Cathodic protection of reinforced concrete

Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy

Reexamination Certificate

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Details

C205S730000, C205S731000, C205S732000, C205S733000, C205S735000, C204S196010, C204S196210, C204S196250, C204S196360

Reexamination Certificate

active

06303017

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the cathodic protection of reinforced concrete.
2. Description of the Related Art
The application of cathodic protection to steel reinforcement in concrete is an accepted method of providing corrosion protection for the metal, particularly where chloride ions are present at significant concentrations in the concrete.
Cathodic protection involves the formation of a circuit with the reinforcement acting as a cathode, electrically connected to an anode, with the circuit being completed by pore solution in the concrete and an electrolyte contacting the anode. When a potential difference exists corrosion of the cathode is prevented or reduced.
It is known to create a potential difference between anode and cathode both by means of impressed current cathodic protection which involves the use of a non-sacrificial anode and an applied electric current using an external DC power supply and by means of a galvanic cell in which the potential arises as a result of the different materials forming a sacrificial anode and a cathode.
Where a galvanic cell is used it is important that the electrolyte contacting the anode is such that sustained active corrosion of the anode can occur. If suitable conditions are not maintained then the cathodic protection will become inefficient.
Furthermore, the electrolyte must be such that its contact with the surrounding concrete does not result in the degradation of the concrete. Of particular significance in this context is the susceptibility of some aggregates, present in concrete, to alkali-silica or alkali-agggregate reactions. These reactions can cause swelling and consequential cracking of concrete.
SUMMARY OF THE INVENTION
According to the invention there is provided a method of cathodically protecting reinforcement in concrete in which a sacrificial anode is galvanically connected to the reinforcement characterised in that the anode is contacted with an electrolyte solution having a pH which is maintained sufficiently high for corrosion of the anode to occur and for passive film formation on the anode to be avoided.
According to a further feature of the invention there is provided a unit for use in the cathodic protection of reinforcement in concrete by the method of claim
1
characterised in that the unit comprises a sacrificial anode in contact with a material containing an electrolyte which in solution has a pH which is sufficiently high for corrosion of the anode to occur and for passive film formation on the anode to be avoided when the anode is galvanically connected to the reinforcement.
According to yet a further feature of the invention there is provided an article of reinforced concrete characterised in that the reinforcement is cathodically protected by the method described above.
To avoid passivation of the anode a suitable pH must be maintained around the anode. Although for zinc a suitable pH value is >13.3, or possibly >13.5, and preferably >14, other materials when used as the anode may require other electrolyte pH limits to avoid passivity. In practice while any pH above the “boundary value” at which passivity is likely may be suitable in the short term, it is advantageous to have a pH well above the “boundary value” to start with. During cathodic protection the pH near the anode is likely to drop and so a higher initial pH acts as a reserve to maintain activity over a long period. pH values of 0.2 above the “boundary pH” may be acceptable, but pH values, 0.5, 0.7 and 1.0 or more units above the “boundary pH” are likely to give a better reserve and a better long term performance.
The anode material selected will determine the electrolyte pH required to maintain active corrosion. In general terms the material chosen must be more reactive, and preferably significantly more reactive, than the material forming the reinforcement.
The anode is preferably zinc or zinc alloy but the anode may be aluminium, an aluminium alloy, cadmium, a cadmium alloy, magnesium or a magnesium alloy or another material which has a more negative electrode potential than the reinforcement under the prevalent conditions.
The electrolyte may be for example sodium hydroxide or potassium hydroxide.
Advantageously, in some circumstances, at least one alkali-silica reaction inhibitor is also present, in at least a portion of the electrolyte.
The high pH of the electrolyte may be due, at least in part, to one or more of the alkali-silica reaction inhibitors.
Preferably at least one of the alkali-silica reaction inhibitors is provided in an hydroxide form. Most preferably the, or one of the inhibitors is lithium hydroxide, which can also function as the electrolyte itself.
The electrolyte solution may be the pore solution of the concrete and/or the pore solution of a mortar, paste or other porous material applied to the concrete being protected.
The method may be practised during the course of repairing reinforced concrete by connecting one or more sacrificial anodes to the reinforcement and applying repair material and the electrolyte to the repair site.
Preferably the anodes are provided in the vicinity of the repair site. If the anode is provided away from the repair site there is likely to be a loss of efficiency due to the extra circuit length required to complete the galvanic cell. Most preferably the anodes are provided near the periphery of the repair site. The anodes are preferably in the new material of the repair site. There may be many anodes. The anode or anodes may have a relatively large surface area and for example could be a mesh or wire (or wires) extending adjacent to the periphery of the repair site.
Preferably each anode is substantially enclosed in repair material containing an electrolyte of high pH. The portion of repair material away from the anode may have a different pH compared with the portion of repair material substantially enclosing the anode. The repair material away from the anode may have a pH that is relatively moderate or low compared with that near the anode.
The whole or any portion of the repair material may also contain one or more alkali-silica reaction inhibitors.
Where only a portion of the material contains an electrolyte of high pH and only a portion contains one or more alkali-silica reaction inhibitors the portions may be the same, distinct or overlapping in extent.
Preferably at least one of the alkali-silica reaction inhibitors also contributes to the high pH of the electrolyte.
As well as introducing sacrificial anodes and an electrolyte of high pH to a structure during a repair, potentially along with an alkali-silica reaction inhibitor, this invention is also applicable to the construction of new reinforced concrete articles or structures and to the improved protection of existing ones.
Just as during repair, anodes and a suitable electrolyte can be provided in electrical contact with the reinforcement to form a galvanic cell, so a similar arrangement can be generated during construction.
The entire structure can be provided with a suitable electrolyte, or merely that portion in the vicinity of the anode can be so provided.
In the construction of new reinforced concrete articles or structures one or more sacrificial anodes can be connected to the reinforcement, a material containing the electrolyte cast around the anode or anodes and concrete then cast around the electrolyte-containing material.
In the improvement of the protection of existing concrete articles one or more sacrificial anodes can be inserted in a hole in a mass of reinforced hardened concrete and connected to the reinforcement and then surrounded by a material containing the electrolyte.
In both methods the material containing the electrolyte can be a non-cementitious material or a cementitious material.
One or more of the sacrificial anodes may be introduced to the repair site as a pre-formed unit comprising an anode in contact in use with a porous material containing an electrolyte of high pH. The material may also contain one or more alkali-silica reaction in

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