Cathodic corrosion protection with current limiter

Abstract

In a method for cathodically protecting and/or passivating a metal section in an ionically conductive material such as steel reinforcement in concrete or mortar, an impressed current or sacrificial anode communicates ionic current to the metal section and a storage component of electrical energy which can be a cell, battery or capacitor is provided as a component of the anode. A current limiter is provided which prevents excess current draining the supply. This can be a semi-conductive device such as a transistor or diode is connected in the path from the anode to the metal section to limit the cathodic protection current to a value of the order of 1 milliamp. When a diode or similar device is used the current can be limited to the reverse leakage current of the diode.

Claims

1. A method for cathodically protecting and/or passivating a steel member in an ionically conductive concrete or mortar material, comprising: providing an anode construction for communication of an ionic current to the steel member in the ionically conductive material; generating a voltage difference between the anode construction and the steel member so as to cause a current to flow through the ionically conductive material between the anode construction and the steel member so as to provide cathodic protection of the steel member; providing at least one electrically conductive circuit between the anode construction and the steel member; and connecting a device in said circuit wherein the device is arranged to pass current in a first direction and which has an insulative mode in a second direction of a type which allows a leakage current when operating in the insulative mode; and applying said voltage difference across the device in the insulative mode such that the leakage current passes through the device in the insulative mode and thus limits the current to a maximum value defined by the leakage current.

2. The method according to claim 1 wherein the device is a semi-conductor.

3. The method according to claim 1 wherein the device includes a P-N junction.

4. The method according to claim 1 wherein the device is a diode.

5. The method according to claim 1 wherein the device is a capacitor.

6. The method according to claim 1 wherein the anode construction comprises a sacrificial anode.

7. The method according to claim 1 wherein the anode construction comprises an impressed current anode.

8. The method according to claim 1 wherein the anode construction is buried in the concrete or mortar material while in an unset condition and the concrete or mortar material is caused to set with the anode construction therein and wherein said device which limits the current to said maximum value acts to restrict formation of gas bubbles in the concrete or mortar material at the steel member and/or at the anode while the concrete or mortar material sets.

9. The method according to claim 1 wherein said anode construction comprises a sacrificial anode and an impressed current anode generating said voltage difference between the sacrificial anode and the steel member so as to cause a first current to flow through the ionically conductive concrete or mortar material between the first sacrificial anode and the steel member so as to provide cathodic protection of the steel member wherein said voltage difference between the impressed current anode and the steel member is generated by a storage component of electrical energy.

10. The method according to claim 9 wherein said storage component is contained within a sleeve or canister defining the anode construction on an exterior surface.

11. The method according to claim 10 wherein the anode construction comprises stainless steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described in conjunction with the accompanying drawings in which:

(2) FIG. 1 is a cross-sectional view of an anode assembly using a sacrificial anode for use in a corrosion protection method according to the present invention.

(3) FIG. 2 is a side elevational view of the anode assembly of FIG. 1.

(4) FIG. 3 is a cross-sectional view of an anode assembly similar to that of FIG. 1 but using a conventional wire wrapping attachment method to the metal section.

(5) FIG. 4 is an enlarged cross-sectional view of an anode assembly of the type using a cell to provide current though an impressed current anode and using the current limiting device and mounting arrangement of FIGS. 1 and 2.

(6) FIG. 5 is an enlarged cross-sectional view of an anode assembly of the type using a cell to provide current though an impressed current anode and a bipolar type transistor to limit the current from the cell to the steel.

(7) FIGS. 6 to 9 show schematic illustrations of four embodiments of current limiting system which uses a gate controlled FET to limit the current in the electrically conductive circuit connecting an anode to the steel member.

(8) In the drawings like characters of reference indicate corresponding parts in the different figures.

DETAILED DESCRIPTION

(9) In the example shown in FIGS. 4 and 5 there is provided a cell which may be rechargeable, as shown in prior PCT Application WO 2017/075699 filed Nov. 2, 2016 and published 11 May 2017, the disclosure of which may be referenced or is incorporated herein by reference, or may be a simple non-rechargeable cell. The cell may form part of the anode structure or the anode and the cell may be physically separated. The anode body 10 is defined by a typical alkaline manganese dioxide-zinc rechargeable cell comprising the following main units: a steel can 12 defining a cylindrical inner space, a manganese dioxide cathode 14 formed by a plurality of hollow cylindrical pellets 16 pressed in the can, a zinc anode 18 made of an anode gel and arranged in the hollow interior of the cathode 14, and a cylindrical separator 20 separating the anode 18 from the cathode 14. The ionic conductivity (electrolyte) between the anode and the cathode is provided by the presence of potassium hydroxide, KOH, electrolyte added into the cell in a predetermined quantity. Other types of rechargeable cells comprise similar main components (can, cathode, anode, separator and electrolyte) but the composition of the components may differ. Some of the types of cell may however be of a different construction such as lead/acid cells or lithium cells.

(10) The can 12 is closed at the bottom, and it has a central circular pip serving as the positive terminal. The upper end of the can 12 is hermetically sealed by a cell closure assembly which comprises a negative cap 24 formed by a thin metal sheet, a current collector nail 26 attached to the negative cap 24 and penetrating deeply into the anode gel to provide electrical contact with the anode, and a plastic top 28 electrically insulating the negative cap 24 from the can 12 and separating gas spaces formed beyond the cathode and anode structures, respectively.

(11) Other types of rechargeable cells may be used. In the present arrangement, the type described above is used in a method for cathodically protecting and/or passivating a metal section such as steel reinforcing bar 40 in an ionically conductive material such as concrete 41. The cell therefore includes a first terminal 42 and a second terminal 43 defined by the outer casing 12. The first terminal 42 is connected to the pin or nail 26 which is engaged into the anode material 18. The terminal 42 connects to a connecting wire 42A which extends from the terminal 42 to threaded connector 53 for eventual connection to the steel reinforcing bar 40 as shown in FIG. 4 through the mounting assembly generally indicated at 50 which mechanically and electrically attaches the anode body to the bar 40.

(12) In FIG. 4, an anode 44 is applied as a coating onto the casing 12 of the cell. In this embodiment the anode 44 is of an inert material so that it is more noble than steel. Examples of such materials are well known. Thus the anode material 44 does not corrode or significantly corrode during the cathodic protection process.

(13) In this arrangement the application of the anode 44 onto the outside surface of the casing 12 provides the structure as a common single unit where the anode is directly connected to the cell and forms an integral element with the cell. Anode 44 may comprise one or more layers and may include a mixed metal oxide (MMO), catalytic or sub-oxide layer.

(14) In this embodiment, as the anode 44 is formed of an inert material which does not corrode in the protection process, the anode and the cell contained therein can be directly incorporated or buried in the concrete or other ionically conductive material without the necessity for an intervening encapsulating material such as a porous mortar matrix. As there are no corrosion products there is no requirement to absorb such products or the expansive forces generated thereby. As the process does not depend upon continued corrosion of a sacrificial anode, there is no necessity for activators at the surface of the anode. As the chemical reaction at the surface of any inert anode during operation generates acid (or consumes alkali) it is beneficial for the anode to be buried in an alkaline material such as concrete or high alkalinity mortar to prevent material near the anode from becoming acidic. If desired, additional alkali may be added to the concrete or other material the anode is in contact with.

(15) The apparatus shown herein includes an anode body generally indicated at 10 which is connected to the reinforcing bar 40 by the mounting assembly generally indicated at 50. In addition, the anode body includes a current limiting system generally indicated at 51 which limits the flow of current from the anode body to the bar 40.

(16) As previously described, the anode body can be defined by a power supply typically in the form of a cell with the anode 44 on the outside surface of the cell and with the other terminal of the cell provided at the end of the cell for connection to the bar 40.

(17) In other embodiments shown in FIGS. 1, 2, 3 and 6 to 8, the cell can be omitted in which case the anode body comprises a sacrificial material which is less noble than the steel rebar, such as zinc where a voltage between the anode and the bar comprises the galvanic voltage between the two metal components.

(18) In yet another embodiment, the anode body can comprise a combination of both an impressed current anode and a sacrificial anode.

(19) In this way the anode body is constructed and arranged so that when the anode is ionically connected to the concrete, a voltage difference is generated between the anode 44 and/or 74 and the bar 40 so as to cause a current to flow through the concrete between the anode and the bar 40 so to provide cathodic protection and/or passivation of the reinforcing bar in the concrete.

(20) In the embodiment shown in FIGS. 1, 2, 4 and 5, the mounting assembly is of the type shown in Published PCT application WO 2019/006540 filed 15 May 2018 and published 10 Jan. 2019, the disclosure of which may be referenced or is incorporated herein by reference.

(21) The mounting 50 comprises a first abutment in the form of a threaded rod 53 which is attached at one end to the anode body 10 and a second abutment 57 for engaging generally the opposed the face of the bar 40. In general the second abutment forms a hook member with two legs 68 and 69 which contact the opposite or rear surface of the bar 40 to provide a stable engagement.

(22) In this embodiment the female threaded portion is provided by a threaded hole through the flange 67. A screw action pulling the second abutment member toward the anode body is therefore provided by rotating the rod 53. This can most effectively be done by grasping manually the anode body and using it as a handle to turn the rod 53.

(23) Of course, this requires a strong connection between the bottom end of the rod 53 and the anode body. This connection is provided by a base plate 71 attached onto the bottom end of the rod 53 and engaged firmly into the upper end of the anode body. The solid anode body 74 includes a conventional covering of a mortar material 75 for purposes of retaining corrosion products and of carrying conventional activating materials described herein before.

(24) Turning now to FIG. 4, there is shown in more detail the connection between the terminal 42 of the cell and the rod 53 which is electrically connected to the bar 40 as described above.

(25) The terminal 42 is connected to a wire 42A which in turn is connected to a diode 51. An output wire 79 of the diode 51 is connected to the base plate 71 connected to the rod 53.

(26) The diode 51 can be a conventional diode connected in reverse polarity so as to prevent flow of current between the anode and the bar 40. In this arrangement, the reverse or leakage current acts to limit the flow of current from the anode to the bar 40 to a value of the order of 0.1 to 1 milliamp. This maximum value is retained regardless of the conductivity between the anode 44 and the bar 40 through the concrete. If the conductivity through the concrete is very high, for example during an initial installation when the concrete is fresh, the current is maintained at the maximum value. As the conductivity through the concrete falls to a lower level (resistivity increases), the current is maintained at the desired level until the voltage drop through the concrete and the circuit (V=IR) at I.sub.Max reaches the voltage of the cell. If the conductivity falls to a yet lower level, the current through the diode or transistor also falls dependent upon the conductivity and is not maintained by the action of the diode or transistor 51. The simple circuit therefore provided by the diode 51 does not act as a regulator but instead merely acts as a current limiter.

(27) FIGS. 1 and 2 show applications of the current limiting device in use with a galvanic anode. In this arrangement, the diode or transistor 51 is connected by wires 51A and 51B connected between the anode 74 and the support plate 71 which is connected to the rod 53.

(28) Limitation of the current to a maximum value set during manufacture by the selection of the diode 51 can ensure that the current remains during the life of the system at a relatively low level so as to dramatically increase the lifetime of the cell from a typical value in the absence of the current limiter which could be of the order of one year up to a more suitable lifetime of 10 years for example. The life of a galvanic anode may be extended from 5 to 10 years to over 50 years for example. In this way the current is maintained at a value which is suitable for cathodic protection but at no time is there any excess current over and beyond this desirable value which may damage the concrete or deplete the cell prematurely or degrade and shorten the life of a galvanic anode such that corrosion protection is not provided for the desired timeframe.

(29) This arrangement is valuable in relation to an arrangement which uses a non-sacrificial impressed current anode and a cell as the power supply for generating the required voltage. In such an arrangement the current generated between the anode 44 and the bar 40 can in some circumstances significantly exceed the desirable value.

(30) In order to connect the terminal 42 to the rod 53, there is provided an insulating or protective collar 83 surrounding the diode 51. The bottom end of the collar is attached to the top end of the cell and the top end of the collar receives the base plate 71 in a suitable receptacle portion. The collar 83 is attached to the cell 44 by a surrounding insulating layer 84 of a suitable plastic material. Inside the collar 83 is provided a conventional potting material 85 which surrounds the diode 51 and wires to maintain connection and to prevent damage from moisture penetration. The structure is thus sufficiently strong to ensure that the base plate 71 is attached to the cell in a manner which allows the cell to be grasped manually and rotated as an operating handle to rotate the rod 53.

(31) In the present method for cathodically protecting and/or passivating a metal section in an ionically conductive material, as shown in FIGS. 1 and 2, a sacrificial anode 74 is provided for communication of an ionic current to the metal section 40 in the ionically conductive material 91. The anode acts for generating a voltage difference between the anode 74 and the metal section 40 so as to cause a current to flow through the ionically conductive material 91 between the anode and the metal section so as to provide cathodic protection of the metal section in the conventional manner.

(32) The current flowing between the anode the metal section is limited to a low selected value by connecting the semi-conductor diode device 51 in an electrically conductive path between the anode and the metal. The semi-conductive device 51 maybe of the type which is arranged to pass current in a first direction and to restrict current in a second direction to a leakage current and is connected so that current between the anode and the metal passes in the second direction and thus limits the current to a maximum value defined by the leakage current.

(33) The semi-conductor device diode 51 forms part of a combined unit including the anode and the mounting arrangement or electrical connector to be inserted in or attached to the ionically conductive concrete or mortar material.

(34) Where there is provided a coating 75 on the anode of a porous absorption material the diode 51 can be located in the coating or in a potting material to provide suitable protection.

(35) The wire or electrical connection 51A must be electrically connected to the anode. The wire or electrical connection preferably will be cast into the anode as indicated at 51C or connected to a connector which is cast into the anode. Less durable connections such as mechanical connections or soldering directly to the exterior of the anode can be made. Wire or connector 51B must be electrically connected to the bar 40. This wire or connector can be soldered or otherwise connected to the support plate 71 which is connected to the attachment mechanism. As the diode is typically supplied with wires which are unsuitable for direct connection to the bar 40, typically the diode needs to be attached to the mounting 71 which provides structural support for the attachment mechanism.

(36) Many types of attachment can be used including the hook and rod system described above and the traditional flexible wire arrangement which is used to wrap around the bar 40 as shown in FIG. 3 where two wires 71A and 71B are connected to the mounting 71 or directly connected to at least one wire or other electrical connector to connect to the bar 40. The sacrificial anode 74 is attached structurally to the mounting plate 71 by an insulating member 78 to form a common unit which can be easily handled and inserted into the material.

(37) In the embodiments shown therefore the anode 74 includes an electrically conductive connector for electrically connecting the anode to the metal section 40 and the diode 51 is located in the electrical connection between the anode and the connector.

(38) Turning now to the arrangements shown in FIGS. 5 to 9 there is method for cathodically protecting and/or passivating a steel member 101 buried in or in contact with an ionically conductive concrete or mortar material 99. Using the constructions shown in FIGS. 1 to 4, there is provided an anode construction 100 for communication of an electrical current to the steel member 101 in the ionically conductive material 99.

(39) By using the sacrificial anodes of FIGS. 6 to 8 or the impressed current anode of FIG. 9, a voltage difference is generated between the anode construction 100 or 104 and the steel member 101 so as to cause a current to flow through the ionically conductive material 99 between the anode 100, 104 and the steel member so as to provide cathodic protection of the steel member. The anode 104 of FIG. 9 is powered by a power supply 105 such as a simple cell connected between the anode and the steel 101.

(40) In accordance with the invention described herein there are provided electrical components 106 which limit the current to a maximum value with the electrical components 106 including at least one electrical conductor 107 connected to the anode construction. As shown schematically in these figures and in more detail in FIGS. 1 to 4, the electrical components including the electrical conductor and the anode construction form components of a common body which is attached to or buried in the concrete or mortar material as a single unit.

(41) Turning now to FIG. 5, there is shown in more detail the connection between the terminal 42 of the cell and the rod 53 which is electrically connected to the bar 40 as described above.

(42) The terminal 42 is connected to a wire 42A which in turn is connected to a transistor 78. An output wire 79 of the transistor 78 is connected to the base plate 71 connected to the rod 53.

(43) The transistor 78 in this embodiment is a conventional or bipolar transistor in which case a base of the transistor 78 has a control current provided by a wire 80 connected through a resistor 81 in turn connected through a wire 82 to the positive terminal of the battery connected to the anode 44.

(44) As the transistor 78 is connected to the steel bar 40 and the wire 82 is connected to the anode 44, the control current to the transistor 78 is determined by the voltage across the cell and the resistance of resistor 81. As this voltage is typically relatively constant at least until the cell is in its later stages of life, this constant control current controls the amount of current flowing through the transistor from the cell to the bar 40. As is well known the resistor 81 can be selected to provide a control base current to the transistor which sets the current flow through the transistor to a maximum value. This maximum value is retained regardless of the conductivity between the anode 44 and the bar 40 through the concrete. As the conductivity through the concrete is very high, for example during an initial installation, the current is maintained at the maximum value. As the conductivity through the concrete falls to a lower level, the current is maintained at the desired level until the maximum voltage of the cell is reached. If the conductivity falls to a yet lower level, the current through the transistor also falls dependent upon the conductivity and is not maintained by the action of the transistor. The simple circuit therefore provided by the resistor and the transistor does not act as a regulator but instead merely acts as a current limiter.

(45) As shown in FIGS. 6 to 9 a current limiting circuit between the anode 100 and the steel member 101 uses a field effect transistor 102 in the electrically conductive circuit 107 which acts to limit the current between the steel member and the anode construction to a maximum value. The current through the transistor is limited by a control voltage applied to a gate of the transistor. The transistor is typically a suitable form of Field effect transistor so that the control terminal acts as a gate. An arrangement is provided in the electrically conductive circuit for generating control voltage from the voltage difference between the anode and the steel member. In FIGS. 6 to 8 this voltage difference is galvanic. In FIG. 8 it is generated in response to the power supply 105.

(46) The anode construction and the transistor form, as shown in FIGS. 1 to 4, components of a common body which is at least partly buried as a single unit in the concrete or mortar material. The transistor uses the voltage difference between anode construction and the steel member and in some cases a resistor to generate a reference voltage or current for the transistor.

(47) In FIG. 6, a resistor R1 is located between the source S and the anode 100. This creates a voltage drop between the gate and the source and acts to enable the voltage at the gate to control the flow of current through the transistor to limit the current to a required value. This is achieved by selection of a suitable transistor having current and control characteristics along with the value of the resistor so as to provide a substantially constant current as described above.

(48) In FIG. 7, the voltage at the gate is set by a voltage generated by a small sacrificial anode 110 also located in the concrete. This anode is separate from the anode 100 and is not provided to directly or significantly assist in the corrosion protection but instead to provide the reference voltage at the gate. The voltage is generated galvanically relative to the steel 101 and remains consistent over time so as to set the current through the transistor at a required restricted value.

(49) In this arrangement typically the anode 110 can be located in the conventional mortar covering around the anode 100.

(50) In FIG. 8, the gate G control line is connected to a location between the drain and the steel. In this location the voltage drop across the transisitor provides a gate voltage which is suitable to set the current flow at a required limited level.

(51) In each of these arrangements, the circuit operates to generate the required gate voltage to maintain the gate voltage above or below a threshold value and to thus control the current passing through the transistor between source S and drain D at the required limited value described herein.

(52) In each of these arrangements of FIGS. 7 and 8 there is no additional resistor in the line from the anode to the steel which, if present, would act to reduce current flow when the system has reached an age and condition when the transistor is no longer acting to limit the current. At that stage the system provides the maximum available current due to the limited voltage drop between the anode and the steel.

(53) The arrangement used n FIG. 9 uses a cell 105 to generate the voltage between an impressed current anode 104 and the steel 101. It will be noted that the cell is located in the line from the anode to the transistor and the gate voltage is set by the voltage drop across the cell.

(54) As a further alternative, not shown, the gate voltage can be provided by a cell provided in the circuit. This arrangement has the advantage that the voltage can be more easily determined and maintained but of course increases cost and complexity.

(55) Typically the transistor 102 is a normally closed transistor so that, if the control voltage or current falls below a threshold, the transistor defaults to a closed position and allows continued passage of current between the anode and the steel member.

(56) The transistor is a normally closed MOSFET transistor with a gate to source voltage of less than 0.7V.

(57) Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same may be made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.