US3182007A - Electrode assembly for the anodic passivation of metals - Google Patents

Description

May 4, 1965 M. HUTCHISON ETAL ELECTRODE ASSEMBLY FOR THE ANODIC PASSIVATION 0F METALS Filed March 9, 1959 3 Sheets-Sheet 3 INVENTORS. Meme- Hum/4750M,

don/u D. Sumac/lav A 7 TOPA/EY United States Patent 3,132,007 ELECTRODE ASSELY FOR THE ANQDIC PASSIVATION 0F METALS Merle Hutchison, Olen L. Riggs, Jr., and John D. Sudbury, Ponca City, Okla, assignors to Continental Oil Company, Ponca City, Okla, a corporation of Delaware Filed Mar. 9, 1959, Ser. No. 797,985 3 Claims. (Cl. 204-147) This invention relates generally to an improved corrosion control system utilizing anodic passivation, and more particularly, but not by way of limitation, to an improved electrode assembly for such a system. This is a continuation-in-part of applicants co-pending application entitled Anodic Protection Against Corrosion, filed December 1, 1958, Serial No. 777,499, now abandoned.

As noted in the above-mentioned application, the principle of anodic passivation of metals has been known for several years, but has not been used commercially to any appreciable extent. In such a system precise control must be maintained over the anodic current passing from the vessel being protected to an electrode in a corrosive solution contained in the vessel, not only to assure that the proper amount of current is available for passivating the exposed surface of the vessel. We have found that if the cathode is exposed around substantially its entire surface and supported near a wall of the vessel, an accelerated corrosion will occur on a portion of the vessel near the cathode. The development of such hot spots in at least some vessels could be prevented by supporting the cathode in the central portion of the vessel a substantially equal distance away from all of the surfaces of the vessel being protected. However, and as it is well known in the art, many processes utilize vessels containing corrosive solutions wherein the liquid level in a vessel frequently changes. Also, in many processes, a solution is violently agitated in its containing vessel, to further complicate the problem of supporting the cathode in a central position in the vessel.

The present invention contemplates a system of anodic passivation for minimizing corrosion of a vessel containing a corrosive solution which employs a novel electrode assembly, such that substantially any desired amount of anodic current may be passed through the solution to passivate the vessel, yet no hot spots will be developed in the walls of the vessel. More specifically, the present invention contemplates a novel electrode assembly wherein the active surface of the electrode is rigidly supported by and adjacent one wall of a vessel containing a corrosive solution, yet the electrode is shielded from the wall in such a manner that current'cannot flow in a straight path between the active portion of the electrode and the adjacent portion of the Wall and develop a hot spot in the wall. We have found that the active portion of an electrode may be supported in close proximity with a wall of a vessel containing a corrosive solution and yet will not develop hot spots, providing no current is passed through the solution along a straight path between the active portion of the electrode and the nearest portion of the wall.

This invention further contemplates the positioning of the electrode assembly in the bottom of a vessel, such that the vessel may be protected against corrosion even though the level of the corrosive solution in the vessel varies to an appreciable extent. Furthermore, the present invention contemplates an electrode assembly which will remain in the desired position and operative even though the solution in the vessel being protected is violently agitated.

An important object of this invention is to provide an economical and eflicient system for minimizing corrosion of a vessel containing a corrosive solution.

Another object of this invention is to prevent the development of hot spots in a vessel being anodically passivated.

A further object of this invention is to provide an efficient distribution of the anodic current in a system of anodic passivation, regardless of the liquid level of the corrosive solution in the vessel being protected.

Another object of this invention is to efliciently passivate vessels containing corrosive solutions wherein the corrosive solutions are violently agitated.

A still further object of this invention is to provide an electrode assembly for a system of anodic passivation which is simple in construction, which will have a long service life, and which may be economically manufactured.

Other objects and advantages of the invention will be evident from the following detailed description, when read in conjunction with the accompanying drawings which illustrate our invention.

FIGURE 1 is a schematic illustration of a system embodying the present invention.

FIGURE 2 is a curve illustrating the relation between the potential of a vessel being protected and the corrosion rate or anodic current required to prevent corrosion at various potentials of the vessel.

FIGURE 3 is a diagrammatic illustration of the potential gradient in a vessel being anodically passivated.

FIGURE 4 is. a vertical sectional view through a preferred electrode assembly embodying the invention.

FIGURE 5 is a vertical sectional view through an electrode assembly particularly adapted for use in a vessel wherein a corrosive solution in the vessel is violently agitated.

FIGURE 6 is a vertical sectional view through a moditied electrode assembly particularly adapted for use in a small diameter vessel.

FIGURE 7 is a vertical sectional view through still another modified electrode assembly.

Referring to the drawings in detail, and particularly FIG. 1, reference character 10 generally designates a vessel containing a corrosive solution 12 and having side walls 14 and a bottom wall 16. The solution 12 is electrolytic and may be either acidic or alkaline. In accordance with the present invention, a novel electrode assembly, generally designated by reference character 18, is supported in the vessel 10, preferably on the bottom wall 16 of the vessel, as will be more fully hereinafter described. The assembly 18 is connected by a conductor 20 to the negative side of a suitable source of direct current energy 22. The source 22 may take any desired form, such as a battery or a source of alternating current and a suitable rectifier, the only limitation being that the necessary amount of direct current energy be made available to passivate the vessel 10. The positive side of the source 22 is connected by a conductor 24 to one wall of the vessel 16.. It may also be observed that the vessel 10 is a metallic vessel, such that the anodic current may be substantially uniformly distributed through the side walls 14 and bottom wall 16 of the vessel. In most processes handling corrosive solutions, the vessel 10 will be constructed out of stainless steel.

A suitable standard electrode 26 is located remote from the walls of the vessel 10 and is connected to the solution 12 in the vessel 10 by a suitable electro-chemical bridge 28. A suitable controller 30 is connected across the standard electrode 26 and the vessel 10 to monitor the difference in potential between the vessel 10 and the standard electrode 26. The standard electrode 26 may take any suitable form, such as a calomel cell, a silver-silver chloride cell, a copper-copper sulphate cell or a hydrogen cell, and the bridge 28 may be any ionic conductor, either liquid or solid, such as KCl or silver chloride. The controller 30 may be of any suitable type which will monitor the difference in potential between the vessel and the standard electrode 26 and which will control the amount of anodic current supplied by the source 22, as by means of a switch 32 in the conductor leading from the electrode assembly 18. A suitable controller of the type which may be used is disclosed in detail in the abovementioned co-pending application. It is to be understood, however, that the controller 30 may be of the type which will constantly vary the anodic current provided by the source 22, as well as a controller of the type disclosed in the above-mentioned co-pending application which provides an alternating supply of anodic current by the source 22.

As it is well known in the art, the rate of corrosion of the vessel 10 may be correlated with the potential of the vessel compared with the potential of a standard electrode 26 located in a more noble position in the standard table than the material out of which the vessel 10 is constructed. We have found this relation to follow a curve of the shape shown in FIG. 2 wherein E designates the potential of the vessel 10 and the horizontal ordinate designates either rate of corrosion or the density of the anodic current required to combat the corrosion.

As will be observed in FIG. 2, corrosion of the vessel 10 increases rapidly as the potential of the vessel first starts moving in a more noble direction from point A (which has been designated the point where corrosion starts). This corrosion decreases rapidly at point B and then stays at a minimum value as the noble voltage increases from point C to point D. An increase in noble voltage above point D results in greatly accelerated corrosion. We have therefore designated any voltage above point D as an excessive noble voltage.

The exact noble voltages at points A, B, C and D, and the exact shape of the curve will vary with materials of construction of the vessel 10 and the composition of the solution 12. The values given in FIG. 2 for the noble voltages were obtained using a calomel cell as the standard electrode and immersing a stainless steel specimen in a 67% by weight sulfuric acid solution. It will be apparent that the corrosion rate is at a minimum between noble voltages of about 0.150 and 1.200, with the safest portion of the range being between about 0.300 and 0.800 volt. It should also be noted that although the relation between potential and corrosion shown in FIG. 2 is ordinarily obtained by use of a standard electrode located remote from any specific portion of the specimen or vessel being corroded, and is the average corrosion, the corrosion rate of various portions of a vessel will also vary and the relation between noble voltage and corrosion shown in FIG. 2 holds true for each minute portion of the vessel.

FIGURE 3 diagrammatically illustrates the variation in the noble voltage of various portions of a vessel spaced at different distances from a cathode 33. The data shown in FIG. 3 was obtained by using a platinum cathode in a stainless steel vessel containing a 67% by weight sulfuric acid solution, and measuring the noble voltages at various portions of the vessel by use of a calomel cell as the standard electrode. In taking a measurement the calomel cell was placed adjacent one portion of the vessel and the noble voltage of the particular portion determined by noting the difference in potential between the calomel cell and the respective portion of the vessel. As will be observed in FIG. 3, the noble voltage of the bottom of a vessel exposed to the cathode 33 decreases with the distance from the cathode. The shaded area directly below the cathode 33 represents the portion of the bottom of the vessel subjected to an excessive noble voltage of 1.4 volts and hence subjected to accelerated corrosion. We have found, however, and as will be more fully hereinafter set forth, that when the cathode 33 is shielded in such a manner as to prevent the passage of anodic current in a straight path between the cathode 33 and those portions of the vessel subjected to such excessive noble voltages, the respective portions of the vessel become passivated. It is also interesting to note that portions of the vessel a distance of 70 feet from the cathode 33 were maintained at a noble voltage within the safe operating range and hence were passivated.

The preferred electrode assembly 18 (FIG. 4) comprises a conductor rod 34 extending loosely through an aperture 36 in the bottom Wall 16 of the vessel 10. The rod 34 is preferably formed out of a relatively high strength conductive material, such as Scovill Brass No. 20, and is sealed in the aperture 36 by a suit-able sleeve of insulating and sealing material 38 of any suitable type which will withstand the temperature, electrical and chemical environment in which the electrode assembly is being used. We have found that a polytetrafiuoroethylene resin, sold under the trademark Teflon, is particularly useful in a high temperature acid environment. It will be understood, however, that any suitable combination insulating and sealing material may be used, it merely being necessary to prevent leakage through the aperture 36 around the rod 34 and to prevent short-circuiting between the rod 34 and the bottom tank wall 16.

A head, generally designated by reference character 33, is formed on the upper or inner end of the conductor rod 34 for conducting anodic current through the solution 12 in the vessel 10. The head 38 preferably comprises a core 40 of the same material as the conductor rod 34, covered with a coating 42 of material which is insensitive to the environment and which is a good conductor. In a preferred embodiment, the core 40 is formed out of Scovill Brass No. 20 integrally with the conductor rod 34 and having a diameter substantially larger than the diameter of the rod 34. The preferred coating material 42 over the outer surface of the core 40 is platinum backed with copper, with the copper and platinum being cladded onto the core 40. It may also be noted that the size of the head 38 must be sufficient to provide passage of the necessary amount of anodic current through the solution 12 to passivate the vessel 10 under the operating conditions encountered. For example, in a process of neutralizing toluene and xylene sulfonic acids with 20 percent caustic at temperatures up to F., and wherein 300 amperes of direct current are required, the core 40 is cladded with copper having a minimum thickness of .025 inch and platinum having a minimum thickness of .0125 inch. About forty square inches of platinum surface were provided.

A tubular insulator 44 is telescoped over the conductor rod 34 between the bottom wall 16 of the vessel and the head 38 to prevent a short circuiting between the conductor rod 34 and the adjacent wall 16 of the vessel. Also, the insulator 44 has a larger outer diameter than the head 38, for purposes hereinafter set forth. A washer of combination sealing and insulating material 46 is provided at each end of the insulator 44 to prevent the corrosive solution 12 from coming into direct contact with the conductor rod 34. In this connection it may be observed that only the sides and top of the head 38 must be covered by the environment insensitive material 42, since the lower end of the head 38 is contacted by the upper sealing washer 46 and is not utilized for the passage of anodic current through the solution 12. However, we prefer to provide a platinum washer 48 between the lower end of the head 38 and the upper sealing washer 46 to enhance the seal at the upper end of the conductor rod 34 and assure that an eflicient distribution of anodic current is provide. The insulator 44 may be of any suitable material which is insensitive to the environment and which will efiiciently insulate the conductor rod 34 from the solution 12. We have found that a ceramic insulator is effective in a vessel used in a neutralizing process as mentioned above.

The lower or outer end 50 of the conductor rod 34 is threaded to receive a nut 52 which holds the electrode assembly 18 in a fixed position on the bottom wall 16. A suitable combination insulating and sealing washer 54 is provided around the conductor rod 34 in contact with the bottom face of the bottom wall 16, and a metal washer 56 is preferably provided between the washer 54 and the nut 52 to prevent damage to the washer 54 when the nut 52 is threaded tightly onto the conductor rod 34. It will be apparent that when the nut 52 is threaded tightly on the conductor rod 34, the head 38 will be urged downwardly against the upper sealing washer 46 and not only tightly hold the assembly 18 to the bottom wall 16, but will also assure that the sealing washers 46 are effective in preventing the corrosive solution 12 from coming into contact with the conductor rod 34. The conductor 28 leading from the direct current energy source 22 is connected to the lower end 50 of the conductor rod 34 below the nut 52 in any suitable manner and may be held on the rod 34 by an additional nut 58, if desired or necessary, it merely being necessary that the conductor 28 have good contact with the conductor rod 34.

The electrode assembly 18 may be secured in any desired position in the vessel to position the head 38 of the electrode within the solution 12, and an er'iicient distribution of the anodic current through the solution 12 will be obtained. However, we prefer to secure the electrode assembly 18 to the bottom wall 16, such that the head 38 will be disposed in the corrosive solution 12, even when only a minor amount of the solution 12 is present in the vessel 10. When the electrode assembly 18 is secured to the bottom wall 16, the vessel 18 may not only be passivated when the solution 12 is at a low level in the vessel, but also facilitates the passivation of the vessel when the vessel is being filled with the solution 12. In other words, when the vessel 10 is passivated during the early stages of being filled with the solution 12, the complete passivation of the vessel is facilitated and the passivation may be accomplished with a minimum amount of anodic current to reduce the overall cost of the system.

In use of the electrode 18, anodic current is passed through the solution 12 in all directions between the side walls 14 and bottom wall 16 of the vessel 14) and the head 38 of the electrode assembly. Therefore, the most even distribution of the anodic current should be obtained when the head 33 is positioned centrally in the body of corrosive solution 12. We have found, however, and as previously indicated, that the head 38 may be positioned relatively close to the respective supporting Wall, providing the anodic current is shielded from passage in a straight path between the head 38 and those portions of the supporting wall subject to excessive noble voltages. In the type of assembly shown in FIG. 4, the portion of the wall 16 subject to excessive noble voltage is around the aperture 36. Therefore, the insulator 44- has a larger outer diameter than the diameter of the head 38 to prevent the passage of anodic current in a straight path between the head 38 and the portion of the supporting wall through which the conductor rod 34 extends. In any specific installation the potential of the supporting wall of the vessel may be surveyed in the manner described in connection with FIG. 3 and the insulator 44 made of suflficient size to shield the anodic current from passage in a straight path between the head 33 and those portions having an excessive noble voltage.

The distance the head 33 must be spaced from the supporting wall will vary with the conditions encountered, and particularly with the conductivity of the solution 12. In the above-mentioned process of neutralizing toluene and xylene sulfonic acids with twenty percent caustic, we have supported the head 38 between eighteen and twenty inches from the bottom wall of the stainless steel vessel in which the neutralizing was accomplished, and no hot spots were developed in the vessel. This distance could be reduced to eight or ten inches without developing hot spots in the bottom wall of the tank. It

may also be noted that in this installation, the diameter of the stainless steel tank being used was approximately twelve feet and the height of the tank was approximately eighteen feet. Thus, even though the head 38 was located substantially closer to the bottom wall of the tank, no hot spots were developed. It should further be noted that when the conductor rod 34 is constructed of a high strength material, and when the insulator 44 is constructed of a relatively high strength material, the corrosive solution 12 may be agitated without interfering with the efiicient operation of the electrode assembly 18. However, when encountering extremely violent agitation of the corrosive solution, we prefer the modified electrode assembly 60 illustrated in FIG. 5.

In the electrode assembly 60 we prefer to use the conductor rod 34 extending through an aperture 36 in the bottom wall 16 of the vessel in the same manner as the electrode assembly 18 shown in FIG. 4. We also prefer to use the same head construction 38, along with a platinum washer 48 and upper seal washer 46 in the same manner as in the electrode assembly 18. To provide additional strength in the electrode assembly 60, we provide a steel tube 62 telescoped over the conductor rod 34 and extending from the upper seal washer 46 downwardly through the aperture 36 in the bottom wall 16. The conductor rod 34 and steel tube 62 are insulated and sealed from the corrosive solution 12 by a tube 64 of any suitable insulating material, such as Teflon, having outwardly extending flanges 66 at the upper and lower ends thereof. Each flange 66 has a diameter larger than the diameter of the head 38 to shield the head 38 from the passage of anodic current in a straight path between the head 38 and the portion of the bottom wall 16 surrounding the aperture 36, substantially in the same manner as in the electrode assembly 18.

A lower sealing washer 68 is positioned between the lower flange 66 and the upper face of the bottom wall 16 around the steel tube 62 to further prevent contact of the corrosive solution 12 with the steel tube 62. A small sleeve 70 of combination sealing and insulating material is positioned in the aperture 36 around the steel tube 62 to both insulate and seal the tube 62 in the bottom wall 16. The usual sealing washer 54, metal washer 56 and nut 52 are provided on the lower end 50 of the conductor rod 34 to hold the electrode assembly 60 rigidly in position on the bottom wall 16 in the same manner described in connection with the electrode assembly 18. It will thus be observed that the steel tube 62 will reinforce the conductor rod 34 and prevent the electrode assembly 60 from bending or otherwise being moved or placed in an inoperative position by violent agitation of the corrosive solution 12.

Another modified electrode assembly 72 particularly adapted for small diameter vessels is illustrated in FIG. 6. In the assembly 72 we again prefer to use the conductor rod 34, head 38, platinum washer 48, sealing washer 54, metal washer 56, and nut 52 in the same manner as in the electrode assembly 18. In order to insulate the conductor 34 in the assembly 72, we provide a tubular shaped insulator 74 which may be formed out of a suitable material, such as Teflon, and which extends from the platinum Washer 48 to the upper face of the bottom wall 16 of the vessel. In this assembly, the insulating material 74 is a combination insulating and sealing material to not only insulate the conductor 34 but also provide a seal with the platinum washer 48 and the upper face of the bottom wall 16 to prevent the corrosive solution 12 from coming into contact with the conductor 34. The lower end 76 of the insulator 74 is reduced in diameter to fit tightly in the aperture 36 around the conductor rod 34 to also insulate the conductor rod 34 from the bottom wall 16.

A cup-shaped insulator 77 is secured around the upper end portion of the insulator 74, as by threads 78, and extends upwardly around the head 38. The insulator 77 may be formed out of any suitable material, such as Teflon, which will shield the anodic current, and may extend above the upper or outer end of the head 38 if desired. With the insulator 77 in the position shown in FIG. 6, anodic current cannot flow in a straight path between the head 38 and the side walls of the vessel 10 adjacent the head 38; therefore, the assembly 72 may be used in small diameter vessels, such as tubes. In other words, the anodic current flowing to the head 38 from a point at either side or below the head will be diverted over the upper end of the insulator 77, and the anodic current will, in effect, be focused along the center line of the vessel 10 away from the bottom wall 10.

When a wall of the vessel 10 is provided with a flanged opening 80 as shown in FIG. 7, and when the solution 12 is not violently agitated, we may use the electrode assembly 82 shown in detail in FIG. 7. The electrode assembly 82 comprises an elongated tubular insulator 84 which may be formed out of any suitable insulating material, such as Teflon, and having threads 86 adjacent the lower end portion thereof for threaded connection with a flange 88 of a size to mate with the flange 80. Again it is preferred that the insulator 84 be formed out of a sealing type of material which will engage the threads in the flange 88 and prevent a leakage of the corrosive solution 12 through the flange 80.

A conductor rod 90 extends upwardly through the tubular insulator 84 and, since high strength is not required, the rod 90 may be formed out of copper. A platinum rod 92 is inserted in the upper end of the insulator 84 and is suitably connected to the upper end of the conductor rod 90, as by being threaded into the upper end of the conductor rod 90, to provide an efficient transfer of current between the conductor rod 90 and the rod 92. We also prefer to provide a suitable sealing ring 94 in the upper end of the insulator 84 around the platinum rod 92 to prevent leakage of the solution 12 into the upper end of the insulator 84 into contact with the copper conductor rod 90.

The head portion 96 of the electrode assembly 82 may be of any suitable construction which will provide the required conducting surface for passage of the desired amount of anodic current through the solution 12. In one installation, we have used platinum baskets salvaged from an electrode plating operation as the head portion 96. However, we may use any design, such as platinum wire or perforated platinum plates as the head 96; the only requirement being that the head 96 be adequately connected to the platinum rod 92 and have the required surface area, as previously indicated. The electrode assembly 82 may be economically manufactured, but, as previously indicated, is useful only in a vessel wherein the corrosive solution is not violently agitated, and is also preferably used when the corrosive solution is not maintained at a high temperature.

From the foregoing it will be apparent that the present invention provides an economical and efficient system for passivating vessels containing corrosive solutions. The electrode assembly of this invention may be easily installed in an existing vessel and may be used under substantially any operating conditions. The active portion of the electrode assembly is positioned relatively close to one wall of a vessel, yet no hot spots will develop in the vessel which would provide an accelerated rate of corrosion. It will also be apparent that the electrode assembly will remain operative, even when the corrosive SOlIlltlOIl in which the electrode is immersed is violently agitated, and the electrode assembly will remain operative at various levels of the solution in the vessel being protected. It will be further apparent that the present electrode assembly is simple in construction, may be economically manufactured and will have a long service life.

Changes may be made in the combination and arrangement of parts or elements as heretofore set forth in the specification md shown in the drawings, it being understood that changes rnay be made in the embodiments disclosed without departing from the spirit and scope of the invention as defined in the following claims.

We claim:

1. In an lanodic passivation system containing a corrosive solution within a metallic vessel, wherein a direct current lpotenial is applied between a cathode disposed in said solution and said metallic vessel, and wherein said cathode is mounted in such proximity to the inner surface of said vessel that excessive positive potential is created at the inner surface of the vessel in the near proximity of said cathode whereby excessive corrosion of said vessel occurs, the improvement which comprises:

providing a zone of electrically insulating material disposed between said vessel wall and said cathode, said zone being adapted to shield said vessel from said cathode and thereby reduce said potential and said corrosion.

2. A method as described in claim 1 and further characterized in that said zone of electrically insulating material is disposed between said vessel wall and said cathode in a manner adapted to focus the current flowing from said cathode away from said vessel wall.

3. A method as described in claim 1 and further characterized in that said zone of electrically insulating material is composed of polytetrafluoroethylene resin.

References Cited by the Examiner UNITED STATES PATENTS 1,506,306 8/24 Kirkaldy 204-196 2,187,143 1/40 Bary 204-196 2,221,997 11/40 Polin 204-196 2,459,123 6/49 Bates et al 204-197 2,483,397 10/49 Bonner 204-196 2,576,680 11/51 Guitton 204-147 2,759,887 8/56 Miles 204-196 2,776,940 1/57 Oliver 204-196 2,803,797 8/57 Cowles 204-196 2,848,658 8/58 Mitchell 317-1485 2,856,342 10/58 Van Der Hoeven et al. 204-197 2,863,819 12/58 Preiser 204-196 2,882,213 4/59 Douglas 204-197 2,903,405 9/59 Sabins 204-196 2,918,420 12/ 5 9 S-abins 204-231 2,941,935 6/60 Miller et al 204-196 2,982,714 5/61 Sabins 204-196 JOHN H. MACK, Primary Examiner. JOSEPH REBOLD, JOHN R. SPECK, Examiners.

Claims (1)

1. IN AN ANODIC PASSIVATION SYSTEM CONTAINING A CORROSIVE SOLUTION WITHIN A METALLIC VESSEL, WHEREIN A DIRECT CURRENT POTENIAL IS APPLIED BETWEEN A CATHODE DISPOSED IN SAID SOLUTION AND SAID METALLIC VESSEL, AND WHEREIN SAID CATHODE IS MOUNTED IN SUCH PROXIMITY TO THE INNER SURFACE OF SAID VESSEL THAT EXCESSIVE POSITIVE POTENTIAL IS CREATED AT THE INNER SURFACE OF THE VESSEL IN THE NEAR PROXIMITY OF SAID CATHODE WHEREBY EXCESSIVE CORROSION OF SAID VESSEL OCCURS, THE IMPROVEMENT WHICH COMPRISES: PROVIDING A ZONE OF ELECTRICALLY INSULATING MATERIAL DISPOSED BETWEEN SAID VESSEL WALL AND SAID CATHODE, SAID ZONE BEING ADAPTED TO SHIELD SAID VESSEL FROM SAID CATHODE AND THEREBY REDUCE SAID POTENTIAL AND SAID CORROSION.

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