WO2004039664A1

WO2004039664A1 - Anti-fouling device

Info

Publication number: WO2004039664A1
Application number: PCT/NL2003/000754
Authority: WO
Inventors: Henk Slebos; Johan Stephan Meijer
Original assignee: Sraf Nederland B.V.
Priority date: 2002-10-31
Filing date: 2003-10-31
Publication date: 2004-05-13
Also published as: NL1021789C2; AU2003279605A1

Abstract

The invention relates to a device for inhibiting the growth of organisms on a submerged surface of a body, such as a ship, which device comprises at least one direct current source and an anode and a cathode connected thereto for obtaining an electrolytic process, wherein neither the anode nor the cathode is connected to the submerged surface of the body. This means that - in contrast to the known anti-fouling system impressed in cooling water systems of ships - the body itself is not a cathode. The anti-fouling device according to the invention can therefore also be employed for polyester, wood and aluminium hulls of boats, yachts, sailing boats and the like.

Description

Anti-fouling Device

The invention relates to a device for inhibiting the growth of organisms on a submerged surface of a body, such as a ship, which device comprises at least one direct current source and an anode and a cathode connected thereto for obtaining an electrolytic process.

Growth and adhesion of fresh-water and seawater organisms in the shipping industry is also referred to with the term "fouling" . Fouling is the result of the growth of, among others, algae, shellfish and mussels on the surfaces of a ship's hull lying under water.

It is known from the prior art to provide the ship's hull with an anti-fouling paint. Such a paint contains toxic pigments, such as cuprous oxides or mercury compounds. It has been found that cuprous oxides leached from the paint have a strong toxic effect on the growth of algae, shellfish, mussels and the like. A drawback of this anti-fouling method is that the effectiveness greatly depends on the oxidation and leaching capacity of the paint. This anti-fouling method is no longer permitted, at least in the Netherlands, because of its great environmental impact. In addition, it is a process which cannot be controlled. Another known anti-fouling method is based on the principle of an impressed current system, wherein copper and aluminium anodes are used. Reduction of anti-fouling can also be achieved to a greater or lesser extent when the ship's hull is embodied in copper/nickel or a solid copper/nickel alloy, or is provided with copper/nickel steel shell plating. The ship's hull serves as cathode in the electrolytic process. The system is based on the same principle as the use of anti-fouling paints, where copper pigments are used to provide a toxic medium. This anti-fouling method finds application in the internal cooling water system of ships for the purpose of keeping this free of organisms.

In order to prevent fouling of submerged surfaces of bodies, for instance the hull of a ship, an independent anode/cathode device is proposed according to the invention. This means that - in contrast to the known anti-fouling system impressed in cooling water systems of ships - the body itself is not a cathode. The anti-fouling device according to the invention can therefore also be employed for polyester, wood and aluminium hulls of boats, yachts, sailing boats and the like.

The use of copper alone as anode is not desirable. Corrosion cells can be formed when dissolved copper comes out of solution and is once again deposited on steel surfaces. However, this possibility does not occur and cannot occur in conditions of use according to the invention since, firstly, the amounts of copper going into solution are negligibly small and, secondly, this copper is not left behind as dissolved copper but immediately oxidizes into copper oxides, which are insoluble.

Both cuprous oxides and cupric oxides are insoluble, so that the quantity of copper ions in water is undetectable and lies far below so-called natural levels (see "Staatscourant", 16 June 2000, no. 114 - Table 1 - page

18. /target value (dissolved) for copper 0.5 μg/1, or maximum permissible risk of 1.5 μg/1) .

According to a preferred embodiment of the invention, an aluminium anode is also used in addition to a copper anode. Direct current dissolution of aluminium alloys results in the formation of colloidal solutions and hydrated aluminium oxides or "floe" which is gelatinous and encapsulates the precipitated copper oxides, so that they can be removed as an "innocent" sludge.

Since cuprous oxides are formed continuously on the anode surface and oxidize immediately into insoluble oxides, they are not free to react with trace chemicals with equivalent reductions in process efficiency. In natural waters, at pH 7 or even in the range of pH 5 to pH 9, there occurs no disadvantageous effect on the efficiency of the anode system.

The device according to the invention has the following advantages. The anti-fouling system is a safe system. Not only because there are no corrosion hazards, but the system will pose no threat to people who come into contact therewith, nor will it have any contaminating effect on the water. The anti-fouling device according to the invention will require little attention and maintenance, and - apart from the replacement of consumed anodes - will operate fully automatically. The device according to the invention is inexpensive and operating costs are low due to a low consumption, since the system operates at low voltage (roughly 1.5-28 Volt DC and max. 1 Ampere for copper and 0.25 Ampere - 0.5 Ampere for aluminium) . Accordingly, the main operating costs involve the replacement of the copper and possibly aluminium anodes, which are relatively low costs compared to other known anti-fouling systems. In anti-fouling paints leaching rates of "copper oxide" can be more than 10-15 μg/cm.24 hours. This means that a surface of a size of 1 m² can be kept clean of biological growth with about 40 grammes of copper oxide per year. Smaller quantities of copper oxide are active in the system according to the invention. While anti-fouling paints are only active during the period that the leaching speed lies above the given quantity, in the system according to the invention cuprous oxides are continuously being formed in controllable quantities and the action against the growth of algae, mussels and shellfish is continuous accordingly.

The annexed drawing shows an exemplary embodiment of the device according to the invention. The drawing shows in perspective a device for inhibiting the growth of organisms on submerged surfaces. The device comprises a housing or rack 1 in which are mounted a copper anode 2 , an aluminium anode 3 and a steel cathode 4. The housing contains a cover 5. The housing and the cover are preferably manufactured from plastic. Anodes 2,3 and cathode 4 are connected to a direct-current source (not shown) arranged in housing 1. Control means (not shown) are further provided to enable remote control of the electrolytic process, for instance whether the anodes are still sufficiently active. According to another embodiment of the invention

(not shown) , the anti-fouling device consists of respectively one or more electrolytic red-copper and similar aluminium rods. Both types of material are "anodes", mounted in electrically insulated manner in a stainless steel rack (316L) serving as "cathode" .

The operation of the anti-fouling device according to the invention is as follows. By connecting the anodes via cables to the positive or "plus" output of a rectifier installation and the separately provided cathode (fig.) or by connecting the stainless steel rack as cathode via cables to the negative or "minus" output there is created, when the rectifier is set into operation, a pre-adjustable continuous direct current from anodes, via the electrolyte as sea water or fresh water, in the direction of the cathode and back to the direct current source. This results in a constant development of sufficient cuprous oxide (0.02 ppm) through the electrolytic dissolution of the red copper anode rod or rods. The active cuprous oxide responsible for combatting sea and fresh water organisms becomes gradually oxidized into insoluble cupric oxide. Cupric oxide is not exactly environmentally-friendly, but the aluminium anode, when dissolved simultaneously, develops a gelatinous aluminium oxide which surrounds and completely encloses the cupric oxide, thereby rendering it harmless. No contamination therefore occurs and the anti-fouling method is environmentally-friendly.

The anti-fouling device is based on an independent cathode whereby ships of aluminium, steel, but also polyester and wooden ships and other random submerged body surfaces can be protected from organisms. Independent cathode means without mechanical and/or electrical contact with the submerged surface. The same applies for the anodes. The device can be placed on the bottom of a (yacht) harbour in order to protect the ships mooring in the harbour against fouling. It is also possible to carry the device on board a ship and to lower this device below the ship as fouling prevention when the ship is at anchor. The anti-fouling system according to the invention is environmentally-friendly because it does not kill any fresh-water and seawater organisms, such as algae and shellfish. It only creates an "unpleasant" environment, thereby preventing the growth on submerged surfaces. According to the invention, anti-fouling and cathodic protection can furthermore be combined in one and the same device. The cathodic protection is employed to protect metal parts lying under water. These may be metal hulls of ships, but also propeller shafts and/or screws of in particular polyester and wooden boats. Compared to the above described system without cathodic protection, the anti- fouling system is modified into a pre-adjustable electrical current generation and is adjustable via a so-called interval system. For objects in fresh water the rectifier can for instance be adjusted to repeat 150 sec ON and 30 sec OFF. In the case of tidal current, the rectifier can for instance be adjusted for objects in seawater to repeat 150 sec ON and 20 sec OFF. The effect of anti-fouling is hereby not disrupted, but even improved due to the so-called "shock effect" . An additional advantage is that the lifespan of the anodes is prolonged relative to a device with only anti-fouling action. Due to the alternating action of anti-fouling a rest period is obtained in which the aluminium oxides have time to enclose the cupric oxides. In the case of excessive tidal current it may be necessary to shorten the time of the rest periods. Finally, immunity is prevented by the alternating anti-fouling. The cathodic protection of metal surfaces lying under water is realized by placing an extra anode in the housing or rack 1, which anode is in electrical contact with the metal surface for protecting. There are two types of system of cathodic protection: — sacrificial anodes (for fresh water usually magnesium anodes and for seawater aluminium or zinc anodes) ; — permanent current impression by impressing direct current to an inert anode (preferably a titanium mixed- oxide anode) . The advantage of the second type relative to the first type of cathodic protection is that the anode does not have to be replaced and that the rectifier already present for the anti-fouling can be used.

The development of an electrical current between a base metal (anode) and a precious metal (cathode = the underwater surface to be protected) ensures that the cathode obtains a lower potential and the corrosion process is stopped. The cathodic protection acts continuously without intervals and is adjusted to a protective current generation which ensures a correct functioning of the cathodic protection, which means that the negative object/ electrolyte potential of at least -800mV to be measured relative to a silver/silver chloride reference electrode is achieved or exceeded up to a maximum of -1050mV.

For the anti-fouling and cathodic protection by means of permanent current application, use is preferably made of the same direct current source.

Claims

1. Device for inhibiting the growth of organisms on a submerged surface of a body, such as a ship, which device comprises a direct current source and an anode and a cathode connected thereto for obtaining an electrolytic process, characterized by at least two anodes of different materials, wherein neither the anodes nor the cathode are connected to the submerged surface of the body.

2. Device as claimed in claim 1, wherein a first anode is a copper anode and a second anode is an aluminium anode .

3. Device as claimed in claim 1 or 2 , wherein the cathode is a cathode of steel.

4. Device as claimed in any of the claims 1-3, further comprising a housing or rack having arranged thereon or therein the anodes and the direct current source .

5. Device as claimed in claim 4, wherein the housing or the rack is the cathode.

6. Device as claimed in any of the claims 1-5, further comprising control means for remote control of the electrolytic process.

7. Device as claimed in any of the claims 1-6, further comprising a third anode which is connectable to a metal surface lying under water for cathodic protection.

8. Device as claimed in claim 7, wherein the third anode is an inert anode connected to the direct current source .

9. Device as claimed in any of the foregoing claims, wherein control means are further provided for causing the anti-fouling system to operate at preset intervals and for causing the cathodic protection system to operate continuously.