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Comparison
testing was performed to measure the relative corrosion rate of the most
commonly used CorrView ® models in 1-1/2 in. and 3/4 in.
ASTM 1018 steel, and commercially available ASTM 1010 and 1018 corrosion
coupons typically used in coupon racks to estimate pipe wall losses.
In addition, comparison was
simultaneously made against the most commonly installed pipe for HVAC and
process water systems - ASTM A53, A106, and A795. Testing was performed by the
accelerated salt spray method.
In order to
provide a more formal and better recognized evaluation of CorrView
® against standard coupons and mild carbon steel pipe, a
salt spray test booth was constructed. Design of the test booth substantially
followed ASTM Designation B 117 - 95: Standard Practice for Operating Salt
Spray (Fog) Apparatus.
Coupons of ASTM
1010 and 1018 mild carbon steel, typically used in the corrosion rate
evaluation of HVAC piping systems, were purchased from Metal Samples.
Samples of actual ASTM A53, A106, and A795 pipe were also acquired on the open
market, and fabricated into the approximate length and width of the corrosion
coupons. Weights of the individual samples were not recorded, since an
ultrasonic measurement of true wall loss would be used as the basis for
corrosion rate, rather than weight loss.
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Side View Of Test
Assembly |
The wall
thickness of one end of each corrosion coupon and pipe sample coupon was
measured ultrasonically at 15 individual locations according to a standard grid
pattern of specific dimension. Both CorrView ® models
were similarly measured according to a specified grid at 15 individual
locations at its center. All wall thickness data was recorded into a
spreadsheet to establish a baseline.
A
glass enclosure was constructed to house the metal test specimens. All
specimens were positioned on a wooden base for isolation and placed into the
fog chamber. Specimens were physically arranged at an angle to the fog spray
according to the requirements of ASTM B 117 - 95.
A 5% salt spray
solution was introduced into the test chamber via ultrasonic atomization and an
overhead header configured to provide uniform and indirect introduction of the
salt fog. Our corrosion simulation procedure took exception to maintaining a
test chamber temperature of 95 ° F or to maintain pH, as the purpose of the
test was to make a side by side comparison of different metal types rather than
produce a standardized corrosion rate environment. All test subjects were
exposed to the same conditions during its entire test
period.
Testing was continued and the
results observed. On the first cycle of exposure at 15 days, the samples were
removed and allowed to dry, then photographed. Deposits from the measured end
of each test sample were then brushed free of rust deposits and rinsed.
Following the removal of this area of deposits, ultrasonic testing was
performed at the same area of each sample, and along the same grid pattern to
provide a second set of wall thickness measurements. Wall loss and corrosion
rates were calculated for each test specimen. Additional photographs were taken
of the cleaned test samples.
Results showed a
general agreement of corrosion rate statistics within what we would consider
reasonable limits. The salt spray environment produced an extremely aggressive
attack at all metal surfaces as evidenced by visual inspection of the exposed
metals, and of their underlying surfaces once deposits were removed (shown
below). Moderate to high pitting was found in all examples, requiring the use
of "echo to echo" ultrasonic measurement technique in order to
accurately measure the base dimension of such
areas.
Corrosion rates of between 19.5 MPY
and 33.3 MPY were measured. The highly corrosive environment produced by the
salt spray chamber would be expected to exaggerate minor differences in metal
chemistry of the steel samples, and therefore produce a wider variation in
result than under more typical conditions of a 1-3 MPY corrosion rate found at
an open cooling tower system. The differences noted between the steel pipe
samples alone help support this
position.
Nevertheless, this series of
tests showed a maximum variance of only 33.8% from the average corrosion rate
measured, which we found reasonable. This highest variance in result was found
at sample # 5 of ASTM A53 pipe. The average variance of all samples tested from
the mean corrosion rate was 13.9% - minor in comparison to the large
discrepancy often found between corrosion coupons and true corrosion
losses.
Test results for the standard
corrosion coupons of ASTM 1010 and ASTM 1018 mild steels showed similar rates
of corrosion to the actual pipe samples measured. Installed in externally
located corrosion coupon racks, however, their reported corrosion rates are
typically far below what actually exists at the interior pipe wall - the result
of their being isolated from most of the corrosive effects existing within the
actual piping system.
A corrosion rate
determination for a condenser water system of 0.5 MPY using coupons, when the
actual measured loss of pipe wall is 0.105 in. over 12 years or 8.75 MPY, is
a substantial 1,750 % difference or under reporting of actual wall loss.
Such extreme error, often found to exist between 100% and 1,000% in actual UT
testing comparisons, shows the current 33.8 % maximum variance in corrosion
rate results of all samples tested to be nearly
insignificant.
The below graph well
illustrates the variation in wall loss for the eight test subjects evaluated
under this exposure period.
The below set of
tables offers a visual and statistical comparison of corrosion rates for the
two corrosion coupons, four pipe sample coupons, and two CorrView
® products tested. Shown at the left is the original metal
sample prior to testing, and in its original form. The center photograph is the
exposed and rusted sample as removed from the salt fog booth. The far right
photograph shows the same exposed sample after wire brushing.
Our testing produced an average corrosion
rate of 28.7 MPY at the four samples of actual steel pipe - ASTM A106, A795,
A53, and A53, and an average corrosion rate of all eight metals tested of 27.3
MPY. For purposes of this evaluation, we then compared the corrosion rates of
the commercially available steel corrosion coupons and CorrView
® products, finding an average 8.1 % and 12.7 % variance in
their corrosion rates from the true pipe samples respectively. Both the
corrosion coupons and CorrView ® products slightly under
reported the corrosion rates found at the steel
samples.
The percentage of variation in
corrosion rate from the average measured value of 28.7 MPY measured at the five
steel pipe samples is also provided below. Further testing is in progress, and
those additional results will be presented when available.
Metal Sample # 1 -
Corrosion Coupon 1010 Steel
Metal Sample # 2 -
Corrosion Coupon 1018 Steel
Metal Sample # 3 -
Steel Pipe ASTM A106
Metal Sample # 4 -
Steel Pipe ASTM A795
Metal Sample # 5 -
Steel Pipe ASTM A53
Metal Sample # 6 -
Steel Pipe ASTM A53
Metal Sample # 7 -
1-1/2 in. NPT CorrView Monitor
Metal Sample # 8 -
3/4 in. NPT CorrView Monitor
Additional
corrosion rate comparison testing is presented on this Internet site under the
above Testing
heading, and is currently underway. A duplicate of this testing procedure has
been commissioned to an independent laboratory. Results will be presented here
as soon as it is available. A second set of results from this group of test
samples will be available on approximately 10/1/04.
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