Method for protecting surfaces against biological macro-fouling

Abstract

The present invention relates to a method for protecting surfaces (S) which are in contact or come into contact with a water-containing medium (M) against biological macro-fouling, wherein 1) S is electrically conducting and 2) such a potential (P) fluctuating over time is applied to S that it inhibits the growth of organisms that live in M and/or propagate therein and which have the tendency to form deposits on S, characterised in that P does not assume values that are higher than the corrosion potential of S in M and the average value of P is lower than the said corrosion potential. The method according to the invention is particularly suitable for the protection of surfaces S where S comprises: a) internal walls of systems through which (M) is fed, such as cooling system tubes, heat exchangers or fluid transport tubes and associated parts which come into contact with (M); b) internal walls and parts of installations, equipment or storage facilities in which (M) is subjected to specific treatments, such as filter installations, purification installations or reaction vessels; c) external walls of vessels and constructions that are in contact with (M).

Description

[0001] The invention relates to a method for protecting surfaces against biological macro-fouling by applying a potential that fluctuates over time.

[0002] The electrochemical protection of ships' hulls against biological macro-fouling by applying alternating potentials is already known. For instance, in U.S. Pat. No. 4,440,611 a method is described for preventing or delaying microbial growth or the build up of limescale on a conducting or semiconducting surface in an aqueous environment by applying such a cathodic voltage and cathodic current to the surface that hydrogen peroxide is formed from the oxygen dissolved in the aqueous environment. The possibility of lowering the current consumption by applying the cathodic voltage and current with interruptions, the surface being kept electrochemically neutral in the intervening periods, is mentioned in this patent in column 2, lines 37-42.

[0003] It is furthermore known to use cathodic protection to protect metal-containing surfaces which come into contact with water against oxidative corrosion. This method is based on the application of a constant negative potential. A description of the effect of cathodic protection can be found in “Cathodic and Anodic Protection”, Chapter 10 in “Corrosion”, Volume 2, edited by L. L. Shreir, R. A. Jarman, G. T. Burstein, 3rd edition (1994), published by Butterworth-Heien Ltd, Oxford, UK. In contrast to the method of active cathodic protection described, with the method according to the present invention use is made of a fluctuating potential. Furthermore, cathodic protection is usually employed on conducting surfaces which are provided with a non-conducting coating, so that the underlying conducting surface is effectively safeguarded against macro-fouling, by cathodic protection, only after the coating has been damaged. In contrast, for the method according to the invention it is essential that the electrically conducting surface to be protected is either in direct contact with the water-containing medium or is provided with a conducting coating.

[0004] Organisms will settle on virtually all materials that are placed in water. This starts with the formation of a very thin biofilm by microorganisms. At a later stage larger organisms will nestle on the material, as a result of which macro-fouling is produced.

[0005] Macro-fouling on ships, constructions and in installations that are in contact with (sea)water constitutes a significant problem. For instance, macro-fouling by, for example, barnacles or mussels can lead to a substantial increase in the resistance of ships in water, to blockages in pipeline systems, to microbiological corrosion, to deposit attack, to erosion/corrosion or to a reduction in heat transfer. There is therefore ongoing research to increase the efficiency of anti-fouling methods.

[0006] At present macro-fouling in open systems is usually combated by applying an anti-fouling coating, an anti-fouling paint. In the majority of cases the anti-fouling action is based on the slow dissolution of a toxic component in these coatings.

[0007] However, the use of an anti-fouling paint is associated with a number of disadvantages. These include, inter alia

[0008] harmful effects on the environment;

[0009] the need for periodic replacement of the paint system that slowly loses its anti-fouling action;

[0010] a coating must be applied with a view to anti-fouling, even if this is not necessary or even undesirable from the corrosion standpoint, with a view to heat transfer or from the structural engineering standpoint.

[0011] In addition to the use of coatings, for open cooling water systems there is the option of metering in biocides. Methods that are widely used are dissolving copper or producing chlorine or hypochloride at the water inlets, which copper, chlorine or hypochloride is also fed through the cooling system and has an anti-fouling effect in this system. However, for these systems as well components harmful to the environment are introduced into the seawater.

[0012] The invention is aimed at overcoming the above disadvantages and relates to a method for protecting surfaces (S) which are in contact or come into contact with a water-containing medium (M) against biological macro-fouling, wherein

[0013] 1) S is electrically conducting and

[0014] 2) such a potential (P) fluctuating over time is applied to S that it inhibits the growth of organisms that live in M and/or propagate therein and which have the tendency to form deposits on S, characterised in that P does not assume values that are higher than the corrosion potential of S in M and the average value of P is lower than the said corrosion potential. The corrosion potential is defined as the potential of a corroding surface in an electrolyte with respect to a reference electrode, as defined in “Principles and Prevention of Corrosion” by D. A. Jones, 2nd edition, Prentice-Hall, Upper Saddle River, N.J. 07458. In this description the values cited for the potential (P) are always with respect to a saturated calomel electrode (SCE).

[0015] The term “biological macro-fouling” is also used to refer to deposits as a consequence of the presence of (micro)organisms which are (can be) present in seawater, brackish water and freshwater or water-containing media or systems.

[0016] Compared with the method as described in U.S. Pat. No. 4,440,611 discussed above, the method according to the invention has the significant advantage that no use is made of potentials that are higher than the corrosion potential. Therefore no precautionary measures have to be taken to prevent accelerated corrosion occurring on parts susceptible to corrosion, such as a ship's hull. The method according to the invention thus offers the significant advantage that it can be employed for simultaneous protection of surfaces in an aqueous environment against macro-fouling by organisms and against corrosion. This is effectively achieved by the use of an adequate negative base potential so as to obtain cathodic protection in combination with fluctuating negative potential pulses which prevent or impede the growth of organisms.

[0017] Although it is not entirely clear how the method according to the invention impedes biological macro-fouling, it appears probable that this is the consequence of the formation of hydroxyl ions at the surface S. These hydroxyl ions are formed as a result of the electrochemical decomposition of water. It has been found experimentally that the pH of the medium in the immediate vicinity of the surface S rises sharply (for example pH>9 in the case of use in seawater, compared with pH 7.8 for untreated seawater), which is a very strong indication of the said formation of hydroxyl ions.

[0018] Although the production of hydroxyl ions appears to be the most important in the method according to the invention, other (reaction) effects can be (co-)determining for the success of the method. The effects concerned here are, for example:

[0019] obstruction of organisms by influencing the bio-electrochemistry of the organisms by the presence of a charge on the surface to be protected against macro-fouling or by varying the charge on said surface in the course of time;

[0020] obstruction (physical obstruction) of organisms by the formation of hydrogen gas at the surface to be protected against macro-fouling;

[0021] obstruction of organisms by oxygen depletion on the surface to be protected against macro-fouling (after all, the oxygen is consumed in the oxygen reduction reaction and is thus no longer available to organisms);

[0022] obstruction of organisms in that a biofilm is not able to form or is able to form to only a limited extent.

[0023] As far as macro-fouling is concerned, it is mainly the following organisms that are of importance: algae, diatoms, ascidians, hydroids, anthozoans, bryozoans, tube worms, bivalve molluscs and crustaceans, especially barnacles, mussels, algae and tubercles.

[0024] According to the invention the settling of microorganisms such as bacteria and the formation of their biofilm (a very thin “slime film” formed by microorganisms) are prevented or controlled. This has the following beneficial effects:

[0025] microbiological corrosion (MIC) is prevented or the risk of MIC occurring is reduced;

[0026] macro-fouling is prevented because there is no biofilm present.

[0027] The surfaces (S) that can be protected according to the invention are, for example:

[0028] a) internal walls of systems through which (M) is fed, such as cooling system tubes, heat exchangers or fluid transport tubes and associated parts which come into contact with (M);

[0029] b) internal walls and parts of installations, equipment or storage facilities in which (M) is subjected to specific treatments, such as filter installations, purification installations or reaction vessels;

[0030] c) external walls of vessels and constructions that are in contact with (M).

[0031] The method according to the invention is particularly suitable for protecting vessels against macro-fouling and corrosion. Despite the fact that when sailing the rate at which the water in the vicinity of the surface (S) is refreshed increases, and thus the inhibitory effect of the electrolytically formed hydroxyl ions on the growth of organisms decreases to some extent, surprisingly good results are nevertheless still obtained if the method is used on ships at sea. Of course, the effectiveness is greater if the vessel is stationary, which is advantageous in view of the act that the rate of growth is highest under these conditions. It is therefore also preferable to employ the present method on vessels irrespective of whether these are stationary or are at sea. The surface (S) to be protected preferably consists of or contains:

[0032] a) iron, copper, nickel, titanium, aluminium or an alloy based on these metals,

[0033] b) electrically conducting or semiconducting coating or top layer, such as a metallic coating, ceramic coating, intrinsically conducting polymer or paint system to which electrically conducting components have been added,

[0034] c) non-metallic, non-conducting structural material or coating to which a conducting component, for example in the form of a filler or fibres, has been added,

[0035] d) non-metallic, conducting or semiconducting structural material. More particularly, the surface (S) to be protected consists of the materials mentioned under a) or b). The best results are obtained with the method according to the invention if S contains steel, and particularly if S contains stainless steel. Preferably, S consists predominantly of steel or stainless steel, it optionally being possible for an electrically conducting coating (for example a coat of paint) also to have been applied to the steel.

[0036] Surprisingly it has been found that the method according to the invention not only protects against biological macro-fouling but also counteracts corrosion of metals present in S that do not belong to the group of noble metals. This effect is even detected under conditions where a potential pulse that is well below the potential that would normally be used in order to achieve cathodic protection is employed for a prolonged period.

[0037] In order to be able to polarise well the surface to be protected—a relatively low potential must be applied—the surface in contact with water must, of course, have a conductivity such that the negative potential can be applied thereto. The method according to the invention will thus not work if electrically insulating paint systems are used. In general it can be stated that the electrical conductivity of (S) must be such that a reduction in potential of at least 300 mV, preferably of at least 500 mV, can be achieved within one minute.

[0038] With the method according to the invention it is preferable that the maximum value of P that is applied by polarisation is lower than the corrosion potential of S in M, since conditions which could lead to corrosion of S, especially in those cases where S contains one or more metals which do not belong to the group of noble metals, are thus prevented. Preferably the maximum value of P is at least 50 mV lower than, more particularly at least 100 mV lower than, the corrosion potential.

[0039] Furthermore it is to be recommended to keep the range of the potential fluctuations within specific limits. Preferably, the fluctuations of P with respect to SCE are within the range of −300 to −3000 mV, and preferably −400 to −2000 mV. Particularly good results are obtained with stainless steel if the said fluctuations are within the range from −800 to −1800 mV. The amplitude of the potential fluctuations is preferably at least 50 mV and more particularly at least 100 mV. The frequency of the fluctuations is preferably at least once per 24 hours. A method in which the amplitude is at least 200 mV and the frequency at least once per 6 hours is to be particularly preferred.

[0040] The desired biological inhibition can he achieved by applying (P) to (S) at intervals (T), or by varying (P) in some way or other over time. All conceivable variations of (P) over time are possible in this context, but in general the method according to the invention will make use of pulse patterns (use of so-called spikes) where the periods in which (P) is applied to (S) have a duration of 0.01-600 seconds, preferably 5-120 seconds, and (T) is 0.1second-48 hours, preferably 10 seconds-4 hours.

[0041] In a particular embodiment the invention relates to the use of a negative potential (P) that fluctuates over time for the protection of electrically conducting surfaces (S) which are in contact or come into contact with a water-containing medium (M), characterised in that P both

[0042] a) impedes the growth of organisms that live, and/or propagate in M and have the tendency to form deposits on S, and

[0043] b) counteracts corrosion of S by means of cathodic protection.

[0044] In the case of active cathodic protection the rate of corrosion of a metal or metal alloy such as steel is very substantially reduced by continuously lowering the potential to approximately −800 to −1000 mV with respect to SCE using a rectifier installation.

[0045] Finally, the present invention also relates to a device for the protection of external walls of vessels and/or other constructions that usually come into long-term contact with water, as well as of internal walls of systems through which water is fed, against biological macro-fouling, which device comprises:

[0046] a) an electrically conducting internal or external wall,

[0047] b) a voltage source that is connected to the internal or external wall and that preferably is able to achieve a reduction in potential of at least 300 mV on the wall within one minute, characterised in that the voltage source is capable of producing a negative polarisation that varies over time on the surface to be protected. against macro-fouling. Usually a counter-electrode, which is positively polarised by the voltage source, is used in this polarisation. This counter-electrode is electrically insulated from (S) and preferably (but not necessarily) is made of a material that is inert in the medium (M). The voltage source is preferably controlled with the aid of a reference electrode to be positioned close to (S).

[0048] The invention is explained in more detail in the following examples.

EXAMPLE I

[0049] In the experiments described below use was made of larvae of the barnacle Balanus amphitrite. These larvae were cultured on a laboratory scale and the effect of the method according to the invention on the so-called settlement behaviour of the barnacles was investigated.

[0050] For the experiments use was made of a test cell in which the settlement behaviour of barnacle larvae can be investigated on a substrate having a potential that can be controlled electrochemically. This test cell consisted of a plexiglas tube that was glued to a test plate. Barnacle larvae were introduced into the plexiglas “inner cell” thus produced and fresh seawater was supplied continuously. Discharge of the seawater was via a very fine filter gauze, so that the barnacle larvae were not able to leave the inner cell.

[0051] The test plate was connected via a time switch to a potentiostat, whilst a reference electrode was fitted in the inner cell. A platinum counter-electrode was fitted on the external wall of the plexiglas cell, in the so-called ‘outer cell’. This means it was possible to polarise the test plate without the reaction products of the counter-electrode being able to influence the barnacle larvae.

[0052] Using this test method it is possible to establish whether barnacle larvae are capable of settling on a test plate, or whether they die because of the lack of a suitable location for settlement, or whether they settle on a location other tan the test plate.

[0053] The test conditions employed were as follows:

[0054] Environment: natural seawater with a refreshment rate of the inner cell of approximately once per hour

[0055] Temperature: 26° C.

[0056] Test duration: 2 days

[0057] Approximately 200-300 larvae added per test cell

[0058] The larvae were not fed in the interim

[0059] The data with regard to the potential pulses used are given in Table 1. During the exposure the maximum output of the potentiostat was used for generation of the pulses. The actual potential level of the pulses was measured during the tests and differs somewhat from one test cell to another. 1 TABLE 1 Settings used per test cell Ref. 1* Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 (Cell 5) Ref. 2 Ref. 3 Pulse frequency ½ hour ½ hour 1 hour 1 hour ½ hour ½ hour None None Pulse duration 2 min. 2 min. 2 min. ½ min. 2 min. 2 min. None None Potential level −1.55 V −1.58 V −1.37 V −1.46 V −1.57 V −1.58 V Ecorr Ecorr *= Stainless steel in outer cell to observe influence of the evolution of chlorine gas alone on the larvae.

[0060] Three reference cells were used during the tests. Reference cell 1 was used to ensure that the reaction products which are produced on the platinum auxiliary electrode in the outer cell (such as chlorine gas) do not have any influence on the barnacles in the inner cell. For this purpose in the case of reference cell 1 instead of the test piece in the inner cell a strip of stainless steel which was fitted around the plexiglas (in the outer cell) was polarised. The reaction products on the auxiliary electrode are consequently the same as in the case of test cells 1, 2, 3, 4 and 5, but there is no polarisation of the test piece whatsoever.

[0061] Barnacle larvae were placed in reference cells 2 and 3 without any potential control whatsoever. If the larvae are in good condition, settlement should take place here as well without any problem. Reference cell 2 is identical to the cells discussed above, whilst reference cell 3 consists of a glass beaker.

[0062] The entire experiment lasted 48 hours. A fairly large quantity of larvae, which partly in view of their mobility were of good quality, was used at the start.

[0063] The results of the settlement experiments are given in Table 2. Here it can be seen that fouling was prevented in all cells in which potential pulses were used. This was found whilst there was substantial fouling in all three reference cells. 2 TABLE 2 Results of the settlement tests Number of larvae Cell no. introduced Temp. Inspection on Jan. 12, 1999 with regard to settling 1 approx. 200-250 approx. 26° C. No larvae settled on the test plate. Substantial adhesion of limescale. Approx. 50 larvae settled on the wall. 2 approx. 200 approx. 26° C. No larvae settled on the test plate. Substantial adhesion of limescale. A few larvae settled on the wall. 3 approx. 250 approx. 26° C. No larvae settled on the test plate. Adhesion of limescale with cavities in the limescale layer. Larvae settled on the wall. 4 approx. 250 approx. 26° C. No larvae settled on the test plate. Slight adhesion of limescale to part of the test plate. Other parts of the test plate were bare. Larvae settled on the wall. 5 approx. 250-300 approx. 26° C. No larvae settled on the test plate. Substantial adhesion of limescale. Large number of larvae settled on the wall. Ref. 1 approx. 250 approx. 26° C. Approx. 40 settled on the test plate. These were very active (mobile). A similar number were still in the course of settling on the test plate. Further settlement on the wall. Ref. 2 approx. 250-300 approx. 30° C. Approx. 50 settled on the test plate. A few free larvae that still have to settle. A large number of larvae settled on the wall. Ref. 3 Remainder of the approx. 30° C. Glass surface completely covered with settled cyprid larvae. population

EXAMPLE II

[0064] The following experiment was carried out to establish how the pH of seawater changes when a negative potential is applied.

[0065] A working electrode (W) made of stainless steel (316 L) was placed, together with a counter-electrode and a reference electrode, in a large glass beaker containing natural seawater. All electrodes were connected to a potentiostat by means of which increasingly more negative potentials were successively applied to the working electrode. The working electrode was partially protected by placing this separately in a smaller glass beaker inside the large glass beaker. A piece of universal pH indicator paper (Riedel-de-Haën (RdH Laborchemikalien GmbH & Co.): PANPEHA (Multi-colour special and universal indicator paper pH 0-14)) was placed both on the base of the large glass beaker and on the working electrode itself. The color change of two areas of the indicator paper (3rd and 4th areas from the top) was observed.

[0066] In addition the pH was also measured using an electronic pH meter that was positioned above the working electrode. The potential of the working electrode was lowered stepwise from −600 to −1400 mV. The observations made during this experiment are given in the table below. 3 Potential of working pH electronic pH indicator paper, electrode (mV) pH meter 3rd/4th colour area −600 7.76 Bottom: somewhat red Top: white/clearly green −800 8.10 Bottom: somewhat red Top: white/tending towards blue −1000 8.11 Bottom: sowewhat red Top: white/tending towards blue −1200 8.15 Bottom: sowewhat red Top: pink/blue −1400 8.20 Bottom: sowewhat red Top: pink/blue

[0067] It can be concluded from the above data that the pH close to the electrode (pH 8.5-9.0) at −600 mV is higher than that of the surrounding water (pH 7.76). This difference increases even further at lower potentials. At −1400 mV the electronic meter indicates 8.20, whilst the pH close to the electrode is about 9.

Claims

1. Method for protecting surfaces (S) which are in contact or come into contact with a water-obtaining medium (M) against biological macro-fouling, wherein

1) S is electrically conducting and

2) such a potential (P) fluctuating over time is applied to S that it inhibits the growth of organisms that live in M and/or propagate therein and which have the tendency to form deposits on S, characterised in that P does not assume values that are higher than the corrosion potential of S in M and the average value of P is lower than the said corrosion potential.

2. Method according to claim 1, characterised in that the maximum value of P is at least 50 mV lower than the corrosion potential of S in M.

3. Method according to claim 1 or 2, characterised in that fluctuations of P with respect to SCE are within the range from −300 to −3000 mV.

4. Method according to one or more of the preceding claims, characterised in that the amplitude of the potential fluctuations is at least 50 mV and the frequency is at least once per 24 hours.

5. Method according to claim 4, characterised in that P is applied to S at intervals (T) or is varied over time in some way or other and the periods for which P is applied to S have a duration of 0.01-600 seconds and T is 0.1 second-48 hours.

6. Method according to one or more of the preceding claims, characterised in that the electrical conductivity of S is at least such that a reduction in the potential on S of at least 300 mV can be achieved within one minute.

7. Method according to one or more of the preceding claims, characterised in that S comprises:

a) internal walls of systems through which (M) is fed, such as cooling system tubes, heat exchangers or fluid transport tubes and associated parts which come into contact with (M);

b) internal walls and parts of installations, equipment or storage facilities in which (M) is subjected to specific treatments, such as filter installations, purification installations or reaction vessels;

c) external walls of vessels and constructions that are in contact with (M).

8. Method according to one or more of the preceding claims, characterised in that S consists of or contains:

a) iron, copper, nickel, titanium, aluminium or an alloy based on these metals,

b) electrically conducting or semiconducting coating or top layer, such as a metallic coating, ceramic coating, intrinsically conducting polymer or paint system to which electrically conducting components have been added,

c) non-metallic, non-conducting structural material or coating to which a conducting component, for example in the form of a filer or fibres, has been added,

d) non-metallic, conducting or semiconducting structural material.

9. Use of a negative potential (P) that fluctuates over time for the protection of electrically conducting surfaces (S) which are in contact or come into contact with a water-containing medium (M), characterised in that P both

a) impedes the growth of organisms that live and/or propagate in M and have the tendency to form deposits on S, and

b) counteracts corrosion of S by means of cathodic protection.

10. Device for the protection of external walls of vessels and/or other constructions that usually come into long-term contact with water, as well as of internal walls of systems through which water is fed, against biological macro-fouling, which device comprises:

a) an electrically conducting internal or external wall,

b) a voltage source that is connected to the internal or external wall and that is able to achieve a reduction in potential of at least 300 mV on the wall within one minute, characterised in that the voltage source is capable of producing a negative polarisation that varies over time on the surface to be protected against macro-fouling.