Sacrificial anode assembly

Abstract

.[.A sacrificial anode assembly for cathodically protecting and/or passivating a metal section, comprising: (a) a cell, which has an anode and a cathode arranged so as to not be in electronic contact with each other but so as to be in ionic contact with each other such that current can flow between the anode and the cathode; (b) a connector attached to the anode of the cell for electrically connecting the anode to the metal section to be cathodically protected; and (c) a sacrificial anode electrically connected in series with the cathode of the cell; wherein the cell is otherwise isolated from the environment such that current can only flow into and out of the cell via the sacrificial anode and the connector. The invention also provides a method of cathodically protecting metal in which such a sacrificial anode assembly is cathodically attached to the metal via the connector of the assembly, and a reinforced concrete structure wherein some or all of the reinforcement is cathodically protected by such a method..]. .Iadd.A sacrificial anode assembly for cathodically protecting and/or passivating a metal section, includes a cell with an anode and a cathode, a connector attached to the anode of the cell for electrically connecting the anode to the metal section to be cathodically protected; and a sacrificial anode electrically connected in series with the cathode of the cell. The cell is otherwise isolated from the environment such that current can only flow into and out of the cell via the sacrificial anode and the connector. A method of cathodically protecting steel in concrete in which such the sacrificial anode assembly is connected to the steel in an initial step of passivation using a higher current and when the first step is terminated the sacrificial anode alone continues to provide protection. .Iaddend.

Claims

1. A sacrificial anode assembly for cathodically protecting and/or passivating a metal section, comprising: a cell, which has an anode and a cathode arranged so as to not be in electronic contact with each other but so as to be in ionic contact with each other such that current can flow between the anode and the cathode; a connector attached to the anode of the cell for electrically connecting the anode to the metal section to be cathodically protected; and a sacrificial anode electrically connected in series with the cathode of the cell; wherein there are provided one or more isolating elements which prevent communication of ionic current from the cell to the environment such that current can only flow between the cathode of the cell and the sacrificial anode and between the anode of the cell and the connector; and wherein the sacrificial anode and the cell are connected together so as to form a single unit such that the sacrificial anode is electrically connected in series with the cathode of the cell.

2. An assembly according to claim 1, wherein the sacrificial anode is of a shape and size corresponding with the shape of at least part of the cell, such that it fits alongside at least part of the cell.

3. An assembly according to claim 1, wherein the sacrificial anode forms a container within which the cell is at least partly located.

4. An assembly according to claim 1, wherein the sacrificial anode is indirectly connected to the cathode of the cell through an electronically conductive separator.

5. An assembly according to claim 4, wherein a layer of a metal is located between the sacrificial anode and the cathode of the cell so as to allow electronic conduction between these components but to prevent direct contact between these components.

6. An assembly according to claim 1, wherein the sacrificial anode is zinc, aluminum, cadmium or magnesium, or an alloy of one or more of these metals.

7. An assembly according to claim 1, wherein the cell is provided with a porous separator located between the cathode and the anode, which prevents direct contact between the anode and the cathode.

8. An assembly according to claim 1, wherein the sacrificial anode forms a container and the cell is located at least partly in the container.

9. An assembly according to claim 8 wherein the sacrificial anode is in the shape of a generally cylindrical can and the cell is at least partly located in this can.

10. An assembly according to claim 1 which is at least partly surrounded by an encapsulating material.

11. An assembly according to claim 10 wherein the encapsulating material is a porous matrix.

12. An assembly according to claim 11 wherein the porous matrix comprises a cementitious mortar.

13. An assembly according to claim 12 wherein the porous matrix comprises a mortar having a pH greater than 12.

14. An assembly according to claim 10 wherein the encapsulating material contains at least one activator to ensure continued corrosion of the sacrificial anode.

15. An assembly according to claim 14 wherein the activator comprises a humectant.

16. A method of cathodically protecting .[.a metal section.]. .Iadd.steel .Iaddend.in an ionically conductive .Iadd.concrete or mortar .Iaddend.covering material comprising: providing a sacrificial anode; generating a voltage between two connections of a power supply such that current can flow between the negative connection and the positive connection; in a first protection step, electrically connecting one of the connections of the power supply to the .[.metal section.]. .Iadd.steel .Iaddend.to be cathodically protected and electrically connecting the sacrificial anode in series with the other connection of the power supply such that the voltage generated by the power supply is added to the voltage generated between the sacrificial anode and the .[.metal.]. .Iadd.steel .Iaddend.to produce a voltage greater than the galvanic voltage generated between the sacrificial anode and the .[.metal section.]. .Iadd.steel .Iaddend.alone; wherein the power supply is otherwise isolated from the environment such that current can only flow into and out of the power supply via the sacrificial anode and the connector; and, in a second protection step, the voltage generated by the power supply is no longer present and a current flows between the sacrificial anode and the .[.metal.]. .Iadd.steel .Iaddend.to continue protecting and/or passivating the .[.metal section.]. .Iadd.steel.Iaddend., where the current is generated solely by the galvanic voltage between the sacrificial anode and the .[.metal.]. .Iadd.steel.Iaddend..

17. The method according to claim 16 wherein the sacrificial anode and the power supply are connected together so as to form a single unit.

18. The method according to claim 17 wherein the sacrificial anode is of a shape and size corresponding with the shape of at least part of the power supply, such that it fits alongside at least part of the anode and cathode.

19. The method according to claim 16 wherein the sacrificial anode forms a container within which the power supply is at least partly located.

20. The method according to claim 16 including surrounding the sacrificial anode by an encapsulating material of a porous matrix.

21. The method according to claim 20 wherein the porous matrix comprises a cementitious mortar.

22. The method according to claim 20 .Iadd.including surrounding the sacrificial anode by an encapsulating material .Iaddend.wherein the .[.porous matrix comprises a mortar having.]. .Iadd.encapsulating material has .Iaddend.a pH greater than 12.

23. The method according to claim 20 wherein the encapsulating material is pre-cast around the anode.

24. The method according to claim 20 wherein the encapsulating material is provided .[.after.]. .Iadd.as .Iaddend.the sacrificial anode is located at its intended position in the concrete or mortar material.

25. The method according to claim 16 wherein the sacrificial anode is activated to ensure continued corrosion of the sacrificial anode.

26. The method according to claim 16 wherein the power supply comprises an electrolytic cell.

27. The method according to claim 16 wherein the .[.ionically conductive material is a concrete or mortar material in contact with which the metal.]. .Iadd.steel .Iaddend.is a steel reinforcing member.

.Iadd.28. A sacrificial anode assembly for cathodically protecting and/or passivating a metal section within an ionically conductive concrete or mortar covering material, comprising: a cell, which has an anode and a cathode with an electrolyte therebetween such that current can flow between the anode and the cathode; a connector attached to the anode of the cell for electrically connecting the anode to the metal section to be cathodically protected; and sacrificial anode material electrically connected with the cathode of the cell; wherein there are provided one or more isolating elements which prevent communication of ionic current from the cell to the environment such that current can only flow between the cathode of the cell and the sacrificial anode and between the anode of the cell and the connector; wherein the sacrificial anode and the cell are connected together so as to form a single unit such that the sacrificial anode is electrically connected in series with the cathode of the cell; wherein the assembly comprises a container containing at least part of the anode, the electrolyte and the cathode of the cell; and wherein the sacrificial anode material forms at least part of an outer surface of the container..Iaddend.

.Iadd.29. An assembly according to claim 28, wherein the sacrificial anode is zinc, aluminum, cadmium or magnesium, or an alloy of one or more of these metals..Iaddend.

.Iadd.30. An assembly according to claim 28 wherein the cell and the container are generally cylindrical..Iaddend.

.Iadd.31. An assembly according to claim 28 wherein the container is at least partly surrounded by an encapsulating material..Iaddend.

.Iadd.32. An assembly according to claim 31 wherein the encapsulating material is a porous matrix..Iaddend.

.Iadd.33. An assembly according to claim 31 wherein the encapsulating material comprises a cementitious mortar..Iaddend.

.Iadd.34. An assembly according to claim 33 wherein the encapsulating material has a pH greater than 12..Iaddend.

.Iadd.35. An assembly according to claim 31 wherein the encapsulating material contains at least one activator to ensure continued corrosion of the sacrificial anode..Iaddend.

.Iadd.36. An assembly according to claim 35 wherein the activator comprises a humectant..Iaddend.

.Iadd.37. A method of cathodically protecting steel in an ionically conductive concrete or mortar covering material comprising: providing a sacrificial anode; generating a voltage between two connections of a power supply such that current can flow between the negative connection and the positive connection; in a first protection step, electrically connecting one of the connections of the power supply to the steel to be cathodically protected and electrically connecting the sacrificial anode in series with the other connection of the power supply such that the voltage generated by the power supply is added to the voltage generated between the sacrificial anode and the steel to produce a voltage greater than the galvanic voltage generated between the sacrificial anode and the steel alone which passivates the steel; wherein the power supply is otherwise isolated from the environment such that current can only flow into and out of the power supply via the sacrificial anode and the connector; and, in a second protection step, subsequent to completion of the first step, the voltage generated by the power supply is no longer present and a current flows between the sacrificial anode and the steel to continue protecting and/or passivating the steel, where the current is generated solely by the galvanic voltage between the sacrificial anode and the steel..Iaddend.

.Iadd.38. The method according to claim 37 wherein a porous material is provided at the sacrificial anode so as to provide pores that absorb expansive forces generated by expansive corrosion products..Iaddend.

.Iadd.39. The method according to claim 37 including providing at least one activator which ensures continued corrosion of the sacrificial anode..Iaddend.

.Iadd.40. The method according to claim 39 wherein said at least one activator provides a pH in the encapsulating material sufficiently high for corrosion of the sacrificial anode to occur and for passive film formation on the sacrificial anode to be avoided..Iaddend.

.Iadd.41. The method according to claim 39 wherein said at least one activator provides a pH in the encapsulating material greater than 12..Iaddend.

.Iadd.42. The method according to claim 39 wherein said at least one activator is provided in a porous material at the sacrificial anode..Iaddend.

.Iadd.43. The method according to claim 37 wherein the sacrificial anode is formed of zinc or zinc alloy..Iaddend.

.Iadd.44. The method according to claim 37 wherein an encapsulating material is pre-cast around the sacrificial anode..Iaddend.

.Iadd.45. The method according to claim 37 wherein an encapsulating material is provided around the sacrificial anode as the sacrificial anode is located at its intended position in the concrete or mortar covering material..Iaddend.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be further described in the following examples, with reference to the drawings in which:

(2) FIG. 1a shows a cross section through a sacrificial anode assembly in accordance with the invention;

(3) FIG. 1b shows a section A-A through the sacrificial anode assembly as shown in FIG. 1a;

(4) FIG. 2 shows a sacrificial anode assembly of the present invention connected to steel in a test arrangement;

(5) FIG. 3 is a graph showing the drive voltage and current density of the sacrificial anode assembly as shown in FIG. 3; and

(6) FIG. 4 shows the potential and current density for the protected steel as connected to the sacrificial anode assembly in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

(7) FIG. 1 shows a sacrificial anode assembly 1 for cathodically protecting a metal section. The assembly comprises a cell, which has an anode 2 and a cathode 3. The cathode 3 is a manganese dioxide/carbon mixture and is in the shape of a can, having a circular base and a wall extending upwards from the circumference of the base, so as to define a cavity. The anode 2 is a solid zinc anode of cylindrical shape, with the solid zinc being cast metal, compressed powder, fibres or foil. The anode 2 is located centrally within the cavity defined by the can shaped cathode 3 and is in contact with electrolyte 4 present in the cavity defined by the can shaped cathode 3, which maintains the activity of the anode. The electrolyte 4 is suitably potassium hydroxide, and may contain other agents such as zinc oxide to inhibit hydrogen discharge from the zinc. A porous separator 5, which is can shaped, is located inside the cavity 3a defined by the cathode 3, adjacent to the cathode 3. Accordingly, anode 2 and cathode 3 are not in electronic contact with each other, but are ionically connected via the electrolyte 4 and porous separator 5 such that current can flow between the anode 2 and the cathode 3.

(8) The anode 2 is attached to a connector 6 for electrically connecting the anode 2 to the metal section to be cathodically protected. The connector 6 is suitably galvanised steel. The cathode 3 of the cell is electrically connected in series with a sacrificial anode 7. Sacrificial anode 7 is solid zinc and is can shaped, with the solid zinc being cast metal, compressed powder, fibres or foil. The cell is located inside the cavity defined by the can shaped sacrificial anode 7. A layer of electrically insulating material 8 is located across the top of the assembly to isolate the cell from the external environment and accordingly current can only flow into and out of the cell via the sacrificial anode 7 and the connector 6.

(9) The sacrificial anode assembly 1 may subsequently be surrounded by a porous matrix; in particular a cementitious mortar such as a calcium sulphoaluminate may be pre-cast around the assembly 1 before use. The matrix may also suitably comprise a reservoir of alkali such as lithium hydroxide.

(10) The sacrificial anode assembly 1 may be utilised by being located in a concrete environment and connecting the conductor 6 to a steel bar also located in the concrete. Current is accordingly driven through the circuit comprising the anode assembly 1, the steel and the electrolyte in the concrete, by the voltage across the cell and the voltage between the sacrificial anode 7 and the steel, which two voltages combine additatively. The reactions that occur at the metal/electrolyte interfaces result in the corrosion of the zinc sacrificial anode 7 and the protection of the steel.

Example 2

(11) FIG. 2 shows a sacrificial anode assembly 11 connected to a 20 mm diameter mild steel bar 12 in a 100 mm concrete cube 13 consisting of 350 kg/m.sup.3 ordinary Portland cement concrete contaminated with 3% chloride ion by weight of cement.

(12) The sacrificial anode assembly 11 comprises a cell, which is an AA size Duracell battery, and a sacrificial anode, which is a sheet of pure zinc folded to produce a zinc can around the cell. This zinc is folded so as to contact the positive terminal of the cell, and a conductor 14 is soldered to the negative terminal of the cell. A silicone-based sealant is located over the negative and positive cell terminals so as to insulate them from the environment.

(13) Prior to placing the sacrificial anode assembly 11 in the concrete cube, potentials were measured using a digital multimeter with an input impedance of 10 Mohm, which showed that the potential between the external zinc casing and a steel bar in moist chloride contaminated sand was 520 mV and the potential between the conductor and the steel was 2110 mV. This suggests that the sacrificial anode assembly 11 would have 1590 mV of additional driving voltage over that of a conventional sacrificial anode to drive current through the electrolyte between the anode and the protected steel.

(14) As shown in FIG. 2, the circuit from the sacrificial anode assembly 11 through the electrolyte in the concrete cube 13 to the steel bar 12 was completed by copper core electric cables 15, with a 10 kOhm resistor 16 and a circuit breaker 17 also being included in the circuit. The drive voltage between the anode and the steel was monitored across monitoring points 18 while the current flowing was determined by measuring the voltage across the 10 kOhm resistor at monitoring points 19. A saturated calomel reference electrode (SCE) 20 was installed to facilitate the independent determination of the steel potential across monitoring points 21.

(15) The drive voltage, sacrificial cathodic current and steel potential were logged at regular intervals. The drive voltage and sacrificial cathodic current expressed relative to the anode surface area are shown in FIG. 3. The anode-steel drive voltage was approximately 2.2 to 2.4 volts in the open circuit condition (circuit breaker open) and fell to 1.5 to 1.8 volts when current was been drawn.

(16) The steel potential and sacrificial cathodic current expressed relative to the steel surface area are shown in FIG. 4. The initial steel potential varied between 410 and 440 mV on the SCE scale. This varied with the moisture content of the concrete at the point of contact between the SCE and the concrete. This negative potential reflects the aggressive nature of the chloride contaminated concrete towards the steel. The steel current density varied between 25 and 30 mA/m.sup.2.

(17) The steel potential decay following the interruption of the current (circuit breaker open) was approximately 100 mV, indicating that steel protection is being achieved. This also means that, of the 1.5 to 1.8 volts anode-steel drive voltage, more than 1.4 volts would be available to overcome the circuit resistance to current flow. This is significantly more voltage than could be provided by a sacrificial anode as currently available to overcome circuit resistance to current flow.

(18) It is therefore clear that in high resistivity environments, i.e. where the circuit resistance to current flow presented by the conditions is high, the sacrificial anode assembly of the present invention has a significant advantage over the more traditional sacrificial anodes currently available.