US20060109012A1 - Corrosion monitoring system - Google Patents
Corrosion monitoring system Download PDFInfo
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- US20060109012A1 US20060109012A1 US10/997,369 US99736904A US2006109012A1 US 20060109012 A1 US20060109012 A1 US 20060109012A1 US 99736904 A US99736904 A US 99736904A US 2006109012 A1 US2006109012 A1 US 2006109012A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N17/00—Investigating resistance of materials to the weather, to corrosion, or to light
- G01N17/04—Corrosion probes
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.
Description
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Therefore, it would be of great advantage if a system could be invented that would use low-power demand while providing acceptable accuracy.
- Another advantage would be if a system could be invented that would avoid offset and thermoelectric potentials at connection points.
- 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.
- Other advantages and features will appear hereinafter.
- 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.
- For a more complete understanding of the invention, reference is hereby made to the drawings, in which:
-
FIG. 1 is a circuit diagram illustrating the operation of a preferred embodiment of the present invention; and -
FIG. 2 is a block diagram illustrating a system approach of the present invention. - In the figures, like reference characters designate identical or corresponding components and units throughout the several views.
- Referring to the figures,
FIG. 1 , which is a schematic diagram, andFIG. 2 , which is a block flow diagram, showing the system generally at 10 in which theoscillator 11 is a 100 Hz symmetrical hysteretic oscillator using a rail-to-rail input and output that produces a symmetricalsquare 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. - The resulting
square wave 13 is filtered by a 4th order Bessel-responselow pass filter 15 comprised of U1b, U1c and associated circuitry. The corner frequency of this filter is 125 Hz. The filteredoutput 17 fromfilter 15 is a sine wave with no second harmonic component because it is symmetrical. The output fromfilter 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. Theoutput sinusoid 17 is stable to within less than 0.1% of steady state magnitude within less than 200 milliseconds. - The
sinusoidal voltage 17 thus produced is presented to a voltage-drivencurrent source 19, comprised of U2 a 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 ½ Vcc so it provides AC current as the drive voltage varies sinusoidally and symmetrically above and below ½ Vcc. - U2
b inverting amplifier 21 inverts the sinusoidal voltage with unity gain. This provides differential drive for thetransformer 23 primary. The current drive's output can only vary from near Vcc to near ground, but since the other end of thetransformer 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 ontransformer 23's primary will depend on the resistance of the leads to the coupon. - Since the
coupons lines Transformer 23 isolation prevents any possibility of exciting ground loop current in thecommon return 31. - The sensor and reference coupons are Kelvin connected as shown in
FIG. 2 . Sense voltage returns onwires 31 separate from thosewires oscillator 11 was chosen as a high enough frequency to result in atransformer 23 of acceptable size and weight (in practice about 2 cm3 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. - The sense voltages from the
coupons instrumentation amplifiers amplifiers coupons amplifiers - The outputs of the
instrumentation amplifiers amplifiers - The post-amplified signals are presented to precision
full wave rectifiers - The resulting full-wave rectified signals are low-pass filtered via
filters 53 and 55 (U4 c and U5 c) respectively, using 2nd order Butterworth-response filters comprised of U4 d for one channel and U5 d for the other channel. These are Sallen-Key filters with unity gain regardless of tolerance in resistor values. Thesefilters -
Filters - 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.
- 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. -
Output 1 andoutput 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 thereference 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=ρL/A, where R is electrical resistance, ρ is the resistivity of the material, L is the element's length, and A is the cross sectional area of the element. Theoutputs 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. - 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.
Claims (20)
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.
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US10/997,369 US7061255B1 (en) | 2004-11-24 | 2004-11-24 | Corrosion monitoring system |
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US10/997,369 US7061255B1 (en) | 2004-11-24 | 2004-11-24 | Corrosion monitoring system |
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US7061255B1 US7061255B1 (en) | 2006-06-13 |
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Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20120007617A1 (en) * | 2010-07-08 | 2012-01-12 | Fisseler Patrick J | Downhole corrosion monitoring |
US8570047B1 (en) * | 2009-02-12 | 2013-10-29 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Battery fault detection with saturating transformers |
Citations (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4217544A (en) * | 1978-10-16 | 1980-08-12 | Shell Oil Company | Method and apparatus for improved temperature compensation in a corrosion measurement system |
US4498044A (en) * | 1981-05-18 | 1985-02-05 | Novasina Ag | Method and circuitry for measuring the impedance of a sensor |
US4587479A (en) * | 1984-07-16 | 1986-05-06 | Rohrback Corporation | Corrosion measurement with multiple compensation |
US5446369A (en) * | 1992-10-09 | 1995-08-29 | Battelle Memorial Institute | Continuous, automatic and remote monitoring of corrosion |
US5854557A (en) * | 1993-04-16 | 1998-12-29 | Tiefnig; Eugen | Corrosion measurement system |
US6831469B2 (en) * | 2003-03-10 | 2004-12-14 | Honeywell International Inc. | Micropower apparatus for low impedance measurements |
-
2004
- 2004-11-24 US US10/997,369 patent/US7061255B1/en not_active Expired - Fee Related
Patent Citations (6)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4217544A (en) * | 1978-10-16 | 1980-08-12 | Shell Oil Company | Method and apparatus for improved temperature compensation in a corrosion measurement system |
US4498044A (en) * | 1981-05-18 | 1985-02-05 | Novasina Ag | Method and circuitry for measuring the impedance of a sensor |
US4587479A (en) * | 1984-07-16 | 1986-05-06 | Rohrback Corporation | Corrosion measurement with multiple compensation |
US5446369A (en) * | 1992-10-09 | 1995-08-29 | Battelle Memorial Institute | Continuous, automatic and remote monitoring of corrosion |
US5854557A (en) * | 1993-04-16 | 1998-12-29 | Tiefnig; Eugen | Corrosion measurement system |
US6831469B2 (en) * | 2003-03-10 | 2004-12-14 | Honeywell International Inc. | Micropower apparatus for low impedance measurements |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US8570047B1 (en) * | 2009-02-12 | 2013-10-29 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Battery fault detection with saturating transformers |
US20120007617A1 (en) * | 2010-07-08 | 2012-01-12 | Fisseler Patrick J | Downhole corrosion monitoring |
US8564315B2 (en) * | 2010-07-08 | 2013-10-22 | Schlumberger Technology Corporation | Downhole corrosion monitoring |
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US7061255B1 (en) | 2006-06-13 |
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Owner name: HONEYWELL INTERNATIONAL INC., NEW JERSEY Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BRUNLING, RUSSELL D.;FOREMAN, DONALD S.;WREST, DARRYL J.;REEL/FRAME:016028/0474;SIGNING DATES FROM 20041013 TO 20041018 |
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Effective date: 20140613 |