Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Patent
1995-08-09
2000-02-08
Tung, T.
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
2041961, 20419621, 20419625, 20419636, C23F 1300
Patent
active
06022469&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This application is the national phase of international application PCT/GB94/01224 filed June 6, 1994 which designated the U.S.
This invention relates to the cathodic protection of reinforced concrete.
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 an anode and a 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 ine the aggregate 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. 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 wherein 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 wherein 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 possible >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 a zinc alloy but the anode may be aluminium, an aluminium alloy, cadmium, a cadmium alloy, magnesium or a magnesium all
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Foller, "Effects of Additives on the Suspension of Products of Discharge of Zinc in Alkaline Solution", Journal of Applied Electrochemistry, vol. 17 (1987) month unavailable, pp. 1296-1303.
Aston Material Services Limited
Tung T.
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