Cathodic Corrosion Protection with Current Limiter

Abstract

In a method for cathodically protecting and/or passivating a metal section in an ionically conductive material such as steel reinforcement in concrete or mortar, an impressed current or sacrificial anode communicates ionic current to the metal section and a storage component of electrical energy which can be a cell, battery or capacitor is provided as a component of the anode. A current limiter is provided which prevents excess current draining the supply. This can be a semi-conductive device such as a transistor or diode is connected in the path from the anode to the metal section to limit the cathodic protection current to a value of the order of 1 milliamp. When a diode or similar device is used the current can be limited to the reverse leakage current of the diode.

1 . A method for cathodically protecting and/or passivating a steel member in an ionically conductive concrete or mortar material, comprising:
 providing an anode construction for communication of an electrical current to the steel member in the ionically conductive concrete or mortar material;   generating a voltage difference between the anode construction and the steel member so as to cause a current to flow through the ionically conductive concrete or mortar material between the anode and the steel member so as to provide cathodic protection of the steel member;   providing at least one electrically conductive circuit between the anode construction and the steel member;   providing a field effect transistor (FET) in the electrically conductive circuit which acts as a current regulating device to limit the current between the steel member and the anode construction to a maximum value.   
     
     
         2 . The method according to  claim 1  wherein the current through the FET is limited by a control current or voltage applied to a control terminal of the FET. 
     
     
         3 . The method according to  claim 2  wherein said control current or voltage is generated from a voltage difference between the anode and the steel member. 
     
     
         4 . The method according to  claim 1  wherein the anode construction and the FET form components of a common body which is at least partly buried in or attached to, as a single unit, the concrete or mortar material. 
     
     
         5 . The method according to  claim 1  wherein the anode construction comprises a sacrificial anode or an impressed current anode. 
     
     
         6 . The method according to  claim 1  wherein the FET is a normally closed transistor so that, if the control voltage or current falls below a threshold, the FET allows continued passage of current between the anode construction and the steel member. 
     
     
         6 . The method according to  claim 1  wherein the FET includes a source and drain with the current through the FET controlled by a gate/source voltage. 
     
     
         7 . The method according to  claim 6  wherein the gate/source voltage is generated by a resistance in the electrical circuit. 
     
     
         8 . The method according to  claim 6  wherein the gate/source voltage is generated by a resistance across the FET. 
     
     
         9 . The method according to  claim 6  wherein the gate/source voltage is generated by a cell. 
     
     
         10 . The method according to  claim 6  wherein the gate/source voltage is generated by a sacrificial anode separate from said anode construction. 
     
     
         11 . The method according to  claim 1  wherein there is provided a resistor in parallel with the current regulating diode. 
     
     
         12 . The method according to  claim 1  wherein the resistor in parallel with the CRD allows current to be increased by application of an increased voltage in the circuit. 
     
     
         13 . A method for cathodically protecting and/or passivating steel components in an ionically conductive concrete or mortar material, comprising:
 providing a plurality of anode constructions at spaced positions in the ionically conductive concrete or mortar material for communication of an electrical current to the steel components in the ionically conductive concrete or mortar material;   providing a DC power supply;   connecting the DC power supply to each of the anode constructions in parallel so as to generate a voltage difference between each anode construction and the steel components so as to cause a current to flow through the ionically conductive concrete or mortar material between each anode construction and the steel components so as to provide cathodic protection of the steel components;   providing between each anode construction and the DC power supply a respective electrically conductive circuit which acts to limit the current through the respective anode construction to a maximum value.   
     
     
         14 . The method according to  claim 13  wherein each electrically conductive circuit comprises a field effect transistor (FET) in which acts as a current regulating diode to limit the current between the steel components and the anode construction to a maximum value. 
     
     
         15 . The method according to  claim 14  wherein the current through the FET is limited by a control current or voltage applied to a control terminal of the FET. 
     
     
         16 . The method according to  claim 15  wherein said control current or voltage is generated from a voltage difference between the anode and the steel member. 
     
     
         17 . The method according to  claim 13  wherein the anode construction and the circuit which acts to limit the current form components of a common body which is at least partly buried in or attached to, as a single unit, the concrete or mortar material. 
     
     
         18 . The method according to  claim 13  wherein the anode construction comprises a sacrificial anode or an impressed current anode. 
     
     
         19 . The method according to  claim 14  wherein the FET is a normally closed transistor so that, if the control voltage or current falls below a threshold, the FET allows continued passage of current between the anode construction and the steel member. 
     
     
         20 . The method according to  claim 14  wherein the FET includes a source and drain with the current through the FET controlled by a gate/source voltage.