Corrosion Coupons

The corrosion of steel piping and its related components is a continuous and virtually unstoppable process.  The end product, which is commonly referred to as rust, is simply the result of an electro-chemical reaction through which the higher energy processed metal is slowly reverted back into its naturally occurring chemical form – metal ore.

Even with the proper application of available countermeasures, the estimated cost for repairing corroded piping systems in the United States alone stands well in excess of $76 billion annually – making corrosion one of the most potentially damaging losses to any commercial, private, or industrial property next to fire.  An estimated one sixth of all steel production worldwide is used to replace corroded metal – much of it at cooling and process water piping systems.

• Different Testing Methods

While various options exist, such as ultrasonic testing, metallurgical analysis, spool pieces, and instantaneous electronic measurement (LPR), corrosion coupons still remain the most widely used and respected form of corrosion measurement and monitoring.  Corrosion coupons are carefully machined small thin bars of various metals, typically mild carbon steel, which are inserted into an external “coupon rack” or zigzag layout of piping to the main circulating loop.

Some racks are made of steel pipe, and especially in areas of higher operating pressures.  More commonly today where pressure permits, prefabricated coupon racks from PVC or other plastics are installed.

• Standard Installation

The coupon itself is reweighed by the manufacturer to an accuracy of four decimal places before installation, and typically left in place for a duration of between one and six months.  The metal coupon is installed on a plastic or phenolic post according to the testing specification, which in turn electrically insulates it from the rest of the piping assembly.  Often, multiple coupons are installed into the same rack, or dual examples of steel and copper.  In some examples where a corrosion consultant is involved, multiple corrosion coupon racks may exist – each attended by a different interest.

After exposure, the coupons are removed and returned to the place of purchase, corrosion consultant, or sent to an independent laboratory for analysis.  Longer or shorter test periods may apply.

Exposed coupons are 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 (MPY) can then be estimated based upon the weight of material lost over its time in service.  This MPY result is actually a weight derived calculation relating to a loss against all 6 surfaces rather than a true wall thickness loss measurement which typically only impacts one side – the inside.

A large variety of metal alloys are available in various physical configurations, although for HVAC and process cooling applications, rectangular bars of ASTM 1010 and 1018 mild carbon steel and soft B88 copper are the primary materials used.

• Benefits Of Coupons

Corrosion coupons offer confirmation that chemical treatment is present to any building owner or plant operator.  If monitoring is continuously maintained and a history of coupon test results accumulated, fluctuations in corrosion activity, or more accurately to the potential effectiveness of one chemical program over another, may be visible.  Although limited in many respects, corrosion coupons will often provide the only indication of corrosion status, and the only suggestion of the conditions and type of deposits existing within a piping system.

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.  Where regular testing under rigorously controlled conditions exists, corrosion coupons will provide an excellent indication of whether the potential for corrosion to occur is increasing or decreasing.

Corrosion coupons will quickly document if a chemical inhibitor is present by an absence of significant wall loss, or similarly show whether the recommended inhibitor is effective for providing protection to a particular metal.  Another great benefit is to provide short term corrosion rate data, such as might be required during a harsh acid cleaning or new chemical program evaluation.  Due to a wide variety of reasons, however, corrosion coupons generally fail to produce corrosion rate values relative to actual pipe wall loss.  At best, and according to their true definition, they offer an estimate of the corrosivity of the fluid, rather than a measurement of the true metal lost from the pipe.

The most severe corrosion related piping failures we have investigated, such as the tenant based condenser water piping system at left, corrosion had been allowed to continue due to the very favorable corrosion coupon rates reported by both the chemical treatment provider as well as the corrosion consultant hired to oversee their program and inhibitor levels.

In many cases, again most notably illustrated by the photograph at left, even the most obvious indications of a corrosion problem such as rust deposits, clogged strainers and condenser tubes, and even leaks, had been ignored or discounted by the building operators, chemical treatment provider and/or consultant due to favorable corrosion coupon testings showing no problems whatsoever.

For many properties, corrosion coupons are relied upon because they still remain the industry standard, and are inaccurately promoted as a reliable means to define the wall loss occurring within the system where installed.  In addition, they require less effort than any other diagnostic method and allow the claim of being diligent and proactive to corrosion issues.

The most basic scientific evaluation, however, clearly defines otherwise.

• Sources Of Their Inaccuracy

Corrosion coupons commonly under report corrosion activity by 10 times or more.  In many examples we have documented an under reporting inaccuracy of 2,000% and greater.  Some of the many factors limiting the accuracy of corrosion coupons are:

• • • • Coupon rack placement

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 and velocity can dramatically influence corrosion estimates by as much as five to ten fold.  In addition, materials of construction, rack layout, pipe size, or filtering of the coupon rack assembly can significantly alter corrosion rate predictions.

Test layouts constructed of PVC will greatly eliminate any possible electrical or galvanic activity with the rest of the system – “elecro-chemical,” of course, being the fundamental definition of corrosion itself.  Even the physical location of the coupon rack at the top or bottom of the system can produce significant differences in measured corrosion rate.

With no water flow available, corrosion coupons cannot be used to measure the always 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.  The exact same 12 in. piping system, having a wall thickness of 0.335 in. where filled, can actually show only 0.125 in. at the roof where drained over the past 25 years.  Given the need for water flow through the coupon rack, this potential for failure will always remain hidden.

Frequently, a high corrosion coupon result will be explained away or reduced by installing filtration to the coupon rack, and/or by altering water flow.  Although an ASTM standard exists to define their construction as well as operation, various manipulations are common should results be higher than desired.

• • • • System dependent

What degree of corrosion activity may exist is greatly dependent upon the type of piping system involved.  Closed systems typically show the lowest corrosion and pitting activity, while open condenser or cooling tower loops show the highest.

An open circulating system also typically exhibits the greatest fluctuation in test result – which means that wall thickness is more likely to vary from top to bottom, at large and small pipe, at supply and return, and at other extremes of the system.  This always present variability for all HVAC piping systems increases the risk that any corrosion coupon testing performed at one area is not representative of the overall system.

• • • • No galvanic influence

By definition, corrosion is an electro-chemical reaction in an aqueous solution.  But 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 electro-chemical reactions always present in an established piping system.

The well recognized steel pipe to brass valve or copper pipe effect is a common example of galvanic forces which always exist to some degree in most piping systems.  Galvanized steel to brass produces likely the most severe galvanic form of deterioration.  Lesser galvanic forces exist where different steels meet as well, and in fact, micro-volt differences have been measured between new and older steel pipe.  As a result, the major corrosion mechanism responsible for a significant amount of material loss is never measured by the corrosion coupon.

• • • • No flow conditions

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

Cooling tower by-pass lines, closed at the downstream end and open at the supply side, are notorious for providing a settlement area for deposits, and then very severe pitting underneath.  With no flow available, corrosion coupon testing is, by definition, impossible in any such area – leaving the most vulnerable areas of the entire piping system unaddressed.  Corrosion coupon reports are commonly reviewed and interpreted without any recognition to the many maximum corrosion locations for which the coupon cannot possibly assess.

• • • • Ignoring the main problem

Failures at most condenser water systems are due to accelerated underdeposit corrosion and the deep pitting caused by heavy deposit settlement at oversized headers, crossovers, by-pass lines and futures – not the lower generalized corrosion more likely at a high velocity pump discharge line.  Yet, the only corrosion activity even barely possible to estimate by corrosion coupons is the less threatening general corrosion condition rarely encountered.

Below left we show the inside of a 12 in. condenser water supply main from the cooling tower after a failure forced its replacement.  Pounds of accumulated heavy iron oxide rust flakes per linear foot of pipe run have completely eliminated any benefit whatsoever from the chemical water treatment program.

To the right, a steel corrosion coupon installed into an electrically isolated side stream loop made of PVC, saw none of that corrosive influence.  While all those involved with the maintenance and management of this building’s condenser water system accepted incredibly low 0.3 MPY corrosion rates for years as fact and without hesitation, the reality of what was really going on inside the pipe, confirmed by a piping failure, soon introduced a new reality.

• • • • Surface differences

The typically mirror smooth polished surface of a corrosion coupon minimizes the adhesion of iron oxide, dirt and microorganisms.  As a result, the metal surface of a corrosion coupon is rarely attacked in the same manner as an aged piping system having an irregularly worn and pitted interior surface.  For an older piping system typically worn and pitted, new corrosion coupons bear no resemblance to the pipe surface – thereby further amplifying reporting error.

• • • • Short 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 of 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 scenarios are well recognized and accepted as factors in the under reporting or over reporting of corrosion activity using corrosion coupons as a test method, yet are all too often used to explain away a high or elevated test result.  Low corrosion rate results even beyond the realm of possibility are rarely, if ever, questioned.

• • • • No surface deposits

By far, the accumulation of interior deposits, or lack thereof, is the greatest limitation to accurate corrosion coupon testing.  Once a solid layer of iron oxide or scale deposits adhere to the pipe’s interior, an entirely new set of corrosion mechanisms typically develop 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.  That very significant limitation, however, is rarely conveyed to the property owner, manager, or engineer.

Mild deposits will, depending upon their thickness, impede contact of the water treatment chemicals to the base metal, and therefore reduce their effectiveness to some degree.  Although most chemical treatment providers will argue that their corrosion control formulations are capable of penetrating through heavy rust deposits to protect the base steel, the results of hundreds of investigations we have performed prove such claims wrong.

Heavy deposit buildup will isolate the pipe from any chemical protection whatsoever.  This often results in random areas of pipe having deep wall loss – often at the lowest areas of the system and at long horizontal runs.  Smallest diameter piping is especially vulnerable.

A 5 MPY corrosion rate against 12 in. schedule 40 steel pipe, for example, will actually remove 64 lbs. of metal into the circulating system for every 100 linear feet of pipe, and for every year of service!  Oxidized, this same steel reverts back into approximately 2.6 cu. ft. of iron oxide that will settle into the system if not filtered out.  And while an open cooling tower system may flush some of those deposits away or at least signal a corrosion problem, closed systems hide their problems.

• • • • Secondary corrosion problems

Accumulated internal deposits often create a localized and severe secondary metal loss known as “concentration cell” or “oxygen cell” corrosion, and may create conditions favorable to micro biologically influenced corrosion or MIC.  Corrosion coupons, however, are never left in service long enough to develop any of those secondary impacts.

• • • • Interrupted monitoring

Should corrosion coupons remain in place for sufficient time to deteriorate toward the surface texture and condition of the actual pipe and accumulate deposits, 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 very beginning.  Corrosion coupons left in service beyond 180 days or installed and forgotten for an extended, are typically discounted as having been exposed beyond their recommended time limit; their now more heavily deteriorated condition misinterpreted as an error in the testing process rather than a more accurate corrosion representation.

This unfortunately negates monitoring one of the most important contributors in all examples of high corrosion loss – the pitted and irregular interior pipe wall surface.

• • • • Further coupon error

• • • • • • Too long or too short of a test interval
          • 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
          • Surface texture of the corrosion coupon
          • Fraud and tampering of the testing process or of the coupon itself
          • Differences in lab analysis procedures, coupon handling, and preparation

• • • • False security

In the majority of ultrasonic investigations CorrView International, LLC has been 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.  In fact, wall losses may actually be 5 MPY and significantly above.  Reported open system corrosion rates of under 0.2 MPY are not uncommon for corrosion coupon testing, yet are 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 information.  Wishful thinking perhaps, and an often mistaken and regretted decision after true corrosion losses have been confirmed.  The sudden appearance of a leak, rust deposits, tower basin chip scale, or other operating problem ultimately signals a corrosion condition hidden over an extended time, and further investigation begins.  Unfortunately, the true problem is usually discovered only after years of concealed and under reported piping damage.

• Significant Limitations To Keep In Mind

Overall, corrosion coupons offer some diagnostic information, though with very clear limitations.  In most examples, the information they provide is virtually worthless.  A comparison of results to other more direct and more reliable testing methods, therefore, is always recommended.

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

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