ANODE ASSEMBLY FOR CORROSION CONTROL OF STEEL REINFORCED CONCRETESTRUCTURESFIELD OF THE INVENTION This invention relates to a sacrificial anode assembly for corrosion control. More specifically, to a sacrificial anode assembly for corrosion control of reinforcing steel embedded in concrete and the like.
BACKGROUND OF THE INVENTION Cathodic protection (CP) of reinforcing steel has been applied to reinforced concrete structures with corrosion damage for over 30 years. Worldwide experience shows that CP prevents further damage in a reliable and economical way for an extensive period of time. CP is particularly suited in cases where chloride contamination is the leading cause of corrosion. The first applications started on bridge decks suffering from corrosion due to de-icing salt penetration which resulted in severe damage to the concrete. Since the 1970's, CP has been applied worldwide to buildings, marine structures, tunnels, bridge decks and substructures.
The normal condition of steel reinforcement in concrete is passivity. This is a state with an almost negligible corrosion rate, thanks to a thin oxide film (passive layer) on the steel surface, which is stabilised by the inherently high pH of approximately 13 in concrete. Basically, this passive layer on reinforcement steel in concrete may be lost by two mechanisms: either carbon dioxide ingress, which reduces the pH to a level of approx. 9 (carbonation), causing an essentially uniform loss of passivation, or the presence of chloride ions, which locally break down the passive film, thus initiating pitting corrosion. Chloride may be either cast in inside the concrete or may penetrate the concrete from the environment.
The type of corrosion that occurs in reinforced concrete is an electrochemical phenomenon, in which the electrochemical potential of the reinforcing steel and the exchange of electrical current between the steel and the surrounding electrolyte, liquid that is present in the pores of the concrete, play important roles.
In the passive state, the potential of the steel is relatively positive, due to chemical reaction between oxygen and the steel surface.
When passivity is lost, iron passes into solution in the form of ferrous ions, leaving an excess of electrons in the steel, which makes the potential of these spots more negative; this reaction is termed ‘anodic’. Potential differences between anodic sites and the remaining, passive, surface of the steel, the so-called cathodic areas, cause currents to flow in the liquid inside the concrete pores, thereby accelerating the steel dissolution reaction.
The corrosion products are significantly more voluminous than the original steel.
The net effect is expansion of the reinforcement, causing tensile stresses in the surrounding concrete.
After relatively small amounts of steel have been transformed into corrosion products, the concrete cover cracks and spalling or delamination occurs.
Cracking and spalling in themselves can be unacceptable, but they also have to be taken as a warning sign of further decay.
When left to corrode, the steel bar diameter may decrease and may become smaller than the structurally required lower limit.
Eventually this can lead to the collapse of a concrete structure.
Therefore, concrete repair may be necessary and the corrosion protection must be reinstated, for example by cathodic protection.
Principles of cathodic protection of steel reinforcement in concrete.
Cathodic protection of reinforcing bars in concrete is based on changing the potential of the steel to more negative values in order to reduce potential differences between anodic and cathodic sites and hence reducing the corrosion current to negligible values.
The change of potential is called polarisation.
In practice, this can be achieved in two ways, either by means of sacrificial anodes or by means of impressed current.
A sacrificial anode cathodic protection assembly is effectuated by mounting an electrode, the anode, on the concrete surface or by embedding it in the concrete and connecting it to the reinforcement steel cage.
Through the steel reinforcement cage, 1043637 | electrons flow to the steel/concrete interface, increasing the so-called cathodic reactions, which produce hydroxide ions from oxygen and water. On the other hand metal atoms are formed at the metal/electrolyte interface and migrate through the electrolyte where they can be oxidised to any metal-complex and electrons. The basic electrochemical reaction at the anode of a sacrificial anode cathodic protection assembly in reinforced concrete is : Me—Me™+ ne The electrons flow to the current source, which closes the electrical circuit. As a result of this current circulation, cathodic reactions are favoured and anodic reactions at the steel surface are suppressed. Even relatively moderate current densities are able to restore passivity of the reinforcing steel and have various beneficial chemical effects. Anode materials At the core of any sacrifical anode assembly for cathodic protection is the anode or electrode material. Such materials are available on the market in various types, forms and shapes. Depending on the application, the anodes comprise metals such as zinc, aluminium, magnesium or any alloy based on one or more of these metals.
Common sacrificial anode materials for cathodic protection systems for reinforced concrete structures comprise for example: e zinc, or any alloy thereof in the form of wire mesh, wire, strip, tape, or tubes, shaped to fit the surface of the structure and to subsequently be covered with a cementitious overlay, such as for example spray crete or shotcrete; e or wires, strips, mesh or tubes placed in holes or slots in the concrete and backfilled with cementitious grouts; e thermally sprayed coatings or otherwise applied coatings covering the concrete surface.
Probably the most important limitation of sacrificial anode based CP systems for the protection of reinforcement in concrete is the limitation in the throwing power of sacrificial anodes. The term throwing power refers to the distance over which the protective current can travel through the concrete, which is a material with a high resistivity.
SUMMARY OF THE INVENTION The invention comprises a sacrificial anode assembly of a cathodic protection system for reinforced concrete structures using electrode materials such as for example zinc, aluminium, magnesium, or any alloy thereof in any form or shape, embedded in, covered by or coated with an electrolyte that keeps the electrode material active, wherein the electrolyte is in direct contact with an electrically conductive material comprising for example a coating, mortar or metallised film or any other material which can be applied on concrete with the purpose of distributing the current coming from the anode over an expanded surface area of the concrete.
BRIEF DESCRIPTION OF THE DRAWINGS The objects and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: - FIG. 1 shows a schematic exploded view of a reinforced concrete slab and the components of an embodiment of an anode assembly according to the invention; - FIG. 2 shows a schematic cross section of a part of a concrete structure that includes another embodiment of the anode assembly according to the invention comprising an embedded sacrificial anode. In this description and in the drawings identical or similar parts have been designated with identical or similar reference numbers. 5
DETAILED DESCRIPTION OF THE INVENTION The present invention comprises an improved method and anode assembly for the application of a sacrificial anode corrosion control system for steel in concrete.
The anode assembly according to the invention comprises an electrically conductive layer for application on the surface of a steel reinforced concrete structure. This electrically conductive layer comprises for example a conductive coating or mortar or a metallised layer or any other electrically conductive material that can be applied on a concrete surface through for example cold or thermal spraying. In addition, the anode assembly according to the invention comprises electrode material, fully or partially, embedded in, covered with or attached to an anode activating electrolyte. The anode activating electrolyte may be solid, semi-solid or liquid whereby the viscosity of the liquid may vary within a wide range.
FIG. 1 shows a schematic exploded view of a reinforced concrete slab and the components of an embodiment of an anode assembly according to the invention.
In this embodiment the assembly comprises an electrically conductive layer 1 which may also be referred to as an electrically conductive coating 1 that is attached to the surface of a reinforced concrete slab 2.
On top of the electrically conductive coating 1 a layer of electrolytically or ion- conductive material 3, hereinafter also referred to as the electrolytic material 3, is applied which only covers a small part of the surface area that is coated with the electrically conductive coating 1. It should be noted that, whereas, an ion-conductive material 3 is electrically conductive, an electrically conductive coating 1 used in the anode assembly according to the invention normally will not comprise ion conductivity. In various embodiments of the anode assembly according to the invention the surface area of the ion-conductive material 3 comprises approximately one hundredth of the surface area of the electrically conductive coating 1. On top of the ion-conductive material 3 a sacrifical anode 4 comprising zinc, aluminium, magnesium and/or any of their alloys in the form of a strip is placed.
The sacrificial anode is not restricted to strip and can have any shape or form that is suitable.
FIG. 2 shows a schematic representation of a cross section of another embodiment of the sacrificial anode corrosion control assembly for reinforced concrete according to the invention.
In this embodiment the anode assembly comprises a sacrificial anode 4 that is installed in a hole 5 in the concrete slab 2. The surface of the concrete slab 2 including the wall of the hole 5 is coated with the electrically conductive coating 1. If the sacrifial anode 4 is not applied directly on the surface of reinforced concrete or in a pre-drilled hole, cavity, slot, slit, recess or any other type of opening in the concrete extending from the concrete surface inwards which opening is suitable for installation of at least a part of an anode in the reinforced concrete it can be used in a pre-fabricated anode assembly wherein the anode and ion-conductive material have been pre-assembled.
In the remainder of this description and/or in the claims the use of the term hole may refer to any opening in the concrete that is suitable for installing an anode in it and such an opening may comprise for example a cavity, slot, slit or recess.
In the above description and in the claims the term electrically conductive coating 1 shall be construed to also include the possibility of a coating or layer in the form of for example an electrically conductive mortar or metallised film.
The advantages of the anode assembly according to the invention compared to sacrificial anodes or anode assemblies for reinforced concrete according to the prior art, can be summarized as follows: e By applying an electrically conductive coating 1 on the concrete surface prior to installation of a sacrificial anode, the anode assembly according to the invention is able to distribute the current over an expanded surface area.
e By applying an electrically conductive coating 1 on the concrete surface, the anode assembly according to the invention is able to reduce the effect of the concrete’s high electrical resistance which normally will restrict the sacrificial anode's throwing power.
e The pre-fabricated anode assembly according to the invention can be applied directly on the concrete surface by virtue of the salt bridge effect of an ion- conductive material 3. In various embodiments the ion-conductive material 3 is applied in the form of an adhesive and no additional concrete pre- or post- treatment is required.
The term “comprise” includes also embodiments wherein the term “comprises” means “consists of”. The term "comprising" may in an embodiment refer to "consisting of" but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species". Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim.
The term “and/or” especially relates to one or more of the items mentioned before and after “and/or”. For instance, a phrase “item 1 and/or item 2” and similar phrases may relate to one or more of item 1 and item 2.
While only certain features of the invention have been described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.
The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The invention further pertains to assemblies comprising one or more of the characterizing features described in the description.
The various aspects discussed in this application can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined.