WO2002103330A1 - Method and device used to detect corrosion in cathodically-protected buried steel, particularly in concrete reinforcements, or to determine if said corrosion is passive - Google Patents

Method and device used to detect corrosion in cathodically-protected buried steel, particularly in concrete reinforcements, or to determine if said corrosion is passive Download PDF

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WO2002103330A1
WO2002103330A1 PCT/ES2002/000295 ES0200295W WO02103330A1 WO 2002103330 A1 WO2002103330 A1 WO 2002103330A1 ES 0200295 W ES0200295 W ES 0200295W WO 02103330 A1 WO02103330 A1 WO 02103330A1
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current
alternating current
corrosion
voltage
passive
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PCT/ES2002/000295
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Spanish (es)
French (fr)
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María Carmen ANDRADE PERDRIX
María José FULLEA GARCIA
Isabel Martinez Sierra
Juan Antonio BOLAÑO RUIZ
Francisco Jimenez Padilla
Alfredo Navarro Segura
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Consejo Superior De Investigaciones Cientificas
Geotecnia Y Cimientos S.A. (Geocisa)
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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
    • G01N17/02—Electrochemical measuring systems for weathering, corrosion or corrosion-protection measurement

Definitions

  • Cathodic protection systems are used, among others, to prevent corrosion of steel reinforcements in concrete structures.
  • the normally established criterion is that cathodic protection is achieved when a potential of -0.85V is reached with respect to a Cu / CuSO reference electrode, called the absolute potential criterion. If the potential of the structure is -0.85 V or more negative, relative to a Cu / CuSO reference electrode, oxidation reactions causing corrosion should not take place and therefore the protected structure should not corrode.
  • the -0.85V criterion is empirical and does not apply to all concrete conditions. For example, in low humidity conditions, potentials more positive than -0.85 V can provide sufficient cathodic protection to mitigate corrosion. At a different point, especially in buried steel structures, where biological corrosion exists where sulfides are present, -0.85 V may not be the level of adequate cathodic protection.
  • the potential that must be applied to bring the potential to the protection region depends on the chemical conditions obtained at the concrete-water interface. These conditions vary from one structure to another and even within the same structure. Consequently, a certain potential value cannot correspond to the different chemical conditions that can occur in reinforced concrete structures.
  • the current density criterion is based on the fact that "theoretically" the current density necessary to achieve protection depends on the mechanism of the polarization process, and can be calculated from the polarization curve.
  • the applied current density depends fundamentally on the corrosion potential, so that its value depends fundamentally on whether the corrosion rate and the potential for future corrosion is low, medium or high.
  • the generally accepted figure is 13 mA / m of concrete surface.
  • Corrosion is a surface phenomenon that takes place at the metal / electrolyte interface due to its action on it, as a consequence of the formation and movement of electrically charged particles in an amount proportional to the current or charge flow caused by the phenomenon. . Consequently, corrosion can be measured and controlled through electrical parameters.
  • the electroneutrality of matter which forces the image of the opposite sign on the other side of the same interface when faced with an arrangement of electrical charges on a surface, causes the double layer around the electrodes, passivating films and other surface layers , behave like capacitors, or more complex electrical circuits that contain capacitors.
  • the transfer of charges, with formation and disappearance of ions and the transport of the same, are processes that limit the flow of current or intensity and act as if they were electrical resistances.
  • Electrochemical systems can be simulated (equivalent circuit) by a combination of resistors, capacitors and coils that, when presented with an electrical signal, reproduce their response with an acceptable approximation.
  • the Randles circuit like all generalizations, is an approximation and a simplification at the same time. Since the resistance offered by capacitors and self-inductions is a function of the speed of variation of the electrical signal, to obtain adequate information from equivalent circuits, apply alternating signals with a wide range of frequencies.
  • the assimilation of electrochemical interfaces means that, although the current of the corrosion cells is continuous, its effect can be studied with sinusoidal excitations such as alternating current.
  • a sinusoidal current with a certain phase angle v is obtained in response.
  • the ⁇ E / ⁇ I ratio is another sinusoidal function; the impedance Z for a parallel RC circuit, such as that of Randles, is given by
  • the impedance is fully defined by simply specifying its magnitude and phase angle, as in Argand diagrams, or its real and imaginary components, Z 'and Z ". If frequency is also included as a variable, in equivalent circuits corresponding to simple metal / medium simple systems, the end of the impedance vector describes a semicircle in the complex plane (figure 1), whose dimensions allow us to estimate the values R e , Rj and C of the equivalent circuit, or attributable to the system under study [S Feliu, JM; Bastidas, and M. Morcillo: Qu ⁇ m. And Ind., 28 (198 ⁇ ), 635; . Hladky, LM Callow and JL Dawson: Br. Corros. J, 15 (1) (1980), 20.].
  • the horizontal axis represents the real part of the electrode impedance, that is, its resistive component and the ordinate axis the imaginary component or capacitive reactance.
  • the capacitor conducts so easily that it short circuits Rx and only the effect of the resistance of the electrolyte and surface layers, R e , remains, a value that corresponds to the left intersection of the semicircle with the abscissa axis.
  • the capacitor When the frequency decreases, the capacitor conducts less and less, since the capacitive reactance is inversely proportional to ⁇ , until at low frequencies or in direct current (zero frequency) the capacitor stops conducting and the impedance equals the sum R e + R ⁇ marked by the right intersection of the semicircle with the real axis.
  • the accuracy of kinetic predictions based on electrochemical techniques requires that the corrosion rate be under activation control, that there be a cause-effect relationship between the applied signal or polarization and the measured response, and that the correlation between that and this is linear.
  • the linearity condition is often forced by resorting to small amplitude signals, whether they are impulses, voltage steps or ramps, or sine waves.
  • the corrosion rate is unambiguously related to the polarization resistance, R p ; in steps with alternating current, semicircles well defined are obtained, which allow simultaneous determination and R and Rx, R + R ⁇ being fully equivalent to R p.
  • the effect of diffusion mass transport is reproduced, in equivalent circuits, by introducing an additional element W, known as the Warburg impedance, in series with Rj.
  • W additional element W
  • the Warburg impedance is a complex number with the real and imaginary parts equal and inversely proportional to the square root of the frequency:
  • the insufficiency of cathodic protection is predicted by the deviation of the Randles model, to which, adding a corrective part of the diffusion of matter (Warburg impedance), a modification is obtained of the semicircle of the Nyquist diagram, consisting in that the final part (corresponding to the low frequency measurements) of the semicircle becomes a line of slope 1.
  • the invention allows to know if a steel is actively corroding or not even if it is not cathodically protected (passivity verification).
  • Figure 1 Representation of the Nyquist diagram corresponding to a system without corrosion according to J.A. González Fernández "Corrosion control: study and measurement by electrochemical techniques". C.S.I.C, Madrid 1989, p. 202.
  • Figure 2 Representation of the Nyquist diagram corresponding to a system subjected to corrosion according to J.A. González Fernández, o.c. p. 207.
  • Figure 4 Block diagram of the procedure to know if a buried metal structure, cathodically protected, is subject to corrosion (in the figure, application to corrosion of a reinforced concrete reinforcement).
  • Unit of measurement and control It contains the CPU and is connected (see block diagram in figure 4) with the confinement and measurement units.
  • Alternating current generation unit It constitutes the essential part of the invention; an alternating current is produced, and the phase difference between said current and the voltage that generates it is measured. It contains the following basic components:
  • Unit of measurement and control It processes the received data and calculates, from the difference between the voltage and the alternating current, whether or not the metal structure (in this case the reinforcement 11 of a reinforced concrete slab 12) is cathodically sufficiently protected.
  • the procedure, object of the present invention, to determine the presence or absence of corrosion in a steel structure protected or not cathodically consists of superimposing the cathodic protection current or the reference potential a current "variable frequency sinusoidal measuring the gap, v, between the voltage and current applied.
  • phase angle v is by determining the parameters of the so-called Lissajous figures, obtained on an oscillograph;
  • the value of the alternating voltage u applied between the metal reinforcement and a reference electrode in contact with the cement surface arrives, as the axis of abscissa, and as the axis of ordinates the corresponding value of i of said alternating current.
  • the determination of the angle v can be carried out in many other ways, for example by means of an impedance bridge. This topic is not emphasized since the measurement of the angle is not the object of the present invention, but how knowing the angle it is possible to know whether or not a circuit is sufficiently protected against corrosion, without the need to suspend the protection during the measurement .
  • a control unit acts on a frequency generator whose current, suitably amplified, is transmitted to the sensor electrodes.
  • the data acquisition unit emits the corresponding signals and receives the corresponding values which it forwards to the control unit. It processes the data that it sends to a screen, prints and / or saves on a disk drive.
  • this block diagram is shown in more detail. As already explained when describing the figures, there is a first unit that allows the confinement of the direct current on which the alternating current to be measured is modulated.
  • This unit which we call confinement unit, contains a signal joining device, 1, which receives on the one hand the signal from the differentiator 3 and, on the other hand, the alternating signal produced at 7; the signal produced by 1 passes to an operational amplifier that generates the confining direct current, I xce , which is measured in ammeter 5 and reaches the external counter electrode 17 of the sensor.
  • this unit receives the signals from two extra reference electrodes 15 and 16, located on the sensor that contains the guard electrode; These signals reach a voltmeter 4, which measures the potential difference, V ss , between the two electrodes, which in turn, in the signal comparator device 2, is transformed into a signal that is sent to 1.
  • alternating current generation unit constitutes the essential part of the invention, since the confinement unit (under certain conditions depending on the dimensioning of the reinforcement to be measured) could be dispensed with; however, since these conditions, in some cases, are unknown to the operator, it does not seem prudent to dispense with the confinement unit in the business model.
  • An alternating current is produced in the generation unit, and the phase difference between said current and the voltage that generates it is measured.
  • It contains an alternating current generator 7 of variable frequency (between 0.01 and 100 Hz), the signal of which is applied, through a signal joining device 8, to an operational amplifier 9, which sends a current of intensity I ce , measured on the ammeter 10, to the counter electrode 14 formed by the central ring of the confinement sensor; in the central part, a reference electrode 13 determines the voltage, E r , at this point which is fed back into the adder 8 and is measured by the voltmeter 6.
  • the block diagram of figure 4 can be embodied in the one presented in figure 6 which gives the schematic of a device based on all the aforementioned.
  • the data acquisition method is based on a conventional circuit based on a microprocessor that is responsible for controlling the various signals and activating the different circuits.
  • 22 Data can be additionally sent and received via a standard RS-232 connection.
  • ADC Analog to Digital Converter
  • a programmable gain amplifier (PGA) is used to adapt them to the ADC.
  • this module converts the logic levels of the microprocessor to a voltage that is subtracted from the input signal by the PGA.
  • a multiplexer device is used that connects the chosen signal to the measurement chain.
  • Data acquisition has four S&H sampling and retention circuits that allow four signals to be retained at the level of a given moment so that they can be analyzed by the measurement chain.
  • D / A modules can reach the structure through three modes i) Directly applying their voltage output to the system. ii) Controlling a potentiostat module that allows maintaining a certain potential based on a reference electrode, iii) Controlling a current source that allows maintaining a certain current through the circuit independent of potential variations.
  • the different circuits and modules that make up the measurement system require various different voltage levels for their operation, which are provided by a power supply.
  • Example n ° 2. Test tubes with moderate corrosion with cathodic protection for 24 h. The results obtained are shown in Table II

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Abstract

The invention relates to a method for detecting corrosion in buried steel which, after containment of a carrier direct current, consists in: modulating an alternating current; determining the phase difference between the intensity of the alternating current and the alternating potential applied in order to ascertain whether or not the cathodic protection is effective, on the condition that the frequency interval of the alternating current is between 0.01 and 100 Hz. The measurement method consists in: (i) generating an alternating current which is applied between the buried steel and a counterelectrode located on the surface of the ground and (ii) using a standard electronic system, measuring the value of the applied voltage u and current i. Said values are subsequently processed electronically using a suitable device in order to determine phase angle n, the value of which indicates the effectiveness of the cathodic protection.

Description

TítuloTitle
Procedimiento y dispositivo para la detección de corrosión en acero enterrado protegido catódicamente, especialmente en las armaduras del hormigón o determinar si está pasivo.Procedure and device for the detection of corrosion in cathodically protected buried steel, especially in concrete reinforcements or to determine if it is passive.
Campo de la técnicaField of technique
* Detección de corrosión de acero en piezas enterradas.* Detection of steel corrosion in buried parts.
* Medida de la corrosión del acero en hormigón armado.* Measurement of the corrosion of steel in reinforced concrete.
IntroducciónIntroduction
Los sistemas de protecciones catódicas se usan, entre otros, para prevenir corrosión de las armaduras de acero en estructuras de hormigón. El criterio normalmente establecido es que la protección catódica se consigue cuando se alcanza un potencial de -0.85V respecto a un electrodo de referencia de Cu/CuSO , denominado criterio del potencial absoluto. Si el potencial de la estructura es de -0.85 V o más negativo, con respecto a un electrodo de la referencia de Cu/CuSO , las reacciones de la oxidación que producen corrosión no deben tener lugar y, por tanto, la estructura protegida no debe corroerse.Cathodic protection systems are used, among others, to prevent corrosion of steel reinforcements in concrete structures. The normally established criterion is that cathodic protection is achieved when a potential of -0.85V is reached with respect to a Cu / CuSO reference electrode, called the absolute potential criterion. If the potential of the structure is -0.85 V or more negative, relative to a Cu / CuSO reference electrode, oxidation reactions causing corrosion should not take place and therefore the protected structure should not corrode.
Sin embargo, hay varias deficiencias con un criterio tan simple: las medidas de potencial están afectadas por la caída de tensión en el hormigón que añade un voltaje negativo al potencial medido; así, por ejemplo, si el potencial medido es de -0.92 V, pero hay una caída de potencial de 0.10 V, el potencial de polarización real es de -0.82 V, con lo que, aunque el potencial medido parece ser inferior a -0.85 V, el potencial de polarización real del sistema de protección catódica no cumple el criterio establecido.However, there are several deficiencies with such a simple criterion: potential measurements are affected by the voltage drop in the concrete that adds a negative voltage to the measured potential; so, for example, if the measured potential is -0.92 V, but there is a 0.10 V potential drop, the actual bias potential is -0.82 V, so even though the measured potential appears to be less than -0.85 V, the actual polarization potential of the cathodic protection system does not meet the established criteria.
Por otra parte, el criterio de -0.85 V es empírico y no aplica a todas las condiciones del hormigón. Por ejemplo, en condiciones de baja humedad, potenciales más positivo que -0.85 V pueden proporcionar protección catódica suficiente para mitigar la corrosión. En un punto diferente, sobre todo en estructuras de acero enterrada, donde existe corrosión biológica donde hay presente sulfuros, -0.85 V no puede ser el nivel de una protección catódica adecuada.Furthermore, the -0.85V criterion is empirical and does not apply to all concrete conditions. For example, in low humidity conditions, potentials more positive than -0.85 V can provide sufficient cathodic protection to mitigate corrosion. At a different point, especially in buried steel structures, where biological corrosion exists where sulfides are present, -0.85 V may not be the level of adequate cathodic protection.
El potencial que debe ser aplicado para llevar el potencial a la región de protección, depende de las condiciones químicas obtenidas en la interfase hormigón- agua. Estas condiciones varían de una estructura a otra e, incluso, dentro de una misma estructura. En consecuencia, un determinado valor del potencial, no puede corresponder a las distintas condiciones químicas que pueden darse en estructuras de hormigón armado.The potential that must be applied to bring the potential to the protection region depends on the chemical conditions obtained at the concrete-water interface. These conditions vary from one structure to another and even within the same structure. Consequently, a certain potential value cannot correspond to the different chemical conditions that can occur in reinforced concrete structures.
Algunos autores han propuesto el denominado criterio de la caída/ desplazamiento del potencial de 100 mV. Este criterio se ha utilizado sobretodo en el campo de la protección de tuberías enterradas; según este criterio la protección es eficaz cuando el potencial de acero se desplaza en la dirección catódica, del potencial natural antes de la polarización, un valor mínimo para lograr la protección. Este desplazamiento según varios autores es de 300 mV a 100 mV.Some authors have proposed the so-called 100 mV potential drop / shift criterion. This criterion has been used especially in the field of protection of buried pipes; According to this criterion, the protection is effective when the steel potential moves in the cathodic direction, from the natural potential before polarization, a minimum value to achieve protection. This displacement according to several authors is from 300 mV to 100 mV.
La aplicación de este criterio a las estructuras de hormigón armado es cuestionable, en tanto que está basado en la premisa de que la corrosión del acero de la armadura está controlada por la "polarización de activación", dato éste que niegan diversos autores.The application of this criterion to reinforced concrete structures is questionable, since it is based on the premise that the corrosion of the steel of the reinforcement is controlled by the "activation polarization", a fact that is denied by various authors.
El criterio de la densidad de corriente está basado en que "teóricamente" la densidad de corriente necesaria para alcanzar la protección, depende del mecanismo del proceso de polarización, y puede calcularse a partir de la curva de polarización.The current density criterion is based on the fact that "theoretically" the current density necessary to achieve protection depends on the mechanism of the polarization process, and can be calculated from the polarization curve.
En la práctica, sin embargo, la densidad de corriente aplicada depende fundamentalmente del potencial de corrosión, de forma que su valor depende fundamentalmente de que la velocidad de corrosión y el potencial para la futura corrosión sea baja media o alta. La cifra generalmente aceptada es de 13 mA/m de superficie de hormigón. finalmente el criterio de E vs. logl propone que la corriente requerida para la protección catódica es aquella que puede determinarse a partir del principio del comportamiento lineal de la curva E = f(logI). Este criterio, sin embargo, está basado solamente en el puro control de activación, pero, el comportamiento lineal (curva teórica de Tafel), puede no tener lugar, si la polarización de concentración y resistiva interfieren en el fenómeno de polarización.In practice, however, the applied current density depends fundamentally on the corrosion potential, so that its value depends fundamentally on whether the corrosion rate and the potential for future corrosion is low, medium or high. The generally accepted figure is 13 mA / m of concrete surface. finally the criterion of E vs. logl proposes that the current required for cathodic protection is that which can be determined from the principle of the linear behavior of the curve E = f (logI). This criterion, however, is based only on pure activation control, but linear behavior (theoretical Tafel curve) may not take place, if the concentration and resistive polarization interfere with the polarization phenomenon.
Por tanto es deseable disponer de una técnica que pueda descubrir la corrosión directamente y establecer si el nivel de protección catódica aplicado es suficiente.Therefore it is desirable to have a technique that can directly detect corrosion and establish whether the level of cathodic protection applied is sufficient.
Estado de la técnica.State of the art.
La corrosión es un fenómeno de superficie que tiene lugar en la interfase metal/ electrolito por la acción de éste sobre aquél, como consecuencia de la formación y movimiento de partículas con carga eléctrica en cantidad proporcional al flujo de corriente o de cargas suscitado por el fenómeno. En consecuencia, la corrosión se puede medir y controlar a través de parámetros eléctricos.Corrosion is a surface phenomenon that takes place at the metal / electrolyte interface due to its action on it, as a consequence of the formation and movement of electrically charged particles in an amount proportional to the current or charge flow caused by the phenomenon. . Consequently, corrosion can be measured and controlled through electrical parameters.
La electroneutralidad de la materia, que ante una disposición de cargas eléctricas sobre una superficie, fuerza la imagen de signo contrario en la otra cara de la misma interfase, hace que la doble capa en torno a los electrodos, las películas pasivantes y otras capas superficiales, se comporte como condensadores, o circuitos eléctricos más complejos que contienen condensadores. La transferencia de cargas, con formación y desaparición de iones y el transporte de los mismos, son procesos que limitan el flujo de corriente o intensidad y actúan como si de resistencias eléctricas se tratara.The electroneutrality of matter, which forces the image of the opposite sign on the other side of the same interface when faced with an arrangement of electrical charges on a surface, causes the double layer around the electrodes, passivating films and other surface layers , behave like capacitors, or more complex electrical circuits that contain capacitors. The transfer of charges, with formation and disappearance of ions and the transport of the same, are processes that limit the flow of current or intensity and act as if they were electrical resistances.
Los sistemas electroquímicos se pueden simular (circuito equivalente) por una combinación de resistencias, condensadores y bobinas que, ante una señal eléctrica, reproducen su respuesta con una aproximación aceptable.Electrochemical systems can be simulated (equivalent circuit) by a combination of resistors, capacitors and coils that, when presented with an electrical signal, reproduce their response with an acceptable approximation.
Un modelo de circuito equivalente que reproduce bastante bien la respuesta de muchos sistemas electroquímicos es el propuesto por Randles [J. E. Randles: Discuss. Faraday Soc, 1 (1947), 11; S. Feliu, J. M., Bastidas, y M. Morcillo: Quím. e Ind., 28 (1982),.'β35] consistente en una resistencia (Rτ) y condensador (Cd) en paralelo, ambos en serie con otra resistencia (Re). La resistencia de transferencia de carga, Rj, determina la velocidad de la reacción de corrosión y es una medida de la transferencia de electrones a través de la interfase [K. Hladky, L. M. Callow y J. L. Dawson: Br. Corros. J, 15 (1) (1980), 20].An equivalent circuit model that reproduces the response of many electrochemical systems quite well is that proposed by Randles [JE Randles: Discuss. Faraday Soc, 1 (1947), 11; S. Feliu, JM, Bastidas, and M. Morcillo: Quím. and Ind., 28 (1982) ,. ' β35] consisting of a parallel resistance (R τ ) and capacitor (Cd), both in series with another resistance (R e ). The charge transfer resistance, Rj, determines the rate of the corrosion reaction and is a measure of the transfer of electrons through the interface [K. Hladky, LM Callow and JL Dawson: Br. Corros. J, 15 (1) (1980), 20].
El circuito de Randles, como todas las generalizaciones, es una aproximación y una simplificación al mismo tiempo. Dado que la resistencia ofrecida por condensadores y autoinducciones, es función de la velocidad de variación de la señal eléctrica, para obtener una información adecuada de los circuitos equivalentes, aplicar señales de alterna con una amplia gama de frecuencias.The Randles circuit, like all generalizations, is an approximation and a simplification at the same time. Since the resistance offered by capacitors and self-inductions is a function of the speed of variation of the electrical signal, to obtain adequate information from equivalent circuits, apply alternating signals with a wide range of frequencies.
La asimilación de las interfases electroquímicas, tales como la superficie de un metal que se corroe, a circuitos equivalentes, hace que, aunque la corriente de las pilas de corrosión es continua, pueda estudiarse su efecto con excitaciones sinusoidales como las de corriente alterna.The assimilation of electrochemical interfaces, such as the surface of a corroded metal, to equivalent circuits, means that, although the current of the corrosion cells is continuous, its effect can be studied with sinusoidal excitations such as alternating current.
Al aplicar un potencial sinusoidal a un sistema metal/medio, o a un circuito equivalente, se obtiene como respuesta una intensidad sinusoidal con un cierto ángulo de desfase v. La relación ΔE/ΔI es otra función sinusoidal; la impedancia Z para un circuito RC en paralelo, como el de Randles, viene dada porBy applying a sinusoidal potential to a metal / medium system, or to an equivalent circuit, a sinusoidal current with a certain phase angle v is obtained in response. The ΔE / ΔI ratio is another sinusoidal function; the impedance Z for a parallel RC circuit, such as that of Randles, is given by
RT jwCR2 τ R T jwCR 2 τ
Z = Re +Z = R e +
1 + w2C2Rτ 1 + w2 C2Rτ1 + w 2 C 2 Rτ 1 + w 2 C 2 Rτ
La impedancia queda totalmente definida sin más que especificar su magnitud y el ángulo de fase, como en los diagramas de Argand, o bien sus componentes real e imaginaria, Z' y Z". Si se incluye también la frecuencia como variable, en circuitos equivalentes correspondientes a sistemas sencillos metal/medio sencillos, el extremo del vector impedancia describe un semicírculo en el plano complejo (figura 1), cuyas dimensiones permiten estimar los valores Re, Rj y C del circuito equivalente, o atribuibles al sistema en estudio [S. Feliu, J. M.; Bastidas, y M. Morcillo: Quím. e Ind., 28 (198Ϊ), 635; . Hladky, L. M. Callow y J. L. Dawson: Br. Corros. J, 15 (1) (1980), 20.].The impedance is fully defined by simply specifying its magnitude and phase angle, as in Argand diagrams, or its real and imaginary components, Z 'and Z ". If frequency is also included as a variable, in equivalent circuits corresponding to simple metal / medium simple systems, the end of the impedance vector describes a semicircle in the complex plane (figure 1), whose dimensions allow us to estimate the values R e , Rj and C of the equivalent circuit, or attributable to the system under study [S Feliu, JM; Bastidas, and M. Morcillo: Quím. And Ind., 28 (198Ϊ), 635; . Hladky, LM Callow and JL Dawson: Br. Corros. J, 15 (1) (1980), 20.].
El análisis de la ecuación que da Z, sustituyendo valores del circuito equivalente, conduce a:The analysis of the equation that gives Z, substituting values from the equivalent circuit, leads to:
Figure imgf000007_0001
Figure imgf000007_0001
que es la ecuación de un círculo de radio Rχ/2, conocido por diagrama de Nyquist o de Colé y Colé [K. S. Colé y R. Colé: J. Chem. Phys., 9 (1941), 341].which is the equation of a circle of radius Rχ / 2, known by Nyquist diagram or by Colé and Colé [K. S. Cole and R. Cole: J. Chem. Phys., 9 (1941), 341].
En el diagrama de Nyquist el eje horizontal representa la parte real de la impedancia del electrodo, es decir, su componente resistivo y el eje de ordenadas el componente imaginario o reactancia capacitiva. A frecuencias elevadas, del orden de 10 kHz o mayores, el condensador conduce tan fácilmente que cortocircuita Rx y permanece sólo el efecto de la resistencia del electrolito y de las capas superficiales, Re, valor que corresponde a la intersección izquierda del semicírculo con el eje de abscisas. Cuando decrece la frecuencia, el condensador conduce cada vez menos, pues la reactancia capacitiva es inversamente proporcional a ω, hasta que a bajas frecuencias o en corriente continua (frecuencia cero) el condensador deja de conducir y la impedancia se iguala a la suma Re + Rτ marcada por la intersección derecha del semicírculo con el eje real.In the Nyquist diagram the horizontal axis represents the real part of the electrode impedance, that is, its resistive component and the ordinate axis the imaginary component or capacitive reactance. At high frequencies, on the order of 10 kHz or higher, the capacitor conducts so easily that it short circuits Rx and only the effect of the resistance of the electrolyte and surface layers, R e , remains, a value that corresponds to the left intersection of the semicircle with the abscissa axis. When the frequency decreases, the capacitor conducts less and less, since the capacitive reactance is inversely proportional to ω, until at low frequencies or in direct current (zero frequency) the capacitor stops conducting and the impedance equals the sum R e + R τ marked by the right intersection of the semicircle with the real axis.
La obtención del diagrama de impedancias en el más amplio dominio de frecuencias constituye un examen analítico (una especie de firma espectral), de los diversos procesos que, de otra manera, intervienen y se manifiestan global y simultáneamente en una solicitación por corriente continua del sistema estudiado [S. Feliu, J. M.; Bastidas, y M. Morcillo: Quím. e lnd., 28 (1982), 635].Obtaining the impedance diagram in the widest frequency domain constitutes an analytical examination (a kind of spectral signature) of the various processes that otherwise intervene and manifest themselves globally and simultaneously in a direct current request from the system studied [S. Feliu, J. M .; Bastidas, and M. Morcillo: Quím. and lnd., 28 (1982), 635].
La exactitud de las predicciones cinéticas basadas en técnicas electroquímicas requiere que la velocidad de corrosión esté bajo control de activación, que exista relación causa-efecto entre la señal o polarización aplicada y la respuesta medida y que la correlación entre aquella y ésta sea lineal. La condición de linealidad se fuerza, a menudo, recurriendo a señales de pequeña amplitud, bien se trate de impulsos, escalones o rampas de tensión, u ondas sinusoidales.The accuracy of kinetic predictions based on electrochemical techniques requires that the corrosion rate be under activation control, that there be a cause-effect relationship between the applied signal or polarization and the measured response, and that the correlation between that and this is linear. The linearity condition is often forced by resorting to small amplitude signals, whether they are impulses, voltage steps or ramps, or sine waves.
En estas condiciones ideales, la velocidad de corrosión está inequívocamente relacionada con la resistencia de polarización, Rp; en las medidas con corriente alterna, se obtienen semicírculos bien definidos, que permiten determinar simultáneamente Re y Rx, siendo R- + Rτ totalmente equivalente a Rp.Under these ideal conditions, the corrosion rate is unambiguously related to the polarization resistance, R p ; in steps with alternating current, semicircles well defined are obtained, which allow simultaneous determination and R and Rx, R + R τ being fully equivalent to R p.
Los sistemas reales se apartan con frecuencia de la situación descrita, a causa de procesos de absorción, difusión, o de otros procesos redox distintos y supeφuestos al de corrosión, que enmascaran a éste, deforman el diagrama de impedancia y complican los circuitos equivalentes (figura 2).Real systems often deviate from the situation described, due to absorption, diffusion, or other redox processes different from and superimposed on corrosion, which mask it, distort the impedance diagram and complicate equivalent circuits (figure two).
El efecto del transporte de masas por difusión se reproduce, en los circuitos equivalentes, introduciendo un elemento adicional W, conocido como impedancia de Warburg, en serie con Rj. La impedancia de Warburg es un número complejo con las partes real e imaginaria iguales e inversamente proporcionales a la raíz cuadrada de la frecuencia:The effect of diffusion mass transport is reproduced, in equivalent circuits, by introducing an additional element W, known as the Warburg impedance, in series with Rj. The Warburg impedance is a complex number with the real and imaginary parts equal and inversely proportional to the square root of the frequency:
Figure imgf000008_0001
Figure imgf000008_0001
siendo σ el llamado coeficiente de Warburg.where σ is the so-called Warburg coefficient.
En un diagrama de Nyquist, consecuentemente, W se traduce en una recta con pendiente unidad, que aparece en el rango de las bajas frecuencias, pues a frecuencias altas el término \Nω se hace despreciable. La figura 2 reproduce un diagrama de impedancias típico de control por difusión que, como era de esperar, es una combinación del semicírculo y una recta de 45°, es decir, de procesos puros de transferencia de carga y difusión. j_Jasadas en lo anteriormente expuesto, pueden citarse, por orden cronológico, las siguientes patentes americanas:In a Nyquist diagram, consequently, W translates into a straight line with unit slope, which appears in the range of low frequencies, since at high frequencies the term \ Nω becomes negligible. Figure 2 reproduces a typical diffusion control impedance diagram which, unsurprisingly, is a combination of the semicircle and a 45 ° straight line, i.e. pure charge transfer and diffusion processes. j_Jasadas in the foregoing, the following American patents may be cited in chronological order:
* US 4.090.170 "Process and apparatus for investigating the activity of a cathodic protection unit (Proceso y aparato para investigar la actividad de una unidad de protección catódica)".* US 4,090,170 "Process and apparatus for investigating the activity of a cathodic protection unit."
* US 4.238.298 "Corrosión rate measuring method and apparatus (Método y aparato para medir la velocidad de corrosión)".* US 4,238,298 "Corrosion rate measuring method and apparatus".
* US 4.591.792 "Method and apparatus for measuring the polarized potential of a buried or submerged structure proteted by impressed current (Método y aparato por medir el potencial polarizado de protección de un una estructura enterrada o sumergida por corriente impresa)".* US 4,591,792 "Method and apparatus for measuring the polarized potential of a buried or submerged structure protected by impressed current." (Method and apparatus for measuring the polarized protective potential of a structure buried or submerged by impressed current).
* US 4.351.703 "Cathodic protection monitoring (Supervisado de protección catódica)"* US 4,351,703 "Cathodic protection monitoring"
* US 4.831.324 "Method and apparatus for analizing the electrode impedance (Método y aparato para el análisis de la impedancia de electrodo)".* US 4,831,324 "Method and apparatus for analizing the electrode impedance".
* US 5.674.375 "Method for detecting the presence or absence of corrosión of cathodically protected estructures (Método para descubrir la presencia o ausencia de corrosión de estructuras protegidas catódicamente)".* US 5,674,375 "Method for detecting the presence or absence of corrosion of cathodically protected structures".
Breve descripción de la invenciónBrief description of the invention
En los procedimientos hasta ahora conocidos, como se acaba de describir, se predice la insuficiencia de protección catódica por la desviación del modelo de Randles, al que, añadiendo una parte correctiva de la difusión de materia (impedancia de Warburg), se obtiene una modificación del semicírculo del diagrama de Nyquist, consistente en que la parte final (correspondiente a las medidas a baja frecuencia) del semicírculo pasa a convertirse en una recta de pendiente 1.In the previously known procedures, as just described, the insufficiency of cathodic protection is predicted by the deviation of the Randles model, to which, adding a corrective part of the diffusion of matter (Warburg impedance), a modification is obtained of the semicircle of the Nyquist diagram, consisting in that the final part (corresponding to the low frequency measurements) of the semicircle becomes a line of slope 1.
Los autores de la presente invención han observado, utilizando un circuito en el que se supeφone una corriente alterna sinusoidal a la corriente continua impresa de protección, que el ángulo de fase entre la intensidad y el potencial de la corriente alterna aplicada, siempre que el intervalo de frecuencias de ésta esté comprendido entre 0.01 aThe authors of the present invention have observed, using a circuit in which a sinusoidal alternating current is superimposed on the protective printed direct current, that the phase angle between the intensity and the potential of the applied alternating current, whenever the interval frequency is between 0.01 to
100 Hz, baja a menos de un determinado valor cuando la corriente continua de protección es suficiente y, por consiguiente, la armadura está protegida frente a la corrosión.100 Hz, drops below a certain value when the direct current of Protection is sufficient and therefore the armor is protected against corrosion.
Igualmente, la invención permite conocer si un acero se está corroyendo activamente o no aunque no esté protegido catódicamente(verificación de pasividad).Likewise, the invention allows to know if a steel is actively corroding or not even if it is not cathodically protected (passivity verification).
Breve descripción de las figurasBrief description of the figures
Figura 1. Representación del diagrama de Nyquist correspondiente a un sistema ausente de corrosión según J.A. González Fernández "Control de la corrosión: estudio y medida por técnicas electroquímicas". C.S.I.C, Madrid 1989, p. 202.Figure 1. Representation of the Nyquist diagram corresponding to a system without corrosion according to J.A. González Fernández "Corrosion control: study and measurement by electrochemical techniques". C.S.I.C, Madrid 1989, p. 202.
Figura 2. Representación del diagrama de Nyquist correspondiente a un sistema sometido a corrosión según J.A. González Fernández, o.c. p. 207.Figure 2. Representation of the Nyquist diagram corresponding to a system subjected to corrosion according to J.A. González Fernández, o.c. p. 207.
Figura 3. Figura de Lissajous correspondiente al oscilograma de un circuito RL sometido a una corriente alterna sinusoidal [Elipse representando i = í(u) y parámetros característicos].Figure 3. Lissajous figure corresponding to the oscillogram of an RL circuit subjected to a sinusoidal alternating current [Ellipse representing i = í (u) and characteristic parameters].
Figura 4. Esquema de bloques del procedimiento para conocer si una estructura metálica enterrada, protegida catódicamente, está sometida a corrosión (en la figura, aplicación a la corrosión de una armadura de hormigón armado).Figure 4. Block diagram of the procedure to know if a buried metal structure, cathodically protected, is subject to corrosion (in the figure, application to corrosion of a reinforced concrete reinforcement).
Figura 5. Dispositivo experimental para conocer si hay corrosión de la armadura en un bloque de hormigón armado protegido catódicamente. En la figura se encierran por una línea de puntos cada una de las unidades que componen el dispositivo:Figure 5. Experimental device to know if there is corrosion of the reinforcement in a cathodically protected reinforced concrete block. In the figure, each of the units that make up the device are enclosed by a dotted line:
* Unidad de medida y control. Contiene la CPU y se encuentra conectada (ver diagrama de bloques de la figura 4) con las unidades de confinamiento y medida.* Unit of measurement and control. It contains the CPU and is connected (see block diagram in figure 4) with the confinement and measurement units.
* Unidad de confinamiento. Contiene los siguientes componentes básicos: 1 Dispositivo de unión de señales.* Confinement unit. It contains the following basic components: 1 Signal joining device.
2 Amplificador operacional.2 Operational Amplifier.
3 Dispositivo comparador de señales. .47 Medida de la diferencia de potencial, Vss, entre los dos electrodos extras de referencia 15 y 16.3 Signal comparing device. .47 Measurement of the potential difference, V ss , between the two extra reference electrodes 15 and 16.
5 Medidor de la corriente Ixce.5 I xce current meter .
* Unidad de generación de corriente alterna. Constituye la parte esencial de la invención; se produce una corriente alterna, y se mide el desfase entre dicha corriente y la tensión que la genera. Contiene los siguientes componentes básicos:* Alternating current generation unit. It constitutes the essential part of the invention; an alternating current is produced, and the phase difference between said current and the voltage that generates it is measured. It contains the following basic components:
6 Medidor de la tensión Er.6 Voltage meter E r .
7 Generador de corriente alterna.7 Alternating current generator.
8 Dispositivo de unión de señales. 9 Amplificador operacional.8 Signal joining device. 9 Operational Amplifier.
10 Medidor de la corriente Ice-10 Current meter I c e-
* Unidad de medida y control. Procesa los datos recibidos y calcula, a partir del desfase entre la tensión y la corriente alterna si la estructura metálica (en este caso la armadura 11 de una losa 12 de hormigón armado) está o no suficientemente protegida catódicamente.* Unit of measurement and control. It processes the received data and calculates, from the difference between the voltage and the alternating current, whether or not the metal structure (in this case the reinforcement 11 of a reinforced concrete slab 12) is cathodically sufficiently protected.
* Sensor para realizar la medida. Contiene el anillo de guarda, el contraelectrodo, el anillo de medida y los electrodos. Esta parte del equipo es esencial para la invención pero no constituye parte de la misma. Comprende:* Sensor to perform the measurement. It contains the guard ring, the counter electrode, the measurement ring and the electrodes. This part of the kit is essential to the invention but does not constitute part of it. Understands:
13 Electrodo de referencia. 14 Contraelectrodo central.13 Reference electrode. 14 Central counter electrode.
15 Electro extra de referencia.15 Electro reference extra.
1 Electro extra de referencia.1 extra reference electro.
17 Contraelectrodo extemo.17 External counter electrode.
18 Fuente de protección catódica.18 Source of cathodic protection.
Figura 6. Diagrama de bloques de una manera de realizar la invenciónFigure 6. Block diagram of a way to carry out the invention
Descripción detallada de la invenciónDetailed description of the invention
El procedimiento, objeto de la presente invención, para determinar la presencia o ausencia de corrosión en una estructura de acero protegida o no catódicamente, consiste en supeφoner a la corriente de protección catódica o al potencial de referencia una corriente" sinusoidal de frecuencia variable y medir el desfase existente, v, entre la tensión y la corriente aplicada.The procedure, object of the present invention, to determine the presence or absence of corrosion in a steel structure protected or not cathodically, consists of superimposing the cathodic protection current or the reference potential a current "variable frequency sinusoidal measuring the gap, v, between the voltage and current applied.
Una manera sencilla de medir el ángulo de fase v, es mediante la determinación de los parámetros de las denominada figuras de Lissajous, obtenidas en un oscilógrafo; al que llega, como eje de abscisas, el valor de la tensión alterna u aplicada entre la armadura metálica y un electrodo de referencia en contacto con la superficie del cemento, y como eje de ordenadas el correspondiente valor de i de dicha corriente alterna.A simple way to measure the phase angle v is by determining the parameters of the so-called Lissajous figures, obtained on an oscillograph; The value of the alternating voltage u applied between the metal reinforcement and a reference electrode in contact with the cement surface arrives, as the axis of abscissa, and as the axis of ordinates the corresponding value of i of said alternating current.
Cuando se trata de una señal sinusoidal u = Uo sen ω t la intensidad de corriente presenta un cierto desfase v, viniendo dada por i = i0 sen (ωt -φ)When it is a sinusoidal signal u = U or sin ω t the current intensity presents a certain phase shift v, given by i = i 0 sin (ωt -φ)
Lafigura de Lissajous obtenida es (figura 3) una elipse, con los ejes rotados respecto a los de coordenadas, que corresponde a la ecuaciónThe Lissajous figure obtained is (figure 3) an ellipse, with the axes rotated with respect to the coordinate axes, which corresponds to the equation
Figure imgf000012_0001
Figure imgf000012_0001
Denominando i' y u' a las ordenadas y abscisas de los puntos de corte de la elipse con los correspondientes ejes de coordenadas, se tieneDenoting i 'and u' the ordinates and abscissa of the ellipse cutoff points with the corresponding coordinate axes, we have
φ = arcsen- u0 φ = arcsen- u 0
Opcionalmente, en el caso de utilizar este sistema de medida, es posible también conocer el valor del argumento, Z , de la impedancia del circuito,Optionally, in the case of using this measurement system, it is also possible to know the value of the argument, Z, of the impedance of the circuit,
1 ' i- 1 'i-
Sin necesidad, por tanto, de dibujar la figura de Lissajous, se puede, mediante un sistema de computerización, medir los valor de u0, i0, u' e i', y utilizando un coprocesador matemático determinas v y Z , a la vez que un frecuentímetro da el valor de ω de la tensión sinusoidal aplicada.Without the need, therefore, to draw the Lissajous figure, it is possible, by means of a computerization system, to measure the values of u 0 , i 0 , u 'and i', and using a mathematical coprocessor you determine v and Z, at the same time that a frequency meter gives the value of ω of the applied sinusoidal tension.
Naturalmente, la determinación del ángulo v puede realizarse de otras muchas manera, por ejemplo mediante un puente de impedancias. No se insiste sobre este tema dado que la medida del ángulo no es el objeto de la presente invención, sino cómo conociendo el ángulo se puede saber si un circuito está o no suficientemente protegido contra la corrosión , sin necesidad de suspender la protección durante la medida.Naturally, the determination of the angle v can be carried out in many other ways, for example by means of an impedance bridge. This topic is not emphasized since the measurement of the angle is not the object of the present invention, but how knowing the angle it is possible to know whether or not a circuit is sufficiently protected against corrosion, without the need to suspend the protection during the measurement .
Los autores de la presente invención han comprobado que, respetando el principio básico de que el desfase entre el voltaje y la intensidad de la corriente alterna modulada determina si la protección catódica es eficaz o si el acero está o no pasivo, existe un factor que mejora la reproducibilidad y precisión de las medidas. Este factor consiste en realizar un confinamiento previo de la corriente mediante el sistema de los anillos de guarda empleado para medir la velocidad de corrosión en armaduras sin protección catódica por Feliú y otros [Feliú, S., González, J.A., Andrade, M.C., Escudero, M.L. y Feliú, Jr., "Procedimiento de determinación electroquímica, no destructiva y cuantitativa, de la velocidad de corrosión metálica en estructuras de hormigón armado de grandes dimensiones, mediante el confinamiento sensorizado de una señal eléctrica que se aplica", Patente española P8901100 (núm. pub. 2013128) solicitada el 29/03/89] y descrito por los autores posteriormente [Feliú, S., González, J.A., Feliú, S. Jr. and Andrade, C. "Confinement of the electrical signal for ín-situ measurements of polarization resistance in reinforced concrete". Mater. J. ACI, September-October 1990, 457-460].The authors of the present invention have verified that, respecting the basic principle that the phase shift between the voltage and the intensity of the modulated alternating current determines if the cathodic protection is effective or if the steel is passive or not, there is a factor that improves reproducibility and precision of measurements. This factor consists in carrying out a previous confinement of the current by means of the guard ring system used to measure the corrosion rate in armor without cathodic protection by Feliú and others [Feliú, S., González, JA, Andrade, MC, Escudero , ML and Feliú, Jr., "Procedure for the electrochemical determination, non-destructive and quantitative, of the speed of metallic corrosion in large-size reinforced concrete structures, by means of the sensorized confinement of an electrical signal that is applied", Spanish patent P8901100 (no. . pub. 2013128) requested on 03/29/89] and described by the authors later [Feliú, S., González, JA, Feliú, S. Jr. and Andrade, C. "Confinement of the electrical signal for in-situ measurements of polarization resistance in reinforced concrete. " Mater. J. ACI, September-October 1990, 457-460].
El procedimiento se presenta en el esquema de diagrama de bloques de la figura 4, que pasamos a describir: Una unidad de control actúa sobre un generador de frecuencia cuya corriente, convenientemente amplificada, se trasmite a los electrodos del sensor. La unidad de adquisición de datos emite las correspondientes señales y recibe los correspondientes valores que reenvía a la unidad de control. Esta procesa los datos que envía a una pantalla, imprime y/o guarda en una unidad de disco. £h la figura 5 se muestra con más detalle este diagrama de bloques. Como ya se ha expuesto al describir las figuras hay una primera unidad que permite el confinamiento de la corriente continua sobre la que se modula la corriente alterna que va a ser medida. Esta unidad, que denominamos unidad de confinamiento, contiene un dispositivo de unión de señales, 1, que recibe por una parte la señal del diferenciador 3 y, por otra parte, la señal alterna producida en 7; la señal producida por 1 pasa a un amplificador operacional que genera la corriente continua, Ixce, de confinamiento, que se mide en el amperímetro 5 y llega al contraelectrodo extemo 17 del sensor. Para el control de esta corriente, en esta unidad se reciben las señales de dos electrodos extras de referencia 15 y 16, situados en el sensor que contiene el electrodo de guarda; estas señales llegan a un voltímetro 4, que mide la diferencia de potencial, Vss, entre los dos electrodos, que a su vez, en el dispositivo comparador de señales 2, se transforma en una señal que se envía a 1.The procedure is presented in the block diagram scheme of figure 4, which we now describe: A control unit acts on a frequency generator whose current, suitably amplified, is transmitted to the sensor electrodes. The data acquisition unit emits the corresponding signals and receives the corresponding values which it forwards to the control unit. It processes the data that it sends to a screen, prints and / or saves on a disk drive. In Figure 5 this block diagram is shown in more detail. As already explained when describing the figures, there is a first unit that allows the confinement of the direct current on which the alternating current to be measured is modulated. This unit, which we call confinement unit, contains a signal joining device, 1, which receives on the one hand the signal from the differentiator 3 and, on the other hand, the alternating signal produced at 7; the signal produced by 1 passes to an operational amplifier that generates the confining direct current, I xce , which is measured in ammeter 5 and reaches the external counter electrode 17 of the sensor. To control this current, this unit receives the signals from two extra reference electrodes 15 and 16, located on the sensor that contains the guard electrode; These signals reach a voltmeter 4, which measures the potential difference, V ss , between the two electrodes, which in turn, in the signal comparator device 2, is transformed into a signal that is sent to 1.
La siguiente unidad, que denominamos unidad de generación de corriente alterna, constituye la parte esencial de la invención, ya que de la unidad de confinamiento (en determinadas condiciones dependientes del dimensionado de la armadura a medir) podría prescindirse; sin embargo, dado que estas condiciones, en algunos casos, son desconocidas del operador, no parece prudente prescindir de la unidad de confinamiento en el modelo comercial.The following unit, which we call alternating current generation unit, constitutes the essential part of the invention, since the confinement unit (under certain conditions depending on the dimensioning of the reinforcement to be measured) could be dispensed with; however, since these conditions, in some cases, are unknown to the operator, it does not seem prudent to dispense with the confinement unit in the business model.
En la unidad de generación se produce una corriente alterna, y se mide el desfase entre dicha corriente y la tensión que la genera. Contiene un generador de corriente alterna 7 de frecuencia variable (entre 0,01 y 100 Hz), cuya señal se aplica, a través de un dispositivo de unión de señales 8, a un amplificador operacional 9, que envía una corriente de intensidad Ice, medida en el amperímetro 10, al contraelectrodo 14 formado por el anillo central del sensor de confinamiento; en la parte central, un electrodo de referencia 13, determina la tensión, Er, en este punto que se retroalimenta en el sumador 8 y se mide en el voltímetro 6.An alternating current is produced in the generation unit, and the phase difference between said current and the voltage that generates it is measured. It contains an alternating current generator 7 of variable frequency (between 0.01 and 100 Hz), the signal of which is applied, through a signal joining device 8, to an operational amplifier 9, which sends a current of intensity I ce , measured on the ammeter 10, to the counter electrode 14 formed by the central ring of the confinement sensor; in the central part, a reference electrode 13 determines the voltage, E r , at this point which is fed back into the adder 8 and is measured by the voltmeter 6.
Aunque todas las operaciones podrían realizarse de forma manual, es aconsejable, como se indica en la figura 5, computarizar el proceso, de forma que automáticamente se inicien las aquellas operaciones, incluso se repitan varias veces las medidas y, a través de los datos obtenidos se hagan los cálculos oportunos para la determinación del ángulo de desfase y se compare con el valor predeterminado (normalmente 10°) y se obtengan los resultados en pantalla, impresora y/o disco.Although all the operations could be carried out manually, it is advisable, as indicated in figure 5, to computerize the process, so that those operations are automatically started, even the measurements are repeated several times and, through the obtained data, make the appropriate calculations to determine the offset angle and compare with the predetermined value (usually 10 °) and obtain the results on screen, printer and / or disk.
Aunque, de acuerdo con la presente invención, es suficiente realizar una sola medida de frecuencia en el rango especificado de 0,01 a 100 Hz, en la práctica es conveniente la realización de 3 medidas para comprobar que todas ellas dan un ángulo inferior al predeterminado (normalmente 10°), ya que por un error operativo podría darse la circunstancia de que, en un caso concreto, accidentalmente, se registrara un ángulo inferior al predeterminado cuando el real fuera mayor. Las probabilidades de que este error accidental ocurra en tres medidas es prácticamente nulo; en cualquier caso, es conveniente comprobar periódicamente el dispositivo sobre un circuito RL de desfase conocido.Although, according to the present invention, it is sufficient to carry out a single frequency measurement in the specified range of 0.01 to 100 Hz, in practice it is convenient to carry out 3 measurements to verify that all of them give an angle less than the predetermined (normally 10 °), since due to an operational error, the circumstance could occur that, in a specific case, an angle lower than the default was registered accidentally when the real one was greater. The probability of this accidental error occurring in three measures is practically nil; in any case, it is convenient to periodically check the device on a known phase shift RL circuit.
Aunque tanto la exposición que acaba de hacerse como los ejemplos que se dan a continuación se refieren a las medidas sobre hormigón armado, todo ello puede aplicarse a tuberías o cualquier tipo de estructuras de acero sumergidas en el terreno.Although both the discussion just made and the examples given below refer to measurements on reinforced concrete, all this can be applied to pipes or any type of steel structures submerged in the ground.
Dispositivo de medidaMeasuring device
El diagrama de bloques de la figura 4 puede materializarse en el que se presenta en la figura 6 que da el esquema de un dispositivo basado en todo lo anteriormente expuesto. El método de adquisición de datos se basa en un circuito convencional realizado en base a un microprocesador que se encarga de controlar las diversas señales y activar los diferentes circuitos.The block diagram of figure 4 can be embodied in the one presented in figure 6 which gives the schematic of a device based on all the aforementioned. The data acquisition method is based on a conventional circuit based on a microprocessor that is responsible for controlling the various signals and activating the different circuits.
Aunque en los bloques de la figura 6 se han presentado los textos, junto a los cuadros más importantes se ha marcado un número que nos servirá de referencia en la siguiente descripción.Although the texts have been presented in the blocks of figure 6, a number has been marked next to the most important tables that will serve as a reference in the following description.
Existen dos tipos de circuitos alrededor del microprocesador (por ejemplo un CPU486) [los números en negrita corresponden a los de la figura 6]: a) Circuitos periféricos del procesador.There are two types of circuits around the microprocessor (for example a CPU486) [the numbers in bold correspond to those in figure 6]: a) Peripheral circuits of the processor.
19 Pantalla para mostrar los textos y gráficos al operador.19 Screen to show texts and graphics to the operator.
20 Teclado, que permite al operador introducir los datos necesarios y activar los procesos de medida a realizar. 21 Los datos y resultados obtenidos se guardan en una memoria que puede utilizarse para trasladar estos datos a un ordenador personal para tratar los resultados. Esta tarjeta respeta la conexión estándar denominado P.C.M.C.I.A.20 Keyboard, which allows the operator to enter the necessary data and activate the measurement processes to be carried out. 21 The data and results obtained are stored in a memory that can be used to transfer this data to a personal computer to process the results. This card respects the standard connection called P.C.M.C.I.A.
22 Los datos pueden ser emitidos y recibidos adicionalmente mediante una conexión estándar RS-232.22 Data can be additionally sent and received via a standard RS-232 connection.
b) Circuitos de interfase ό unión con el sensor, que pueden introducir una excitación (tensión o corriente) y obtener la respuesta:b) Interface circuits or sensor connection, which can introduce an excitation (voltage or current) and obtain the response:
23 Las tensiones y corrientes involucradas en el proceso se obtienen por parte del microprocesador mediante un sistema, que convierte las señales eléctricas en niveles lógicos (bits), llamado ADC (Convertidor analógico a digital).23 The voltages and currents involved in the process are obtained by the microprocessor through a system that converts the electrical signals into logical levels (bits), called ADC (Analog to Digital Converter).
24 Puesto que los niveles de las señales pueden ser muy dispares y algunas muy débiles, para adaptarlas al ADC se utiliza un amplificador con ganancia programable (PGA).24 Since the signal levels can be very different and some very weak, a programmable gain amplifier (PGA) is used to adapt them to the ADC.
* 25 Las señales débiles pero supeφuestas a niveles continuos elevados se adaptan mediante un convertidor digital analógico (D/A) este módulo convierte los niveles lógicos del microprocesador en una tensión que se resta de la señal de entrada mediante el PGA. * 25 Weak but superimposed signals at high continuous levels are matched by a digital analog converter (D / A) this module converts the logic levels of the microprocessor to a voltage that is subtracted from the input signal by the PGA.
26 Para poder utilizar un solo sistema de medida con varias tensiones y corrientes se utiliza un dispositivo multiplexor que conecta la señal elegida a la cadena de medida.26 In order to use a single measurement system with various voltages and currents, a multiplexer device is used that connects the chosen signal to the measurement chain.
27 La adquisición de datos dispone de cuatro circuitos de muestreo y retención S&H que permiten retener cuatro señales en el nivel de un determinado instante para que puedan ser analizadas por la cadena de medida.27 Data acquisition has four S&H sampling and retention circuits that allow four signals to be retained at the level of a given moment so that they can be analyzed by the measurement chain.
28 La aplicación de tensiones y/o corrientes en el sistema para medir su respuesta se realiza mediante dos sistemas D/A (mencionados anteriormente).28 The application of voltages and / or currents in the system to measure its response is done using two D / A systems (mentioned above).
Estos módulos D/A pueden alcanzar la estructura mediante tres modos i) Directamente aplicando su salida de tensión al sistema. ii) Controlando un módulo potenciostato que permite mantener un determinado potencial en función de un electrodo de referencia, iii) Controlando una fuente de corriente que permite mantener una determinada corriente a través del circuito independiente de las variacio- nes del potencial.These D / A modules can reach the structure through three modes i) Directly applying their voltage output to the system. ii) Controlling a potentiostat module that allows maintaining a certain potential based on a reference electrode, iii) Controlling a current source that allows maintaining a certain current through the circuit independent of potential variations.
29 Todas las señales introducidas o medidas del sistema se realizan respecto de la conexión que se realiza a la estructura metálica mediante una pinza conectada a la barra.29 All the signals entered or measured in the system are made with respect to the connection made to the metal structure using a clamp connected to the bar.
Los diferentes circuitos y módulos que componen el sistema de medida necesitan para su funcionamiento varios niveles distintos de tensión que son proporcionados por una fuente de alimentación.The different circuits and modules that make up the measurement system require various different voltage levels for their operation, which are provided by a power supply.
Ejemplos Para realizar las medidas objeto de la presente invención se han utilizado una serie de probetas de hormigón armado de las siguientes características:Examples A series of reinforced concrete specimens with the following characteristics have been used to carry out the measurements object of the present invention:
* Cemento 1-45 A de Valderrivas 380 kg/m3 * Valderrivas cement 1-45 A 380 kg / m 3
* Arena de río (tamaño máximo 4 mm) 771 kg/m* River sand (maximum size 4 mm) 771 kg / m
* Gravilla de machaqueo (tamaño máximo 12,5 mm) 1170 kg/m * Agua 152 kg/m3 * Crushing gravel (maximum size 12.5 mm) 1170 kg / m * Water 152 kg / m 3
Relación agua/cemento: 0,4Water / cement ratio: 0.4
Con esta mezcla se recubrió una armadura de acero construcción formada por redondos de 5 mm 0, constituyendo el conjunto una losa de hormigón armado de 1x2 m de superficie. * Tiempo de vibrado: 2 min 15 sWith this mixture, a steel reinforcement made of 5 mm rounds was covered, the whole constituting a reinforced concrete slab with a surface of 1 x 2 m. * Vibration time: 2 min 15 s
Utilizando el dispositivo de medida descrito, se han recogido una serie de datos correspondientes a las probetas de la losa en diferentes estados de protección Los resultados han sido los siguientes: Eiemplo.n0 1. Probetas con corrosión moderada sin protección catódica. Los resultados obtenidos se muestran en la tabla IUsing the described measuring device, a series of data has been collected corresponding to the slab specimens in different protection states. The results have been as follows: Example.n 0 1. Test tubes with moderate corrosion without cathodic protection. The results obtained are shown in Table I
Ejemplo n° 2. Probetas con corrosión moderada con protección catódica durante 24 h. Los resultados obtenidos se muestran en la tabla IIExample n ° 2. Test tubes with moderate corrosion with cathodic protection for 24 h. The results obtained are shown in Table II
Ejemplo n° 3. Probetas con corrosión moderada con protección catódica durante 48 h. Los resultados obtenidos se muestran en la tabla IIIExample n ° 3. Test tubes with moderate corrosion with cathodic protection for 48 h. The results obtained are shown in Table III
TABLA ITABLE I
Figure imgf000018_0001
TABLA III
Figure imgf000018_0001
TABLE III
Figure imgf000019_0001
Figure imgf000019_0001

Claims

REIVINDICACIONES
1. Procedimiento para conocer si un acero enterrado (especialmente en las armaduras del hormigón) está protegido catódicamente contra la corrosión o si está pasivo o no, sin tener que interrumpir ésta, caracterizado porque comprende las siguientes etapas: a) Se aplica entre el acero y un contraelectrodo, situado sobre la superficie exterior del terreno o del hormigón, una corriente alterna cuya frecuencia está comprendida entre 0,01 y 100 Hz. b) Mediante un dispositivo convencional, se mide el desfase entre esta corriente y la diferencia de potencial medida entre el acero y un electrodo de referencia situado próximo al contraelectrodo. c) El ángulo medido se compara con un valor previamente fijado de forma que cuando el ángulo medido es inferior al fijado, se sabe que la protección es eficaz.1. Procedure to know if a buried steel (especially in concrete reinforcements) is cathodically protected against corrosion or if it is passive or not, without having to interrupt it, characterized in that it comprises the following stages: a) It is applied between the steel and a counter electrode, located on the exterior surface of the ground or concrete, an alternating current whose frequency is between 0.01 and 100 Hz. b) By means of a conventional device, the gap between this current and the measured potential difference is measured between the steel and a reference electrode located near the counter electrode. c) The measured angle is compared with a previously set value so that when the measured angle is less than the set angle, it is known that the protection is effective.
2. Procedimiento para conocer si un acero enterrado está protegido catódicamente o si está pasivo, según la reivindicación 1, caracterizado porque, en su mejor realización, el sensor de medida empleado es el conocido como anillo de guarda, formado por dos anillos metálicos concéntricos, siendo el interior el contraelectrodo de la reivindicación 1 y el exterior el contraelectrodo extemo por el que se hace pasar la corriente continua de confinamiento, definida porque su valor es tal que, la diferencia de potencial entre dos electrodos de referencia situados entre ambos anillos, es nula.2. Method for knowing if a buried steel is cathodically protected or if it is passive, according to claim 1, characterized in that, in its best embodiment, the measurement sensor used is known as guard ring, formed by two concentric metal rings, the inside being the counter electrode of claim 1 and the outside being the external counter electrode through which the continuous confinement current is passed, defined because its value is such that the potential difference between two reference electrodes located between both rings is void
3. Procedimiento para conocer si un acero enterrado está protegido catódicamente o si está pasivo, según la reivindicación 2, caracterizado porque la corriente alterna se modula sobre la corriente continua confinada.3. Method for knowing if a buried steel is cathodically protected or if it is passive, according to claim 2, characterized in that the alternating current is modulated over the confined direct current.
4. Procedimiento para conocer si un acero enterrado está protegido catódicamente o si está pasivo, según la reivindicación 1, caracterizado porque la diferencia de fase entre la tensión y corriente alterna, se hace llevando la tensión alterna u medida con el electrodo de referencia y otra tensión proporcional a la corriente i aplicada, a un oscilógrafo de rayos catódicos, midiendo la figura de Lissajous que se dibuja en pantalla. 4. Method for knowing if a buried steel is cathodically protected or if it is passive, according to claim 1, characterized in that the phase difference between the voltage and alternating current is made by carrying the alternating or measured voltage with the reference electrode and another voltage proportional to the current i applied to a cathode ray oscillograph, measuring the figure of Lissajous that is drawn on the screen.
5. Dispositivo para conocer si un acero enterrado está protegido catódicamente o si está pasivo, según las reivindicaciones 1 a 3, caracterizado porque la diferencia de fase entre la tensión y corriente alterna, se hace por un sistema formado por: a) Una unidad de medida y control. Que contiene la CPU y se encuentra conectada con las unidades de confinamiento y medida. b) Una unidad de confinamiento. Que contiene los siguientes componentes básicos: Dispositivo de unión de señales.5. Device for knowing if a buried steel is cathodically protected or if it is passive, according to claims 1 to 3, characterized in that the phase difference between the voltage and alternating current is made by a system consisting of: a) A unit of Measurement and control. It contains the CPU and is connected to the confinement and measurement units. b) A confinement unit. It contains the following basic components: Signal joining device.
Amplificador operacional. Dispositivo comparador de señales.Operational Amplifier Signal comparator device.
Medidor de la diferencia de potencial, Vss, entre los dos electrodos extras de referencia, situado entre los dos anillos que actúan de contraelectrodos. Un medidor de la corriente Ixce. c) Una unidad de generación de corriente alterna. Que produce una corriente alterna de intensidad i, y mide el desfase entre dicha corriente y la tensión que la genera.Potential difference meter, V ss , between the two extra reference electrodes, located between the two rings that act as counter electrodes. A meter of the current I xce . c) An alternating current generation unit. It produces an alternating current of intensity i, and measures the offset between said current and the voltage that generates it.
Contiene los siguientes componentes básicos:It contains the following basic components:
Medidor de la tensión Er.Voltage meter E r .
Generador de corriente alterna.AC generator.
Dispositivo de unión de señales. Amplificador operacional.Signal joining device. Operational Amplifier
Medidor de la corriente Ice.Current meter I ce .
Unidad de medida y control. La unidad de medida y control procesa los datos recibidos y calcula, a partir del desfase entre la tensión y la corriente alterna, si la estructura metálica (en este caso la armadura de una losa de hormigón armado) está o no suficientemente protegida catódicamente.Unit of measurement and control. The measurement and control unit processes the received data and calculates, from the gap between the voltage and the alternating current, if the metal structure (in this case the reinforcement of a reinforced concrete slab) is or is not sufficiently cathodically protected.
6. Procedimiento y dispositivo para conocer si un acero enterrado está protegido catódicamente o si está pasivo, según las reivindicaciones 1 a 5, caracterizado porque en el ángulo de desfase que determina si hay o no protección, es menor de 10 sexagesimales. 6. Procedure and device for knowing if a buried steel is cathodically protected or if it is passive, according to claims 1 to 5, characterized in that in the offset angle that determines whether or not there is protection, it is less than 10 sexagesimal.
PCT/ES2002/000295 2001-06-19 2002-06-14 Method and device used to detect corrosion in cathodically-protected buried steel, particularly in concrete reinforcements, or to determine if said corrosion is passive WO2002103330A1 (en)

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