as pipe surface deposits increase, the correlation between the actual corrosion rate and the corrosion coupon measured rate significantly decreases.

Mild deposits may, depending upon their thickness, impede the interaction between any water treatment chemicals and the base metal, and therefore reduce their effectiveness to some degree.  Heavy deposit buildup, however, will likely isolate the pipe from any chemical protection whatsoever.

• Secondary Corrosion

Accumulated surface deposits often create a localized and severe secondary metal loss known as “concentration cell” or “oxygen cell” corrosion, and may create conditions favorable to microbiologically influenced corrosion, or MIC.

While the metal lost in mils per year may be acceptable, often overlooked is the consequence of high volumes of iron oxide settling into the piping system.

For any circulating system, therefore, the removal of all interior pipe surface deposits should be a priority.  It is our opinion (as well as of others) that it is virtually impossible to provide adequate corrosion protection to any piping system already heavily fouled with iron oxide deposits, and that the preliminary and total removal of such deposits is mandatory to reducing high corrosion and pitting rates.

• MPY From Weight Loss

Extrapolating a thickness loss in mils per year (MPY) based upon the weight loss of a six sided metal coupon adds even further error to the entire process.  The calculation of a corrosion rate based upon the weight loss of the coupon does not directly relate to the loss of metal from the single side of the interior pipe wall.

• Interrupted Monitoring

Should corrosion coupons remain in place for a sufficient time to accumulate the surface texture and condition of the actual pipe, they are rarely re-weighed and returned in that worn and pitted condition.  Instead, they are typically replaced with new test coupons and the entire testing process started over from the beginning.

• Different Metal

The mild steel of a typical ASTM 1018 corrosion coupon may have quite a different chemical composition than most ASTM specified pipe steels – and can vary between highly corrosion resistant A 72 wrought iron, and easily corroded A 795 foreign pipe.

Dozens of chemically different mild steel alloys are offered by corrosion coupon manufacturers to simulate the losses of the ASTM A 53 pipe most commonly used in HVAC and process applications.  However, testing to select the most appropriate alloy is rarely performed.

Copper pipe corrosion losses present far less of a concern.

The above listing highlights the most significant limitations in attempting to measure piping losses via standard corrosion coupons.  Of those, the actual surface conditions at the interior pipe wall itself often provide the most serious obstacle to an accurate corrosion rate measurement.

• More Coupon Error

Some additional sources of corrosion coupon error include:

• • • Too long or too short of a test intervals
    • Varying time intervals between successive tests
    • Seasonal or water temperature variations
    • Actions of the operating engineer
    • Different corrosion coupon manufacturers
    • The use of different corrosion coupon alloys
    • Tampering of the testing process or of the coupon itself
    • Differences in lab analysis procedures, coupon handling, and preparation

In a majority of ultrasonic investigations we are involved, a property owner or plant operator will, for years, mistakenly believe they have a corrosion rate of well under 1 MPY based entirely upon corrosion coupon results, when in fact wall losses may actually be 5 MPY and above.  Reported corrosion rates in the hundredths of a mil per year are not uncommon for corrosion coupon results, though not even remotely feasible.

Often, when presenting conflicting MPY statistics between corrosion coupons and ultrasonic testing, building or plant owners and operators will choose to rely on the less reliable coupon based result.  The sudden appearance of iron oxide deposits, chip scale, a leak, or an operating problem ultimately signals a problem hidden over an extended time, in such cases, and further investigation begins.  Unfortunately, this is usually discovered only after years of concealed and under reported piping damage.

• Different Services Same Procedures And Results

Most larger chemical water treatment contractors utilize corrosion coupon testing as a performance benchmark of their corrosion protection program, and clearly benefit from this under reporting.  While many facility and plant operators also contract independent corrosion monitoring consultants as a check of their chemical treatment program, or submit their own samples to independent laboratories, such services inevitably rely upon the same error prone technique of using corrosion coupons to establish piping system and corrosion rate status.

CorrView ® corrosion monitors generally presents a more realistic, albeit higher corrosion rate assessment.  This is due to its being located within the piping system itself and subject to all the same environmental forces – rather than existing in an isolated rack assembly.  In general, CorrView is a long term monitoring device not intended to take the place of corrosion coupons, but to provide one additional tool for the property owner and plant engineer in safeguarding reliable and trouble free system operation.

You can view and download our two page handout on this subject below.

© Copyright 2024 – William P. Duncan, CorrView International, LLC