What’s Wrong With These Pictures?

This Is Not What A Corrosion Coupon History Of 0.4 MPY Should Look Like At A 6 Year Old Condenser Water System


The below photographs were taken of pipe removed from an approximate 6 year old open condenser water system following multiple through wall failures.

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Corrosion is the second most serious threat to any facility or building property next to fire – yet a far more likely occurrence.  With pipe corrosion losses on the rise, the reliable and accurate monitoring of any corrosion control program becomes critical to efficient and trouble free operation.

Often it is not the absence of a corrosion monitoring program which is traced back as the event allowing high corrosion activity to produce a piping failure, but the actual corrosion monitoring test method itself.  Corrosion coupons have traditionally served as the standard monitoring tool by facility managers, engineers, corrosion experts, and water treatment firms alike.  But with one obvious fault no one recognizes, or admits, until a piping failure occurs.  That is:

Corrosion coupons are nearly worthless as a measurement of actual pipe wall loss.  After nearly 25 years of making such statements, our comments have never been criticized or refuted by anyone; only agreed quietly behind doors.

In over three decades of involvement in the corrosion field, we have documented the unquestioned reliance on corrosion coupon results as the underlying cause of most major corrosion related piping failures.  In such cases, an absolute belief and blind trust in the information provided by corrosion coupon testing has allowed years of runaway corrosion activity to often destroy the piping system beyond repair.

Had low corrosion activity not been reported, at least a closer look into piping conditions been might have been pursued.  Trust in the results reported by corrosion coupons, often a hopeful belief, is typically so strong that the most obvious visual signs of a problem are ignored.  Blatantly obvious indicators pointing to a high corrosion problem such as rust deposits, thread leaks, and clogged strainers are quickly discounted by favorable corrosion coupon results – and all too often by those responsible for maintaining good corrosion control!

The gross under estimating of corrosion activity by corrosion coupons, often by at least 10 times or greater, is not an entirely unwelcome outcome given that they are the report card upon which the effectiveness of the entire chemical water treatment program and / or oversight program is judged.  An obvious conflict of interest exists in promoting the total reliance upon a testing method so clearly flawed in its under reporting of true corrosion conditions.  This is especially of issue where years of hands-on experience in the corrosion control field has likely made this fact known, or at very least should have.

In fact, most corrosion authorities are well aware that corrosion coupons do not indicate the wall loss occurring at the pipe itself – they just haven’t informed their clients of this fact!

Of the many factors preventing corrosion coupons from reporting a reasonably accurate corrosion rate measurement, its electrical isolation from the main piping system is the most significant.  With corrosion formally defined as an “electro-chemical” reaction, electrical isolation of the corrosion coupon on a plastic insulating post eliminates measurement of the major deteriorating forces within any steel piping system.  In addition to the coupon being isolated from any contact with other metal via its plastic mounting post, it is totally isolated from the main piping in a side stream loop.  Today, most corrosion coupon racks are pre-fabricated from PVC or some other form of plastic to even further isolate them from any form of electrical or galvanic activity which is, in fact, the corrosion mechanism.  Furthermore, the water is filtered of particulates and restricted down in flow – unlike the piping itself, to produce a near sterile environment.

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Frequent replacement with a new polished steel coupon every 30 to 120 days prevents any aging of the coupon surface, as well as minimizes any particulate deposition or micro biological influence common in the real world.  Of course, this information is rarely known or made available to the clients and property managers so dependant and reliant upon corrosion coupon test results, and whom are under the impression that corrosion coupon results represent the corrosion activity acting against their piping system..

New  coupon test reports every 30 to 120 days are accepted as fact and filed as testimony to an effective and well maintained corrosion control program.  Never called into question until an unknown corrosion problem produces a piping failure or otherwise becomes too great to ignore.  Often, multiple corrosion coupons racks are maintained by different independent water treatment consultants; resulting in follow-up discourse and debate as to whether the 0.25 MPY, 0.28 MPY, or 0.3 MPY corrosion rate was closer to that of the piping system when in reality, all were likely in error by 1,000 % or more.

A typical chronology of events leading to the realization that years of corrosion coupon testing have been totally wrong and completely worthless typically begins with an unexpected piping failure.  Such failure then prompts the interest to determine the cause and extent of the problem.  A review of the coupon reports show excellent results, offering the reassurance that a localized and not system wide corrosion condition is at fault.   Ultrasound, the most accurate and cost-effective method of evaluating pipe condition, is often called upon, and if performed properly, will produce highly accurate and valuable wall thickness data.

A competent and comprehensive ultrasonic investigation and analysis with then often produce wall thickness measurements in direct conflict with years of very favorable corrosion coupon results to initiate further debate.  In many examples, a through wall failure of 10 year old 6 in. schedule 40 condenser water pipe having an initial wall thickness of 0.280 in. is proof enough that the years of reported 0.3 MPY corrosion coupon rates, in which case only 0.03 in. of wall thickness would have been lost, have been, to say the very least – wrong!

Nevertheless, the chemical treatment company will side with their own coupon reports that corrosion is under control and all too often find their client very willing to believe such claims.  Where independent corrosion consultants are involved, multiple series of parallel corrosion coupon results, generated by different laboratories but typically showing the same low rates, combine as even stronger evidence against a corrosion problem existing.

Ultimately, removed sections of pipe or metallurgical investigation confirms a severe corrosion loss, and at that time a completely new definition of what a corrosion coupon means surfaces.

With irrefutable evidence to a major corrosion problem, chemical treatment representatives and consultants will finally, and only then, admit to their bewildered property owner a new understanding of the situation.  That unfortunately, building owners and engineers somehow misunderstood all along what their corrosion coupons actually measured and what their test results really meant.  No, as the explanation typically starts, corrosion coupons were never meant to indicate corrosion activity actually occurring in the HVAC cooling system, as the building owner somehow misunderstood, but only to indicate the potential corrosiveness of the treated water against a new clean steel surface.

And that’s the unfortunate reality far too many property owners have learned after its too late!  Ultimately, corroded pipe is replaced and the true extent of the corrosion problem is realized.  Interior rust deposits are typically present with large scale tuberculation revealing deep under deposit pitting underneath.  Metallurgical investigation of failed piping, if performed, will then confirm ultrasonic measurements.  In many examples, such severe deterioration has occurred, at rates of 50 MPY in some examples, that saving the system may not be possible.


After 5.5 years of operation, a 17 story New York City luxury condominium property begins experiencing pinhole leaks at the 2 in. schedule 40 threaded steel connections between main condenser water risers and isolation valves to the individual air conditioning units.  When the first failed pipe sections are replaced, heavy internal rust deposits and near through thread losses are found.  Such deposits are explained away by the chemical treatment company, and confirmed by the mechanical contractor as a localized problem; the result of a galvanic reaction between the black steel pipe and brass shut-off valve – two dissimilar metals.  The lack of any dielectric fittings is blamed as the cause.

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This speculation toward an installation deficiency is further supported by prior corrosion coupon results indicating low corrosion activity at below 0.5 mils per year (MPY).  All threaded pipe fittings are replaced with dielectric fittings and the problem is stated by the piping contractor to have been solved.

One year passes with continued low coupon results until a more serious through wall piping failure occurs at a section of larger 3 in. schedule 40 pipe having a wall thickness of 0.216 in.  The replaced section of pipe is heavily laden with tuberculation,  and with thick rust deposits at a uniform depth of over 1/2 in. against the interior pipe wall.  All removed pipe samples are heavily constricted wih iron oxide tuberculation.  No brass or copper fittings are involved to again cite a galvanic corrosion condition.  With a documented wall loss of 237 thousandths (0.237 in.) over 6.5 years, or 33 MPY, additional concerns are raised and an ultrasonic piping investigation is conducted by CorrView International, LLC.

A final ultrasonic report relating to over 65 examples of condenser water pipe representative of the building and addressing both largest diameter risers and smallest run-out piping identifies a very high system wide average corrosion rate of near 20-24 MPY, but with areas of random deep pitting to near 45 MPY.  All smaller diameter pipe of 3 in. and under is recommended for replacement, as well as the need to establish an aggressive and more effective chemical water treatment program to save the larger condenser water risers.

Our report concludes that high internal deposits now prevent any benefit from the chemical water treatment program to protect the underlying steel pipe.  All small diameter threaded pipe is recommended for replacement and prior to an effective chemical cleaning.  The isolation of the entire system from the cooling towers via plate and frame heat exchangers is also recommended.  In response, a corrosion consultant is hired by the property owners to review all known facts and issues relating to the property; the consultant reaching the conclusion that the ultrasonic report is totally in error and should be ignored.

The consultant recommends changing the chemical water treatment provider and to install a new corrosion coupon rack.  A mild TSP chemical cleaning is performed after which the building is advised that the rust issue has been solved and all internal rust deposits have been removed.  Corrosion coupon reports from the newly installed coupon rack are provided at 60 day intervals and report low corrosion activity in the 0.2 MPY to 0.4 MPY range.  The water treatment consultant reports to the building owners that corrosion activity within their condenser water system is now at the lowest rates realistically possible, and that their corrosion problem has been resolved.

Exactly one year following establishment of the new chemical program and new corrosion coupon rack installation, additional failures occur to the larger 6 in. diameter condenser water risers.  Building owners remove more heavily deteriorated and rust laden condenser water piping from throughout the building and begin a massive staged pipe replacement program using extra heavy Type K copper.

CorrView International, LLC is again contracted to perform a follow-up ultrasonic investigation whereby a higher average corrosion rate of near 26 MPY is documented at all areas tested.  As part of the investigation, ultrasonic testing is actually performed at the one year old steel corrosion coupon rack itself, and documents a 0.042 in. or 42 thousandths wall loss at the 1 in. schedule 40 steel pipe housing the steel coupon samples.

articles casehistory 12From an original wall thickness of 0.133 in. for 1 in. ASTM schedule 40 pipe, low wall thickness measurements at near 0.091 in. are recorded – now at or below minimum acceptable values and approaching the 0.072 in. thread cut limit.  Closer examination of the coupon rack shows leaks beginning at the threads, in support of the ultrasonic results.  Yet despite leaking threads and the conflict between a year long average reported corrosion coupon rate of 0.3 MPY vs. a true measured wall loss of 43 MPY at the steel pipe itself which is housing the corrosion coupons, the corrosion coupon results are presumed by building management and their consultant as accurate.

The same chemical water treatment program and corrosion coupon monitoring is continued for another year while selective pipe replacement is performed.

Additional piping failures prompt a third ultrasonic investigation by CorrView International, LLC,  whereby severe pitting identified at the largest 6 in. and 8 in. risers spells doom for the entire condenser water system.  With massive pitting throughout the building, no further options exist other than replace the entire condenser water piping system!


Such total reliance on corrosion coupon results is very common.  And in fact, most individuals relying on corrosion coupons as an indicator of true corrosion activity system  have no comprehension to their true technical role.   In some cases, the obvious conclusion that corrosion coupons are in error is not accepted; resulting in the installation of additional coupon racks after the first ones so thoroughly failed.

In fact, corrosion coupons can provide indication to whether the water is more or less corrosive over time, and to whether chemical inhibitors under use are effective.  However, they offer very little to no useful information regarding the actual corrosion loss actually occurring at the piping system itself.

Yet, as long as they remain the officially recognized report card upon the services which those recommending its use are graded, and until property owners and operators begin to realize such limitations in its corrosion reporting capability, far more similar corrosion failures will occur.