US3152059A

US3152059A - Sacrificial zinc anode

Info

Publication number: US3152059A
Application number: US32028A
Authority: US
Inventors: Wellington John Richard
Original assignee: Teck Metals Ltd
Current assignee: Teck Metals Ltd
Priority date: 1960-05-24
Filing date: 1960-05-26
Publication date: 1964-10-06
Anticipated expiration: 1981-10-06

Description

O 1964 J. R. WELLINGTON 3,

SACRIFICIAL ZINC ANODE Filed May 26, 1960 lnven tor JOHN R. WELLINGTON Attorney United States Patent 3,152,059 SACRIFICIAL ZINC ANGDE John Richard Wellington, Trail, British Columbia, Cauada, assignor to The Consolidated Mining and Smelling Company of Canada Limited, Montreal, Quebec, Canada, a corporation of Canada Filed May 26, 196i), Ser. No. 32,028 4 Claims. (Cl. 294-197) This invention relates to a method of making sacrificial or consumable zinc anodes for use in inhibiting corrosion by galvanic action and sacrificial Zinc anodes produced thereby.

The use of sacrificial or consumable zinc anodes is well known for the protection from corrosion of metal structures and equipment such as heat exchangers, pipe lines, ships hulls, storage tanks and the like. In some in stances, sacrificial or consumable anodes are made in the form of cast blocks or slabs with steel inserts to provide electrical contact between the anode and the metal structure to be protected and, also, to provide means for fastening the anode to the structure.

In other instances, long, narrow anodes are required or preferred because of their particular characteristics such as high current output per unit of volume, ease of installation and more streamlined design. Such long, narrow anodes are also made by casting methods in which molten zinc is poured into moulds around a metal core, or insert. The core extends the length of the anode and provides the necessary electrical contact and, also, the means for attaching the anode to the structure.

Casting methods are not completely satisfactory for the production of long, narrow zinc anodes. The cast anodes are not flexible and, therefore, they are not easily adapted to types of service in which curved anodes are desired. Furthermore, it is difiicult to cast long anodes of small cross-section with the necessary core, particularly with an exposed core. Long anodes with exposed cores have advantages in cases where the anode is to be fastened in position by bolting to the structure to be protected.

It is known that zinc can be formed in long, narrow shapes by extrusion, but a major diificulty arises in the formation by extrusion of zinc anodes with the insert or core required for electrical contact and support. The core must be efiectively bonded to the Zinc along the length of the anode to ensure that a positive and perma nent electrical contact between the Zinc and the core material is maintained throughout the life of the anode. If the bond is imperfect, corrosive solution, for example, sea water, can penetrate along the junction of the core and the zinc metal when the anode is in service and the resultant corrosion products will insulate the core from the zinc and thus render the anode inefiective for its intended purpose. Also, the core member must extend the full length of the anode as otherwise the anode would break as it became corroded in service, thereby severing the electrical connection between the anode and the protected structure and the anode would cease to function.

The difficulties encountered in the manufacture of sacrificial anodes of elongated shape can be overcome by extruding a composite billet comprised of a cylindrical block of zinc with a central positioned aluminum core. Extrusions made in this manner prior to this invention have consisted of zinc anode stock with a central alumi- "Ice num core, and the cross-section of both anode and core usually has been cylindrical.

Cylindrical zinc anodes with a central aluminum core have a number of important applications, for example, they can be conveniently suspended in tanks and ships holds and are readily adapted to pipe fittings. However, they cannot be easily bolted to structures with plane surfaces, such as steel plate. A flat, ribbon-shaped anode with an exposed core would be preferable for such service, and also for use with moving structures, such as ships hulls.

I have found that flat, ribbon-shaped anodes with exposed cores, which otfer a number of important advantages in service, can be made from a composite billet comprised of a block of zinc with a centrally positioned aluminum core by extruding the billet through a two-hole die, the centre of the billet corresponding with the centre of the die, and the two holes of the die being similar in size and shape and being symmetrically positioned at right angles to a diameter of the die at equal distances from its centre.

The product obtained from the composite billet in the extrusion step of this invention is two similar strips, or ribbons, of zinc anode stock, each with an exposed surface of aluminum core along one longitudinal face, the aluminum core being securely bonded to the Zinc as an integral part of the anode stock. The anode stock can be cut into suitable lengths to form sacrificial zinc anodes, each with an exposed aluminum core.

An understanding of the method of this invention and the product formed thereby can be obtained from the following description, reference being made to the accompanying drawing in which:

FIGURE 1 is an end view of a composite billet composed of zinc with an aluminum core of a shape suitable for extrusion by this method to form a zinc strip with an exposed aluminum core;

FIGURE 2 is a schematic view, in perspective, of the composite billet in the course of being extruded;

FIGURE 3 is a section taken along the line 3-3 of FIGURE 2;

FIGURE 4 is a front end view of the die showing the arrangement of the die openings;

FIGURE 5 is a perspective view of a finished sacrificial anode; 'ld

FIGURE 6 is a section which illustrates a modification of the anode and core assembly.

Like reference characters refer to like parts throughout the description and drawings.

The invention is described with particular reference to the billet, core, die openings and anode illustrated in FIGURES l to 5, but it will be understood that suitable modifications in size and shape of these parts can be made if desired.

The composite billet illustrated in FIGURE 1 is comprised of a cylindrical block of zinc It with a centrally positioned aluminum insert or core 11 which extends the length thereof. The aluminum core is in the form of a body having generally similarly arcuately curved bottom and top portions 12-13 separated by diametrically opposed grooves lei-14a defined by converging sides 15-1541 and l6l6a. The converging sides terminate in inner walls 1747a, which can be rounded, as shown, or flat, or the converging sides can be extended to their line of intersection.

The billet preferably is formed by casting zinc around the aluminum core or insert. Before the zinc is cast around it, the insert is degreased, if necessary, and coated with a flux for aluminum surfaces. Suitable fluxes include those based on zinc chloride and which contain fluorides, such as those used in normal aluminum fluxing operations.

The flux coated aluminum core can be heated, for example, in an oven at a temperature of from 1000 F. to 1050 F., to melt the flux and thus form a continuous film of flux on the surfaces of the core.

It is preferred to apply a thin prime coating of molten zinc to the aluminum core before casting the main body of the zinc around the core, thus to ensure a positive and permanent bond between the main body of the zinc and the core. This prime coating of zinc can be applied by dipping the flux coated core into a bath of molten zinc at a temperature of, for example, from 900 F. to 950 F. This dipping step ensures the formation of a continuous coating of zinc on the aluminum core with a zinc-aluminum alloy at the interface between the aluminum and zinc.

In the casting step, the aluminum core, preferably freshly dipped and hot from the preceding step, is positioned vertically and centrally in a billet mould which is then filled with molten zinc, preferably at a temperature within the range of from 1000 F. to 1050 F.

The cast billet is placed in the chamber of a conventional extrusion press indicated by the numeral 20 wherein pressure is applied to its rearward end, such as by a hydraulically actuated ram 21. The die of the extrusion press is characterized by the arrangement of the two openings 22-23, which are separated by a bridge 25. These openings 22-23 are generally rectangular in shape, are equidistant from the centre of the die, and are symmetrically positioned at right angles to a diameter of the die, the openings being bisected by this diameter. The billet is positioned in the container of the extrusion press with its centre coinciding with the centre of the die and with the grooves 14-14:: in alignment with the bridge 25. That is, the centre plane of the grooves 14-14:: is at right angles to the diameter bisecting the die openings.

When pressure is applied by the ram against the base of the billet, it is extruded through the die openings 22-23 in the form of two strips with embedded alum num cores 11 exposed on the faces opposing each other, see FIGS. 2 and 5.

The extrusion operation slices the core along its narrow mid-section, resulting in a relatively narrow exposed core surface along the centre line of, and substantially flush with the inner face of each zinc strip, and a wide, flat, inner core surface embedded within the zinc strip parallel to the outer face of the strip. The core is thereby effectively keyed into the zinc strip and provides for convenient attachment of the anode to steel work, as described hereinafter.

lllustrathig the operation of the extrusion step, a composite billet, having a diameter of 4% inches and a length of 15 inches and weighing about 60 pounds, was placed in the extrusion press at a temperature of 330 F. An extrusion pressure of 520 tons was applied against the base of the billet with a maximum pressure of 640 tons. The resulting strips were 15 feet long, 1% inches wide and had a thickness of 7 inch. The temperature at which the billet is extruded is optional, having regard to several factors. At temperatures above 500 F., the resulting strip has brittle characteristics which may be undesirable. However, the required extrusion pressure is reduced as the initial temperature of the billet is increased from atmospheric to 500 F. It is found that lower temperatures, below 300 F., produce a better finish on the surfaces of the strip but higher extrusion pressures are required.

4 The dimensions for the core illustrated in FIGURE 1 are:

Radius of curvature of convex protrusion at base of groove 0.135

1 Same as billet.

The dimensions of the core in each extruded zinc strip, as illustrated in FIGURE 3, are:

Inch Width of exposed face 7 Width of embedded face 1 Distance between faces Usually the first few inches and the last foot of each extruded strip is cut ofi and discarded as the core is not consistently uniform in dimensions and is not always completely bonded in these regions. Throughout the remaining length of the anode stock, the core is uniform in size and shape and is permanently bonded to the zinc.

The aluminum used in the core was commercial grade of about 99.9% purity and the Zinc used was special high grade (99.99%) alloyed with about 0.4% aluminum. However, other suitable compositions can be used. For example, high purity zinc which contains less than 0.0015% iron is generally satisfactory and other zinc alloys are also known in the art. Similarly, other grades of aluminum can be used.

The desired shape and size of the anode core determine the shape and size of the aluminum insert in the composite billet. If modifications in the shape and size of the anode core are required, suitable changes can be made in the design for the aluminum insert for the composite billet.

The size and shape of the die openings are selected to provide the required cross-sectional size and shape of the anode. The distance between die openings should be sufiicient that the bridge 25 is strong enough to withstand the stresses to which it is subjected during the extrusion operation. In FIGURE 4, the width of the bridge 25 is the same as the width of each die opening but the bridge width can be varied Without appreciably affecting the desired shape of the core in the extruded strip. For example, the width of the bridge can be from one-half to twice the width of each die opening without significant change in the shape of the core in the anode stock.

The extruded anode stock is cut into convenient lengths for use as anodes. The usual length is about 6 feet, but lengths of from a few inches up to 12 feet and more can be used.

FIGURE 5 illustrates an anode prepared for attachment to a structure to be protected. Passageways 30 have been drilled completely through the anode, and countersunk holes 31 around the passageways 30 have been drilled from the outer face of the anode to the embedded surface of the aluminum core. The anode is placed in position with the inner face and its narrow exposed surface of aluminum against the surface of the structure to be protected. Bolts previously fastened to tlns structure project through the passageways 30, and washers and nuts are threaded on these bolts and tightened firmly against the embedded aluminum surface at the base of the countersunk holes 31. The anode is thus securely fastened and electrically connected to the structure. Passageways 30 can be drilled at any desired intervals along the length of the anode according to the degree of security of attachment required.

The anode illustrated in FIGURES 3 and 5 has a core which is keyed into the zinc metal. FIGURE 6 shows a cross-section of an anode in which the core is shaped differently although the anode is made in the same way. To make the anode of FIGURES 3 and 5, the composite billet is carefully aligned in the container of the extrusion press to ensure that the core in the anode stock will have the desired shape. The anode of FIGURE 6 is made from a composite billet with a cylindrical aluminum insert, so there is no need for careful alignment of the billet. The billet is extruded through a two-hole die, as described above, but the die openings are shaped to provide the channel type cross-section indicated. A billet 4% inches in diameter with a cylindrical core 1% inches in diameter produces anodes of the cross-sectional dimensions, 1% inches by inch, noted above. The core of the anode provides the desired exposed outer face and the wide, flat inner face for convenience in mounting, and the anode is attached to the structure to be protected in the same manner as described for the anode of FIG- URES 3 and 5.

The core of the anode of FIGURE 6 does not key into the Zinc, but if care is taken when preparing the composite billet, the resulting zinc-aluminum alloy interface ensures permanent bonding between core and anode metal, and there is no likelihood of zinc falling away from the core during service.

In both designs, the electrical connection between anode and structure to be protected is provided by direct contact between the exposed core surface of the anode and the structure, and/ or between the embedded core surface and bolts attached to the structure.

The anode of the present invention is suitable for use in protecting iron, steel and aluminum structures, and is especially suited for attachment to more or less plane surfaces.

The sacrificial Zinc anode of this invention and the process of making it possess several important advantages. Extrusion is relatively simple compared with casting as a method of making long, narrow shapes with exposed cores.

The extruded strip can be cut into any desired length, and the anodes can be readily fastened to the structure to be protected. As the means for attaching the anodes is independent of the zinc metal, the fastening remains secure throughout the life of the anodes. As the zinc is effectively bonded to the aluminum core, the anode life continues until all or substantially all the zinc has been consumed. The fiat ribbon shape of the anode provides flexibility so the anode easily can be bent or curved to conform with the protected surface. The high de gree of security of attachment is particularly helpful in cases where the anode cannot be readily examined in service. The fiat cross-section of the anode reduces resistance to liquid flow, and this feature of low resistance combined with security of attachment makes this anode ideal for the protection of steel hulls from sea-water corrosion.

The foregoing description has referred to the manufacture of the anode of the present invention by means of a two-hole die. Three or more openings could be used,

arranged symmetrically around the centre of the die. However, although a multiple-opening die with more than two openings can be used, the two-hole die is simpler to construct and operate and is preferred for the production of consistently uniform anode stock.

The described anode is generally rectangular in crosssection. Other cross-sectional shapes can be used. For example, the outer face of the anode can be curved, and the anodes can be produced with the exposed surface of the aluminum projecting beyond the inner sur face of the zinc by cutting appropriate lands in the die openings. However, according to the present invention, the core of the anode must have an exposed surface for contact with the structure to be protected, and a relatively Wide, flat embedded surface substantially parallel to the exposed surface to permit bolting of the anode to the structure independently of the zinc metal.

It will be understood that modifications can be made in the preferred embodiment of the invention described herein without departing from the scope of the invention as defined by the appended claims.

What I claim as new and desire to protect by Letters Patent of the United States is:

l. A sacrificial zinc anode which comprises a composite extrusion product of zinc bonded to an aluminum core embedded therein, said aluminum core being exposed throughout the length of said anode along one face thereof.

2. A sacrificial zinc anode which comprises a composite extrusion product of zinc bonded to an aluminum core embedded therein, said aluminum core being exposed throughout the length of said anode anode along one face thereof, said extrusion having a zinc-aluminum alloy at the interface between the aluminum and the zinc.

3. A sacrificial zinc anode which comprises a composite extrusion product of zinc bonded to an aluminum core embedded therein, a surface of said core being exposed throughout the length of said anode along one face thereof, said exposed core surface being substantially flush with said one face of said anode, said exposed flush core surface being narrower than the core surface embedded in said zinc.

4. A sacrificial zinc anode which comprises a composite extrusion product of zinc bonded to an aluminum core embedded therein, said core having a surface exposed throughout the length of said anode along one face thereof, and a wide, flat surface embedded in said zinc substantially parallel to said exposed core surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,478,479 Grebe et a1. Aug. 9, 1949 2,571,062 Robinson et a1. Oct. 9, 1951 2,619,455 Harris et a1. Nov. 25, 1952 2,763,907 Douglas Sept. 25, 1956 2,798,604 Todd July 9, 1957 2,841,546 Robinson July 1, 1958 2,947,680 Preiser Aug. 2, 1960 2,986,273 Bardgett May 30, 1961

Claims

1. A SACRIFICIAL ZINC ANODE WHICH COMPRISES A COMPOSITE EXTRUSION PRODUCT OF ZINC BONDED TO AN ALUMINUM CORE EMBEDDED THEREIN, SAID ALUMINUM CORE BEING EXPOSED THROUGHOUT THE LENGTH OF SAID ANODE ALONG ONE FACE THEREOF.