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Corrosion coupons
are the most widely used form of corrosion measurement and monitoring today.
They are carefully machined small 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, 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 remote to the actual piping system.
Each corrosion
coupon is pre-weighed by the manufacturer to an accuracy of four decimal
places. They are usually left in place for a duration of between one month to
one year, and are then removed and sent to an independent laboratory for
analysis. Longer or shorter test intervals may apply. The 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 can then be
estimated based upon the weight of material lost over its time in
service.
By providing a corrosion estimate
in mils per year (MPY), corrosion coupons offer invaluable information
regarding the interior conditions of any piping or other metal containing
system subject to the negative effects of corrosion. While limitations clearly
exist, they often provide the only information possibly available.
Corrosion coupons
are an excellent source of 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. 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.
Corrosion coupons
can document if a chemical inhibitor is present 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.
Our own product
testing has shown a good relative uniformity of wall loss between actual pipe
and corrosion coupons when equally exposed under a salt spray fog. The wide
differences often seed in comparing corrosion coupon results to actual pipe
loss we attribute Review corrosion
statistics comparing standard coupons to actual steel pipe.
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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.
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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. See Technical Bulletin
# C-3 regarding accelerated corrosion in drained pipe.
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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.
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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.
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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.
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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.
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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.
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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.
View a table of pipe
deposits created for different corrosion rates.
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.
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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.
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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.
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Very important, 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.
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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.
Some additional
sources of corrosion coupon error include:
- 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
- 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.
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You can
download and print out a condensed two page handout on this subject in Adobe
PDF format from our Documents download
page, or view the full article by clicking the image at the
left.
We offer certain topics and some of
our Technical
Bulletins in condensed format for your conveniece. Further documents
will be added as time allows. |
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 ®
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.
©
Copyright
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