EP2809830A1

EP2809830A1 - Device for the cathodic protection of a metal wall against corrosion in a saline environment

Info

Publication number: EP2809830A1
Application number: EP13705736.0A
Authority: EP
Inventors: Serge Prigent
Original assignee: Alstom Renewable Technologies Wind BV
Current assignee: GE Renewable Technologies Wind BV
Priority date: 2012-02-01
Filing date: 2013-01-31
Publication date: 2014-12-10
Also published as: CA2862349A1; FR2986241A1; FR2986241B1; WO2013113777A1; CL2014002004A1; US20140332373A1; AU2013214235B2; CN104080953A; JP2015505583A; KR20140122739A; AU2013214235A1

Abstract

A device for the cathodic protection of a metal wall (1) against corrosion in a saline environment, comprising an anode and means (4, 5) for connecting said anode to said wall (1), said anode having a higher electrochemical potential than said wall (1), characterised in that said anode is placed in a compartment (6) delimited by a wall permeable to electrons and, optionally, to water, comprising: - a porous outer layer (7) made from a material selected from: polymeric materials, ceramic materials or hydrated inorganic materials; - and at least one porous layer (9, 10) having the ability to collect the cations emitted by the anode during the dissolution of same, the material forming said at least one layer being selected from osmotic membranes, active carbon, a cation exchange resin such as a zeolite, a cation-collecting polymer with nanofillers, cation-collecting mineral compounds such as phyllosilicates and inosilicates, cation-retaining nanofiltering semi-permeable organic microporous membranes.

Description

Device for cathodic protection of a metal wall against the

corrosion in a saline environment

The invention relates to the cathodic protection of immersed installations in a brackish or saline environment.

The protection of metal hulls of boats or metal parts against corrosion by seawater is conventionally often provided by a sacrificial anode, consisting of a more electronegative metal than the one to be protected. When the boat and its sacrificial anode are immersed in seawater, a polarization occurs, the shell becoming the cathode of an electrochemical cell and the sacrificial anode becoming the anode of the same cell. As a result, it is the metal components of the sacrificial anode that undergo the effects of corrosion by seawater and not those of the shell. These constituents M are released in seawater in the form of cations according to the reaction M → M ^{n +} + ne.

Sacrificial anodes protecting the submerged steel parts of a boat or other fixed gear are usually aluminum-zinc (2-6%) - indium (0.01-0.05%), or aluminum -gallium (0.01%) or zinc alloy-AI (0.1 -0.5%). The disadvantage of this protection is that it releases in the marine environment ions of the metals constituting the sacrificial anodes. If this disadvantage is relatively minimal in the case of boats moving on the open sea, it is to be considered significantly more seriously when the boats are immobilized in a port, because the metals of the anode will accumulate in the water and in the water. the seabed of the parking areas, where they are going to be absorbed by the living beings who stay there. The problem also arises for fixed installations such as offshore oil platforms and wind turbines. It is therefore imperative to find effective and economical solutions to avoid as much as possible the release of harmful metals in the external environment, especially as changes in the environmental protection legislation could make the use of control solutions possible. mandatory sacrificial anode discharges under certain circumstances.

An alternative solution to the use of a sacrificial anode as just described is to make this anode in a material not necessarily more electropositive than the material to be protected (it can be steel, cast iron, graphite, metal oxides ...), but to which is imposed, constantly or cyclically, an electric potential by means of a DC generator or rectified. This potential makes the anode more likely to be corroded than the wall to be protected. This technique is cumbersome to implement, especially in areas of the facility that are difficult to access, but is effective for large installations mainly. This technique is known as "cathodic current protection" (abbreviated PCCI).

The object of the invention is to provide a solution for preventing the release into the external environment of the cations resulting from the dissolution of an anode of a cathodic protection device.

For this purpose, the subject of the invention is a device for cathodic protection of a metal wall against corrosion in a salt medium, comprising an anode and means for connecting said anode to said wall, said anode being at an electrochemical potential. higher than said wall, characterized in that said anode is placed in a compartment delimited by an electron-permeable wall and, possibly, water, comprising:

a porous outer layer made of a material chosen from: polymer materials, ceramic materials or hydrated inorganic materials;

and at least one porous layer having the capacity of capturing the cations emitted by the anode during its dissolution, the material constituting said at least one layer being chosen from osmotic membranes, activated carbon, a cation exchange resin such as a zeolite, a nano-cations cation-capturing polymer, cation-capturing mineral compounds such as phyllosilicates and inosilicates, microporous organic semi-permeable nanofiltration membranes of a cation-retaining type.

Said wall may also include a membrane trapping negatively charged contaminants.

The anode may be a sacrificial anode whose electrochemical potential is naturally greater than that of the metal wall.

Otherwise, the device may comprise means for imposing on the anode an electrochemical potential greater than that of the metal wall.

The connection means of the anode to the metal wall may be in contact with the wall outside the compartment and pass through the porous wall of the compartment.

The means for connecting the anode to the metal wall may be in contact with the metal wall inside the compartment, and the porous wall of the compartment is sealingly connected to the metal wall.

The device may comprise means for keeping the anode away from the metal wall. It may comprise a plurality of layers having the capacity to capture the cations emitted by the anode during its dissolution, each layer preferably capturing cations different from those picked up in a preferred manner by the other layers.

As will be understood, the invention consists in placing the anode in a compartment delimited by a series of porous membranes permeable to at least electrons, or even to water, arranged in layers. The outer layer is constituted by a porous non-metallic membrane, intended to reduce the hydraulic flow in the vicinity of the anode. The other porous layer (s) act as a cation barrier or trap, which prevents cations from dissolution of the anode from escaping out of the compartment into the external environment.

If the membranes are all permeable to water, the space between the anode and the wall of the compartment fills with water naturally. If at least one of the membranes is not permeable to water but only to electrons, it is necessary, during installation of the device, to fill the compartment with water, preferably with water. seawater to obtain good electrical conductivity, in order to bathe the anode in an electron-conducting medium and to give the compartment its operational form, with internal and external pressures that balance out.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

- Figure 1 which shows schematically in longitudinal section a first example of implementation of the invention;

- Figure 2 which shows schematically in longitudinal section a second example of implementation of the invention.

In Figure 1, there is shown a metal wall 1 belonging to an installation located in a marine environment 2, such as an oil rig or a wind turbine. But this wall 1 could also be the hull of a boat, or any other flying craft. In known manner, a sacrificial anode 3 is disposed in the vicinity of the wall 1, to which it is connected by electrical connectors 4, 5. In general, as in the prior art, the sacrificial anode 3 is made around a single steel connector, and in this case, the connectors 4, 5 shown are in fact the two ends of this single connector. The constituent material of the sacrificial anode 3 is conventional for this purpose (Al-In, Al-Ga or Zn alloy for example), and its choice is not a characteristic of the invention. According to the invention, the sacrificial anode 3 is included in a compartment 6 which is delimited by a set of membrane forming layers, and surrounds the anode 3 at a distance, for example, of the order of 1 cm.

The outermost layer 7 is a porous electron-permeable layer and, preferably, also water, intended to reduce the hydraulic flow between the external environment and the interior space 8 of the compartment. The material constituting it is selected from polymeric materials, ceramic materials or hydrated inorganic materials.

Examples of such materials include, but are not limited to, thermoplastic polymers of the polyethylene or high density polyethylene type, or industrial mullite or alumina type porcelains.

If this material is electrically insulating, it must have porosity. Indeed, the polarization of the anode corresponds to the establishment of a small electrochemical circuit, which can only work if the electrons circulate. The open porosity allows the electrons to pass into the liquid even though the layer 7 is insulating.

This outermost layer 7, whose thickness is generally of the order of mm, serves to protect the anode and the other membranes of the compartment 6 against hydraulic abrasion. It must have suitable properties of resistance to wear and impacts, resistance to deformation in the presence of a fluid in motion.

The permeability of the layer 7 is, for example, of the order of 10 ml / min for 1 cm ² of anode surface.

The other layer or layers of the wall of the compartment 6 (there are two, 9, 10, in the example shown) is or consist of one or more materials serving as cation traps, and which capture the cations emitted by the sacrificial anode 3 to prevent them from being found in the marine environment 2. For this purpose, various types of materials may be suitable: osmotic membranes, activated carbon powder or granules, a cation exchange resin such as a zeolite, a cation-attracting negative nanocharging cationic polymer, mineral cation-capturing compounds such as phyllosilicates and inosilicates. Such materials are among those commonly used in treating and softening water to trap or exchange cations. An activated alumina membrane or a functionally equivalent compound can be added to them, which in turn traps negatively charged contaminants, such as As and fluorides, that could decrease the efficiency of cation-trapping membranes. Semipermeable membranes used in electrolytic ion exchange processes can also be used.

Microporous organic semi-permeable nanofiltration membranes of a cation-retaining type may also be suitable.

The number of layers of cation sensor materials can be arbitrary, at the user's choice. These layers may advantageously be of multiple natures, each layer species being able, for example, to preferentially absorb one or more of the chemical species that the sacrificial anode 3 is capable of releasing.

For example, we can provide:

an outer layer 9 permeable to chemical elements with a radius less than

1, 5A and impervious to chemical elements of radius greater than 1.5A such as Ca, K, Mg, Na;

and an inner layer 10 permeable to chemical elements with a radius of less than 1, 1 Å such that O, Cl, N (which will therefore be able to enter the internal space 8 of the compartment 6 or leave it, which does not present disadvantages), and impermeable to chemical elements of radius greater than 1, 1A such that Al, Zn, Ga, In, thus the main elements that can emit the anode 3 in the form of cations, and of which it is not desired that they are released into the surrounding environment; the thickness of the layers, in particular of the layer 10, can vary from about 1 mm to a few cm, depending on the size of the anode 3 and, therefore, the amount of cations to trap.

In the case where the entire wall defining the compartment 6 is permeable to water, the water penetrates inside the compartment 6 and a pressure balance is reached between the inside and the outside of the compartment 6 The compartment 6 thus permanently takes its nominal form and its wall does not undergo crushing which could lead to its rupture.

As has been said, it is not mandatory that the entire wall defining the compartment 6 is permeable to water. It may be permeable only to electrons, but then pre-filling compartment 6 with water, preferably seawater, is necessary during commissioning of the device according to the invention.

Thanks to the invention, the progressive dissolution of the sacrificial anode 3 is carried out without pollution of the external medium by the cations resulting from this dissolution, these being captured by the layer or layers 9, 10. These must have advantageously a total absorption capacity of the various cations and an absorption volume sufficient to not reach saturation before the end of life of the sacrificial anode 3.

In the example shown in Figure 1, the electrical conductors 4, 5 are in contact with the metal wall 1 in areas outside the compartment 6. It is necessary to therefore ensure a sealing of the wall of the compartment 6 in the areas where it is crossed by the conductors 4, 5. But alternatively, as shown in Figure 2, the contacts between the conductors 5, 6 and the metal wall 1 may be located inside the compartment 6. The wall of the compartment 6 is then in sealing contact with the metal wall 1 to be protected.

As shown in the figures, it is preferable that the sacrificial anode 3 is not in direct contact with the wall 1 to be protected. This avoids creating short circuits between the anode 3 and at least the area of the wall 1 which is opposite it. In this way, a greater part of the surface of the wall 1 can be protected under the best conditions by the same anode 3. Means for holding the anode 3 away from the wall 1 are therefore provided, preferably ( not shown in the figures). But in practice, they can often be constituted by the connectors 4, 5, which are generally made of steel and have due to their material and their dimensions, a sufficient rigidity to achieve the maintenance of the anode 3 at a distance from the wall 1.

In a variant, the invention is also applicable in the case where the anode is not a sacrificial anode in the sense that it naturally has a higher electrochemical potential than that of the wall 1 to be protected, but is placed at this potential by a DC or rectified current generator to which it is connected by conductors which pass through the wall of the compartment 6 sealingly.

Claims

1. - Device for cathodic protection of a metal wall (1) against corrosion in a salt medium, comprising an anode and connection means (4, 5) of said anode to said wall (1), said anode being at a potential electrochemical higher than said wall (1), characterized in that said anode is placed in a compartment (6) delimited by an electron-permeable wall and, optionally, water, comprising:

a porous outer layer (7) of a material chosen from: polymeric materials, ceramic materials or hydrated inorganic materials;

and at least one porous layer (9, 10) having the capacity of capturing the cations emitted by the anode during its dissolution, the material constituting said at least one layer being chosen from osmotic membranes, activated carbon, a resin cation exchanger such as a zeolite, a nano-cations cation-capturing polymer, mineral cation-capturing compounds such as phyllosilicates and inosilicates, microporous organic semi-permeable nanofiltration membranes of a cation-retaining type.

2. - Device according to claim 1, characterized in that said wall (1) also comprises a membrane trapping negatively charged contaminants.

3.- Device according to claim 1 or 2, characterized in that the anode is a sacrificial anode (3) whose electrochemical potential is naturally greater than that of the metal wall (1).

4. - Device according to claim 1 or 2, characterized in that it comprises means for imposing on the anode an electrochemical potential greater than that of the metal wall (1).

5. - Device according to one of claims 1 to 4, characterized in that the connecting means (4, 5) of the anode to the metal wall (1) are in contact with the wall outside the compartment (6) and in that they pass through the porous wall of the compartment (6).

6.- Device according to one of claims 1 to 4, characterized in that the connection means (4, 5) of the anode to the metal wall (1) are in contact with the metal wall (1) to the inside the compartment (6), and in that the porous wall of the compartment (6) is sealingly connected to the metal wall (1).

7.- Device according to one of claims 1 to 6, characterized in that it comprises means for holding the anode away from the metal wall (1).

8.- Device according to one of claims 1 to 7, characterized in that it comprises a plurality of layers (9,10) having the ability to capture the cations emitted by the anode during its dissolution, each layer ( 9, 10) preferentially capturing different cations from those picked in a privileged way by the other layers.