EP1063321A2

EP1063321A2 - Improvements in cathodic protection for concrete and masonry structures

Info

Publication number: EP1063321A2
Application number: EP00304730A
Authority: EP
Inventors: Frits Gronvold
Original assignee: Gronvold & Karnov As
Current assignee: GRONVOLD & KARNOV AS
Priority date: 1999-06-17
Filing date: 2000-06-05
Publication date: 2000-12-27
Also published as: GB9914198D0; EP1063321A3

Abstract

A method for the cathodic protection of reinforced concrete or masonry structures comprises inserting a number of pin anodes (11, 12) in said structure (13) and connecting the pin anodes to a source of direct current. A high frequency current interruption is imposed on the output of the DC circuit supplying current to the anodes. The interruption of the DC current gives enhanced cathodic protection for reinforced concrete or masonry structures.

Description

This invention relates to cathodic protection for reinforced concrete or masonry structures and, in particular, cathodic protection of such structures using internal anodes. The invention is particularly concerned with the prevention or reduction of corrosion of reinforcing elements in concrete and masonry structures using impressed current cathodic protection.
Cathodic protection for reinforced concrete structures is well-known and has for the last two decades been used to protect steel in concrete. These systems typically use an anode which feeds current, a cathode or metal structure to be protected and an electrolytic conductor between the anode and cathode. The current is supplied by a power source feeding a continuous direct current (DC) and a reference electrode is utilised to indicate the required current.
The principle of cathodic protection is to transfer the natural surface potential of the structure to a more negative value so that the corrosion rate is slowed. If the potential of carbon steel is commonly taken to a more negative value than around -850mV Cu/CuSO₄, the corrosion is stopped. For steel in concrete the depolarisation is also commonly used to denote reduction in corrosion rate.
The normal reason that cathodic protection is applied to steel in concrete structures is that in the presence of chlorides, the passivity of the steel is reduced and active corrosion can occur with consequent reduction in strength of the reinforcement and often spalling or cracking of the concrete cover. The same problems arise with steel in masonry.
Numerous solutions have been suggested for cathodic protection and it has commonly been used for protecting reinforced concrete. One way is the use of a pin anode connected to the concrete by an electrically conductive couplant. This anode has to be placed at numerous locations around the concrete structure in order to spread the current evenly.
The present invention aims to reduce the number of pin anodes required and yet still maintain the level and spread of the protection current. This gives a significant advantage both economically in material and installation cost and also causes significantly less disruption to the original structure.
According to one aspect of the invention, there is provided a method for the cathodic protection of reinforced concrete or masonry structures in which a number of pin anodes are inserted and connected to a source of direct current, wherein a high frequency current interruption is imposed on the output of the DC circuit supplying current to the anode(s).
Typically, the high frequency current interruption is imposed in the range of 100 Hz to 10 MHz and a duty cycle typically of 1 to 99.9%.
It has been found that when this current interruption is introduced, there is an increase in the throwing power or spread of the cathodic protection current and a reduction in the current necessary to obtain cathodic protection on the reinforcing steel being protected.
The interruption can be caused by the imposition of a specially developed circuit on the output of a DC circuit. This circuit is sufficiently specified to be able to switch the currents being delivered and will normally be between 1 mA and 5A, depending on the application.
The present invention also provides a circuit for performing the above-described method.
The invention will now be described in greater detail, by way of example, with reference to the drawings in which:-
Fig. 1 is a diagram showing one embodiment of a current interruption circuit according to the invention;
Fig. 2 shows an electrode installed on a reinforced concrete deck slab; and
Figs. 3 and 4 are graphs showing, respectively, the results of trials using interrupted and non-interrupted current.
Reference will first be made to Fig. 1 of the drawings in which a current interruption circuit according to the invention comprises two unstable multivibrators M1 and M2 making a square pulse with adjustable frequency and pulse width. This pulse is used to control a semiconductor switch (power MOS) which switches the input from DC to interrupted current.
As shown in Fig. 2, two pin anodes 11 and 12 were installed in a concrete deck slab 13 which was reinforced with a network of steel reinforcing rods 14 having a diameter of approximately 16mm. A series of first reference electrodes MMO consisting of mixed metal oxide on a titanium substrate were installed in the reinforced concrete slab 13 at regular spaced intervals between the pin anodes 11 and 12 and a series of second reference electrodes G consisting of pure graphite were also installed in the reinforced concrete slab at regular spaced intervals between the pin electrodes 11 and 12. The MMO electrodes were placed away from the reinforcement 14 while the G electrodes were placed close to the reinforcement. The MMO electrodes measured approximately 6mm x 50mm while the G electrodes measured approximately 4mm x 20mm. A third series of combined MMO and G electrodes (MMO+G) were also located on the reinforced concrete slab, again between the pin electrodes 11 and 12. The pin anodes 11 and 12 had an active length of about 90mm and were surrounded by an electrically conductive couplant (not shown). As indicated in Fig. 2, the pin anodes were located close to one another but were electrically isolated from each other.
The results of continuous and interrupted current passed through other pin anodes (not shown) on the reinforced concrete structure 13 were recorded. Simultaneously, the two pin anodes were energised at 8 V. The effect on the steel reinforcement was measured at increasing distances from the anodes using buried reference electrodes MMO, G and MMO+G. The levels of depolarisation and hence the levels of polarisation on the steel at increasing distances from the respective pin anodes 11 and 12 were measured. This was done by taking "instant off" readings when the current was interrupted for approximately 0.2 seconds, keeping the current disconnected and measuring the change in electrical potential which occurs over a 24 hour time period. This measurement is commonly used in the cathodic protection industry and is termed "potential decay measurement".
The results of the trials are given in Figs. 3 and 4. These figures demonstrate that there is a difference in the attenuation between the interrupted and continuous currents. The lines indicating the best fitting curves show the reproducibility of the experiments. The interrupted current provides a substantial depolarisation effect at a distance, which is substantially greater than the continuous current. At 100 mV depolarisation, which is BS 7361:Part 1:1991 Cathodic Protection advised minimum level, the interrupted current has a reach of 800mm with the continuous current reach of only 500mm.
It will thus be seen that the method according to the invention of using interrupted DC current gives enhanced cathodic protection for reinforced concrete or masonry structures using internal anodes.

Claims

A method for the cathodic protection of a reinforced concrete or masonry structure in which a number of pin anodes (11, 12) are inserted in said structure (13) and are connected to a source of direct current, characterized in that a high frequency current interruption is imposed on the output of the DC circuit supplying current to the anodes.
A method according to Claim 1, characterized in that the high frequency current interruption is imposed in the range of 100 Hz to 10 MHz.
A method according to Claim 2, characterized in that the high frequency current interruption is imposed at a duty cycle of 1 to 99.9%.
A method according to any one of the preceding claims, characterized in that a circuit (M2, M2) is imposed on the output of the DC circuit which is able to switch the currents being delivered, preferably between 1 mA and 5A.
A circuit for performing the method claimed in any one of the preceding claims, characterized in that said circuit comprises a number of pin electrodes (11, 12), means for connecting the pin electrodes to a source of direct current and a current interruption circuit (M1, M2) connected between the pin electrodes and the said means for connecting the pin electrodes to a source of DC current.