FI77269C - Method and apparatus for cathodic protection of steel structures operating in sea conditions. - Google Patents

Description

77269

METHOD AND APPARATUS FOR THE CATHODIC PROTECTION OF STEEL STRUCTURES OPERATING AT MARINE CONDITIONS - FÖRFARANDE OCH ANORDNING FÖR KATODISKT SKYDD AV I HAVSFÖRHALLRNDEN FUNGERANDE STÄIKONSTRUK-TION

The invention relates to a method according to claim 5, recital 5, for cathodic protection of systems operating under marine conditions from corrosion, and to an anode structure according to claim 9, according to the preamble, for applying the method.

Cathodic protection prevents corrosion of the metal in an electrically conductive medium, such as seawater. Cathodic protection can be provided by passively sacrificing anodes, the operation being based on arranging two metals of different degrees of nobility in contact with each other, i.e. forming a galvanic pair in which the object to be protected acts as a cathode. In active protection (ICCP, Impressed Current Cathodic Pretection), a power supply connected between the structure to be protected and the anode supplies direct current from the anode through the medium to the surface to be protected, thus forming a cathode of the generated electric field 20. The protective current then lowers the corrosion potential of the surface to be protected to the so-called immunity zone, whereby corrosion of this surface is prevented at the expense of the anode. The anode may be a device separate from the structure to be protected, or alternatively a device insulated to the structure to be protected .25. The latter arrangement, to which the invention is also directed, provides corrosion prevention with a lower current requirement. Moreover, such a combined structure is a necessity for mobile systems. In automatically controlled, active cathode protection systems tel-30 where the potential of the surface to be protected is monitored by a so-called reference electrode. The control unit monitors the potential difference between the surface to be protected and the reference electrode and determines the amount of current supplied by the power supply based on this.

__ - · 17 _ _ 2 77269 Such externally powered cathodic protection devices are currently installed in a significant proportion of new construction vessels and offshore structures. The main component of the power supply anode is then a metal rail or plate 5, through which power is supplied to the seawater. The plate material is often platinum-plated titanium or niobium or a lead-alloy. The power supply board must not be in direct conductive contact with the surface to be protected. Therefore, an insulating material must be used between them, for example 10 so that the power supply plate is attached to plastic, reinforced plastic or epoxy resin. An insulated cable is attached to the power supply board, through which power is supplied from the power supply. The structure of the reference electrodes is basically similar to that of the anodes, however, the operation is based on the potential difference of the electrode with respect to seawater remaining constant. Usually, metallic zinc or a silver-silver chloride material is used as such an electrode. The electrode is isolated from the object in the same way as the anode, and an insulated cable 20 is attached to it, which leads to the control unit. In the following, anode equally means a reference electrode according to the above.

Currently, there are mainly two different methods of mounting the anodes (or reference electrodes): surface mounting and flush mounting.

25 In surface mounting, anodes embedded in plastic or similar insulation are attached to the surface of the structure to be protected, for example the hull of the ship, so that they are completely outside the surface profile. The anode material sheet is often made of zinc plates or pieces, i.e. the operation is based on the use of so-called sacrificing anodes. Fastening is done, for example, with bolts welded to the surface or by gluing. Such installation and fixing is relatively inexpensive and easy to implement. However, a significant disadvantage is the poor ability of the anode installed in this way to withstand 35 large mechanical stresses, such as the abrasive effect of ice, without being detached. The prior art surface-mounted 3 77269 anodes are therefore not at all suitable for cathodic protection of icebreaker bodies.

In flush mounting, housings for anodes are first welded to the surface to be protected. The anode plates 5 mounted on plastic or the like are then fixed to these housings, for example, by bolts and glue welded to the bottom of the housing. According to this installation method, the anodes are entirely inside the surface profile, whereby the significant advantage is achieved that the durability of the anodes under ice conditions, for example, is considerably improved when ice is unable to remove the anodes. The disadvantage, on the other hand, is the cumbersome implementation of the method and the associated high cost. The protection current distribution provided by the flush-mounted anode is also disadvantageous compared to surface-mounted anodes. A gap is left between the anode and the housing welded to the structure, which is filled with plastic putty or epoxy resin. Puttying is difficult to perform when the surfaces are vertical or curved downwards, such as the hull of a ship. If the power supply plate is damaged, the anode Gears-20 is a method, which is quite a laborious operation due to the high strength and adhesion of the plastic putty. The development of the longest recessed anodes has led, for example, in the method according to Finnish patent application no. 841632, in which the recessed anode is prefabricated from an element which is directly welded to an opening made in the structure to be protected.

Compared to surface-mounted anodes, such a structure is also quite expensive to implement, and in addition, the replacement of a damaged anode requires the entire housing to be detached during docking and replaced with a new corresponding element. It is also difficult to weld the prefabricated element to the structure to be protected without damaging the element due to the different thermal expansion between the steel and the epoxy derivatives or fiberglass-based fillers.

35 An additional disadvantage of prior art anode structures is the susceptibility of the insulating material required by the anode to destruction during use __ - rz. ____ 77269 4, which results in a short-circuit of the protective current in the protected structure so that the cathodic protective effect disappears. Such destruction is caused, for example, by the porosity of the surface of the insulation materials used, the cracks contained therein and / or easily formed during installation, the tendency of the glass fiber materials to absorb water, resulting in a change in volume, etc. Water penetration between the anode and the porous insulation mass causes titanium in platinum-plated titanium anodis-10 sa. In addition to the high cost, the disadvantage of flush-mounted anodes is the large space requirement of the housing, which leads to difficulties in fitting the housing with the protective tubes between the arches of the ship. The dimensions of the housing are often of the order of 300 x 900 mm. In addition, the dimensions of the inner protective tube or space 15, the so-called cofferdam, to be provided to ensure the tightness of the housing are of the same size.

The object of the invention is to create a new, more efficient and economical method for providing active cathodic protection in systems operating under marine conditions, and an anode structure for implementing the method. The aim is to create an anode structure for use in marine conditions, especially in the Arctic, which is well suited for prefabrication and the installation of which is as easy as possible. Another goal is to create an anode structure that is as easy to replace as possible, either during docking or by a diver. It is a further object to provide an anode structure having an improved ability of the insulating material to withstand corrosion and / or mechanical stress, in addition to making the insulating material more effective, whereby the thickness of the insulating material can be reduced.

The features of the invention are set out in the characterizing part of claim 1. By installing the anode plate in a ceramic electrically insulating material, e.g. aluminum or zirconia or the like, the prior art has the considerable advantage that the insulating material at the same time acts as an effective and impact-resistant holder for locking the anode plate in place. The advantage of such a ceramic insulating material is its quite good corrosion resistance and insulating ability. In addition, the strength properties of the material 5, in particular the compressive strength, are quite high, making such an anode structure excellent for use in arctic or subarctic conditions where the mechanical stress caused by moving ice cubes puts the anode duration to a severe test.

The described anode structure is equally suitable for surface and flush mounting of anodes, in a preferred embodiment in addition to a mounting method with features for both surface and flush mounting.

When using such an anode structure consisting of a ceramic material in an immersion installation, the anode is better suited for prefabrication. The ceramic filler and insulator can be arranged in advance in a metal housing, the sealing of which in the structure to be protected is easier to carry out than in the case of known anode elements, since the thermal expansion of a ceramic, e.g. ZrO2 », is of the same order as steel. For this reason, the thermal effect of welding does not produce a cracking or pressure effect in the material, but the welding can be performed in the immediate vicinity of the anode structure.

When a ceramic insulating plate is used for surface mounting, a compact and strong structure is achieved in which the insulating material surrounds the circumference of the anode plate from the sides so that the mechanical force exerted by the ice pieces on the anode structure is primarily applied to the high compressive strength ceramic. The fastening of the anode structure to the structure to be protected can then be carried out in a known manner, for example with fastening screws or gluing. The suitability of such a surface-mounted anode structure for ice conditions is considerably better than known surface anodes.

___- - · T- __ 77269 6

Particularly preferred is an application in which the washer formed by the ceramic insulating material has a protrusion which is located in a correspondingly shaped hole or cavity in the structure to be protected. In this case, the high compressive strength of the ceramic material 5 can be fully utilized in attaching the anode structure to the substrate. Although the outer surface of such a partially immersed anode extends beyond the surface profile of the structure to be protected, a structure with a mechanical stress resistance of at least 10 as good as the properties of the best known immersed anodes is achieved.

Ceramic insulating sheets, cones or the like can be produced by a known method, either by sintering or slurry casting. The shape of the insulating plate is freely selectable; for manufacturing reasons, insulating plates in the form of a rotating body are generally the most advantageous.

The starting point for the above solutions is the use of an anode plate made of a conventional material, for example platinum-plated titanium. However, a very preferred solution is to use a known current-conducting ceramic material, for example lithium ferrite, magnesium aluminum ferrite or metal oxides such as Fe 2 O 3, NiO, CO 3 O 4 or the like, as the current-supplying anode of the anode structure. The corrosion resistance and strength properties of an anode plate composed of such a material are superior to metal structures. In addition, the current-supplying anode can then be cast, sintered or welded in one piece with another electrical material that insulates the electricity.

The invention will now be described in more detail with reference to the accompanying drawing, in which Figure 1 is a side sectional view of one embodiment of the invention; 7 77269 Fig. 2 is a side sectional view of another embodiment of the invention; Fig. 3 is a side sectional view of a third embodiment of the invention; Fig. 4 is a side sectional view of a fourth embodiment of the invention; and Fig. 5 is a side sectional view of a variation of the embodiment of Fig. 1.

10 In the drawing, reference 1 refers to the outer shell of a structure to be protected, which structure may be any system operating under marine conditions, e.g. a hull side plate, an offshore support leg or other corrosive metal structure. Reference 2 refers to a current-supplying anode 15 constructed of a suitable inert, electrically conductive substance. It is usually a platinum-coated titanium plate, but niobium, tantalum or zirconium are equally conceivable. The anode material may also be an electrically conductive ceramic material, as described in more detail below. The anode is preferably plate-shaped, although other shapes, e.g. rail-shaped anodes, can be used. An electric wire is welded to the inner surface of the anode to a power supply 15, e.g. a titanium rod 13 connected to a cable 14. The power supply 25 and the attachment of the supply devices to the anode can be performed in a known manner, so these systems are shown in the drawing only for reference. An opening Α, Α ^ Α '' is machined in the structure 1 to be protected for the power supply equipment. Inside the structure to be protected, a watertight protective space or pipe 10, the so-called cofferdamm, required by the classification systems is arranged.

An insulating plate 3; 3 '; 4,5 is made between the anode plate 2 and the structure 1 to be protected.

8 77269 Al 2 O 3, ZrO 2 or the like. Such a material is very hard, and has a very high compressive strength. The shear strength is also in the class of structural steels, although the material is very brittle, ie there is almost no yield. According to Fig. 1, the ceramic insulating plate 3 extends laterally beyond the anode plate 2, and vertically to the level of the upper surface of the anode plate. In this way, the anode plate, which is easily damaged by external forces, is encased in a ceramic insulating material 3, which thus at the same time acts as an effective holder and 10 protection for the anode. The insulating plate 3 has sloping and preferably rounded edges so that the force acting on it is transmitted to the material as a compressive stress. The anode plate 2 is attached to the insulating plate 3 with an adhesive, e.g. epoxy resin, in the same way the insulator 3 is also attached to the structure 1 to be protected 15. The holding of the anode plate can be further facilitated by an electrically conductive titanium rod attached to the inside. In the solution according to the invention, in which the anode is encapsulated in a ceramic insulating material, the problem of the anode remaining in place cannot be formed, since the external force effect is either applied to the insulator or tends to press the anode plate towards its support.

In the application according to Figure 1, which is particularly well suited for icebreakers operating in arctic conditions, for example, an opening 25 A is machined in the structure to be protected, the dimensions of which correspond to the protrusion 8 of the insulating plate. In order to achieve this goal, the planar dimension 30 of the opening A and the protrusion 8 must be large enough, at least considerably larger, than the opening required for the power supply of the anode in the structure to be protected. Within the scope of the invention, instead of the opening A, a recess corresponding to the projection 8 can be used in the structure to be protected. Instead of one protrusion 8, it is also possible to use several protrusions in the insulating plate 3. The location of the protrusion 8 does not necessarily have to be arranged symmetrically with respect to the insulating plate 3, but within the scope of the invention

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9 77269 any location that is economically advantageous in terms of the stress present can be selected.

In the embodiment according to Figure 2, the ceramic insulator 4 is arranged around the anode 2 in a circumferential shape. Compared to the application according to Fig. 1, a saving of the collecting material is thus achieved. The attachment of the insulating ring to the structure 1 to be protected, as well as the attachment of the anode plate 2 to the insulating material, is carried out as described above. The space between the anode 2 and the structure 1 is preferably filled with an adhesive and an elastic filler. The retention of the ceramic insulator 4 is facilitated by a steel support plate 7 arranged in the form of a locking cone, which provides a favorable stress distribution in the insulator 4 when an external force action occurs on the anode structure.

In the embodiment according to Fig. 3, the fixing of the ceramic insulating plate 3 'to the structure to be protected is carried out with a suitable high-shear adhesive, for example epoxy resin. In addition, countersunk mounting screws 20 11,12 passing through the ceramic material can be used for fastening, either below or on the sides of the anode plate.

The use of fastening screws is of course also possible in the applications of Figure 1 or 2. The mounting screws are preferably ceramic or otherwise insulated. The advantage of the application according to Fig. 3 is a structurally simpler insulating plate, in addition, the dimensions of the opening A 'to be machined in the structure 1 to be protected can be chosen considerably smaller than in the solution according to Fig. 1.

Figure 4 shows a flush-mounted embodiment in which the anode plate 2 and the ceramic insulating material 5 are placed in a metal housing 9 to be welded to the body. At the bottom of the housing there is an opening A '' for power supply systems and a protective space 10 securing the opening. A suitable sealant, e.g. epoxy resin, is arranged between the ceramic insulator 5 and the steel housing 9. The advantage of such a solution for known flush-mounted anodes is mainly that a heat-resistant ceramic material with a coefficient of thermal expansion such as steel, e.g. ZrO 2, allows the elements 2,5,9,10 to be prefabricated so that welding can be performed very close to the anode structure, whereby the dimensions of the opening to be machined into the structure to be naturally protected can be correspondingly reduced. In order to reduce internal stresses during welding, the anode structure and its surroundings should preferably be preheated before welding. Thanks to the high-strength insulating material 5, the anode plate remains better in place in this solution during use than in the prior art.

Fig. 5 shows a variant of the anode according to Fig. 1, which is particularly well suited for underwater installation by a diver. The protrusion 8 'is conical in that a wedge-shaped spacer 16 is mounted between the intermediate protrusion and the protective space 10. The anode is locked in place from the interior of the ship by means of a crimp connection provided by a screw or screws 17.

.20 Prior art solutions, e.g. platinum-plated titanium, can be used as the material of the anode plate 2 within the scope of the invention. Alternatively, a suitable electrically conductive ceramic may be used, for example magnesium-aluminum ferrite, lithium ferrite or metal oxides such as Fe 2, NiO and CO 3 O 4. In this way, the frequent anode corrosion associated with metallic anodes is avoided. In addition, when both the insulating material and the anode are ceramic, a very compact, strong and uniform anode structure is achieved. The ceramic anode plate 30 can be made separately for subsequent placement in the insulating material. In a preferred solution, the anode and insulator are made into an integrated structure by sintering the materials together or performing slurry casting in the same mold immediately after succession. The anode plate can also be obtained by plasma spraying the anode material directly onto the surface of the insulating material, or by making an anode of a metal body.

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Claims (11)

1. A method for cathodic protection of offshore operating structures, such as ship hulls or off-shore structures, by using an external current source (15) which provides direct current through an anode (2) attached to the construction bay (1) to be protected , characterized in that the current-carrying anode clamp (2), rail or the like is secured to the construction liner (1) which is to be protected by a fastening unit, for example a supporting slab (3,3 '), a support (4) , a socket (5) or the like, which fixing unit consists of a ceramic material having a high compressive strength and also insulating electrical current, such that said ceramic material insulates the current producing anode (2) from the construction liner (1) to be protected and all together, the anode encapsulates at least the anode sealing edges for protecting it, for example when impacted by ice blocks, so that the attachment unit mainly receives the outer force impact in form a v compressive voltage. 20 2. A method according to claim 1, characterized in that the anode clamp (2) or the like is attached to the construction structure which is to be protected by means of so-called. surface attachment, such that the ceramic attachment unit (3,3 ', 4) is substantially outside the surface profile of the structural liner (1) to be protected. 3. A method according to claim 1 or 2, characterized in that the ceramic fastening unit (4) is arranged to surround the periphery of the current generating anode clamp (2) or correspondingly so that between the anode clamp and the structure to be protected a gap ( 6) which is preferably filled with an elastic and / or adhesive insulating material, for example plastic, epoxy resin or II ice 77269 correspondingly, and that in the intermediate runner (6), there is also provided a reinforcing reading cone (7) of steel, if necessary. Method according to claim 1 or 2, characterized in that the ceramic fastening unit (3 ') extends in its entirety also below the anode plate or the like, the area between it and the structural layer to be protected (1). 5. A method according to any of the preceding claims, characterized in that one or more projections (8) are arranged in the bottom of the ceramic insulating plate (3) which acts as a fastening unit, which in the attachment of the anode structure is arranged in a corresponding depression or heel 1 of the structural liner (1) to be protected, preferably such that an external force impact relative to the anode structure is directed as effectively as possible to the region of said burst 1 in the form of compressive stress 1 in the ceramic material. 6. A method according to claim 5, characterized in that the bottom of the insulating disc (3 '') is provided with a preferably conical projection (8 ') such that the anode structure is read in place from the construction hole to be protected by means of a screw connection by means of a screw ( 17) and a wedge-shaped intermediate piece (16) or by means of the corresponding cow-reading method. 25 Method according to any of the above claims, characterized in that the ceramic fastening unit is attached to the construction layer (1) which is to be protected with a glue of high shear strength, eg epoxy resin, and / or with 1 surface submerged ceramic or insulated screws (11). . 30 8. A method according to claim 1, characterized in that the anode is arranged in the construction bay (1) to be protected with so-called. recessed mounting says that the anode structure is pre-fabricated as an element which likes to weld on the structural liner ie 77269 to be protected, which element consists of a fixing unit consisting of a ceramic insulating material (5), in the surface of which a current-driven anode clamp (2) or equivalent is submerged. 5 9. A method according to any of the preceding claims, characterized in that as the current-generating anode, a conductive ceramic material, for example LiFe, F & 2 3 3 NiO or CO 3 O 4, is preferably used, so that the anode and the fixing unit consisting of insulation material are formed. a uniform, compact construction either by sintering, diffusion welding or plasma spraying of the conductive ceramic to the ceramic insulating material. 10. Anode construction intended for applying some of the above methods for cathodic protection of offshore operating structures, such as ship's hulls or off-shore structures, using an external power source which provides direct current through an anode (2) secured to the structure (1) to be protected, characterized in that the current-producing anode plate (2), rail; or the like is arranged to be attached to the structure (1) which is to be protected by a fastening unit (3,3 '). , 4,5) consisting of a ceramic electrical current insulating material, and that the anode plate (2) or equivalent is arranged to be immersed in the fastening unit (3,3 ', 4,5), which is provided with such lateral and vertical extension that the side surfaces of the anode plate (2), the rail or the like are encapsulated in the insulating material. 11. Anode structure according to claim 10, characterized in that the side edges of the fastening unit (3,3 ', 4,5) which consists of ceramic insulating material have an oblique or conical profile.