GB2535847A

GB2535847A - Cathodic protection of a hollow metal structure against corrosion

Info

Publication number: GB2535847A
Application number: GB1600204.0A
Authority: GB
Inventors: Barbouchi Sami; Kemp Henry
Original assignee: Electricite de France SA
Current assignee: Electricite de France SA
Priority date: 2015-01-06
Filing date: 2016-01-06
Publication date: 2016-08-31
Anticipated expiration: 2036-01-06
Also published as: FR3031347B1; DE102016000029B4; DE102016000029A1; GB2535847B; FR3031347A1; GB201600204D0

Abstract

A device for the treatment of a hollow metal structure ST containing an aqueous medium against corrosion, including a tank 10 supplying the structure ST with metal hydroxide, an electric power generator G for applying electrical energy to promote an oxidation-reduction reaction in which metal ions react with carbonate generated by the aqueous medium to form a layer of metal carbonate CM, to protect against corrosion, on an internal wall of the structure fulfilling the role of an electrode in the redox reaction, at least one sensor 13, 11 for measuring at least one parameter of the aqueous medium connected to a control module 12 of the generator G and/or to a means VA for opening the tank 10 in order to apply an additional supply of metal hydroxide to the medium and/or to apply an additional supply of electrical energy on the basis of the measurement of said parameter. The application of electrical energy may comprise an impressed current supply of electrons to the internal wall CA of the structure, and also may comprise applying a difference in potential DDP between -850 mV and -1100 mV between the internal wall CA and reference electrode (REF). A potential difference and/or pH of the aqueous medium may be measured and if these values are below a threshold value, at least an application of electrical energy and/or supply of metal hydroxide is repeated. The pH threshold may be between 8 and 10, and the potential difference threshold may be between -900 mV and -1100 mV. The metal hydroxide may be Ca(OH)2. Generator G may be supplied by a converter of a wind turbine EOL.

Description

Cathodic protection of a hollow metal structure against corrosion This invention relates to the field of protecting hollow metal structures against corrosion, in particular structures bearing offshore wind turbines, for example, which are subjected to a high degree of corrosion at sea.

This type of steel foundation, known as a monopile, necessarily has sealing defects and the hollow structure ultimately becomes filled with an aqueous medium (for example seawater). The metal of the structure combines with the hydroxide ions H30+ of the aqueous medium to form a metal hydroxide precipitate, thus causing the structure to corrode.

One approach would be to provide a galvanic protection device to protect the internal wall of the structure against corrosion by an oxide reduction reaction. To this end, a sacrificial anode made from aluminium, for example, could be submersed in the structure. Such an anode then undergoes corrosion in place of the steel of the structure: the Ala ion from the anode reacts with hydroxide ions (OW of the medium which sufficiently reduces the level of dissolved oxygen in order to reduce the quantity of corrosive elements in the medium contained in the structure. However, the anode, primarily made of aluminium, produces Ala' ions which react with the medium to form the compound Al(OH)3, but also the FL ion (more specifically 1130+), making the medium inside the structure more acid and thus increasing the speed of corrosion.

Inside the structure, the solution acidified in this manner can then consume electrons at the cathode, which renders the galvanic protection less effective and reduces its service life. Furthermore, steel is known to be less resistant to corrosion in acid medium unless it is protected (which increases the speed of corrosion).

The objective of this invention is to improve the situation To this end, it proposes a method of treating a hollow metal structure containing an aqueous medium against corrosion, this method comprising the steps: - supplying the structure with metal hydroxide, - and applying electrical energy to promote an oxidation-reduction reaction in which metal ions react with carbonate generated by the aqueous medium to form a layer of metal carbonate, to protect against corrosion, on an internal wall of the structure fulfilling the role of an electrode in the oxidation-reduction reaction.

The risks associated with corrosion from galvanic protection involving a sacrificial anode are thus reduced because acidity in the medium remains low.

Furthermore, based on one advantageous embodiment: - at least one parameter of the aqueous medium is measured and, depending on said measurement, it is determined whether it is necessary to supply additional metal hydroxide and/or whether it is necessary to apply additional electrical energy, and - if necessary, electrical energy is applied again and/or metal hydroxide is supplied again.

This measured parameter may be the pH directly but also, in addition or as an alternative, the resistivity of the layer of protection, which also provides an indication of the effectiveness of the treatment by oxidation reduction.

Based on one particular embodiment, the electrical energy is applied by supplying electrons to the internal wall of the structure serving as a cathode in the form of impressed current. Such an embodiment will be referred to hereafter as "impressed current galvanic protection" Based on one example of an embodiment, the application of electrical energy comprises applying a difference in potential of between -800mV and -1100m V between the internal wall of the structure sewing as a cathode and a reference electrode submersed in the structure.

Based on one example of an embodiment in which a resistance at the surface of the internal wall is measured as a parameter of the aqueous medium, if this measured resistance is below a threshold value, at least an application of electrical energy is repeated and/or a supply of metal hydroxide is repeated.

This resistance measurement provides both an indication of the thickness of the layer of protection (and hence its effectiveness) and an indication of the need for additional electrical energy to be applied.

Furthermore, in addition or as an alternative, a difference in potential between the internal wall of the structure and a reference electrode submersed in the structure may also be measured as a parameter of the aqueous medium and if this difference in potential is below a threshold value, in terms of absolute value, at least an application of electrical energy is repeated and/or a supply of metal hydroxide is repeated.

The above-mentioned threshold value may typically be between -900mV and -1100mV.

Based on an additional or alternative embodiment, a pH value is measured as a parameter of the aqueous medium and at least the step of supplying metal hydroxide is repeated if the measured value is below a pH threshold value.

Based on an embodiment where the resistivity (or resistance) of the layer of protection and/or the difference in potential with the reference electrode is also measured, an appropriate difference in potential may be imposed first of all and before also repeating a supply of metal hydroxide, a pH value is measured as a parameter of the aqueous medium: this step of supplying metal hydroxide is repeated if the measured pH value is below a pH threshold value.

This pH threshold value is between 8 and 10, for example.

The metal hydroxide supplied may be calcium hydroxide (Ca(OH)2) or other variants of calcium hydroxide, such as barium hydroxide.

This invention naturally also relates to a device for implementing the method proposed by the invention for treating a hollow metal structure containing an aqueous medium against corrosion, the device comprising: - a tank for supplying the structure with metal hydroxide, - an electric power generator for promoting an oxidation-reduction reaction in which metal ions react with carbonate generated by the aqueous medium to form a layer of metal carbonate, protecting against corrosion, on an internal wall of the structure fulfilling the role of an electrode in the oxidation-reduction reaction.

The device advantageously further comprises: -at least one sensor for measuring at least one parameter of the aqueous medium connected to a control module of the generator and/or a means for opening the tank so that an additional supply of metal hydroxide is applied to the medium and/or an application of additional electrical energy is supplied on the basis of the measurement of said parameter.

Based on one advantageous embodiment where the structure supports a wind turbine, the above-mentioned generator may be directly supplied with electric power by a converter of the wind turbine likewise supported by the structure.

This invention therefore enables the interior of the structure to be "automatically" maintained without the need for human intervention (apart from topping up the tank with metal hydroxide on a monthly basis, for example).

Other features and advantages of the invention will become clear from the detailed description given below and the appended drawings, of which: - figure 1 illustrates an example of an embodiment of a device proposed by the invention for protecting against corrosion; - figure 2 illustrates an example of an embodiment of a method proposed by the invention for protecting against corrosion.

Turning to figure 1, the device proposed by the invention as a means of protecting against corrosion is based on (impressed current or, more generally, impressed electrical energy) cathodic protection. To this end, the device comprises at least one anode AN (preferably more than one, for example between three and six) that is inert (for example made from graphite or titanium) and connected to a generator G of direct current (generated, for example, by a converter of a wind turbine EOL). The generator G, on the other hand, is connected to the internal wall of the metal structure ST thus constituting a cathode CA. The anode AN may be submersed in the interior of the structure or may be adhered against a wall of the structure with electrical insulation provided between the anode and this wall.

Metal ions XN can be introduced into the interior of the structure in the form of metal hydroxide X(OH)N. In the example illustrated, this metal hydroxide X(OH)N is initially contained in a tank 10 suspended above the aqueous medium contained in the structure ST. Once the metal hydroxide X(OH)N has been discharged into the structure ST and the generator G is in service, a protective layer CM of metal carbonate is formed on the internal wall CA of the structure. By way of example, calcium hydroxide Ca(OH)2 is contained in the tank 10 in this instance. However, other materials such as barium hydroxide are also possible as an alternative. When poured into the aqueous medium of the structure, calcium hydroxide Ca(OH)2 tends to release metal ions Cat-which combine with the carbonate (CO3)2-of the aqueous medium to form a layer of calcium carbonate CaCO3 on the internal wall CA which protects against corrosion. This is accompanied by another advantageous effect: ON ions are released by the reaction of the calcium hydroxide and contribute to an increase in pH in the medium (between 8.2 and 9 in the case of calcium), which keeps the speed of corrosion of the steel at a low level.

Accordingly, it is clear that a reduction in pH (increase in the acidity of the medium) corresponds to a deficiency of metal hydroxide in the medium and hence a risk of limiting the oxidation-reduction reaction needed to form the layer of protection. As a result, am indication that there has been a reduction in pH means that there is a risk of the medium shifting from a basic pH to an acid pH which will promote corrosion, whilst at the same time the layer of protection is no longer being formed.

This invention proposes introducing metal hydroxide into the structure on a metered basis, controlled in particular by a pH sensor 11 of the pH meter type submersed in the structure. In the example illustrated in figure 1, the introduction of metal hydroxide X(OH)N from the tank 10 into the medium contained in the structure is controlled by a valve and/or a shut-off VA controlled by the pH meter 11. The introduction of hydroxide is interrupted when the pH is within a range of between 8 and 10 for example, and preferably equal to about 9 (to prevent an overdose and a waste of metal hydroxide), and is resumed when the pH drops to within a range of between 7 and 9, and preferably equal to about 8.

Furthermore, based on an additional or alternative embodiment, an ohmmeter 13 measures the resistance of a cathodic protection circuit formed from the surface of the internal wall CA to a reference REF connected to the generator G, passing in particular via the protective layer CM. Accordingly, the ohmmeter 13 provides an indication of the resistivity of the surface of the wall CA and hence the thickness of the layer CM. In particular, a resistance measurement which is high in this circuit characterises the formation of a protective layer which is thick at the surface of the cathode CA.

Consequently, cathodic protection based on applying electrical energy (impressed current for example) consumes less power, and even may not be necessary at all. If resistance in the circuit decreases, it is assumed that the layer of metal carbonate CM is deteriorating and the impressed current cathodic protection must be activated again to prevent corrosion and promote regeneration of the layer of metal carbonate.

Based on one embodiment, the ohmmeter 13 may be connected to the control module 12 of the generator Gin order to place the latter in service if the value of the resistance is below a selected threshold or alternatively to increase a difference in potential between the anode AN and the cathode CA, as will be explained below.

Furthermore, in addition or as an alternative, it is also possible to measure the difference in potential DDP between the cathode CA and a reference electrode REF directly (for example a saturated calomel electrode) and to control the generator G on the basis of this measurement in order to adjust (and most often increase) the difference in potential DDP between the cathode CA and this reference electrode REF.

Under the conditions created in the case of a steel cathode, this difference in potential must be less than -900mV (greater than 900mV in terms of absolute value). If it is found that there has been an increase in this value (decrease in terms of absolute value), then it is necessary to apply electrical energy again by impressing a current supplying electrons to the cathode CA until a value DDP of between -900mV and - 1100mV has been restored. Optionally, metal hydroxide may also be injected in, as will be described below.

As may now be seen from figure 2, during a first step Sl, the method starts with a first injection of metal hydroxide and the application of electrical energy by the generator G is activated during concomitant step S2 until a difference in potential DDP of about 900mV is reached in order to initiate the impressed current cathodic protection at step S3 by forming the layer CM of metal carbonate.

The sensor or sensors 11, 13 is/are then activated. Accordingly, if the resistance increases (arrow OK at the end of test S4), the quantity of energy applied can be progressively decreased at step S5 and the introduction of metal hydroxide can be slowed down or stopped at step S6 until the measured resistance (or resistivity) has reached a maximum asymptotic value. The resistance value is then monitored again at step S4.

If the resistance starts to decrease (arrow KO at the end of test S4), the application of impressed current energy is activated again at step S7 until the resistance starts to increase again. It is also possible to inject a dose of metal hydroxide in order to further enhance growth of the layer CM at step S9.

However, the addition of metal hydroxide is advantageously operated on the basis of the pH measurement taken of the medium at step S8. Accordingly, if the pH in the interior of the structure is high, for example higher than 9 (arrow KO at the end of test S8), it is possible that the medium contains sufficient metal ions to form a protective layer CM and it is not necessary to implement step S9 of injecting another dose of metal hydroxide. Consequently, when the resistance increases, the supply of impressed current electrical energy may be used solely to attract the metal ions towards the cathode, enabling the layer of carbonate to form again.

Based on one example of an embodiment, the structure is made from 355NL steel. A potential of between -800mV and -1100mV is applied to the cathode. The anodes are platinised titanium anodes in order to minimise the corrosion products of the anode. Graphite may be used as an alternative (a cheaper solution) but generates more corrosion products (albeit less galvanic).

Based on one embodiment mentioned here as an example, a structure of the monopile type with a diameter of 4m and an average seawater height of 12m (a value which changes with the tide) contains on average 150m' of seawater which is not evacuated or which is replaced by only an infinitesimal amount. As a result of this only slight exchange with water from outside, the level of hydroxide is found to decrease after a few weeks and the level of corrosive species increases. It is then necessary to re-inject a dose of hydroxide into the structure periodically. Calcium hydroxide may be used with a view to obtaining the formation of a layer of calcium carbonate, which is advantageously a very effective insulating material. Furthermore, calcium hydroxide is not toxic (the only risk to the environment being a high pH). It also has the advantage of being very cheap. A structure of the type defined above requires on average 3kg of calcium hydroxide per week.

Based on one example of an embodiment, the tank 10 contains calcium hydroxide above the structure. It is preferably sealed with a robust containment system capable of holding all the hydroxide needed to fill the tank. This tank is sealed and fluid-tight so that the hydroxide does not overflow in the event of submersion. The dosage is automatically controlled on the basis of the information uploaded from the shutoff/valve VA via the pH meter or meters 11. If the measuring system fails, the default position is preferably the "shut-off closed" mode so as to prevent too great an excess of hydroxide from being released into the structure. The tank 10 has a capacity of approximately 25kg. It is possible to impose a high potential in terms of absolute value (more or less in the vicinity of -t000mV) during periods in which the layer of carbonate is not sufficiently thick (for regions of the structure that are not as yet protected).

The invention advantageously obviates the need for an operator to fit a layer of anticorrosion protection manually. It also enables autonomous regeneration over time, thereby enabling maintenance work to be reduced.

Naturally, the invention is not restricted to the embodiments described above by way of example and includes other variants.

Amongst the conceivable variants, one option is to use other metal hydroxides to create a protective layer on the steel of the structure. The choice is made on the basis of a material that is not harmful to the environment and which creates a strong layer. Other alternatives may also be used for the material of the anode. Nevertheless, current conventional systems normally use steel, titanium or graphite.

Typically, one approach may be to impose a constant difference in potential to the cathode and to measure the pH of the medium in particular as a basis for deciding when to inject successive doses of metal hydroxide, based on one embodiment included within the scope of the invention.

The invention may be implemented in a hollow metal structure of any type which requires protection against a corrosive aqueous medium contained in it, provided the chemical composition of the solution can be changed (by adding hydroxide). Accordingly, one application of the invention might be the treatment of internal foundations of the monopile type, made from steel, for wind turbines at sea (offshore). However, the invention may also be applied to a metal structure of any type exposed to a relatively low flow of water entering and leaving a confined space in which the phenomenon of corrosion is observed or suspected.

Claims

Claims L Method of treating a hollow metal structure containing an aqueous medium against corrosion, characterised in that it comprises the steps: -supplying the structure with metal hydroxide (S1), - applying electrical energy (S2) to promote an oxidation-reduction reaction in which metal ions react with carbonate generated by the aqueous medium to form a layer of metal carbonate, to protect against corrosion, on an internal wall of the structure fulfilling the role of an electrode in the oxidation-reduction reaction, and in that: - at least one parameter of the aqueous medium (S4, S8) is measured and, on the basis of said measurement, it is determined whether it is necessary to supply additional metal hydroxide and/or whether it is necessary to apply additional electrical energy, and -if necessary, electrical energy is applied again (S7) and/or metal hydroxide is supplied again (S9).
2. Method as claimed in claim 1, characterised in that the application of electrical energy (S2) comprises an impressed current supply of electrons to the internal wall (CA) of the structure serving as a cathode.
3. Method as claimed in one of claims 1 and 2, characterised in that the application of electrical energy (S2) comprises applying a difference in potential (DDP) of between -800mV and -1100mV, between the internal wall (CA) of the structure serving as a cathode and a reference electrode (REF) submersed in the structure.
4. Method as claimed in one of the preceding claims, characterised in that a resistance at the surface of the internal wall (CA) is measured as a parameter of the aqueous medium (S4) and if said resistance is below a threshold value, at least an application of electrical energy (S7) and/or a supply of metal hydroxide (S9) is repeated.
5. Method as claimed in one of the preceding claims, characterised in that a difference in potential between the internal wall (CA) of the structure and a reference electrode (REF) submersed in the structure is measured as a parameter of the aqueous medium and if said difference in potential is below a threshold value, in terms of absolute value, at least an application of electrical energy (S7) and/or a supply of metal hydroxide (S9) is repeated.
6. Method as claimed in claim 5, characterised in that the threshold value is between -900 mV and -1100mV.
7. Method as claimed in one of the preceding claims, characterised in that a pH value is measured (S8) as a parameter of the aqueous medium and in that at least the step of supplying metal hydroxide (59) is repeated if the measured value is below a pH threshold value.
8. Method as claimed in one of claims 4 to 6, characterised in that, before repeating a supply of metal hydroxide, a pH value is measured (88) as a parameter of the aqueous medium and the step of supplying metal hydroxide (S9) is repeated if the measured pH value is below a pH threshold value.
9. Method as claimed in one of claims 7 and 8, characterised in that the pH threshold value is between 8 and 10.
10. Method as claimed in one of the preceding claims, characterised in that the metal hydroxide is Ca(OH)2.
11. Device for treating a hollow metal structure containing an aqueous medium against corrosion, characterised in that it comprises: -a tank (10) supplying the structure with metal hydroxide, -an electric power generator (G) to promote an oxidation-reduction reaction in which metal ions react with carbonate generated by the aqueous medium to form a layer of metal carbonate, to protect against corrosion, on an internal wall of the structure fulfilling the role of an electrode in the oxidation-reduction reaction, and in that the device further comprises: -at least one sensor (13, 11) for measuring at least one parameter of the aqueous medium connected to a control module (12) of the generator (G) and/or to a means (VA) for opening the tank (10) in order to apply an additional supply of metal hydroxide to the medium and/or to apply an additional supply of electrical energy on the basis of the measurement of said parameter.
12. Device as claimed in claim 11, characterised in that the generator (G) is electrically supplied by a converter of a wind turbine (EOL) supported by the structure (ST).