US2756204A - Devices for cathodic protection of metallic structures - Google Patents

Description

July 24, 1956 w. F. HIGGINS 2,756,204

DEVICES FOR CATHODIC PROTECTION OF METALLIC STRUCTURES Filed Dec. 21. 1953 1 F 1 G. 4. BM

W/L LIAM FREDERICK H/eoms United States Patent DEVICES FOR CATHODIC PROTECTION OF METALLIC STRUCTURES William F. Higgins, Clifton Junction, near Manchester, England, assignor to F. A. Hughes & Co. Limited, London, England Application December 21, 1953, Serial No. 399,362

Claims priority, application Great Britain January 27, 1953 4 Claims. (Cl. 204-197) This invention relates to devices for the cathodic protection of ferrous metallic structures, e. g. piers and pipe lines, against corrosion. Such devices generally consist of an anode of a suitable metal such as magnesium or magnesium alloy connected electrically to the structure, the anode and structure being in contact with a common electrolytic medium e. g. seawater. When this is done the surface ofthe anode corrodes and the supply of electrons generated thereby flows into the structure to be protected and prevents or reduces the tendency of metal particles of that structure to pass into solution as positively charged ions, which process is commonly known as cor- IOSlGll. difiicult to arrange that the corrosion of the sacrificial surface shall proceed uniformly or in a regular manner; this is of less consequence where a massive anode of large volume to surface ratio is under consideration, for in such a case a slightly increased rate of consumption at one region or another will not usually seriously interfere with the general progress of the reaction, but in the case of a very thin anode of the type which may be used to provide rapid initial protection and is intended to be consumed quickly, uncontrolled corrosion may cause a large area of the anode to fall out of circuit and be lost. The electric current generated by an anode is reduced if a second anode is brought into operation close to the first, and is reduced still further as successive anodes are brought into operation in close proximity. This interference elfect is generally to be avoided as far as possible in that it limits the output of anodes if they are placed too close together. We, however, utilise this effect to control the manner of corrosion of booster anodes.

In accordance with the invention we provide an anodic device comprising a plurality of elongated members of metal suitable for sacrificial anodes said members having a length of at least six inches and greater than six times their width, said members being connected closely together at one part of their length and extending away from the position of connection in directions diverging from each other or being deformable into such diverging directions whereby a strong interference effect is produced when the device is connected to said structure in the electrolyte.

The invention also includes a protective system com- PllSlllg a ferrous metal structure immersed in an aqueous electrolyte and a plurality of elongated members immersed in said electrolyte and in connection with said structure, said members being made of a metal suitable for sacrificial anodes and having a length of at least six inches and greater than six times their width, and connected so close together as to cause a strong interference eifect between but diverging from each other progressively from the position of their connection together. At and adjacent the positions of attachment to each other distance apart of the members may be from nil up to a distance equal to ten per cent (or in special cases up to fifty per cent) of the distance from their position of attachment to their outer edge. Thus members in the form of strips may be In the practice of galvanic protection it is often attached together in bundles e. g. at least three in a bundle. The strips may then be bent outwards so that a fan efiect is achieved, the ends of the individual strips being made to branch out like the spokes of wheel while the central portions remain as a compact mass. Thus each pair of adjacent strips are curved so as to be at a progressively increasing distance from each other from the centre to their outer free ends on one or both sides of their position of connection together where the strips are in contact with each other. The strips or plates may have a thickness of 0.020" to 2".

The strips may have a length of 6 to 36 times (e. g. 6 to 16 times) the width, e. g. a length of 6 to 72 inches and a width of 0.5 to 12 inches. If wires are used in place of strips they may have a minimum width (thickness) of 0.05 inch.

When immersed in an electrolyte the effect is that the divergent ends interfere little with each other, current generation is high, and metal consumption rapid in conse quence. Further along the strips, towards the central mass, the interference effect becomes more and more pronounced so that less current is generated in this region and less corrosion occurs. Hence the output of the anode is controlled and the device is forced to corrode from the outside edges inwards, which is the effect desired.

It should be understood that the interference eliect is an elfect resulting from the generation of current and only occurs when the anode is attached to the cathode and current is flowing. it is not due merely to the reduced volume of electrolyte in the narrow angles between the strips. If magnesium alloy strips are immersed in a corrosive electrolyte, disposed as in the anode described but not connected to any cathode, then no interference effect is observed and the strips corrode as they would have done had they been immersed individually.

The invention will now be further described by way of example with reference to the accompanying diagrammatic drawings wherein:

Figure l is a perspective view of an anodic device made in accordance with the invention;

Figure 2 is a front view of the device when deformed ready for use;

Figure 3 is a side view of the device connected to a ferrous structure to be protected; and

Figure 4 is a view of a modified construction.

Referring to Figures 1 to 3, two bundles of rolled or extruded strips 10 of magnesium base alloy are surrounded at and adjacent their middle positions with a wrapping 11, 12 of zinc or aluminium foil or sheet. By zinc and aluminium we mean either the pure metals or alloys constituted to at least 85 per cent by these metals. The major length of each end of the bundle protrudes from the wrapping and the protruding ends are each at least six inches in length and at least six times their width. Two or more holes are drilled through the thickness of the strips and the zinc or aluminium wrapping, and the anodes, when installed, are attached to a supporting bar or wire rope 14, or any suitable connection to the cathode, by securing the two bundles together with the wire rope clamped between them by means of bolts 15, 16 through the aforesaid holes. The bar or the like 14 is screwed into or otherwise connected to the ferrous structure 18 (Figure 3) to be protected. A clamping nut 19 may be used to prevent inadvertent unscrewing of the bar.

The use of zinc or aluminium for wrapping is important. We find that either of these two metals may be used in contact with magnesium alloys, used as anodes,

- without seriously increasing the corrosion of the magnesium, by galvanic effects-due to the intimate contact, which usually occurs when this metal is coupled closely to a dissimilar metal. We find, further, that despite their large electro-chemical potential, neither zinc nor aluminium tends to corrode rapidly themselves when coupled to steel work, that is, they do not themselves behave as active anodes especially when coupled with magnesium-base alloys. As far as we are aware attention has not previously been directed to this phenomenon.

The invention enables the spacing of the strips, plates or discs of which the composite anode is made to be regulated at the time of assembly or installation to suit varying requirements of galvanic protection while using a few standard components. In particular, the blades or projecting strips of the bundles first described can be bent out into fan shape structures (as shown inFigure 2) of diiferent angle, during actual installation, to provide a varying degree of cathodic protection on the one hand, or of duration of efiect on theother hand. The smaller the angle which each strip subtends to the next, the greater the over-all interference eifect, the lower the current output but the longer the time the device will remain in operation before it is completely consumed. This feature is of considerable practical value in installing a cathodic protection system and an anode has not previously been available insuch a convenient form. Thinand rapidly consumed anodes are frequently used for achieving high current densities for shortperiods, so bringing about the initial polarisation found desirable when applying galvanic protection by means of more massive anodes to provide permanent freedom from corrosion, and the device herein described allows this purpose to be achieved with the greatest facility. By varying the spread of the fan shaped ends any required output can be achieved at will, whereas, if the bundles are used with the blades not bent out at all, maximum life is secured andlong term protection can be achieved with the same device.

In the modification shown in Figure 4 the anode membets are in the form of wires welded or otherwise fixed together at one end 21. The welding or other fixing may be disposed at some other position along the wires. Zinc may be wrapped around the weld to avoid attack on the weld and wires at and adjacent the weld. Any suitable bolt, clamp or the like can be used for attaching the device to the structure to be protected.

Iclaim: 1

l. A device for the protection of ferrous metal structures immersed in an aqueous electrolyte against corrosion 'comprising a plurality of elongated members of metal suitable for sacrificial anodes of the class consisting of magnesium and magnesium alloys, said members having a length of at least six inches and greater than six times their width, said members being connected closely together at one part of their length by a body of zinc parallel to each other and extending away from the 'pOSiw tion of connection parallel to each other, said members being deformable by bending into directions diverging from each other whereby a strong interference effect is produced when the device is connected to said structure in the electrolyte.

2. A device for the protection of ferrous metal structures immersed in an aqueous electrolyte against corrosion comprising a plurality of separate strips of wrought magnesium, connected together at their middle positions where they are parallel to each other with a wrapping of zinc metal and having a length on each side of the position of connection of at least six inches and greater than six times their width, said strips being deformable by bending into directions diverging from each other whereby a strong interference eltect is produced when the device is connected to said structure in the electrolyte.

3. A device for the protection of ferrous metal structures immersed in an aqueous electrolyte against corrosion comprising a plurality of separate strips of magnesium base alloy, connected together at their middle positions where they are parallel to each other and having a length on each side of the position of connection of at least six inches and greater than six times their width, said strips being deformable by bending into directions diverging from each other whereby a strong interference effect is produced when the device is connected to said structure in the electrolyte, a metal element for attachment to said structure and connected to said strips, and sheet wrapping made of a metal selected from the group consisting of zinc and aluminium, said wrapping being wrapped around said strips at and adjacent their position of connection to said element.

4. A device for the protection of ferrous metal structures immersed in an aqueous electrolyte against corrosion comprising a plurality of elongated members of metal suitable for sacrificial anodes of the class consisting of magnesium and magnesium base alloys, said members having a length of at least six inches and greater than six times their width, said members being connected closely together at one part of their length parallel to each other and extending away from the position of connection parallel to each other, said members being deformable by bending into directions diverging from each other whereby a strong interference efiect is produced when the device is connected to said structure in the electrolyte; said members being in the form of wires Welded together at their ends and a wrapping of zinc around the welded ends of the wires.

References Cited inthe file of this patent FOREIGN PATENTS 11,216 Great Britain 1906

Claims (1)

1. A DEVICE FOR THE PROTECTION OF FERROUS METAL STRUCTURES IMMERSED IN AN AQUEOUS ELECTROLYTE AGAINST CORROSION COMPRISING A PLURALITY OF ELONGATED MEMBERS OF METAL SUITABLE FOR SACRIFICIAL ANODES OF THE CLASS CONSISTING OF MAGNESIUM AND MAGNESIUM ALLOYS, SAID MEMBERS HAVING A LENGTH OF AT LEAST SIX INCHES AND GREATER THAN SIX TIMES THEIR WIDTH, SAID MEMBERS BEING CONNECTED CLOSELY TOGETHER AT ONE PART OF THEIR LENGTH BY A BODY OF ZINC PARALLEL TO EACH OTHER AND EXTENDING AWAY FROM THE POSITION OF CONNECTION PARALLEL TO EACH OTHER, SAID MEMBERS BEING DEFORMABLE BY BENDING INTO DIRECTIONS DIVERGING FROM EACH OTHER WHEREBY A STRONG INTERFERENCE EFFECT IS PRODUCED WHEN THE DEVICE IS CONNECTED TO SAID STRUCTURE IN THE ELECTROYLTE.

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