CA2073530A1 - Corrosion protection system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of cathodic corrosion protection of a conductive objective and a system therefore, this system comprising at least one electrode for contact with an electrolyte, a control unit connected to the electrode and for connection to the object to complete a circuit through said electrolyte, and a means for providing power to the control unit. The control unit comprises a switch means for interrupting current flow in the circuit at predetermined intervals, each time interval comprising a first time segment during which said current flow remains interrupted and a succeeding second time segment, a comparator means for measuring the potential difference between the electrode and the object when there is no current flow in the circuit and for comparing the measured potential to a predetermined voltage value, the control unit closing the circuit during the second time segment only if and when the comparison indicates that additional corrosion protection of the object is required.

Description

BP File No. 3491-011 3,a~

Title: CORROSIO~ PROTECTION SYSq~EM

FIELD OF THE I~lVENTION
The present invention relates to a self monitoring corrosion protection system and a method of corrosion protection. In particular, the present invention relates to a self-monitoring system which protects a conductive object by controlling the duration of current flow through a circuit comprising said object, an electrode and an electrolyte, without adjusting the intensity of the current.

BACRGROUIJD OF 1~ INVEN~rION
Corrosion of conductive metallic objects in contact with corrosive agents, or electrolytes, such as soil, water or an aqueous solution, is commonplace.
Metallic objects subject to corrosion include: metal pipes placed on river beds or buried underground; metal structural supports in contact with water for piers or offshore oil rigs; metal tanks containing corrosive agents; reinforcing rods embedded in concrete for use in roads, bridges, dams and buildings; and boats or ships.
Hence, corrosion control of metallic objects is often not only desirable but necessary.
Methods of corrosion control of such metallic objects are well known. It is generally understood that corrosion is an electro-chemical phenomenon, hence a standard method of protecting metallic objects in contact with electrolytes is termed "cathodic protection~'. In cathodic protection, a current (usually direct current rather than alternating current) is sent from an anode through the electrolyte to the object to be protected (i.e. the cathode), thereby altering the voltage or potential of the object and achieving a measure of protection against corrosion.
In the usual case, it is desirable to keep the potential low to reduce to zero or at least to minimize ~ ~ 7 the rate of corrosion. The potential of the object is monitored and compared against a suitable reference value, namely a "corrosion protection criterion value~' (termed CPCV). If the object's potential is above the CPCV, then it is inadequately protected and protective current must be applied. If the potential is equal to or somewhat below the CPCV, then the object is adequately protected against corrosion. If the potential is well below the CPCV, then the object is over-protected. Over-protection is undesirable because other types of damage (such as removal of paint from the object's surface) may occur and there is wastage of protective current and electrode material.
An error in measuring the object~s potential can occur due to resistance of the electrolyte. This so called "IR drop" error across the electrolyte is related to the magnitude of the current flow (~ ) and the resistance ("R") of the electrolyte to said current. An IR drop error may make an object appear to be protected when in fact it is not.
One practice to combat IR drop error is to interrupt the flow of protective current to the object, such as a buried pipeline, and take a potential measurement prior to restoring the protective current flow. Such an interruption will not necessarily terminate the object's cathodic protection. It is known that upon interruption of protective current flow, the potential of the protected object may rise relatively slowly, and so the object may remain protected (i.e. the potential will remain below the CPCV) for minutes or even hours after interruption.
An alternate method of avoiding IR drop error without the need to interrupt protective current flow is the use of a special instrument which automatically allows for or cancels out IR drop error.
Several cathodic protection systems have been proposed in the past for controlling the flow of .. ~ . , . .. ". ~ -~c~

protective current to an object and to combat IR drop error. Typical known systems are shown in U.S. patent 2,759,887 (Miles) issued August 21, 1956 and U.S. patent 4,755,267 (Saunders) issued July 5, 1988. A disadvantage of these prior art systems is that they require elaborate mechanisms to measure the protected object's potential, including the use of separate "reference" electrodes. Such mechanisms are also required to vary the intensity of protective current applied in the system to control the object's potential.
It is also known that an electrode (and to some extent the object~ subjected to current flow becomes polarized. With current flow interrupted to the electrode, some time must pass before the polarization disappears. A polarized electrode will not function as an accurate reference. Hence, the prior art systems use reference electrodes which are separate from the protective circuit for potential measurements.
What is therefore desired is to provide a protection system which minimizes or eliminates IR drop error and overcomes the disadvantages of the prior art systems. Preferably, the system should include a simple mechanism to control the flow of current, so as to maintain the desired potential of the object, whilst minimizing the current flow. Preferably it should not be as costly to make, install, and maintain as existing systems, and may be adapted for use in residential water heaters to reduce the rate of corrosion therein.

SUMNARY OF r~ P~ESENT INVENTION
According to the present invention, there is provided a corrosion protection system for a conductive object, the system comprising:
(a) at least one electrode for contact with an electrolyte;
(b) a control unit connected to the electrode and for connection to the object to 4 ~ 2~ J~J~'o~

complete a circuit through said electrolyte, said control unit comprising:
(i) switch means for interrupting current flow in the circuit at predetermined intervals, each time interval comprising a first time segment during which said current flow remains interrupted and a succeeding second time segment;
(ii) comparator means for measuring the potential difference between the electrode and the object and comparing the measured potential difference to a predetermined voltage value; and (iii) means for closing the circuit during the second time segment when said comparison indicates that additional corrosion protection of the object is required; and (c) means for providing power to the control unit.

In this system, the potential difference is measured when there is no current flow in the circuit and is compared to the predetermined voltage value, the current flow being resumed during the second time segment when said comparison indicates that additional corrosion protection of the ob~ect is required. Preferably, the electrode is the reference for measuring the potential of the object.
The present invention further includes mounting means for an electrode for use with said protection system comprising:
(i) an insulating collar mounted over a first end of the electrode in sealing engagement therewith;
(ii) a bushing mounted over the collar in '~ .

sealing engagement therewith, the bushing being electrically insulated from the electrode by the collar and being adapted for sealing engagement with an opening in the object to prevent the escape of electrolyte from the object;
(iii) a first terminal located on the first end of the electrode for receiving a first connection from a control unit; and (iv) a second terminal located on the bushing for receiving a second connection from the control unit.

According to the present invention there is also provided a method of corrosion protection of a conductive object in contact with an electrolyte, the method comprising:
(a) providing at least one electrode for contacting the electrolyte;
(b) connecting a control unit to the object and to the electrode to form a circuit through said electrolyte;
(c) providing power to the control unit;
(d) interrupting current flow in the circuit at predetermined intervals, each time interval comprising a first time segment during which said current flow.remains interrupted and a second time segment;
(e) measuring the potential difference between the electrode and the object;
(f) comparing the measured potential difference to a predetermined voltage value; and (g) resuming the current flow in the circuit during the second time segment only when said comparison indicates that additional ,. ,. -,. .
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corrosion protection of the object is required.
An advantage of the present invention is that, where the total amount of protective current flow available is limited, said flow is not wasted, and so corrosion protection is available over a longer period of time. A further advantage is that the possibility of over-protection is reduced or eliminated, therefore reducing the risk of other types of damage to the protected object.

BRIEF DESCRIPTION OF THE DRAWING FIGURES
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:
Figure 1 is a schematic sectional view showing one embodiment of the present invention;
Figure ~ is a graph showing sample time intervals according to the present invention;
Figure 3 is a sectional view of an object to be protected, typically a water storage and heating tank, showing another embodiment of the present invention; and Figure 4 is an electrical circuit schematic for the control electronics for use with one embodiment of the invention.

DESCRIPTIO~ OF THE PREFERRED EMBODIMENT
The description of the present invention focuses on cathodic protection of an object, although it is also applicable to protection by anodic passivation.
Figure 1 shows a basic embodiment of a cathodic corrosion protection system employing an "impressed current~'. An alternate embodiment of the system employing a l'sacrificial anode" to provide current therein will be discussed later, althouyh this invention also provides for use of sacrificial anodes with the impressed current system.

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In Figure 1 an electrode 2 and an object 4 to be protected are shown in contact with an electrolyte 6 which may comprise a soil, water or aqueous solution, capable of transmitting an electric current. The electrode 2 is connected to a control unit 8 via electrically insulated wire 10, and the object 4 is connected to the control unit 8 by electrically insulated wire 12 to complete a circuit through said electrolyte 6. In said electrolytic circuit, the electrode 2 is connected to a positive terminal 14 of the control unit 8 and so may be referred to as an "anode"; and the object 4 is connected to ~ negative terminal 15 of control unit 8 and so may be referred to as a 'Icathodell. It will be understood that the term "cathode~ will refer to any object subjected to cathodic protection. It will also be appreciated that in an impressed current system the anode 2 may be constructed of platinum or other suitable material (for example, a magnesium or ferrite rod). In another version of this invention (not shown in the Figures), the electrode 2 may be in the form of a wire (a tantalum wire, for example) which has a platinum coating. The platinum coated wire is in contact with the electrolyte 6 and has the dual function of transmitting current to the electrolyte and acting as a reference electrode.
The protection system includes a means for providing power for operation of the control unit 8. Said power means may comprise a standard alternating current ("AC") power supply, an internal or external battery, a solar collector, a heat conversion electrical source or the like. In this embodiment, the control unit 8 employs a suitable source of AC power, be it 110 volt or 220 volt, designated by the numeral 16. The AC power source 16 also provides power to a current supply unit 9 incorporated within the control unit 8. The current supply unit 9 converts the AC current into a direct current and provides "impressed" direct current flow or protective current flow to the electrolytic circuit, in this case to the anode 2 z~

as shown by the reference numeral 18. Arrow 20 indicates the direction of current flow through the electrolyte 6.
Hence, in this embodiment, the control unit 8 is also a source of direct current to the circuit. It will be appreciated that the current supply unit 9 need not be incorporated within the control unit and that the current supply unit 9 and control unit 8 can have separate power sources. It will also be appreciated that for large objects to be protected, one or more current supply units and control units, each with one or more anodes, may be necessary.
The control unit 8 further comprises a switch 30 for interrupting the current flow 18 in the circuit at predetermined intervals of time. Each time interval comprises a first and a succeeding second time segment.
Through the first time segment the switch 30 keeps the circuit open and the current flow 18 remains interrupted in the circuit. During the second time segment the switch 30 may close the circuit and resume the current flow 18 if and when the potential of the cathode 4 exceeds a preset maximum voltage value. The time intervals are selected appropriately in relation to the material of construction of the electrode, the properties of the electrolyte and in relation to the operating cycle of the conductive object.
The time intervals selected should also eliminate polarization of the electrode for it to act as an accurate reference.
To provide for adequate protection of the cathode, the preset voltage value sh~uld be set at or slightly below a suitable corrosion protection criterion value ("CPCV"), by 0.1 volts for example. The preset voltage value is typically set within .001 and 1.0 volts of the CPCV. As long as the cathode's potential is equal to or somewhat below the CPCV, it is protected against corrosion. Potentials far below the CPCV would render the cathode 4 "over-protected'l, increasing the chances of other types of damage occurring to the cathode, such as 2 ~ ? C~

the peeling of paint. Current flow would also occur needlessly. On the other hand, potentials above the CPCV
would leave the cathode under-protected, and so current 18 should be applied for longer periods of time--i.e. for a larger portion, or all, of the second time segment. It is understood that if the negative of the CPCV and the negative of the measured potential are used, then current flow would be resumed if the absolute value of the potential rises above, or exceeds, the absolute value of the CPCV.
The CPCV is determined on the basis of several factors, including the nature of the electrolyte, the temperature and pressure of the electrolyte, and the material of construction of the electrode.
The control unit 8 includes a comparator 50 for measuring the above discussed potential of the cathode 4 with respect to a reference electrode, in this case the anode 2, and comparing the potential to the preset (or predetermined) maximum voltage value. It will be appreciated that a separate reference electrode immersed in the electrolyte could be used for reference purposes, although this is not preferred because it is desired to keep the mechanism as simple as possible.
In the present invention, the potential of the object is measured and compared to the preset voltage value when there is no current flow 18 in the circuit.
The switch 30 resumes the current flow 18 in the circuit during the second time segment only if and when said measured potential exceeds said preset voltage value.
Once the current flow resumes, the measurement and comparison process is disabled.
The operation of the control unit 8 is best illustrated by way of example and with reference to the graph in figure 2. The vertical axis represents impressed current supplied by the unit, with the reference "I"
representing a preset intensity of current flow 18. The horizontal axis represents time "t" (in seconds). The switch 30 is set to interrupt the current flow I in the circuit at appropriate time intervals, say 90 seconds, and so the duration of each successive time period A1, A2 is 90 seconds. The duration of each successive first time segment B1, B2 is set for 30 seconds and each successive second time segment C1, C2 is set for 60 seconds. These times are for illustrative purposes only. The length of the time segments is selected based on the nature of the electrolyte, material of construction of the electrode, and other factors.
At time t=0 the switch 30 opens the circuit so that the magnitude of the current 18 is zero for the duration of time segment B1. In one embodiment, no measurement and comparison of the potential takes place during time segment B1. In another version of the invention, the comparator 50 proceeds to automatically measure and compare the potential of the cathode 4 to the preset voltage value during time segment Bl. To combat IR
drop error, the potential measurements take place only when there is no current flow. This first time segment also gives the reference electrode an opportunity to reduce or eliminate its polarization due to the prior current flow. The reference electrode functions as an accurate reference when the polarization has disappeared.
During the second time intervals indicated as C1, C2, the potential is measured and compared. The switch 30 will resume the current flow I if and when the measured potential exceeds the preset voltage value (i.e.
measurement and comparison of the potential continues as long as there is no current flow I). If said measured potential remains at or below the preset value throughout the time intervals C1, C2, switch 30 will not xesume the current flow I during said time intervals. If at some point during segment Cl the measured potential rises above the preset value, then the current flow I will be automatically resumed in the circuit and the monitoring of the potential will cease.

1 1 ~ 7 ~ A7 ~ ~ 7 ~

At time t=90 seconds (i.e. at the end of period Al and segment Cl), the switch 30 will again go to the non-conducting state to prevent current flow in the circuit whether or not current flow was resumed during segment Cl. During time period A2, the control unit 8 follows the same process as for period Al.
It is noted that the current flow could remain interrupted for many successive time periods until the measured potential rises above the preset value. It is known that the potential of the cathode may remain below the preset value, and hence the cathode will remain protected, for minutes or even hours after current flow is interrupted. It is also noted that the intensity of the current flow 18 is not adjusted by the control unit 8 at any point; the applied current flow is always at one of two levels: a reasonably constant non-zero level I, or zero. It will be appreciated that the present invention may be set to operate on a wide variety of time schedules.
Rather than using impressed current in the system, the electrode 2 of the present invention may comprise a sacrificial anode for cathodic protection.
The sacrificial anode is constructed of a suitable material (magnesium, for example) to provide protective current to the circuit. A sufficient number of said anodes should be used to provide enough current flow to reduce the potential of the object below the CPCV. For present purposes it will be assumed that one anode is sufficient. The sacrificial anode is itself subject to corrosion, or is "sacrificed~, to maintain current flow in the circuit.
The control unit in a sacrificial anode system performs in the same way as in the impressed current system (see description relating to figure 2). Although the control unit does not include any current supply unit for providing impressed current into the circuit, a power unit for providing power to the control unit is still necessary for the switch and comparator to function.

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Here, the switch simply opens or closes the circuit between the anode 2 and the cathode 4.
A commercial version of the present invention appears in figure 3 showing a ~hot water" tank 60 for storing a corrosive electrolyte 62 therein, typically domestic tap water containing low levels of dissolved substances. The tank 60 is adapted to heat the water using electrlcal coils or the like (not shown). The tank 60, typically constructed of steel or other metal subject to corrosion, is circular in plan and comprises sidewalls 64, a roof 66 and a base (not shown). The tank has a cold water inlet (not shown) which effectively supplies cold water to the lower portion of the interior of the tank 60.
The tank also has a hot water outlet 68 in the roof 66.
The interior surface (i.e. the portion in contact with the electrolyte) of the tank may be coated with anti-corrosive material.
An electrode or anode 72 is mounted from the roof 66 into the electrolyte 62. In this embodiment, the anode 72 is connected by electrically insulated wire 10 to the positive terminal 14 of control unit 8 (like reference numbers are used for like elements of prior discussed embodiments), while the tank 60, the cathode, is connected to the negative terminal 15. Means for mounting said anode 72 in the tank comprises a circular insulating collar 74 mounted over a first end 76 of the anode 72 in sealing engagement therewith. The first end 76 is preferably of a reduced diameter to accommodate the collar 74 as shown. A circular metal bushing 80 is mounted over the collar 74 in sealing engagement therewith, the bushing 80 being electrically insulated from the electrode 72 by the collar 74. The outer surface of the bushing 80 is threaded and adapted to engage cooperating threads on an opening 78 in the tank 60. When the anode 72 is inserted into the tank 60 to contact the electrolyte 62 as shown in figure 3, the bushing 80 is screwed into the opening 78 in sealing engagement therewith to prevent the electrolyte 62 _ 13 - 2 h ~ 3~

from escaping out of the tank 60 through the opening. The anode 72 should be mounted a selected distance from each wall of the tank to substantially balance the protection of each portion of the tank.
The first end 76 of the anode 72 has a first terminal 82 for receiving the wire 10. A second terminal 84 is located on the bushing 80 to receive the wire 12 from the negative terminal 15, although the second terminal 84 could be located on the body of the tank 60.
Hence a current flow path or an electric circuit is created from the anode 72, through the water or electrolyte 62 to the tank 60, through the bushing 80, terminal 84 and wire 12 to the control unit 8, and back to the anode 72 through the wire 10 and terminal 82.
Whether the current flow in the above circuit is impressed current provided by a current supply unit or is provided by a sacrificial anode alone, the circuit or current flow path remains the same because the anode 72 is insulated from the metal bushing 80. In prior art sacrificial anode systems the current flow path differs.
Typically, the anode is not insulated from the bushing and so the current flow in the circuit proceeds from the anode, through the electrolyte, to the tank, and then returns directly through the bushing back to the anode.
An impressed current cannot be provided to such a circuit.
Hence, unlike prior art protection systems, the present invention allows for stopping current flow in a sacrificial anode circuit, when current flow is not needed.
One embodiment of the electrical circuit for the control unit 8 is shown in Figure 4. References in primed numerals here indicate features corresponding to the embodiments of the invention shown in Figures 1 and 3.
The control unit 8' comprises a power supply 140, a switch 30l, a voltage comparator 141 and a constant current source 132.
The power supply 140 is connected to an 110 volt - 14 - Z~ 3~

A.C. power source 16', and generates positive 5 volts and negative 5 volts outputs. Regular diodes 100 and 101, zener diodes 102 and 103, smoothing capacitors 104, 105, 106, 107, 108, 109, and current limiting resistors 110 and 111 constitute this simple half-wave rectified power supply.
The switch 30~ contains an 8-pin LN555 timer chip 117. Capacitors 114 and 115 and resistors 112 and 113 are used to configure this timer chip 117 as an astable multi-vibrator. In the embodiment shown in figure 4, the output 150 of this timer 117 will be low for approximately 30 seconds, and then high for the remaining duration (i.e. 60 seconds~ of a cycle of 90 seconds. The switch 30' is used to enable or d~sable the output 130 of the voltage comparator 141.
The voltage comparator 141 is an 8-pin LM311 comparator chip. It performs the functions of the comparator 50 shown in Figure 1. The non-inverting input 129 of the comparator 141 is connected to the positive terminal 14' of the control unit 8 which in turn is connected to the anode 2 or 72 (a magnesium anode in one embodiment). The inverting input 128 is connected to a reference voltage 124 and the output of the diode 122.
The reference voltage 124 is about 0.1 volt higher than the negative of the CPCV. In this embodiment, if the CPCV is 0.8 vol$, the reference voltage would be -0.7 volt. This negative voltage is generated using resistors 125 and 126 which are connected to the positive and negative 5 volts lines.
The output 130 of the voltage comparator 141 is controlled by pins 5 and 6. As shown, pins 5 and 6 are tied together and connected to the output 150 of the timer 117 through a resistor 116 and a diode 118. The output 130 of the comparator 141 is enabled when there is no current flowing through the resistor 116 and diode 118.
The output 130 is at approximately -5 volts when the non-inverting input 129 is lower in potential or more negative :

e~ r ~o - lS -than the inverting input 128. In this caser the cathode 4 is not sufficiently protected and a current flow is required. Conversely, the output 130 is at approximately +5 volts when the non-inverting input 129 is higher in potential or more positive than the inverting input 128.
Here the cathode 4 is adequately protected and no more current is needed.
The output 130 of the voltage comparator 141 is disabled or goes to high impedance when current flows out of pin 5 and 6 of the voltage comparator 141 through resistor 116.
The capacitor 120 between the inverting input 128 and the non-inverting 129 and the connection between pins 5 and 6 of the voltage comparator 141 are to stabilize the comparator 141 and prevent oscillation.
The constant current source 132 is a TIP32 transistor. The transistor and the power supply 140 constitute the current supply unit 9 as shown in Figures 1 and 3. When the output 130 of the voltage comparator 141 is negative, the constant current source 132 turns on and there is a current flow 18' to the anode 2 or 72.
When the output 130 is at approximately +5 volts or at high impedance, the constant current source 132 turns off and any current flow 18' stops. A resistor 121 limits current flow in the output 130 of the comparator 141.
In this embodiment, the protected object or cathode 4 is connected to the negative terminal 15 (not shown) of the control unit 8 which is tied to the ground of the circuit. Therefore, the potential of the anode 2 or 72 measured by the comparator 141 is with respect to the cathode 4, i.e. ground, in this embodiment.
With reference to Figure 2, the operation of the control unit 8' will now be explained.
Starting at t=0 second, the first time segment Bl commences and the output 150 of the timer chip 117 ~oes low. Current flows out of pins 5 and 6 of the voltage comparator 141 and through the resistor 11~. Thus the 7 ~ r output 130 of the comparator 141 is disabled, and there will be no current flow 18~ to the anode 2 or 72.
During the firs~ time segment which lasts approximately 30 seconds, the potential of the anode 2 or 72 is allowed to stabilize. This permits decay of any polarization of the anode 2 or 72 which may have developed if a current was flowing prior to the first time segment.
Furthermore, no measurements of the anode 2 or 72 are made during the first time segment in this embodiment.

At the end of the first time segment B1, the second time segment Cl begins and lasts for approximately seconds. The second time segment Cl is both the measurement phase and the protection phase of the cycle.
Starting at appro~imately time t=30 seconds, the output 150 of the timer chip 117 becomes high. There will not be any current flow out of pins 5 and 6 of the voltage comparator 141 and through the resistor 116 because of the diode 118. The output 130 of the voltage comparator 141 is thus enabled. At this time, a measurement of the voltage of the anode 2 or 72 is made, and the voltages of the anode 2 or 72 and the reference voltage 124 are compared.
If the potential of the anode 2 or 72 falls below the reference voltage 124 which is negative, the cathode 4 is not sufficiently protected, and the output 130 will be at about -5 volts. The constant current source 132 will turn on and there will be a current flow 18' through the two series diodes 122 and 123 to the anode 2 or 72.
On the other hand, if the potential of the anode 2 or 72 is above the reference voltage 124, the cathode 4 is sufficiently protected, and the output 130 will be at about +5 volts. The constant current source 132 will turn off and there will not be any current flow 18'.
Measurements of the anode 2 or 72 and comparisons will resume.

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The diodes 122 and 123 are necessary because when the current 18 is flowing, a positive IR drop is developed across the resistance of the electrolyte which would have the effect of turning off the current 18'. The diode 122 also serves another purpose since its output is connected to the inverting input 128 of the voltage comparator 141. Whenever there is a current flow 18', this diode keeps the inverting input 128 at a higher voltage than the non-inverting input 129 by one diode voltage drop. This forces the output 130 of the comparator 141 to remain negative thus enabling further curre~t flow 18'. As a result, no more measurements of the anode 2 or 72 will be taken once the current flow 18' has begun. This current flow will only stop when the output 150 of the timer 117 goes low again.
At the end of the second time segment C1, the cycle will repeat. The output 150 of the timer chip 117 will become low again and the output 130 of the voltage comparator will be disabled. Thus there will not be any current flow 18'.
An embodiment of the present invention similar to that shown in figure 3 has been field tested, and the test results are reported below.
The field tested version of the present invention used the impressed current method of cathodic protection in combination with a magnesium anode commonly used as a sacrificial anode. There are two reasons for choosing the magnesium anode. The first is that it provides a direct comparison of the present invention with .-existing sacrificial anode cathodic protection systems.
The second reason is that the magnesium anode is known to exhibit little or no polarization, or at least the rapid disappearance of any polarization, following the interruption of current flow in a circuit. This is particularly desirable in this case since the measurement of the voltage or potential of the tank being protected is made by the control unit with reference to the anode.

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Other types of anodes exhibit varying rates of polarization disappearance after the interruption of current flow. Although the voltage or potential measurement can be calibrated to take account of such polarization phenomena, it is desirable to dispense with the need for such calibration. Therefore, the choice of the magnesium anode simplified the test program and allowed for better evaluation of the performance of the control unit.
Although the test program ~ocused on the prevention of internal corrosion of the metal tanks of domestic water heaters, it will be appreciated that the present invention can be applied to many different types of metal objects or structures in contact with an electrolyte (such as bridge piers, inground pipes, and the like as discussed previously). In the test program, the metal tank was exposed to hot water containing low levels o~ dissolved substances. In this case, the CPCV of the object to be protected, in contact with the electrolyte, or hot aqueous solution, was estimated to be +0.80 volts with reference to the magnesium anode.
In the test program, technicians visited the test sites periodically to monitor the operation of the invention. During the visits, the technicians disconnected the control units (and hence the current supply unit incorporated in said units) from the source of electric power to obtain a longer interruption of current flow than might occur with the protection system in normal operation. The cathode voltage was then manually measured and recorded. This procedure provided independent information as to whether the invention was working correctly, and whether the tank was properly protected against corrosion.
Early in the test program, the preset maximum voltage value was set rather close to the estimated CPCV
of +0.80 volts. It was considered that :if the potential of the tank is +0.80 volts or less when the magnitude of the current flow is zero, then the tank is protected from corrosion. Later in the test program, the preset voltage value was changed to approximately +0.70 volts to determine more clearly whether the control units were operating properly. (It was noted that if a tank is properly protected for several months, and then the corrosion protection equipment is completely removed, the potential of the tank will rise very slowly and after some weeks may only be at +0.82 volts. Therefore it was extremely difficult to detect any failure of the invention in field test work. On the other hand, if the preset voltage value is set at +0.70 volts, resulting in slight over-protection of the tank, any failure of the invention would be readily seen. The potential of the object would rapidly rise past +0.70 volts, and the rate of rise would not slow until the potential approached the +0.80 level.) A test unit built according to the invention was installed at each of the four locations listed below. The first date in each case indicates the date of installation. Subsequent dates indicate when the unit was monitored.
It is noted that it would require in the order of 10 years or so for corrosion to occur in the tested water tanks under existing water conditions, even if only elementary corrosion protection were in place. Therefore, a program to test an improved corrosion protection system must continue over an extended period of time. The test program described herein was intended to investigate the present invention's ability to keep the tested tanks at a suitable potential, without under or over-protection, and was not necessarily intended to produce information concerning corrosion of the tanks.

Location Erin, Ontario 2~

Date Tank Potential, Volts Auyust 14, 1989 0.67 August 28, 1989 0.36 March 30, 1990 0.846 June 1, 1990 0.814 July 12, 1990 0.82 August 9, 1990 0.77 March 22, 1991 0.715 July 31, 1991 0.723 10 January 10, 1992 0.696 Test 1 findings show a tank that was initially slightly over-protected from the previous corrosion control system. The reason for the over-protected status (on August 28, 1989) is not known. It appears that the procedure for taking readings may not have been correct.
Otherwise, the operation appears to have been correct on the basis of recent readings.

Location Orangeville, Ontario Date Tank Potential Volts September 12, 1989 0.61 March 16, 1990 0.92 25 March 30, 1990 0.91 June 1, 1990 0.83 July 12, 1990 0.815 August 9, 1990 0.761 ~arch 22, 1991 0.55 30 July 31, 1991 0.68 January 10, 1992 0.695 Test 2 findings show that the tank was initially slightly over-protected from the previous corrosion control system, and that the control unit failed in March 1990 and was replaced. The operation appeared to have been correct until Narch 1991 at which point the tank was found to have been over-protected. As a result, the control unit was replaced.

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Location Newmarket, Ontario (Park Avenue) Date Tank Potential, Volts September 29, 1989 1.104 October 6, 1989 0.72 March 8, 1990 0.68 April 26, 1990 0.662 Nay 30, 1990 0.716 September 14, 1990 0.72 March 21, 1991 0.70 July 19, 1991 0.70 January 8, 1992 0.55 Test 3 findings show a brand-new tank which was not protected at the time of the initial install~tion.
The operation of the control unit appears to have been generally correct in that the potential has been kept at or slightly below the CPCV. The exception is the last reading which indicates a malfunction of the control unit.
A new control unit was installed to continue the test program.

- I,ocation Holland Landing, Ontario 25 Date Tank Potential, Volts December 7, 1989 0.834 Narch 15, 1990 0.708 April 26, 1990 0.722 May 30, 1990 0.738 September 18, 1990 0.739 October 11, 1990 0.70 Narch 21, 1991 0.65 July 19, 19gl 0.70 January 8, 1992 0.723 At the time of the March 1991 visit, the tank appeared to have been slightly over-protected but otherwise the operation of the control unit appeared to have been correct.
Although the present invention has been ?i5~) described in reference to a preferred example thereof, it will be apparent to those skilled in the art that various alterations and modifications may be carried out without departing from the scope of the invention. For instance, the anode 72 in the present embodiment could be installed in a sidewall 64 of the tank 60 rather than the roof 66 prior to the tank being filled with water. As well, unlike what is shown in figure 1, when the present invention is used with underground pipes, the control unit need not be above ground as shown but may be adapted for burial underground. Some other modifications have been discussed previously.

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Claims (36)

1. A corrosion protection system for a conductive object, the system comprising:
(a) at least one electrode for contact with an electrolyte;
(b) a control unit connected to the electrode and for connection to the object to complete a circuit through said electrolyte, said control unit comprising:
(i) switch means for interrupting current flow in the circuit at predetermined intervals, each time interval comprising a first time segment during which said current flow remains interrupted and a succeeding second time segment;
(ii) comparator means for measuring the potential difference between the electrode and the object and comparing the measured potential difference to a predetermined voltage value; and (iii) means for closing the circuit during the second time segment when said comparison indicates that additional corrosion protection of the object is required; and (c) means for providing power to the control unit.

2. The protection system of claim 1 wherein the potential difference is measured and compared to the predetermined voltage value only when there is no current flow in the circuit.

3. The protection system of claim 2, wherein the potential of the object is measured with reference to said electrode.

4. The protection system of claim 2 wherein the potential of the electrode is measured with reference to said object.

5. The protection system of claim 1, 2, 3 or 4, wherein the electrode comprises a sacrificial anode for providing a protective current for the object, and wherein the control unit operates to open and close a connection between the anode and the object.

6. The protection system of claim 1, 2, 3 or 4, further including a current supply means for providing impressed direct current in the circuit.

7. The protection system of claim 2 wherein the comparator means operates only during the second time segment.

8. The protection system of claim 1, further including electrode mounting means comprising:
(i) an insulating collar mounted over a first end of the electrode in sealing engagement therewith; and (ii) a bushing mounted over the collar in sealing engagement therewith, the bushing being electrically insulated from the electrode by the collar, and being adapted for sealing engagement with an opening in the object to prevent the escape of electrolyte from the object.

9. The protection system of claim 8 wherein the bushing's outer surface is threaded for engaging an opening having a complementary thread.

10. The protection system of claim 9, in combination with an object comprising a container, wherein said electrolyte comprises one of water or an aqueous solution, said container being adapted to heat the water or aqueous solution.

11. The protection system of claim 10, wherein the electrode is mounted within said container a selected distance from each wall of the container to substantially balance the protection of each portion of the container.

12. The protection system of claim 11 wherein at least two control units are connected to the object, and each control unit is connected to at least one respective electrode.

13. The protection system of claim 5, wherein the electrode is a magnesium anode.

14. The protection system of claim 6, wherein the electrode includes a coating of platinum.

15. The protection system of claim 7, wherein the comparator means comprises: an operational amplifier means having two inputs, one of which inputs is connected to the electrode, and having an output; an adjustable reference voltage connected to the other input of the operational amplifier means; a current supply means connected to the electrode and having a control input connected to the output of the operational amplifier means; and a timer which times the first and second time segments and is connected to and controls the operational amplifier means, the timer only permitting operation of the operational amplifier means during the second time segment;
and said means for closing the circuit is provided by the current supply means.

16. The protection system of claim 15, wherein the current supply means comprises a transistor having its base connected to the output of the operational amplifier means, and having its other terminals connected to the electrode and a positive DC source, and being configured to provide a constant current source to the electrode when turned on.

17. The protection system of claim 16, wherein the output of the transistor is coupled to at least one input of the operational amplifier means, for maintaining the transistor turned on.

18. The protection system of claim 15, wherein the operational amplifier has a non-inverting input connected to the electrode, and an inverting input connected to the reference voltage source.

19. The protection system of claim 18, wherein the output of the current supply means is coupled to at least one input of the operational amplifier means, for maintaining the current supply means turned on.

20. The protection system of claim 19, wherein the current supply means comprises a transistor having a base connected to the output of the operational amplifier means, with the other terminals of the transistor connected between a positive DC source and the electrode, the transistor being configured to provide a constant current source, when turned on by the operational amplifier means; and wherein the transistor is connected to the electrode via a line including a pair of diodes in series, with a point of that line between the diodes being connected to the inverting input of the operational amplifier means.

21. The protection system of claim 15, 17 or 19, wherein the timer has an output connected to the operational amplifier means, which output is switched between a first level during the first time segment and a second level during the second time segment, the operational amplifier being disabled when the output is at the first level.

22. A method of corrosion protection of a conductive object in contact with an electrolyte, the method comprising:

(a) providing at least one electrode contacting the electrolyte;
(b) connecting a control unit to the object and to the electrode to form a circuit through said electrolyte;
(c) providing power to the control unit;
(d) interrupting current flow in the circuit at predetermined intervals, each time interval comprising a first time segment during which said current flow remains interrupted and a second time segment;
(e) measuring the potential difference between the electrode and the object;
(f) comparing the measured potential difference to a predetermined voltage value; and (g) resuming the current flow in the circuit during the second time segment only when said comparison indicates that additional corrosion protection of the object is required.

23. The method of claim 22, wherein the potential difference is measured and compared to the predetermined voltage value only when there is no current flow in the circuit.

24. The method of claim 23, having only one electrode wherein the potential of the object is measured with reference to the electrode.

25. The method of claim 23, having only one electrode wherein the potential of the electrode is measured with reference to the object.

26. The method of claim 23, wherein the electrode comprises a sacrificial anode for providing a protective current for the object.

27. The method of claim 22, 24 or 25 wherein the durations of the time segments are selected to reduce or eliminate errors due to polarization in measuring said potentials and to account for the operating cycle of the conductive object and the properties of the electrode and electrolyte.

28. The method of claim 27 wherein the duration of the first time segment is between 3 seconds and 3 hours.

29. The method of claim 28 wherein the duration of the second time segment is between one-half and five times the duration of the first time segment.

30. The method of claim 22 wherein the predetermined voltage value is set within .001 to 1.0 volts of a corrosion protection criterion value.

31. Mounting means for an electrode for use with a corrosion protection system for a conductive object as claimed in claim 1, said means comprising:

(i) an insulating collar mounted over a first end of the electrode in sealing engagement therewith;
(ii) a bushing mounted over the collar in sealing engagement therewith, the bushing being electrically insulated from the electrode by the collar and being adapted for sealing engagement with an opening in the object to prevent the escape of electrolyte from the object;
(iii) a first terminal located on the first end of the electrode for receiving a first connection from a control unit; and (iv) a second terminal located on the bushing for receiving a second connection from the control unit.

32. The mounting means of claim 31 wherein the bushing's outer surface is threaded for engaging an opening having a complementary thread.

33. A control unit for use with a corrosion protection system for a conductive object as claimed in claim 1, said unit being connectable to the object and at least one electrode to complete a circuit through said electrolyte, said control unit comprising:
(i) switch means for interrupting current flow in the circuit at predetermined intervals, each time interval comprising a first time segment during which said current flow remains interrupted and a succeeding second time segment;
(ii) comparator means for measuring the potential difference between the electrode and the o b j e c t a n d comparing the measured potential difference to a predetermined voltage value; and (iii) means for closing the circuit during the second time segment when said comparison indicates that additional corrosion protection of the object is required.

34. The control unit of claim 33 wherein the potential difference is measured and compared to the predetermined voltage value only when there is no current flow in the circuit.

35. The control unit of claim 34 wherein the comparator means operates only during the second time segment.

36. The control unit of claim 35 wherein the comparator means comprises an operational amplifier means having two inputs, one of which inputs is connected to the electrode, and having an output; an adjustable reference voltage connected to the other input of the operational amplifier means; a current supply means connected to the electrode and having a control input connected to the output of the operational amplifier means; and a timer which times the first and second time segments and is connected to and controls the operational amplifier means, the timer only permitting operation of the operational amplifier means during the second time segment;
and said means for closing the circuit is provided by the current supply means.