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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 electrochemical 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 $65 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.
While various
options exist, such as ultrasonic testing, metallurgical analysis, spool
pieces, and instantaneous electronic measurement, corrosion coupons still
remain the most widely used form of corrosion measurement and monitoring today.
Corrosion coupons are carefully machined small thin bars of various metals
which are inserted into an external "coupon rack" or zigzag layout of
piping to the main circulating loop.
Each coupon is
pre-weighed by the manufacturer to an accuracy of four decimal places, and
typically left in place for a duration of between one and six months. After
exposure, they 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. 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.
Corrosion coupons
offer an excellent source of information to any building owner or plant
operator - especially if monitoring is continuously maintained and a history of
coupon test results accumulated. Though limited in many respects, coupons will
often provide the only indication of corrosion status, and the only inside look
at 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 chemical cleaning or 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, they offer an estimate of
the corrosivity of the fluid, rather than a measurement of the true metal lost
from the pipe.
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.
In addition, materials of
construction, rack layout, pipe size, or filtering of the coupon rack assembly
can significantly alter corrosion rate estimates. Test layouts constructed of
PVC will greatly eliminate any possible galvanic activity. Even the physical
location of the coupon rack itself, 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 20 years. Yet with only the water filled
area tested, this potential for failure will always remain hidden.
See Technical Bulletin
# C-3 about increased roof level corrosion activity during winter or temporary
drain down.
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
shows 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 increases the risk that
any corrosion coupon testing performed at one area is not representative of the
overall system. See Technical Bulletin
# C-10 regarding some common corrosion trends.
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 a common example
of galvanic forces which always exist to some degree in most piping systems.
Lesser galvanic forces exist where different steels meet as well. As a result,
a major corrosion mechanism responsible for a significant amount of material
loss is never measured. View a photo gallery
of galvanic corrosion examples.
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, 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 - leaving the most vulnerable areas of the entire piping
system un-addressed.
The typically
mirror smooth polished surface of a corrosion coupon minimizes the adhesion of
iron oxide, dirt and micro organisms. As a result, they are 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.
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 are
rarely questioned.
By far, the
accumulation of interior deposits is the greatest limitation in 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
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 will, depending upon their thickness,
impede contact of the water treatment chemicals to 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.
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. See Technical Bulletin
C-4 about the problems associated with interior deposits.
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. While the volume of metal lost in MPY
may be often viewed as acceptable, often overlooked are the consequences 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 fundamental to reducing high corrosion and pitting rates.
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. 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.
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
- Surface texture
of the corrosion coupon
- 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 CVI 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 under one tenth of a mil per year 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, chip scale, or other operating problem ultimately signals a corrosion
condition hidden over an extended time, and further investigation begins.
Unfortunately, this is usually discovered only after years of concealed and
under reported piping damage.
Overall,
corrosion coupons offer some excellent diagnostic information, though with very
clear limitations. A comparison of results to other more direct testing
methods, therefore, is always recommended.
©
Copyright
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