GB2233981A

GB2233981A - Cathodic corrosion protection of metal surfaces in liquid electrolyte

Info

Publication number: GB2233981A
Application number: GB9015716A
Authority: GB
Inventors: Guenter Pietsch; Ernst-Peter Von Bergen
Original assignee: Blohm and Voss GmbH
Current assignee: Blohm and Voss GmbH
Priority date: 1989-07-21
Filing date: 1990-07-17
Publication date: 1991-01-23
Anticipated expiration: 2010-07-17
Also published as: DE3924158A1; SE9002368L; CN1049533A; SE9002368D0; GB2233981B; NL9001663A; KR910003153A; GB9015716D0; JPH03104890A

Abstract

An apparatus for the cathodic corrosion protection of metal surfaces immersed in a liquid electrolyte, preferably seawater, the metal surfaces preferably being provided at the stern tube seal of a ship, by means of at least one sacrificial anode 7 which is secured to a component which is moved relatively fast relative to the electrolyte, preferably at the propeller shaft 1 of a ship, is characterised by the use of a housing 8 which surrounds the sacrificial anode and is preferably electrically insulated relative thereto, with the wall of the housing being spaced from the sacrificial anode at least in the region of one of the side surfaces thereof and being provided there with apertures for the passage of the electrolyte, with the spacing of the passage openings from the sacrificial anode and/or the clear area of the passage openings being so selected that during the operational movement, preferably rotation, of the housing together with the sacrificial anode, the speed of flow of the electrolyte in the interior of the housing does not amount to more than 0.3 m/sec. <IMAGE>

Description

v Apparatus for the cathodic corrosion protection of metal surfaces

immersed in a licruid electrolvte, Dreferablv seawater, preferably metal surfaces at the stern tube seal of a shiio The invention relates to an apparatus for the cathod corrosion protection of metal surfaces immersed in a liquid electrolyte, preferably seawater, preferably metal surfaces at the stern tube seal of a ship, by means of at least one sacrificial anode and having the features set forth in the preamble of claim 1.

ic In apparatuses of this species, which have become known from DE-OS 25 20 948 and US-PS 3 726 779 the cathodic protection is only ensured for a relatively short period of time, since the anode which is secured to the rapidly moved part transmits considerably more current, as a result of the pronounced flow of the surrounding electrolyte than would be necessary for the protection of the metal surfaces which are to be protected.

This current flux can rise to a maximum 15-fold value with stron gly flowing media, so that the anodes are rapidly consumed.

In these known systems it is necessary, for example when a period of protection of four years is aimed at with zinc anodes, to provide sacrificial anodes of 163 kg per m 2 of metal surface to be protected.

These masses can frequently not be arranged at the named surfaces for space reasons, i.e. the protection can only be ensured over a relatively short period of time. When the anodes are consumed it is necessary to take the relevant plant out of use and to equip it with new anodes.

This has unfavourable effects, particularly with ships in which the stern tube seal is to be cathodically protected and the housing with the sacrificial anode co-rotates with the chrome steel sleeve at the seal, since these ships must then be docked. If this is not possible corrosion at the chrome steel sleeve and a failure of the stern tube seal must be expected.

Moreover, practice has shown that the insulation of the chrome steel sleeve relative to the propeller is very problematic. The sacrificial anode thereby mainly also protects the propeller region lying in the seal area beneath the rope guard, i.e. the life of the sacrificial anodes will be further shortened hereby.

The invention is based on the object of restricting the current flux in an apparatus in accordance with the preamble of claim 1 in such a way that the required life of the sacrificial anodes of approximately four years is obtained despite the high speed of flow of the electrolyte outside of the housing.

This object is satisfied in accordance with the invention by the use of a housing which surrounds the sacrificial anode and which is preferably electrically insulated relative thereto, with the wall of the housing being spaced from the sacrificial anode at least in the region of one of the side surfaces thereof and being provided there with apertures for the passage of the electrolyte, with the spacing of the wall of the housing having the apertures from the sacrificial anode and/or the clear opening of the apertures being selected such that during the operational movement, preferably rotation, of the housing together with the sacrificial anode the speed of flow of the electrolyte in the interior of the housing does not amount to more than 0.3 m/sec.

It is admittedly known from DE-PS 1 182 929 to surround a sacrificial anode with a housing which is electrically insulated relative to the sacrificial anode and to space the walls of this housing from the sacrificial anode in the region of one of the side surfaces of the sacrificial anode and to provide it with apertures there for the passage of the electrolyte, however these known devices are those in which no or only relatively little relative movement takes place between the housing containing the sacrificial anode and the surrounding electrolyte, and accordingly only a minor restriction of the flow of current is necessary.

Moreover, the encapsulation there in a housing and the spacing and perforation of the front wall serves the purpose of protecting the sacrificial anode from erosion under chemical loading by the seawater and not, as in the subject of the-invention, to reduce the speed of flow at the surface of the sacrificial anode.

The invention will now be explained in further detail with reference to an embodiment as shown in the accompanying drawings in which Fig. 1 shows a part longitudinal section through a seal arrangement with a sacrificial anode secured to the shaft sleeve, and Fig. 2 shows a section through the housing or cage for the anode.

4 In the stern tube seal shown in Fig. 1 a running sleeve 2 is mounted on the shaft 1. The shaft 1 is journalled in a bearing sleeve 3 in a stern boss 4 to which the seal housing 5 is secured. The seal housing 5 consists of several housing rings and several seal rings 6. A sacrificial anode 7, which is located in a cage, is secured to the sleeve 2. More particularly the cage is secured to the radial flange at the end of the sleeve where it is secured to the propeller hub. The cage 8 is so constructed that the flow through the cage cannot change the current flux of the anode.

The cage, which can be regarded as an annular housing concentric to the axis of the propeller shaft with an annular ring-shaped apertured cover is shown in cross-section in Fig. 1 and to an enlarged scale in Fig. 2. The sacrificial anode 7 which is typically of zinc is located within the housing 8 which may for example be of plastic (for example a strong engineering fiber reinforced plastic). The running sleeve 2 will normally be bolted to the hub of the propeller and the sacrificial anode and/or its housing will typically be bolted to the radial flange of the running shield at intermediate positions between the bolts connecting the radial flange to the propeller hub. Instead of an annular ring-shaped housing 8 with an annular ring-shaped sacrificial anode one could also use discrete block or parallelopiped shaped housings which would have the advantage that they could be exchanged without having to remove the propeller hub and running sleeve. A composite construction of the housing 8 and sacrificial anode 7 (in the form of two semicircular halves) is also straightforwardly possible.

Claims

P a t e n t c 1 a i m s

1. Apparatus for the cathodic corrosion protection of metal surfaces immersed in a liquid electrolyte, preferably seawater, the metal surfaces preferably being provided at the stern tube seal of a ship, by means of at least one sacrificial anode which is secured to a component which is moved relatively fast relative to the electrolyte, preferably at the propeller shaft of a ship, characterised by the use of a housing which surrounds the sacrificial anode and is preferably electrically insulated relative thereto, with the wall of the housing being spaced from the sacrificial anode at least in the region of one of the side surfaces thereof and being provided there with apertures for the passage of the electrolyte, with the spacing of the passage openings from the sacrificial anode and/or the clear area of the passage openings being so selected that during the operational movement, preferably rotation, of the housing together with the sacrificial anode, the speed of flow.of the electrolyte in the interior of the housing does not amount to more than 0. 3 m/sec.

2. Apparatus in accordance with claim 1, characterised in that with a ship having a bearing sleeve for the propeller shaft, the point of mounting for the housing of the sacrificial anode is located at the mounting flange for the chrome steel sleeve of the seal.

3. Apparatus in accordance with claim 1 or claim 2, characterised in that the housing consists of metal and the electrical insulation is formed relative to 1 the sacrificial anode by an intermediate layer of electrically insulating material.

4. Apparatus in accordance with claim 1 or claim 2, characterised in that the housing consists of an electrically insulating material, preferably plastic, which simultaneously forms the electrical insulation relative to the sacrificial anode.

Apparatus in accordance with one of the claims 1 to 3, characterised in that, when forming the housing in the manner of a parallelopiped, the spacing of the wall of the housing provided with the passage openings amounts to approximately 30% of the total height of the housing and the clear total area of the passage openings amounts to approximately 30% of the total area of the housing cover provided with the passage openings.

6. Apparatus in accordance with one of the claims 1 to 5, characterised in that the anode is secured to the propeller.

7. Appa ratus in accordance with one of the claims 1 to 5, characterised in that the anode is secured to the cylindrical sleeve part.

8. Apparatus in accordance with one of the claims 1 to 5, characterised in that the anode is secured to the seal housing.

9. Apparatus substantially as herein described and illustrated with reference to the accompanying drawings.

Published 1991 at The Patent Office. State House. 66/71 HighHolborn. London WCIR47P. Further copies may be obtained from Sales Branch. Unit 6. Nine Mile Point. Cwmfehnfach. Cross Keys, Newport NPI 7HZ- Printed by Multiplex techniques lid, St Mary Cray, Kent.

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