Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-10-19
2001-07-03
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C205S766000, C106S795000, C106S805000
Reexamination Certificate
active
06254752
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the electrochemical removal of chloride from concrete and to a composition for use in the process.
BACKGROUND OF THE INVENTION
The presence of chloride ions in reinforced concrete is known to be deleterious and it has been previously proposed to remove chloride ions by a process which has become known as desalination. This process has been described in European Patents Nos. 200,428 and 398,117 and involves passing an electric current through the concrete by applying a voltage for example 3 to 15 volts between an external temporary anode and the steel reinforcement in the concrete. The external temporary anode is usually covered in a layer of electrolyte which often includes in addition to the electrolyte an adherent coating material. The effect of the electric current is to cause chloride ions within the concrete to migrate through the concrete towards the surface and into the electrolyte layer. European Patent No. 398,117 describes the use of a fibrous cellulose pulp in the coating and the mixing of the pulp fibres with a liquid to form a self adherent material which is applied to the exterior surface of the concrete by spraying whereby an anode which has been previously applied to the concrete surface is embedded in a fibrous cellulose coating. Loss of alkalinity in reinforced concrete is also known to be deleterious. The loss of alkalinity is caused by carbon dioxide and other atmospheric gases such as sulphur dioxide. A process for increasing the alkalinity of concrete whose pH has been reduced by atmospheric gases has been previously described in European Patent No. 264,421 and U.S. Pat. No. 4,865,702. In this process an electric current is passed between an externally applied electrode which is in contact with an alkaline solution and the internal reinforcement of the concrete as cathode. The result of the process is to increase the concentration of hydroxyl ions in the region of the reinforcement and thereby increase the alkalinity of the concrete. The benefit of doing this is that corrosion of the reinforcement is reduced or prevented. The process has become known as realkalization.
Problem to be solved by the Invention
The anode employed in desalination may be made of either an inert material for example expanded titanium mesh coated with electrocatalytic oxides of platinum metals, or made of a consumable metal such as reinforcing steel net.
The passage of the electric current produces acid around the anode. The concrete surface may be stained or damaged and in the case of a consumable electrode, corrosion of the anode may occur.
Also in a desalination process discharge of the chloride ion at the anode produces chlorine gas which, in enclosed spaces can lead to unpleasantly high or even hazardous levels of the gas. Corrosion of metal fittings may also occur. Previously the formation of acid and chlorine has been combated by adding to the electrolytic layer substances which buffer the pH.
This can to some extent reduce chlorine formation because the latter tends to be suppressed with increasing pH. The materials which have been previously used have been mixtures of fire retarding agents such as hydrated alumina and soluble pH buffers such as sodium borate. The amount of pH buffer which can be added is not normally sufficient to last throughout the usual treatment period. Also the pH which can be attained in practice is not high enough to significantly suppress the production of chlorine gas.
European Patent No. 200,428 describes an electrochemical process for the removal of chloride from reinforced concrete in which the anode is embedded in a coating of a material capable of absorbing chloride such as a retarded gunite. Gunite is a finely graded concrete (ie a mixture of cement and sand) which is suitable for application by spraying. The gunite is retarded to prevent it hardening before completion of the electrochemical process. However gunite is a heavy and unpleasant material to work with and presents a disposal problem when removed from the concrete at the end of the process. It is now preferred to use adhesive materials which are lighter, are also able to retain larger amounts of water and are more easily disposed of at the end of the process.
U.S. Pat. Nos. 5,198,082 and 5,228,959 describes the use as the anode of a ferrous metal such as steel particularly in the form of a grid, the effect of which is to react with the chloride ions and corrode thereby reducing the liberation of chlorine gas. These patents disclose that in most cases the development of corrosion products can be compensated for by increasing the voltage level up to 40 volts but that in severe cases it may be necessary to replace the anode grid.
A solution to the above described problem of chlorine formation has now been invented in which calcium hydroxide or barium hydroxide is employed as a chlorine scavenger in the adherent coating.
SUMMARY OF THE INVENTION
According to the present invention a process for the electrochemical removal of chloride ions from chloride-containing reinforced concrete comprises: passing a direct current between (i) an anode in electrical contact with a coating containing a water retaining material and an aqueous electrolyte applied to an external surface of the concrete and (ii) a cathode which is located internally in the concrete, which process causes chloride ions to migrate to the anode and wherein to reduce the production of chlorine gas, the coating contains, as a chlorine scavenger, solid calcium hydroxide or barium hydroxide or mixtures thereof, the chlorine scavenger being present in an amount of at least 8% by weight based on the dry weight of the water retaining material.
ADVANTAGEOUS EFFECT OF THE INVENTION
The advantage of the present invention is that the production of chlorine gas is reduced.
REFERENCES:
patent: 5407543 (1995-04-01), Miller
patent: WO 9321130 A1 (1993-10-01), None
Fosroc International Limited
Nixon & Vanderhye P.C.
Phasge Arun S,.
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