US2779729A

US2779729A - Cathodic protection assembly

Info

Publication number: US2779729A
Application number: US171710A
Authority: US
Inventors: Roy C Jorgensen
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1950-07-01
Filing date: 1950-07-01
Publication date: 1957-01-29
Anticipated expiration: 1974-01-29

Description

M8 Wm a 7 R 5 v--- 4 1 Z V e 2 U M W W J m m w 6 W JUN g 5 Mw/ M a a. a 2 w B 2 a 1 5 1 2 25 J I B J 9 Z 7 z Q 0L -.I) Mm l w w 9 p o Z W I I w a a L 5 9 0 2 1 1 2 v a W 1 Jan. 29, 1957 United States Patent C F CATHODIC PROTECTION ASSEMBLY Roy C. Jorgensen, Lake Jackson, Tex., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application July 1, 1950, Serial No. 171,710

Claims. (Cl. 204-197) This invention relates to cathodic protection, and, more particularly, to an improved cathodic protection assembly whose structure includes a combined compression and electrical resistance member.

During recent years cathodic protection of corrodible metal structures has been used as a way of substantially decreasing losses due to the corrosive action of aqueous solutions. When cathodic protection is employed it is desirable to obviate anode waste by avoiding unnecessarily high current densities. However, when employing galvanic anodes as a current source, control of current density by varying either the potential developed between the anode and the metal to be protected, or the electrical conductivity of the aqueous solution to which the assembly is subjected, is not ordinarily feasible. The potential developed is determined principally by the metals of which the anode and the device to be protected are constructed, and is usually approximately fixed for a given application. The electrical conductivity of the aqueous solution is that of the available supply: sea water, for example, is the only cooling water available for certain condensers; its extremely corrosive nature makes galvanic protection highly important in structures subjected to its action, but its extremely high electrical conductivity results in a high current flow and consequent anode waste.

In order to decrease the cost of cathodic protection in highly conductive waters, the current flow has ordinarily been limited by insulating the anode from the metal to be protected, and interposing a resistor as the sole electrical connection therebetween. Such a resistor has been provided as merely a loose resistance wire with one end attached to the anode, and the other to the structure to be protected, or as a relatively expensive device that provides a protected resistance for use with special types of anodes. Satisfactory electrical results are achieved using a loose wire, but the latter is undesirable because considerable care is required in attaching it, and it is extremely vulnerable in the course of ordinary maintenance. Special devices are available for such use with certain types of anodes, but, so far as is known, none has heretofore been provided for use with anodes designed to be rigidly bolted to a structure, e. g. fiat-faced anodes of hemi-cylindrical cross-section. These anodes are particularly advantageous for such applications as the protection of condenser separator plates.

The principal object of the present invention is to provide a combined compression and electrical resistance member in the nature of a washer that is readily instal ed, comparatively inexpensive and quite rugged, and serves to limit the current density of a cathodically protected system to a value within the optimum range.

Another object of the invention is to provide a cathodically protected system comprising, as a means for limiting the current density thereof, a combined compression and electrical resistance member.

More specific objects and advantages are apparent from 2,779,729 Patented Jan. 29, 1857 the description and drawings which illustrate and disclose, but are not to be construed as limiting, the invention.

The invention will be more clearly understood by reference to the accompanying drawings, in which:

Fig. 1 is a fragmentary vertical partial sectional View of a hemi-cylindrical magnesium anode bolted in place on a condenser separator plate, showing one form of the resistance washer of the invention in section;

Fig. 2 is a partial sectional view along the line 22 of Fig. 1, showing the resistance washer in elevation;

Fig. 3 is a detailed sectional view of the washer of Figs. 1 and 2;

Fig. 4 is an elevation of the body portion of the washer of the preceding figures; and

Fig. 5 is a detailed sectional view of an alternative washer according to the invention in which the body portion is composed of a conducting resin.

Figs. 1 and 2 show a magnesium anode 11 bolted to a steel condenser separator plate 12 to protect it against corrosion by sea water, not shown. The anode may preferably be of rectangular or hemi-cylindrical cross-section, the latter being illustrated. The anode rests on but is electrically insulated from the separator plate by a sheet rubber gasket 13 greater in area than the base of the anode. Water seepage between the anode and the plate is prevented by an insulating sealer coating, not shown. The anode is formed with bolt holes 14 and 14' extending centrally through it near the ends, perpendicular to the fiat face of the anode. Each hole is provided with a

counterbore

15 or 15 at its end remote from the fiat face. lvionel stud bolts 16 and 16' threaded into the separator plate project outwardly to enter the holes 14 and 14' in the anode to support the latter. A rubber sleeve 17 or 17' surrounds each

bolt

16 or 16 for its entire portion below the counterbore to insulate the bolt from the anode.

The anode 11 is held firmly against the plate 12 by means of Monel hex nuts 18 and 18' threaded on the outer ends of the

bolts

16 and 16 within the

counterbores

15 and 15, each nut being drawn up against a steel washer 19 or 19'. As shown in Fig. 1, the bolt 16 is fully insulated from the anode by an insulating washer 20 of Micarta or other non-conducting material fitting over the bolt 16' between the steel washer 19 and the end of the counterbore 15 and seating firmly against the shoulder at the inner end of the latter.

The other bolt 16 is relied upon to establish the sole electrical connection between the anode and the separator plate. In this case, the steel washer 19 against which the nut 18 is drawn seats on a combined compression and electrical resistance member, or resistor washer, 21, more fully described hereinafter, which surrounds the bolt 16 and in turn seats on the inner end of the counterbore 15. In operation, the protective electrical current draining from the separator plate 12 flows through the bolt 16, nut 18, washer 19 and resistor washer 21 to the anode 11.

In installing the anode 11, the

studs

16 and 16 are positioned in the condenser separator plate so that the anode is in contact with the corrosive medium during normal operation of the condenser. After the anode is installed, the nuts 13 and 18' are tightened to hold the anode firmly in place. The surface inside the holes 15 and 15' and all exposed portions of the bolts and washers are then coated with a glyptal resin, and all voids filled with tar, to prevent water seepage.

In an optional construction, not illustrated, the anode may be strengthened by forming it with tubular sleeves of metal stronger than that of the anode cast integrally in position to line the holes 14 and 14'.

The details of the resistor washer 21 of Figs. 1 and 2 are shown particularly in Figs. 3 and 4. This member comprises a pair of spaced metal washer 22, an electrical resistance wire 23 interconnecting the washers, and a body 24 of electrically insulating material separating the washers, in firm contact with each, having an opening therethrough which registers with the opening of the washers, and completely embedding the resistance. The body may be considered as consisting of a tubular central portion 26 having an annular enlarged shoulder or disc-like collar 27 central thereon. This collar is of the same outside diameter as the end washers 22 and from each of its plane surfaces two bosses 28 protrude. The metal washers 22 fit snugly over the tube and seat on opposite sides of the collar flush with the end of the tube. than the axial length of the tubular portion. The body may be formed of any insulating material which will withstand compression, e. g., polystyrene, polymethylmethacrylate, hard rubber, or Micarta, and is easily made by compression molding a powdered thermoplastic or thermoset resin. Each metal washer is provided with a pair of holes 25 which slip over the bosses 2'8 and pre vent movement between the metal washers and the body when the finished resistor washer is subjected to a twisting stress, e. g., during the tightening of the nut 18.

The details of an alternative resistor washer 21 according to the invention in which the body portion is composed of a conducting resin are shown in Fig. 5. This member comprises, in addition to the pair of spaced metal washers 22, a body portion 29 of electrical resistance material separating the washers, but in firm contact with each. The material forming the body 29 consists of a matrix of electrical insulating material, such as a synthetic resin, in which i dispersed fine graphite or other con-ducting material in sufiicient concentration to render the body electrically conducting but of substantial resistivity.

A composite member of the type shown in Fig. 3 can be produced in any of several ways. For example, the body portion can be molded in the desired shape around the resistance wire as an insert, with one end of the wire protruding from each face of the annular collar. The washers, provided with additional holes to receive the protruding wire ends are then slipped in place with the wires extending through the additional holes, and solder is applied to complete the contacts. Alternatively, such a member can be produced by attaching one end of the resistance wire to the inner face of each metal washer, placing the resulting assembly in a mold with suitable means for introducing the desired plastic and molding the member. When such a method is employed, the matrix flows through the holes 25 in the washers and spreads at the outer surfaces of the washers to provide a flange that binds the composite member firmly together. Compression molding is preferred for this purpose.

A composite member of the type shown in Fig. can be produced by methods similar to those described in the preceding paragraph, except that the resistance wire is not required, and a conducting resin is used. The resistance of the member depends upon the amount and character of the conducting filler used in the resin, the matrix employed, and the dimensions of the member.

For the protection of steel structures in seawater, using a magnesium anode, it is usually preferred that the slicetive resistance of the composite member be from A to 5 ohms. The exact value depends upon operating conditions, and can, in extreme cases, be out-side the indicated range.

The invention has been described with particular reference to cathodic protection using as the consumable anode a body of magnesium or magnesium-base alloy, all such metals being contemplated by the term magnesium metal in the claims. However, the principle of the invention is equally applicable to the protection in aqueous media of corrod-ible metals, both ferrous and non-ferrous, using as the anode a metal no less anodic The diameter of the collar is preferably greater than zinc and more anodic than the structure. Most commonly, the metals protected are not substantially more anodic than iron.

I claim:

1. A combined compression and electrical resistance member comprising a body of electrically insulating material consisting of a tubular central portion having an annular collar central thereon and at least one boss protruding from each plane surface of the collar, a pair of metal washers fitting over the tube, and seating on opposite sides of the collar with their outer faces flush with the ends of the tube, each washer being provided with a hole encircling each boss and electrical resistor means embedded in the collar and electrically interconnecting the washers.

2. A combined compression and electrical resistance member as claimed in claim 1 in which the diameter of the annular collar is greater than the axial length of the tubular central portion.

3. In combination with a ferrous metal structure immersed in an aqueous medium, cathodic protection means comprising a body of magnesium metal having a boltiole extending centrally t-herethrough provided with a counterbore at its end remote from the structure, a bolt secured to the structure and projecting into the bolt-hole in the anode, the bolt being so placed on the structure that th anode is in position to contact the aqueous medium, electrical insulation separating the anode from the structure and from the bolt, a compression washer as defined in claim 1 encircling the bolt within the counterbore, and a nut threaded on the bolt within the counterbore compressing the washer against the seat of the counterbore to provide the sole electrical connection between the anode and the structure.

4. In combination with a corrodible metallic structure immersed in an aqueous medium, cathodic protection means comprising a body of consumable metal at least as anodic as zinc and more anodic than the structure having a bolt-hole extending centrally therethrough provided with an enlarged counterbore at its end remote from the structure, a bolt secured to the structure and projecting into the bolt-hole in the anode, the bolt being so placed on the structure that the anode is in position to contact the aqueous medium, electrical insulation separating the anode from the structure and from the belt, a compression and electrical resistance washer encircling the bolt and wholly within the counterbore, and a nut threaded on the bolt within the counterbore compressing the washer against the seat of the counterbore to provide the sole electrical connection between the anode and the structure.

5. In combination with a ferrous metal plate structure immersed in an aqueous medium, cathodic protection means comprising a body of consumable metal at least as anodic as zinc having a flat face and a bolt-hole extending from the face through the body and provided with an enlarged counterbore at its end remote from the face, a bolt secured to the structure and projecting into the bolt-hole in the anode to a depth beyond the seat of the counterbore but not beyond the outer end thereof, the bolt being so placed on the structure that the anode is in position to contact the aqueous medium, a sheet of electrical insulating material between the structure and the face of the anode and of greater area than the latter, electrical insulation separating the anode from the bolt, a compression and electrical resistance washer encircling the bolt and wholly within'the counterbore, and a nut threaded on the bolt within the counterbore compressing the washer against the seat of the counterbore to force the face of the anode firmly against the structure with the insulating sheet therebetween, the said washer being the sole electrical connection between the anode and the structure,

(References on following page) 5 References Cited in the fue of this patent 2,375,178 UNITED STATES PATENTS 2,486,871 757,687 Storey Apr. 19, 1904 841,232 Dana et a1 Jan. 15, 1907 5 621,746 901,809 Harris et a1 Oct. 20, 1908 749,636

6 Ruben May 1, 1945 Osterheld Nov. 1, 1949 FOREIGN PATENTS Germany Oct. 24, 1935 France May 8, 1933