U.S. patent application number 10/997369 was filed with the patent office on 2006-05-25 for corrosion monitoring system.
Invention is credited to Russell D. Braunling, Donald S. Foreman, Darryl J. Wrest.
Application Number | 20060109012 10/997369 |
Document ID | / |
Family ID | 36460366 |
Filed Date | 2006-05-25 |
United States Patent
Application |
20060109012 |
Kind Code |
A1 |
Foreman; Donald S. ; et
al. |
May 25, 2006 |
CORROSION MONITORING SYSTEM
Abstract
A system for monitoring corrosion in metal by comparing a test
sample exposed to a corrosion causing environment and a reference
sample exposed to a protected environment. An AC voltage source
generates a square wave signal oscillating between ground and
voltage Vcc and a filter is positioned to filter the signal to
produce a sine wave with no second harmonic component. A
voltage-driven current source and inverting amplifier produce a
current referenced to 0.5 Vcc to provide an AC current from the
drive voltage driven sinusoidally and symmetrically above and below
0.5 Vcc. A transformer steps up the AC current and thereafter
transmits the current through the samples to an amplifier for
amplifying the current to provide outputs in a ratio representing
the degree of corrosion of the reference sample. The system can
operate in situ for on site measurement and uses relatively low
current to permit long operation.
Inventors: |
Foreman; Donald S.;
(Fridley, MN) ; Braunling; Russell D.; (Eden
Prairie, MN) ; Wrest; Darryl J.; (O'Fallon,
MO) |
Correspondence
Address: |
Kris T. Fredrick;Patent Services
Honeywell International Inc.
101 Columbia Road
Morristown
NJ
07962
US
|
Family ID: |
36460366 |
Appl. No.: |
10/997369 |
Filed: |
November 24, 2004 |
Current U.S.
Class: |
324/700 |
Current CPC
Class: |
G01N 17/04 20130101 |
Class at
Publication: |
324/700 |
International
Class: |
G01R 27/08 20060101
G01R027/08 |
Claims
1. A system for monitoring corrosion in metal, comprising: a test
sample exposed to a corrosion causing environment and a reference
sample exposed to a protected environment,; an AC voltage source
adapted to generate a square wave signal oscillating between ground
and voltage Vcc; a filter positioned to receive said signal and
filter said signal to produce a sine wave with no second harmonic
component; a voltage-driven current source and inverting amplifier
adapted to receive said sine wave and produce a current referenced
to 0.5 Vcc to provide an AC current from the drive voltage driven
sinusoidally and symmetrically above and below 0.5 Vcc; a
transformer for receiving said AC current and stepping up said AC
current and thereafter transmit said stepped up current through
said test sample and said reference sample in series connection;
and an amplifier for amplifying AC voltage that is the result of
said AC current and respective resistances of test sample and said
reference sample to provide a test output and a reference output
voltage in a ratio representing the degree of corrosion of said
reference sample.
2. The system of claim 1, where said square wave signal operates at
a frequency ranging from about 50 Hz to about 150 Hz.
3. The system of claim 2, where said frequency is about 100 Hz.
4. The system of claim 1, where said transformer operates at a
current step up ratio of from about 5:1 to about 15:1.
5. The system of claim 1, where said transformer operates at a
current step up ratio of about 10:1.
6. The system of claim 1, wherein said amplifier is a pair of
amplifiers in series and each is adapted to amplify one of said
test output and sample output by an identical gain of from about
500 to about 2000,
7. The system of claim 6, wherein said gain is about 1000.
8. The system of claim 1, which further includes full-wave
rectifiers for rectifying said amplified test output signal and
said reference output signal to produce rectified amplified
signals.
9. The system of claim 1, which further includes low pass filters
for filtering said rectified amplified signals prior to use of said
signals to calculate the degree of corrosion of said output
signals.
10. The system of claim 1, wherein said test output and a reference
output voltage ratio representing the degree of corrosion of said
reference sample is measured in situ.
11. A system for monitoring corrosion in metal, comprising: a test
sample exposed to a corrosion causing environment and a reference
sample exposed to a protected environment,; AC voltage source means
for generating a square wave signal oscillating between ground and
voltage Vcc; filter means for receiving said signal and filter said
signal to produce a sine wave with no second harmonic component;
voltage-driven current source means and inverting amplifier means
adapted to receive said sine wave for producing a current
referenced to 0.5 Vcc to provide an AC current from the drive
voltage driven sinusoidally and symmetrically above and below 0.5
Vcc; transformer means for receiving said AC current and stepping
up said AC current and thereafter transmit said stepped up current
through said test sample and said reference sample in series
connection; and amplifier means for amplifying the AC voltage that
is the result of said AC current and respective resistances of test
sample and said reference sample to provide a test output and a
sample output voltage in a ratio representing the degree of
corrosion of said reference sample.
12. The system of claim 11, where said square wave signal operates
at a frequency ranging from about 50 Hz to about 150 Hz.
13. The system of claim 12, where said frequency is about 100
Hz.
14. The system of claim 11, where said transformer operates at a
current step up ratio of from about 5:1 to about 15:1.
15. The system of claim 14, where said transformer operates at a
current step up ratio of about 10:1.
16. The system of claim 11, wherein said amplifier is a pair of
amplifiers in series and each is adapted to amplify one of said
test output and sample output by an identical gain of from about
500 to about 2000,
17. The system of claim 16, wherein said gain is about 1000.
18. The system of claim 11, which further includes full-wave
rectifiers for rectifying said amplified test output signal and
said reference output signal to produce rectified amplified
signals.
19. The system of claim 11, which further includes low pass filters
for filtering said rectified amplified signals prior to use of said
signals to calculate the degree of corrosion of said output
signals.
20. The system of claim 11, wherein said test output and a
reference output voltage ratio representing the degree of corrosion
of said reference sample is measured in situ.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to a system for
measuring corrosion in materials by comparison of a material being
corroded with an essentially identical material in a protected
environment. More particularly, the present invention relates to a
system in which AC voltage at low current levels is used to measure
corrosion without high energy drain.
BACKGROUND OF THE INVENTION
[0002] The use of the electrical resistance technique is widely
applied in monitoring material loss occurring in industrial plant
equipment and pipelines. This technique operates by measuring the
change in electrical resistance of a metallic element immersed in a
product media relative to a reference element sealed within the
probe body. Since temperature changes affect the resistance of both
the exposed and protected element equally, measuring the resistance
ratio minimizes the influence of changes in the ambient
temperature. If the corrosion occurring in the vessel under study
is roughly uniform, a change in resistance is proportional to an
increment of corrosion. Although universally applicable, the
electrical resistance method is uniquely suited to corrosive
environments having either poor or non-continuous electrolytes such
as vapors, gases, soils, "wet" hydrocarbons, and non-aqueous
liquids.
[0003] An electrical resistance monitoring system consists of an
instrument usually with data logging functions connected to a
probe. The instrument may be permanently installed to provide
continuous information, or may be portable to gather periodic data
from a number of locations. The probe is equipped with a sensing
element having a composition and material processing history
similar to that of the process equipment of interest.
[0004] Electrochemical noise is a useful, sensitive and
non-intrusive technique for corrosion monitoring. Fluctuations of
potential or current of a corroding metallic specimen are monitored
to gage and understand the corrosion process. Electrochemical noise
is used to investigate localized corrosion processes such as
pitting or stress corrosion cracking, exfoliation, and
erosion-corrosion in either laboratory or diverse and complex
industrial environments. During localized corrosion, film
formation, passivation breakdown or pit propagation processes
generate the electrochemical noise that is observed. The most
traditional way to analyze electrochemical noise data has been to
transform time records in the frequency domain in order to obtain
power spectra with FFT methods.
[0005] Inductive resistance probes are similar to ER probes. The
weight loss in the sensor element is detected by measuring changes
in the inductive resistance of a coil, located inside the element.
The inductive measurement technique provides greatly improved
sensitivity and earlier detection of corrosion rate changes
compared to conventional electrical resistance probes. Inductive
resistance probes require temperature compensation, similar to ER
probes. Like ER probes, the sensors can be used in a broad range of
environments such as low conductivity and non-aqueous environments,
where electrochemical techniques are generally unsuitable.
[0006] Polarization resistance is particularly useful as a method
to rapidly identify corrosion upsets and initiate remedial action,
thereby prolonging plant life and minimizing unscheduled downtime.
The technique is utilized to maximum effect, when installed as a
continuous monitoring system in almost all types of water-based,
corrosive environments. The measurement of polarization resistance
has very similar requirements to the measurement of full
polarization curves.
[0007] One drawback all prior art systems and devices have is that
the power required to operate them is far too high for sustained,
long-term battery operation. The resistances of corroding test
coupons are very low, typically on the order of 10 milliohms.
Because low average power consumption is a requirement in many
applications of corrosion measurement, high-current excitation of
the coupons is not an option, while low-current excitation results
in signals of microvolt magnitude. Presently available commercial
instruments fail to meet the low-power requirement.
[0008] Therefore, it would be of great advantage if a system could
be invented that would use low-power demand while providing
acceptable accuracy.
[0009] Another advantage would be if a system could be invented
that would avoid offset and thermoelectric potentials at connection
points.
[0010] Yet another advantage would be a system for measuring
corrosion that is more accurate due to elimination of noise and
offset from high gain amplification.
[0011] Other advantages and features will appear hereinafter.
SUMMARY OF TIE INVENTION
[0012] The present invention provides a system for monitoring
corrosion in metal. A test sample is exposed to a corrosion causing
environment and a reference sample is exposed to a protected
environment. In its simples form the system of this invention uses
an AC current to excite the samples or coupons to avoid DC offset
errors in amplifiers. The system uses a 10:1 current step-up
transformer in the drive circuit to gain a tenfold increase in
power efficiency in driving the very low-impedance load. Very
low-noise, low-offset, high-gain instrumentation operational
amplifiers are used in the first signal-processing stage.
Ratiometric measurement is accomplished by current driving the
reference and sensor coupon in series, sensing and
signal-processing their responsive voltages, and taking the ratio
of these voltages, using conventional digital signal processing, to
provide an accurate, in situ measurement of the corrosion of the
test sample.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a more complete understanding of the invention,
reference is hereby made to the drawings, in which:
[0014] FIG. 1 is a circuit diagram illustrating the operation of a
preferred embodiment of the present invention; and
[0015] FIG. 2 is a block diagram illustrating a system approach of
the present invention.
[0016] In the figures, like reference characters designate
identical or corresponding components and units throughout the
several views.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] Referring to the figures, FIG. 1, which is a schematic
diagram, and FIG. 2, which is a block flow diagram, showing the
system generally at 10 in which the oscillator 11 is a 100 Hz
symmetrical hysteretic oscillator using a rail-to-rail input and
output that produces a symmetrical square wave 13 oscillating
between ground and Vcc. This type of oscillator has the advantage
over sine wave oscillators, such as, for example, Wein bridge,
state-variable, phase shift or twin-tee oscillators, in that it is
capable of starting immediately at fill magnitude. It has been
found that this is important to minimize the total time required
from application of power to availability of stable measurement
data. It has been found that post-filtering of such a square wave
signal provides much faster response than was possible with
alternative oscillators, while also providing acceptable spectral
purity.
[0018] The resulting square wave 13 is filtered by a 4.sup.th order
Bessel-response low pass filter 15 comprised of U1b, U1c and
associated circuitry. The corner frequency of this filter is 125
Hz. The filtered output 17 from filter 15 is a sine wave with no
second harmonic component because it is symmetrical. The output
from filter 15 also has been found to have less than a 1%
third-harmonic component. Filter 15 has been found to have
excellent transient response while providing acceptable spectral
purity. The output sinusoid 17 is stable to within less than 0.1%
of steady state magnitude within less than 200 milliseconds.
[0019] The sinusoidal voltage 17 thus produced is presented to a
voltage-driven current source 19, comprised of U2a and associated
circuitry. It drives its load with a sinusoidal current of 5 mA
peak regardless of load impedance or voltage required to produce
the intended current. The current source is referenced to 1/2 Vcc
so it provides AC current as the drive voltage varies sinusoidally
and symmetrically above and below 1/2 Vcc.
[0020] U2b inverting amplifier 21 inverts the sinusoidal voltage
with unity gain. This provides differential drive for the
transformer 23 primary. The current drive's output can only vary
from near Vcc to near ground, but since the other end of the
transformer 23's primary is connected to a voltage source out of
phase with the drive current, drive voltage approaches.+-.Vcc in
magnitude if necessary to reach peak values of .+-.5 mA. The actual
voltage appearing on transformer 23's primary will depend on the
resistance of the leads to the coupon.
[0021] Since the coupons 27 and 37 are current driven in series,
they will be excited with identical current via lines 29 and 39.
Transformer 23 isolation prevents any possibility of exciting
ground loop current in the common return 31.
[0022] The sensor and reference coupons are Kelvin connected as
shown in FIG. 2. Sense voltage returns on wires 31 separate from
those wires 29, 39 conducting the drive current. 100 Hz for the
oscillator 11 was chosen as a high enough frequency to result in a
transformer 23 of acceptable size and weight (in practice about 2
cm.sup.3 and about 4 grams) but low enough to minimize the effects
of inductive coupling between drive wires and sense wires, in the
wiring between the instrument and the coupons.
[0023] The sense voltages from the coupons 33 and 35 respectively,
are supplied to instrumentation amplifiers 41 and 43, which in
practice are U3a and U3b. These amplifiers have very low noise and
very low DC offset. Low DC offset is necessary to keep the
amplifiers out of saturation, given the high gain and the low
supply voltage available. The amplifiers 41 and 43 are set to a
gain of 1000 (60 dB). They are arranged such that the
phase-opposite voltages from coupons 27 and 37 are amplified in
phase to minimize crosstalk, although crosstalk isolation between
amplifiers 41 and 43 exceeds 120 dB.
[0024] The outputs of the instrumentation amplifiers 41 and 43 are
AC-coupled to post amplifiers 45 and 47 respectively, or U4a and
U5a respectively. AC coupling is used to remove any amplified DC
offset error from the instrumentation amps.
[0025] The post-amplified signals are presented to precision full
wave rectifiers 49 and 51 respectively, comprised of U4b and U5b
and associated circuitry. The theory of operation of this block is
straightforward and well documented by those skilled in the
art.
[0026] The resulting full-wave rectified signals are low-pass
filtered via filters 53 and 55 (U4c and U5c) respectively, using
2.sup.nd order Butterworth-response filters comprised of U4d for
one channel and U5d for the other channel. These are Sallen-Key
filters with unity gain regardless of tolerance in resistor values.
These filters 53 and 55 have a 3 dB corner frequency of 5 Hz. This
form of filtering has been found to be superior over conventional
R-C post-detection filtering to achieve fast response with good
rejection of post-rectification ripple. Their DC output is
proportional to the average value of the rectified AC input. It is
settled to within 0.2% within 200 milliseconds of oscillator
startup, including delay in the oscillator filter. At 1 volt
output, ripple is about 500 .mu.V RMS.
[0027] Filters 53 and 55 limit system bandwidth to 5 Hz, which
gives the system very good immunity to electromagnetic interference
and to Johnson noise in the low-level stages.
[0028] The inputs are protected against transients by resistors 57
and 59, followed by diode clamps 61 to ground 63 and Vcc. DC offset
from leakage currents of the diode 61 is negligible over the full
military temperature range due to the low impedances involved. In
addition, the instrumentation op amps used are internally protected
for overvoltage up to 40 volts.
[0029] U6 is a charge-pump voltage inverter operating at about 35
KHz to produce a negative bias voltage for the instrumentation op
amps. The input signals are within less than a millivolt of ground
potential so the first stage amplifiers require negative bias.
Power supply rejection of these amplifiers exceeds 120 dB at the
frequencies of interest and the 5 Hz low pass filter 15 will
eliminate any noise significantly above that frequency, so the
negative bias is not regulated.
[0030] Output 1 and output 2 produce signals that are compared to
determine the degree of corrosion measured by the system of this
invention. The test sample output, 75 and the reference sample
output 77 are both representative of the condition of the
respective coupons or samples. Electrical resistance is directly
related to the degree of corrosion, based on the formula
R=.rho.L/A, where R is electrical resistance, .rho. is the
resistivity of the material, L is the element's length, and A is
the cross sectional area of the element. The outputs 75 and 77 are
compared and the difference in surface area calculated represents
the degree of corrosion. In the present invention, the current
driving the reference and sensor coupons, in series as shown in
FIGS. 1 and 2, uses a ratiometric measurement such that the ratio
of the outputs is an accurate measurement of the degree of
corrosion. Tests show that on site measurement of corrosion using
the present invention corresponds with data from physical
measurements of corrosion using weight-loss calculations. The
present invention operates in situ, and does not require removal of
the sample to determine the degree of corrosion. Not only is the
present invention useable on site, on a continuous basis, it
consumes low power, thus allowing for battery operation over long
periods of time.
[0031] While particular embodiments of the present invention have
been illustrated and described, they are merely exemplary and a
person skilled in the art may make variations and modifications to
the embodiments described herein without departing from the spirit
and scope of the present invention. All such equivalent variations
and modifications are intended to be included within the scope of
this invention, and it is not intended to limit the invention,
except as defined by the following claims.
* * * * *