CM-07:   So What’s Wrong With This Picture?

Actually, the above picture is not wrong this time!

Forty years of involvement in the business of investigating, monitoring, and reducing corrosion related losses to commercial building properties has resulted in certain well documented and irrefutable findings.

Our ultrasonic inspection of 1,129 building properties in 252 cities across the United States, addressing 2,961 piping systems, 63,796 individual sections of pipe, and the taking of 4,919,955 wall thickness measurements, substantiates our experience and authority in this field.

• Increasing Losses

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

Yet in most investigations, we find that 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. Rather, it is the corrosion monitoring method itself.

Corrosion coupons have traditionally served as the standard corrosion monitoring tool of 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:

They Are Nearly Worthless

After nearly three decades of involvement in the corrosion monitoring 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 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 heavy rust deposits, in tower pans or condenser heads, thread leaks, and clogged strainers are quickly discounted by favorable corrosion coupon results. All too often, in fact, by those same individuals responsible for maintaining good corrosion control!

• An Obvious Conflict

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 possibly an oversight program is judged.

Quite obviously, a conflict of interest exists in promoting a 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 that fact!

• Few Testing Options

Unfortunately, few other options exist. LPR is excellent, and is at least installed within the actual pipe. Yet it will still only address one, two, or three piping locations at most. Once covered with the rust and other deposits, which cause the greatest piping damage, they lose their accuracy and require cleaning and re-calibration.

Spool pieces are excellent if monitored correctly, and likely represent the most accurate assessment possible, although they cannot be established in the most vulnerable locations.

Ultrasound, properly performed and interpreted, provides extremely accurate wall thickness measurement across a large number of locations to provide the best fitness for service evaluation available. However, it is rarely utilized prior to a failure taking place.

• No Relevance

  

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 within an aqueous solution,” 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 in a side stream loop, the rack itself is typically fabricated of PVC, the water is filtered of particulates, and restricted down in flow. It is a nearly ideal environment, unlike the piping system which it is intended to simulate.

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 rust deposition or microbiological influence – all real life corrosion contributors.

Of course, this information is rarely known or made available to the property managers and engineers so dependant upon corrosion coupon test results; existing under the impression that corrosion coupon rates represent the corrosion activity actually impacting their pipe.

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, and are never called into question until an unknown corrosion problem produces a piping failure or otherwise becomes too great to ignore.

In some cases, 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 more accurate.

In reality, however, all were likely in error by 500 % or more.

• Failure Alters Perception

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 a highly accurate and valuable piping assessment

A competent and comprehensive ultrasonic investigation and analysis with then often produce wall thickness measurements and corrosion rate estimates 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 water treatment provider 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 run corrosion coupons, generated by different laboratories but typically showing the same low rates, combine as even stronger evidence against a corrosion problem existing.

• Reality Confirmed

Ultimately, removed sections of pipe or the metallurgical investigation of a failed pipe section will confirm both a severe corrosion loss as well as the ultrasonic results. And it is 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 that they have unfortunately and somehow misunderstood all along what coupons actually measured.

No, as the explanation typically starts, corrosion coupons were never meant to indicate corrosion activity actually occurring within the HVAC cooling system, but to only indicate the potential corrosiveness of the chemically treated water against a new steel surface. Ultimately, the corroded pipe is replaced, whereupon the true extent of the corrosion problem is realized.

And that’s the unfortunate reality far too many property owners have learned after its too late!

• A Case History Of Failure

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 take-off connections between the 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 thread failures and rust deposits are explained away by the chemical treatment provider, and confirmed by the mechanical contractor as the result of a very well known galvanic reaction between the black steel pipe and brass valve – two dissimilar metals. Both advisors indicate that the failures are caused by the absence of dielectric insulators.

This speculation toward an installation deficiency is supported by prior corrosion coupon results indicating low corrosion activity at below 0.5 mils per year (MPY). All threaded steel pipe connecting to a brass valve or copper pipe are replaced with dielectric fittings.

One year passes with continued excellent corrosion coupon results until a more serious through wall piping failure occurs at a section of larger 3 in. schedule 40 grooved clamped pipe having a wall thickness of 0.216 in. The replaced section of pipe is heavily laden with tuberculation, and exhibits thick rust deposits at a uniform depth of about 1/2 in. against its interior wall.

With a documented wall loss of 237 thousandths (0.237 in.) over 6.5 years, or 33 MPY, additional concerns are raised by the building’s chief engineer, 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 8 in. risers and smallest 2 in. 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 almost 45 MPY.

All smaller diameter pipe of 3 in. and smaller is recommended for replacement. In addition, a review of the chemical treatment program is recommended as well as a chemical cleaning once the widespread weakness at the smaller diameter pipe has been eliminated.

Explained is that high internal deposits now prevent any benefit from the chemical water treatment program to protect the underlying steel pipe. Effective chemical cleaning and the isolation of the entire system from the open cooling towers via the installation of plate and frame heat exchangers are recommended as the only possible solution.

An independent corrosion consultant is hired by the property owners to review all known facts and issues relating to the problems experienced. He quickly reaches the conclusion that the CorrView ultrasonic report is in error and should be ignored. The consultant further recommends changing the chemical water treatment provider and to install a new corrosion coupon rack.

The consultant performs a mild chemical cleaning of the condenser water system, after which the building is advised that 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 0.3 MPY range. With corrosion coupon reports in hand, the water treatment consultant reports to the building owners and managers that corrosion activity at their condenser water system has now been reduced to the lowest rates realistically possible. Corrosion problem solved!

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

CorrView is again contracted by building management to perform a follow-up ultrasonic investigation whereby a higher average corrosion rate of near 26 MPY is documented throughout all areas tested.

As part of the investigation, and in order to prove the inaccuracy of corrosion coupons to all involved, ultrasonic testing is performed at the one year old steel corrosion coupon rack itself. Results document a 0.042 in. or 42 thousandths wall loss at the 1 in. schedule 40 steel pipe housing the steel corrosion coupon samples.

From an original wall thickness of 0.133 in. for 1 in. schedule 40 pipe, low wall thickness measurements at near 0.091 in. are measured – now at or below minimum acceptable limits and approaching the 0.072 in. thread cut itself.

Closer examination of the coupon rack identifies 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 housing, the corrosion coupon results are argued to be accurate by the corrosion consultant.

CorrView again recommend that the only possible solution to their problem is to close the system from the open cooling tower by installing plate and frame heat exchangers, effective filtration, and significantly higher dual inhibitors.

The consultant disagrees entirely, and 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, 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, no further options exist other than to replace the entire condenser system!

• Lessons Not Learned

Such a total reliance on corrosion coupon results is very common. And in fact, most individuals relying on corrosion coupons as an indicator of corrosion activity within their piping system have no idea to their true technical role.

In most cases, the obvious conclusion that corrosion coupons are in error is not believed simply based upon their long history of use and acceptance. Such trust then results in the installation of additional coupon racks after the first one so thoroughly failed.

Where buildings have had their piping system destroyed by inaccurate corrosion coupons, they have corrected the issue by installing two or more corrosion coupon racks with their new pipe.

In fact, corrosion coupons can provide indication to whether the water is more or less corrosive over time, and to whether the chemical inhibitor employed is effective. However, they offer very little to no useful information regarding the actual corrosion loss occurring at the piping system itself.
Yet, as long as they remain the report card upon the services which those recommending its use are graded, and until property owners and operators fail to realize such limitations in its corrosion reporting capability, far more similar corrosion failures will occur.

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

© Copyright 2005 – 2025 – William P. Duncan, CorrView International, LLC