GB2167770A - Cathodic protection system - Google Patents

Abstract

A cathodic protection system for protecting a metallic body such as a reinforcing steel bar from corrosion as a result of contact with an electrolyte comprises a probe formed from a material which is electronegative relative to the bar 2 positioned adjacent the bar so as to contact the electrolyte, and anode 4 positioned to contact the electrolyte, the probe and bar being connected to an electrical circuit arranged such that in the absence of anode current the probe is at a lower electrical potential than the bar. Current is supplied to the anode so as to minimise the probe current. <IMAGE>

Description

SPECIFICATION (:cathodic protection system The present invention reFatesto a cathodic protection system forprnt'esti-ng metaflicbodies from corrosion using impressed currents.

Various methods have been employed to control the applied voltage and/or current between anodes and protected cathodes in impressed current cathodic protection systems. In such systems an electrical potential is applied between a cathode (the body to be protected) and an anode such that the cathode is protected against corrosion. In some circumstances where conditions vary little it is possible to apply a fixed voltage and currentto ensu refull protection with no need forfurther control or monitoring other than periodic maintenance checks.Where conditions such as electrolyte composition, moisture content and electrode conditions are likely to vary, as in the case for example ofthe protection of buried structures or reinforcing steel in concrete, some means of automatic voltage or current control is desirable to ensure full protection without applying unnecessarily high vol tagesand/orcurrentswhich can in themselves be damaging.

One method ofcompensating for variable conditions is to em ploy a constant current voltage source which maintains a predetermined current calculated on the basis of an optimum current density atthe surface ofthe structure which ensures protection. This method, however, takes no account of cathode surface conditions which may alter with time (e.g.

chalking) or the changes in surface current density required for protection whii occurwith variations in electrolyte resistivity, moisture content etc. This can lead to excessive cathode/electrolyte voltages causing damage to paint films or, in concrete, damage to the bond between reinforcement and concrete.

A more satisfactory method relies on the empirically proven fact that minimum metal/electrolyte voltages do exist atwhich corrosion cannot occur. The voltages are measured between protected metal and a suitable reference electrode immersed in the electrolyte and are used to control the voltage applied at the protective anode(s). In this way the metal/electrolyte potential is maintained at a level which ensures protection but does not exceed that atwhich damage might be caused to coatings, the bond or surrounding material.

This method suffers from the disadvantage that reference electrodes are, in the main, fragile and unreliable. Asimplewire electrode cannot be used as the wire becomes polarised with the result that surface potentials on the wire are unpredictable and mask the true electrolyte potential. Thus it is necessaryto use fragile electrodes such as copper sulphate halfcellsthatcan be easily damaged in use. Electrodes of th is type are paru:c'tarIyvu l nerabiewhen used in systems forthe cathodic protecion- of concrete reinforcement wltere the concrete can form part of for example a road bridge and be subjected to severe mechanical loading and vibration.

U.S. Patent No. 4160171 sets out in some detail the problems experienced in accurately determining the required reference voltage in impressed current protection systems and proposes the use of a half cell reference electrode. Another system using a half cell reference electrode is described in British Patent No. 1 589 739, the disclosure mentioning copper/copper sulphate, silver/silverchlorideand zinc reference electrodes.

It is an object of the present invention to provide a system in which the protected metal/electrolyte potential can be maintained within desired limits by automatic control of the applied voltage, but which avoids the use of vulnerable reference electrodes.

According to the present invention, there is provided a cathodic protection system comprising a metallic body which is to be protected from corrosion as a result of contact with an electrolyte, a probe positioned adjacent the metallic body so asto contact the electrolyte, an anode positioned to contact the electrolyte, meansforsupplying currenttotheanode, a circuit connected to the probe and the metallic body and arranged such that in the absence of anode currentthe probe is at a lower electrical potential than the metallic body, and means for controlling the current supplied so as to minimise the probe current.

The above arrangement enables the use of a simple metallic probe which can be easily positioned adjacent the metallic body to be protected in contrastto the conventional systems which use fragile electrode structures. A high gain, high input impedance differential amplifier can be arranged to supplycurrentto the anode so as to minimise the probe current. If only a minimal currentflows through the probe, an impressed cu rrentflowsfrom the electrolyte through the metallic body to earth. This prevents corrosion of the metallic body. If a simple electronegative probe is used, the impressed current will be a function of the degree to which the probe is electronegative to the metallic body.The impressed current can be increased or decreased however whilst using the same probe and whilst maintaining the probe current zero by adding a source of offsetting electrical potential to the probe circuit. The source of offsetting potential could be positioned remote from the probe/electrolyte interface and therefore the provision of such a source of offsetting potential is not disadvantageous.

Embodiments ofthe present invention will now be described, by way of example, with reference to the accompanying drawings in which Fig. lisa schematic diagram explaining the principle of the present invention; and Figs. 2 illustrates an embodiment of the present invention.

Referring to Fig. 1, a probe 1 and steel bar2 are shown immersed in an electrolyte 3 contained in an electrically conductive tank 4, the tank 4 being connected as an anode to a current sou rce 5. In practical embodiments of the invention, the electrolyte will befor example a salt solution contained within a mass of concrete reinforced by the bar 2. The bar 2 will be close to the probe 1 and relatively remote from the anode 4, and the anode will be for example an electrically conductive coating on the surface of the concrete.

The bar 2 is electrically connected to a source of electric potential 6, the potential of which is the earth of the system. The probe 1 is made from a material which is electronegative relative to the material ofthe bar 2. Thus when no current is supplied to the anode 4, a current will flow through resistors 7 and 8 from he bar 2 to the probe 1, that is to saythe potential of the probe will be negative relative to the potential ofthe bar. In these circumstances the probe 1 will actasa sacrificial anode and corrode, thereby providing some galvanic protection to the bar 2.

In accordance with the invention howeverthe currentthrough the probe 1 and the series resistors 7 and 8 is reduced to substantially zero. This is achieved by connecting a current comparator9 across the resistor 8, and controlling the current source5 using the comparator9 so asto supply currenttothe anode 4. The comparator 9 thus senses the currentthrough resistor 8. An impressed current is supplied to the bar 2, the magnitude of the current being a function of the difference between the electrochemical properties of the probe 1 and the bar 2. Thus it is not necessary to accurately measure the electrolyte potential as with prior art impressed current systems.

Fig. 2 illustrates the circuit of an embodiment of the invention corresponding in operation to the schematic arrangement of Fig. 1. Equivalent components carry the same reference numerals in Figs. 1 and 2.

The current source 5 comprises a transistor 10 which supplies currentto the anode 4when the transistor base is positive relative to earth. A series resistor 11 limits the base current and a diode 12 limits the negative bias applied to the transistor base. The comparator9 comprises a high input impedance, high gain operational amplifier 13 the positive and nega tiveterminals ofwhich are connected by diodes 14 of opposed polarities. The diodes 14 limitthevoltage which can appear across the amplifier inputs to the cut-in voltage of the diodes. It will be noted thatthere is no component shown equivalenttothe resistor 8 of Fig. 1.This component is in effect a substantially infinitely large resistance representing the resistance of the diodes 14 which in normal operation are non-conducting. The resistor7 merely limits abnormal currents through the probe.

By way of example, the various illustrated componentscould be as follows:- resistor7 :100 ohm amplifier 13 : 3140 family integrated circuit transistor 10: TIP 121 or equivalent diode 12: IN 4001 diodes: IN 4148 In the circuit of Fig. 2, the current supplied to the anode 4 is entireiy dependent upon the potential difference between the probe 1 and bar2 resulting from the difference in their electrochemical properties. It may be desirable however, to offsetthe magnitude ofthe current supplied to the anode 4 by adjusting the effective potential difference between the probe and the bar.This can be done by connecting a source of voltage (not shown) in series with the probe/electrolyte interface, i.e. between the probe 1 and resistor 7, or between the resistor7 and the amplifier 13.

The material ofthe probe 1 will of course be selected to suit the circumstances found in particular applications. Byway of example,a probe which has been used successfully to prevent corrosion of steel in brine comprised 99.5% by weight of aluminium, 0.17% by weight of iron, and 0.23% by weight ofsilicon and traces of elements such as copper, tin, lead, nickel, manganese, magnesium, titanium and/or chromium.

This probe was found to provide a shift of approximately 300mV in the potential between a steel reinforcing bar and the probe in an electrolyte to which the bar and probe are exposed. This is generally considered to impartfull protection against corrosion ofthe bar. In addition, this material, because of its iron impurities, forms a high resistance passivating film on its su rface. This helps to prevent corrosion of the probe, which would otherwise occur due to anodic and cathodic areas on the probe itself, but has little appreciable effect on the operation of the illustrated circuits. Such a probe has been found to corrode when used to protect steel in a reinforced concrete beam however, and for such applications probes of tin/zinc alloy are preferred.

Claims (6)

1. Acathodic protection system comprising a metallic body which is to be protected from corrosion as a result of contact with an electrolyte, a probe positioned adjacentthe metallic body so as to contact the electrolyte, an anode positioned to contactthe electrolyte, meansforsupplying currentto the anode, a circuit connected to the probe and the metallic body and arranged such that in the absence of anode currentthe probe is at a lower electrical potential than the metallic body, and meansforcontrolling the current supplied so asto minimise the probe current.

2. A cathodic protection system according to claim 1, wherein the probe is electronegative relative to the metallic body.

3. A cathodic protection system according to claim 1 or2, in which the metallic body is connected to earth.

4. A cathodic protection system according to any preceding claim, wherein the probe comprises a source of electrical potential connected in series with the probe/electrolyte interface.

5. A cathodic protection system according to any preceding claim, wherein a differential amplifier is connected to control the supply of currentto the anode, the differential amplifier having one terminal connected to the probe and its otherterminal connected to the metallic body.

6. A cathodic protection system substantially as hereinbefore described with reference to the accompanying drawings.