Dispositif de protection cathodique d'un structure métallique contre la corrosion

Abstract

A device for cathodic protection against corrosion of at least one metal structure in contact with an electrolytic medium comprising a sedimentary soil, the protective device dispensing with the need for a sacrificial metal and means for connection to an electrical distribution network and including at least two microbial anode systems including microorganisms and an electrode configured to be in contact with the microorganisms and the electrolytic medium, the microorganisms having the ability to supply electrons to the electrode by degradation of oxidizable ambient resources according to oxidation-reduction reactions, the electrode of each microbial anode system being configured to be buried at least partially in the sedimentary soil of the electrolytic medium and the free electrochemical potential of the microbial anode system is lower than the free electrochemical potential of the metal of the metal structure to be protected.

Claims

1. A device for cathodic protection against corrosion of at least one metal structure in contact with an electrolytic medium comprising a sedimentary soil, said protective device dispensing with the need for a sacrificial metal and means for connection to an electrical distribution network, wherein said protective device comprises: at least two microbial anode systems each comprising microorganisms and an electrode configured to be in contact with said microorganisms and said electrolytic medium, said microorganisms configured to supply electrons to the electrode by degradation of oxidizable ambient resources following oxidation-reduction reactions, the electrode of each microbial anode system being configured to be buried at least partially in said sedimentary soil of the electrolytic medium and the free electrochemical potential of each microbial anode system being lower than the free electrochemical potential of the metal of said at least one metal structure to be protected; at least one connecting means configured to connect each microbial anode system to said at least one metal structure and configured to let the electrons circulate from the electrode of each microbial anode system up to said at least one metal structure; so that a protective galvanic current is applied through the electrolytic medium from at least one of the microbial anode systems up to said at least one metal structure when said at least one microbial anode system is in contact with said electrolytic medium and is connected by said at least one means for connection to said at least one metal structure, wherein said protective device further comprises: a system for measuring the current of electrons circulating in each connecting means allowing determining the variations in depletion of the oxidizable ambient resources which are in the environment close to the microbial anode system to which the connecting means is connected; and a control system allowing stopping and activating the circulation of the current of electrons circulating in each connecting means.

2. The device according to claim 1, wherein the microorganisms of the microbial anode system are at least microorganisms present in the electrolytic medium.

3. The device according to claim 1, wherein at least one microbial anode system comprises a chamber comprising the electrode and the microorganisms, said chamber being configured to receive oxidizable resources which are degradable by the microorganisms of the microbial anode system.

4. The device according to claim 1, wherein the protective galvanic current is distributed in the metal structure at a density comprised between 0.2 mA/m.sup.2 and 100 mA/m.sup.2, preferably between 0.2 mA/m.sup.2 and 20 mA/m.sup.2.

5. The device according to claim 1, wherein at least one microbial anode system is connected to a flotation device floating at the surface of the electrolytic medium.

6. A method of cathodic protection against corrosion of at least one metal structure in contact with an electrolytic medium implementing the cathodic protection device according to claim 1, said method comprising the steps of: setting up several microbial anode systems each comprising microorganisms and an electrode in contact with said microorganisms and the electrolytic medium, the microbial anode system being configured to supply electrons to the electrode by degradation of oxidizable ambient resources carried out by said microorganisms, the electrode of each microbial anode system being buried at least partially in said sedimentary soil of the electrolytic medium and the free electrochemical potential of said microbial anode system being lower than the free electrochemical potential of the metal of said at least one metal structure to be protected; connecting said at least one metal structure with each microbial anode system with at least one connecting means configured to let the electrons circulate from the electrode of each microbial anode system up to said at least one metal structure; activating the circulation of electrons alternately over time between each electrode of bacterial anode systems and said at least one metal structure, said activating comprising the sub-steps of: activating the circulation of electrons in at least one connecting means connecting at least one microbial anode system to the metal structure so that a protective galvanic current is applied through the electrolytic medium from said at least one microbial anode system up to said at least one metal structure; stopping the circulation of electrons in said at least one connecting means connecting at least one microbial anode system to the metal structure when a previously determined threshold of depletion of electrons circulating in said at least one connecting means is reached, said previously determined threshold reflecting a threshold of depletion in oxidizable ambient resources in the environment close to said at least one microbial anode system, and activating the circulation of electrons in at least one connecting means connecting at least one other microbial anode system to the metal structure.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] The invention will be better understood upon reading the following description, given as a non-limiting example, and made with reference to the figures which show:

[0060] FIG. 1 illustrates a diagram of an embodiment of the cathodic protection device object of the present invention comprising two microbial anode systems;

[0061] FIG. 2 illustrates a diagram of an embodiment of the cathodic protection device object of the present invention comprising three microbial anode systems;

[0062] FIG. 3 illustrates a diagram of an embodiment of the cathodic protection device object of the present invention comprising switches on the connecting means and a system for continuously measuring the current circulating in each connecting means.

[0063] FIG. 4 shows a cathodic protection device object of the present invention in which the metal structure to be protected is a reinforced concrete bridge pier;

[0064] FIG. 5 illustrates the cathodic protection device shown in FIG. 4 with a curve of the distribution of the density of the protective galvanic current as a function of the height of the metal structure.

[0065] In these figures, identical reference numerals from one figure to another designate identical or similar elements. Moreover, for clarity, the drawings are not plotted to scale, unless stated otherwise.

DESCRIPTION OF THE EMBODIMENTS

[0066] FIG. 1 illustrates a diagram of the device 100 for cathodic protection against corrosion of a metal structure 101 in contact with an electrolytic medium 102 comprising a sedimentary soil 107, according to an embodiment of the present invention, wherein the cathodic protection device 100 further includes two microbial anode systems 103 arranged in contact with the electrolytic medium 102 and configured to supply electrons by biodegradation of oxidizable ambient resources (oxidation of chemical molecules), the free electrochemical potential of each microbial anode system 103 being lower than the free potential electrochemical of the metal of the metal structure 101 to be protected.

[0067] Each microbial anode system 103 includes microorganisms. These microorganisms are at least those naturally present in the electrolytic medium 102 and can be supplemented by microorganisms supplied for example in the form of a biofilm 104 of microorganisms as shown in FIG. 1. Each microbial anode system 103 also includes an electrode 105 which serves as an anode in the cathodic protection device 100. The electrode 105 is buried at least partially in the sedimentary soil 107 of the natural electrolytic medium 102 so that the microorganisms contained in the microbial anode system 103 are at least those of said soil 107. In turn, the metal structure 101 serves as a cathode in the protective device 100.

[0068] Each microbial anode system 103 is capable of generating electricity autonomously from oxidation reactions of ambient oxidizable resources (chemical molecules) carried out by the microorganisms it contains. Indeed, said oxidation reactions result in the release and transfer of electrons to the electrode 105 included in the microbial anode system 103, thereby generating an electrical current in the electrode 105.

[0069] The sedimentary soil 107 of an electrolytic medium 102 (in general a natural electrolytic medium) is composed of microorganisms and sediments rich in oxidizable resources representing a target substrate for the biodegradation carried out by the microorganisms to create electricity.

[0070] In some embodiments, at least one of the microbial anode systems 103 includes a chamber 106 comprising the biofilm 104 of microorganisms and the electrode 105. This chamber 106 is configured so that microorganisms present in the biofilm 104 of microorganisms are in contact with the electrolytic medium 102 and preferably with the sedimentary soil 107. For example, such a configuration corresponds to the fact that the chamber 106 is permeable to the electrolytic medium 102. The chamber 106 is also configured to receive resources oxidizable by the microorganisms of the microbial anode system 103. This has the advantage of enabling an operator of the cathodic protection device 100 to bring oxidizable resources into the chamber 106 to supply the microorganisms that it includes in addition to the oxidizable resources present in the electrolytic medium 102 and the sedimentary soil 107 with which they are in contact.

[0071] The cathodic protection device 100 further includes a means 108 for connection between each microbial anode system 103 and the metal structure 101. This connecting means 108 is configured to let the electrons circulate from said microbial anode system 103 up to the metal structure 101. Of course, the connecting means 108 includes an electron-conducting material, for example a metal. For example, the connecting means 108 may be a cable including an electron-conducting core. By a means for connection 108 between each microbial anode system 103 and the metal structure 101, it should be understood that either the cathodic protection device 100 includes one single connecting means 108 ensuring connection between all of the microbial anode systems and the metal structure 101 (not illustrated in the figures) or that the cathodic protection device 100 includes several connecting means 108, and for each connection between a microbial anode system 103 and the metal structure 101, a distinct unique connecting means 108 is used (as illustrated in the figures).

[0072] The connecting means 108 is in contact with the electrode 105. The electrons originating from the degradation by oxidation of the chemical molecules (oxidizable ambient resources) present in the electrolytic medium 102 and in particular the sediments which is carried out by the microorganisms of the microbial anode system 103 are transferred to the electrode 105. Afterwards, the electrons pass from the electrode 105 to the connecting means 108 and circulate in said connecting means 108 to get into the metal structure 101.

[0073] Thus, a protective galvanic current 109 is created through the electrolytic medium 102 between each microbial anode system 103 and the metal structure 101. Advantageously, this galvanic current 109 protects the metal structure 101 against corrosion.

[0074] If used indiscriminately, powering a cathodic protection device with a biogalvanic current could lead to the depletion of oxidizable ambient resources. An insufficient current would not allow protection of the structure against corrosion; conversely, an excessive current could lead to an unnecessary depletion of the microbial resource. Advantageously, the present invention allows controlling the current delivered by one or more microbial anode system(s).

[0075] The cathodic protection device 100 illustrated in FIG. 1 also includes a control apparatus 110 comprising a system for measuring the current of electrons circulating in the connecting means 108 or for measuring the current of electrons circulating in each means for connection and for determining the variations in depletion of the oxidizable ambient resources, as well as a control system allowing stopping and activating the circulation of the current of electrons circulating in the connecting means 108. After having stopped the circulation of the current of electrons thanks to the control system, an operator of the cathodic protection device 100 will be able, using the measuring system, to advantageously measure the depolarization of the metal structure in order to assess the performance of the cathodic protection device within the meaning of the standard EN ISO 12696.

[0076] The exact number of microbial anode systems is calculated based on the kinetics of regeneration of oxidizable ambient resources. Control of the cathodic protection device (alternation of operation between the microbial anode systems) is based on two elements which are compliance with the performance standard criteria (cut-off current potential and depolarization) and compliance with a current criterion (a predetermined threshold not to exceed) in anticipation of depletion of oxidizable ambient resources (anticipation of non-compliance with the standard criteria).

[0077] According to an embodiment of the present invention, illustrated in FIG. 2, the cathodic protection device 100 includes several microbial anode systems 103 in contact with the electrolytic medium 102, preferably at least three. A first microbial anode system 103A does not include any chamber 106 whereas a second microbial anode system 103B and a third microbial anode system 103C include a chamber 106 comprising microorganisms preferably in the form of a biofilm 104 of microorganisms and an electrode 105. The cathodic protection device 100 comprises a means 108 for connection between each microbial anode system 103 and said metal structure 101, said connecting means 108 being configured to let the electrons circulate from the electrode 105 of the microbial anode system 103 which it connects to the metal structure 101 up to the latter. Each connecting means 108 is in contact with the electrode 105 of the microbial anode system 103 which it connects to the metal structure 101.

[0078] Thus, a protective galvanic current 109 is created through the electrolytic medium 102 between each microbial anode system 103 and the metal structure 101.

[0079] Each microbial anode system 103A, 103B, and 103C comprises an electrode 105 buried at least partially in the sedimentary soil 107 of the electrolytic medium 102 so that the microorganisms included in the microbial anode systems 103 are at least those of said electrolytic medium 102 and in particular those of the soil sediments 107. The third microbial anode system 103C is connected to a flotation device 111 configured to float in the electrolytic medium 102, preferably at the surface of the latter. For example, such a flotation device 111 may be a buoy. This has the advantage of facilitating access to the microbial anode system 103C if an operator of the cathodic protection device 100 wishes to access it, for example to perform a maintenance operation or a supply of resources oxidizable by the microorganisms or even a supply of microorganisms, for example a replacement of the biofilm 104.

[0080] The cathodic protection device 100 illustrated in FIG. 2 also comprises a control apparatus 110 comprising a system for measuring the current of electrons circulating in each connecting means 108 or means for measuring the current of electrons circulating in each means for connection and for determining the variations in depletion of the oxidizable ambient resources, as well as a control system allowing stopping and activating the circulation of the current of electrons circulating in each connecting means 108. Thus, the operator of the device can measure the current of electrons circulating in each connecting means 108 and he/she can also choose to stop or activate said current of electrons in each connecting means 108, which respectively stops or activates the protective galvanic current 109 between the anode system 103 and the metal structure 101 connected by said connecting means 108 in which the current is stopped or activated. Advantageously, this allows activating the circulation of current in the connecting means 108 alternately and therefore operating the cathodic protection device 100 while alternating from one microbial anode system 103 to another so as not to deplete the oxidizable ambient resources present in the close environment of a microbial anode system 103.

[0081] Whether the microorganisms are present in the electrolytic medium 102, for example in the sedimentary soil 107, or the microorganisms are supplied by the user into a microbial anode system 103, during the operation of the cathodic protection device 100, said microorganisms will progressively form a biofilm 104 of microorganisms at the surface of the electrode 105 which is reflected by a progressive increase in the current circulating in the connecting means 108 connecting said electrode 105 to the metal structure 101 up to a peak value. The local consumption of the oxidizable ambient resources of the electrolytic medium 102, in particular the sediments, carried out by the microorganisms of the microbial anode system 103, leads to a drop in the current circulating in the connecting means 108 connected to the electrode 105 of said microbial anode system 103. It is then relevant to change the microbial anode system 103 via the control system (for example electronically-controlled switches or relays) whose control is guided according to the aforementioned two criteria (standard performance criterion and current criterion in anticipation of depletion of the oxidizable ambient resources). As regards the current criterion, it may consist of a predetermined threshold value for depletion of the current (electrons) circulating in the connecting means 108. For example, this threshold value is a percentage of the assessed reduction in the current circulating in the connecting means 108 with respect to the peak value. When the threshold value is reached, the control system stops the circulation of current in the considered connecting means 108, thereby allowing stopping the microbial anode system 103 to which it is connected and therefore the natural renewal of the oxidizable ambient resources found in the environment close to the latter. In parallel or upstream, or still after, the control system activates the circulation of current in a connecting means 108 connecting another microbial anode system 103 to the metal structure 101 so that the cathodic protection against corrosion of the latter continues.

[0082] FIG. 3 illustrates a diagram of an embodiment of the cathodic protection device 100 which is the object of the present invention. The protective device 100 includes several microbial anode systems 103A, 103B and 103C buried at least partially in the sedimentary soil 107 of the electrolytic medium 102. The protective device 100 comprises a connecting means 108 comprising a main line 108P connected to the metal structure 101 and comprising a connection line 108A, 108B and 108C for each anode system, each connection line being connected to the main line 108P so that each microbial anode system 103 is connected to the metal structure 101. In this embodiment, the system for measuring the current of electrons circulating in each connecting means 108 or for measuring the current of electrons circulating in each means for connection and for determining the variations in depletion of the oxidizable ambient resources comprises an ammeter 114 arranged on each connection line 108A, 108B and 108C. Thus, it is possible to continuously measure the current circulating in each connection line 108A, 108B and 108C, which continuously indicates the current delivered by each microbial anode system 103 and allows determining the rate of depletion of the oxidizable ambient resources for each microbial anode system 103. Optionally, the system for measuring the current of electrons circulating in each connecting means 108 or for measuring the current of electrons circulating in each means for connection and for determining the variations in depletion of the oxidizable ambient resources also comprises an ammeter 114 arranged on the main line 108P.

[0083] The protective device 100 comprises a control system allowing stopping and activating the circulation of the current of electrons circulating in the connecting means 108. The control system comprises a switch 115 arranged on each connection line 108A, 108B and 108C. Thus, it is possible to cut (interruption of the flow of the current) and close (passage of the current) the switches 115 so as to preserve the weakest microbial anode systems 103 whose oxidizable ambient resources are about to be depleted and enable regeneration thereof by allowing them to rest. This also allows regulating the current by activating or deactivating some microbial anode systems 103 according to a predefined setpoint to better protect the metal structure 101. Thus, in a configuration with two microbial anode systems 103, the protective device 100 allows alternating the activation and deactivation of the microbial anode systems 103. In a configuration with more than two microbial anode systems 103, it is not only possible to preserve the microbial anode systems 103 and the oxidizable ambient resources from its close environment but also regulate the current per quantum of a microbial anode system 103.

[0084] Optionally, instead of switches 115 or in addition to each switch 115, the control system may comprise a variable resistor (not illustrated in the figures) thereby allowing varying the intensity of the current reaching the metal structure 101 from each microbial anode system 103.

[0085] Advantageously, the control system and the system for measuring the current of electrons circulating in each connecting means 108 or for measuring the current of electrons circulating in each means for connection and for determining the variations in depletion of the oxidizable ambient resources, may be automated and implemented into a control apparatus 110 such as a programmable controller or an industrial computer. In this case, the switches 115 may be replaced by programmable relays and the ammeters 114 by analog-to-digital converters. In particular, the measuring system may comprise a resistor shunted (shunt in English terminology) on each connection line 108A, 108B and 108C so as to measure the intensity of the current in each of these connection lines.

[0086] The cathodic protection obtained with the present invention has been tested with a particular embodiment of the invention, given herein for illustrative and non-limiting purposes. It consists of a cathodic protection device 100 according to the invention used for the protection of a metal structure 101 which is a reinforced concrete bridge pier in contact with an electrolytic medium 102, which is a marine environment, comprising a sedimentary soil 107. This bridge pier is a 25 m high concrete element having a 33 m.sup.2 section and reinforced with 76 longitudinal steel rebars having a 32 mm diameter (FIG. 4). The metal structure 101 is immersed in sea water (the electrolytic medium 102) to a depth in the range of 18 m and rests on a seabed (the sedimentary soil 107) consisting of sediments rich in oxidizable resources for the microorganisms of the microbial anode systems 103, only one of these being illustrated in the FIG. 4. In this configuration type, the atmospheric concrete area or emerged area 112 is subject to the greatest levels of damage, due to a strong contamination by chlorides, concrete partially saturated with water and high oxygen availability. This area should be the priority target of the cathodic protection device 100. The drawdown area 113, which is an area that is partially submerged and partially emerged because of the difference in water level depending on the tides, goes down to a few meters below the water level (down to about 3 to 7 meters below the water level). The drawdown area 113 is also an area subject to high levels of damage. This vulnerability of the drawdown area is related to the problem of the water/air interface with a high oxygen availability, the reduction contributes to the corrosion phenomenon. The rest of the metal structure 101 immersed more deeply is very little subject to corrosion due to the very low content or absence of oxygen in the electrolytic medium 102.

[0087] One of the microbial anode systems 103 is positioned at the foot of the metal structure 101, at a depth of 18 m, buried in the sedimentary layer and connected to the steel rebar network of the bridge pier by means of a connecting means 108 which herein consists of an electric cable.

[0088] One could observe the protective galvanic current lines 109 (in light gray) in FIG. 4, naturally exchanged between the microbial anode system 103 and the steel rebar network of the reinforced concrete bridge pier. It should be noted that, because of the absence of oxygen at depth, the galvanic current lines 109 rise vertically and penetrate the concrete near the water surface.

[0089] FIG. 5 illustrates the distribution of the protective galvanic current 109 as a function of height in the bridge pier. For the above-mentioned reasons, one could observe that the protective current tends to concentrate in the oxygen-rich areas, which also correspond to the emerged 112 and drawdown 113 areas, which should be protected in priority.

[0090] The test has been reproduced with various depths for the positioning of the microbial anode system 103, without any significant impact on the amount and distribution of the protective galvanic current 109 towards the emerged 112 and drawdown 113 areas, which should be protected in priority.

[0091] More generally, it should be noted that the implementations and embodiments of the invention considered hereinabove have been described as non-limiting examples and that other variants are consequently possible.