EP0630426B1

EP0630426B1 - Process for mantaining a cathodic protection against corrosion and device for carrying out said process

Info

Publication number: EP0630426B1
Application number: EP92919515A
Authority: EP
Inventors: Gino Cadignani
Original assignee: Cgr Di Cadignani Pi Gino
Current assignee: Cgr Di Cadignani Pi Gino
Priority date: 1991-08-14
Filing date: 1992-08-12
Publication date: 1995-11-15
Anticipated expiration: 2012-08-12
Also published as: EP0630426A1; GR3018985T3; ITVR910070A1; IT1253258B; WO1993004218A1; DE69206157T2; DK0630426T3; ITVR910070A0; ES2082500T3; DE69206157D1

Abstract

The cathodic protection process against corrosion according to the invention features a single electrode (12), the anode, which is connected to the positive pole of an electrical power supply, whose negative pole is connected to the object to be protected, e.g. to a boiler. According to an essential feature of the invention, the intensity of the current necessary for mantaining the process is periodically raised or lowered for a predetermined lapse of time about a predetermined percentage value relative to the steady condition value, and during this phase in which a higher or a lower current is delivered the potential difference between boiler and anode is gauged. This potential difference is compared, by means of a comparator, with a predetermined reference value, and the deviation is used for determining the current intensity during the next steady condition phase.

Description

The present invention relates to a process for mantaining a cathodic protection agaist corrosion.
More particularly, the present invention relates to a galvanic process for mantaining a cathodic protection against corrosion on metallic objects, particularly on boilers, using an electrode acting as anode which is crossed by an external current and an electronic control potentiometer.
Furthermore, the invention relates to a device suitable for carrying out said process.
The invention may be mainly applied in the fields of thermo-mechanical and electrochemical industries.
Cathodic protection processes against corrosion in boilers or water heaters are known since long time.
Italian patent n. IT-A-1.079.586 discloses a device for carrying out a method which provides for the use of two electrodes, respectively an anode and a reference electrode which, together with the boiler wall, are connected to the three input terminals of an electrical generator; more precisely, the anode is connected to the positive pole, the boiler wall is connected to the negative pole, and the reference electrode is connected to the connecting pole of a reverse feedback circuit.
The system continuously gauges the potential difference appearing between the reference electrode and the boiler wall, without reducing nor interrupting the current.
The gauged potential difference is compared with a reference value which is seeked for by means of suitable adjustments of the current which is delivered between the anode and the boiler wall.
This system implies some disadvantages and drawbacks, since the measure of potential difference is carried out under current, comprises an ohmic term which is variable with water conductivity and with temperature, and which may therefore not be determined in advance.
In order to overcome this problem, the reference electrode is provided with a particular structure, according to which said electrode should be mounted close to the boiler wall.
However, this causes further drawbacks for assemblying the boiler structure.
EP-A-0 018 522 discloses a further cathodic protection method according to which a single electrode is used, acting as anode, which is connected to the positive pole of an electrical generator whose negative pole is connected to the boiler.
According to this method, the current which is delivered is periodically interrupted (i.e. its value is set to zero) and, during this phase, the potential difference existing between anode and boiler wall is gauged.
During the zero current phase, the anode acts as reference electrode.
In a similar way as that disclosed in the italian patent cited above, the gauged potential difference is compared with a reference value by means of a comparator, and the deviation between the values is used for determining the intensity of the current to be delivered during the next phase.
Measuring the potential difference between anode and boiler wall during a zero current phase aims to avoid the cited drawbacks due to the presence of a ohmic drop in the water.
However, this method provides for disadvantages and drawbacks too; in fact, when the delivered current is interrupted, the anode potential quickly drops from its value under current towards the zero current value, which is remarkably lower than the former.
This drop depends on intermediate desorbing processes which are carried out during the normal current operation, and on a superficial pH variation, the pH passing from an acid value, which is typical of the situation when current is flowing, to an alkaline value, which is typical of water.
This potential drop, which has a remarkably high value and which is very fast, causes the measurement which is carried out during the zero current phase to be scarcerly reproduceable; thus, the following control of the current intensity is not fully reliable.
Document WO 88/08462 discloses a process for measuring the polarised potential of a buried pipeline having impressed current cathodic potential. In particular, it relates to the measurement of the potential of a pipe which has more than one protection circuit.
This document mentions the problems due to IR drop and inductive transients associated with the measurement of the polarised potential.
In the process of this document each of the protection circuits are periodically pulsed on and off. The voltage is sampled over both the on and off pulse periods. The sampling technique used enables the problems of IR drop and inductive transients to be overcome. Furthermore, it permits an accurate measurement of the polarise potential, even when current is flowing in the pipe from other protection circuits. However, this document does not disclose adjusting the protection current to a value between 10 and 50% or 200% of the steady condition value. Furthermore, the use of altered currents as opposed to zero current to determine the polarised potential, which makes it possible to reduce the effect of the IR drop and transients on the measurements is not suggested in this document; rather, WO 88/08462 overcomes these problems by using a complex pulsing and sampling technique.
The present invention aims to obviate to the disadvantages and drawbacks of the background art, and to provide, thus, for a cathodic protection process which, while making use of a single electrode, allows a reproduceable current measurement to be carried out, and which therefore allows the intensity of the protection current to be reliably controlled.
This is achieved by carrying out a cathodic protection process against corrosion having the features disclosed in claim 1.
The dependent claims (2-5) disclose particularly advantageous forms of embodiment of the invention.
Furthermore, claims 6 to 11 describe a device suitable for carrying out the process according to the invention.
According to the invention, the cathodic protection process against corrosion features a single electrode, the anode, which is connected to the positive pole of an electrical power supply, whose negative pole is connected to the object to be protected, e.g. to a boiler.
According to an essential feature of the invention, the intensity of the current necessary for mantaining the process is periodically raised or lowered for a predetermined lapse of time about a predetermined percentage value relative to the steady condition value, and during this phase in which a higher or a lower current is delivered the potential difference between boiler and anode is gauged.
This potential difference is compared, by means of a comparator, with a predetermined reference value, and the deviation is used for determining the current intensity during the next steady condition phase.
In this way several advantages may be achieved relative to the known processes.
In fact, measuring the potential difference during a phase in which a lower current is delivered allows the ohmic drop value in the water to be substantially decreased up to a negligible value; this value is otherwise difficult to be gauged, since it varies from place to place and it depends on temperature.
Furthermore, the fact of simply reducing, and of not interrupting the intensity of the protection current, greatly reduces the dropping effect of the anode potential towards the zero current steady value; this fact allows a continuously reproduceable measurement to be carried out and a highly reliable controlling operation of the current intensity to be obtained.
According to another form of embodiment of the process according to the invention, the intensity of the protection current is periodically reduced to a fraction relative to the steady condition value for a predetermined lapse of time, and a voltage measurement is carried out during this phase.
By means of a suitable control electronics, the result of the measurement is linearly corrected by extrapolating the zero current value, and then compared with a reference value in order to obtain a value for controlling the current during the next steady condition phase.
According to a further form of embodiment of the process according to the invention, the intensity of the protection current is periodically doubled during a predetermined lapse of time, and the corresponding potential is gauged.
Yet, an extrapolation of the resulting value is carried out, and the corrected value is compared with a predetermined reference value in order to carry out the current control during the next cycle.
Other features and advantages of the invention will become apparent by reading the folowing description of a form of embodiment of the invention, given as a non-limiting example, with the help of the figure illustrated in the attached sheet, which shows a block diagram of a device suitable for carrying out a process for mantaining a cathodic protection current against corrosion according to the present invention.
In the figure, reference sign 10 generally indicates a block diagram representing the component parts of a device suitable for carrying out a process according to the invention.
Device 10 generally cooperates with an object 11 to be protected against corrosion, having a metallic surface which contacts water.
Commonly, object 11 is constituted by a boiler; however, the process according to the invention may be also applied to other metallic surfaces which are suitable for being corroded by water, such as boat parts, pipelines, gutters etc..
Boiler 11, which is filled with water consituting the electrolyte solution, houses an electrode 12 belonging to device 10, and which is connected to the positive pole of a current generator 20, whose negative pole is connected to the wall of boiler 11.
This electrode is generally constituted by a titanium bar activated by noble metals.
The electrical power supply for the whole circuit is delivered by a suitable DC source (not shown in the figure), which is generally constituted by a feeder which is connected to the AC mains and which is provided with a transformer, a rectifier circuit and suitable filters.
Furthermore, current generator 20 and electrode 10 are connected to a pair of Sample and Hold cells 14, 15, which are push-pull controlled by an oscillator 19.
According to this form of embodiment, cell 15 is rendered conductive, by means of a suitable control, relative to a current intensity equal to 100% of the steady condition current, while cell 14 is rendered conductive relative to a current intensity equal to 50% of the steady condition current.
Thus, in cell 15 a value V_100% (V₁) is stored, which represents the potential value which is present on anode 12 when the current intensity is equal to the steady condition current I_100% (I₁), while in cell 14 a value V_50% (V₂) is stored which represents the potential value which is present on anode 12 when the current intensity is equal to 50% of the steady condition current I_50% (I₂).
According to a feature of the invention, values V₁ and V₂ are used for carrying out a linear correction according to the following formula: $V₃=V₁-2(V₁-V₂)$
Value V₃ corresponds to an extrapolated value of the potential corresponding to zero current, and is compared with a potential reference value V_r which is predetermined and stored in a memory.
In order to practically implement said formula, device 10 comprises a first differential amplifier 16 which carries out the substraction (V₁-V₂) and which, by a suitable choice of the resistance values, is set in order to obtain a total gain equal to two; in this way, the output voltage of differential amplifier 16 is equal to 2(V₁-V₂).
This potential is applied to one of the input terminals of a second differential amplifier 17, which substracts said value from the steadi condition potential V₁.
Thus, a potential equal to V₃ may be found at the output of amplifier 17.
Thereafter, potential V₃ is applied to an input terminal of a third differential amplifier 18, at the other input terminal of which being applied reference potential V_r, which is obtained from a divider module 22 by dividing the supply voltage; the output voltage of amplifier 18 is then applied to current generator 20.
As a consequence thereof, module 20 generates a current which is proportional to the output voltage of amplifier 18.
The circuit is completed by an electronic switch 21 which, by means of a control signal generated by oscillator 19, and timed with the conduction period (Sample) of cell 14, halves the output current of generator 20 and sets it to a value corresponding to 50% of the steady condition value.
The process described above is then periodically repeated and the current intensity is suitably controlled.
According to this form of embodiment, the potential value V₂ corresponds to a 50% reduction of the current intensity.
This fact does not involve a limitation of the process according to the invention, since value V₂ may be choosed within wide limits, corresponding to values of current intensity comprised between 10% and 50% of the steady condition current intensity; in any case, very reliable controls of the current intensity are achieved.
According to a second form of embodiment of the invention, it is possible to invert the inputs of differential amplifier 16, and to set it in order to obtain a gain equal to one, as well as to connect electronic switch 21 in such a way that the inversion causes the current which is generated at the output to be doubled.
In this case, the current is delivered to one cell with an intensity equal to 100% relative to the steady condition value, and to the other cell with an intensity equal to 200% relative to the steady condition value, for a short period; at the output terminal of amplifier 17 is then present a potential equal to $V₄=V₁-(V₂-V₁)$
and this value is compared with reference value V_r.
According to a third form of embodiment of the invention, divider 22 is connected to a water temperature sensor (not shown in the figure).
The insertion of a water temperature sensor has the purpose of compensating the potential variation on the anode, due to a water temperature change.
In the applications which are not provided with this sensor, reference potential V_r is generated by divider 22, which reduces the potential which is stabilized by a Zener diode (not shown in the figure); since V_r is fixed, it happens that, by controlling the current delivered by the circuit, the potential between anode and cathode varies in accordance with the different behaviour of the electrolyte solution interposed between anode and cathode at different operating temperatures.
However, when V_r is also controlled, by providing a water temperature sensor cooperating with the divider circuit 22, this reference potential may be varied in accordance with temperature changes in the electrolyte solution.
Therefore, by suitably rating the sensor in order to obtain a variation of reference potential V_r, the potential between anode and cathode is characterised by a more constant behaviour; this limited possibility of ranging of the potential allows a more reliable and homogeneous protection to be achieved; the danger of corrosion due to an overprotection in the case where passive protection materials (enamels, paintings etc.) are used, is also greatly reduced.
According to a fourth form of embodiment of the invention, a indicating device (not shown in the figure), which is generally constituted by a LED display, is connected to current generator 20.
This device has the purpose of delivering a visible indication of the operation of the protection circuit against corrosion.

Claims

Process for maintaining a cathodic protection agaist corrosion, in which an electrode (12) acting as anode in which flows an external current is immerged in an electrolyte solution contacting a metallic object (11) to be protected, acting as cathode, said process using a module (22) at the terminals of which a reference potential (V_r) is present, and an electronic circuit suitable for comparing data relative to the potential difference which is present between anode and cathode with said reference potential (V_r), characterised in that it comprises:
- a phase, which is periodically repeated in the time, in which the steady condition of the intensity of the protection current (I_100%) is raised or lowered to a predetermined value which is never equal to zero, said value being either comprised between 10% and 50% relative to the value of the steady condition current value, or equal to 200% relative to the steady condition current value;

- a phase in which the potential difference (V₂) which is present between anode (12) and cathode is gauged during this condition in which the current intensity is varied relative to the steady condition intensity, and

- a phase in which said potential difference and said reference potential (V_r) are compared to each other, the difference of values obtained in this way being used as control quantity for controlling the intensity of the protection current.
Process according to claim 1, characterised in that the potential values which respectively correspond to a steady condition current intensity (V₁) and to a reduced current intensity (V₂) are processed by said electronic circuit by operating a linear correction extrapolation of the potential value (V₃) corresponding to a current equal to zero in the anode (12), said potential (V₃) being then compared with the reference potential (V_r).
Process according to claim 2, characterised in that during the phase in which the steady condition intensity of the protection current is lowered to a value comprised between 10% and 50%, the potential value (V₃) corresponding to a current intensity equal to zero in the anode (12) which is extrapolated by said electronic circuit is obtained by implementation of the following formula: $V₃=V₁-2(V₁-V₂)$
in which V₁ represents the potential corresponding to the steady condition current intensity and V₂ represents the potential corresponding to the reduced current intensity.
Process according to claim 1, characterised in that the potential values which respectively correspond to a steady condition current intensity (V₁) and to a reduced current intensity (V₂) are processed by said electronic circuit by operating a linear correction extrapolation of the potential value (V₄), this potential (V₄) being then compared with the reference potential (V_r).
Process according to claim 4, characterised in that said linearly corrected potential value (V₄) is obtained by the implementation of the following formula: $V₄=V₁-(V₂-V₁)$
in which V₁ represents the potential corresponding to the steady condition current intensity and V₂ represents the potential corresponding to the raised current intensity.
Device (10) for carrying out a process according to anyone of the preceding claims, comprising an electrode (12) acting as anode immerged in an electrolyte solution contacting an object (11) to be protected against corrosion which acts as cathode, characterised in that it further comprises a pair of Sample and Hold cells (14, 15) for recording and storing data relative to the potential difference existing between anode and cathode under different conditions of intensity of the current which flows through the anode, a reference potential (V_r) source (22), first means (16, 17) for processing said data, second means (18) suitable for comparing the processed data with said reference potential (V_r), a current generator (20) controlled by said second means (18) and switchable to a current intensity which is respectively higher or lower than 100% steady condition current, and third, timing, means (19, 21) suitable for respectively raising or lowering the intensity of current at predetermined instants of time.
Device (10) according to claim 6, characterised in that said first means (16, 17) comprise a pair of differential amplifiers which are serially connected with respect to each other and which are suitably connected to said cells (14, 15).
Device (10) according to anyone of claims 6 and 7, characterised in that said second means comprise a differential amplifier (18), an input terminal of which is connected to the output terminal of said first means (16, 17), the other input terminal of which is connected to said reference potential (V_r) source, the output terminal of said amplifier (18) being connected to said current generator (20).
Device according to anyone of claims 6 to 8, characterised in that said third, timing, means (19, 21) comprise an electronic switch (21) and an oscillator (19).
Device (10) according to anyone of claims 6 to 9, characterised in that it is further provided with an electrolyte solution temperature sensor cooperating with said reference potential (V_r) source (22) in order to control the value of said reference potential (V_r) in accordance with temperature changes intervening in the electrolyte solution during operation.
Device (10) according to anyone of claims 6 to 10, characterised in that it is further provided with an indicating device for displaying the state of the protection process.