Anode Containing Pin-type Inserts

Abstract

An anode for use in a cathodic protection system or other electrolytic process includes a body of lead or a lead alloy and a plurality of pins or wire inserts of tantalum, titanium, niobium, zirconium, vanadium or a similar metal coated with a thin outer coating of platinum or other noble metal from Group VIII of the Periodic Table. The inserts will normally range from about 50 to about 250 mils in diameter and the coating will generally be from about 1 to about 10,000 microinches in thickness.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to anodes for use in impressed current cathodic protection systems and other electrolytic processes and is particularly concerned with lead-type anodes containing wire or pin inserts.

2. Description of the Prior Art

Lead and lead alloy anodes used in impressed current cathodic protection systems and similar electrolytic processes tend to deteriorate in the presence of brines. This deterioration is manifested by the formation of coatings of lead chloride and other salts which eventually deactivate the anodes and prevent further electrolytic action. It is known that this difficulty can be alleviated by providing such anodes with small pins or inserts of platinum which will serve as nucleation sites and promote the formation of lead peroxide in lieu of lead chloride and other undesirable divalent salts. The improvements which can be obtained in this manner are limited because of the high cost of platinum and practical problems normally encountered in installing the pins and holding them in place. Experience has shown that conventional platinum pins are easily dislodged and are often lost from the anodes long before the lead has been consumed. As a result of these and related difficulties, efforts to prolong the useful life of lead-type anodes through the use of platinum pins or inserts have been only moderately successful.

SUMMARY OF THE INVENTION

The present invention provides an improved lead-type anode for use in impressed current cathodic protection systems and similar electrolytic processes which at least in part eliminates the difficulties outlined above. This improved anode comprises a lead metal anode body and a plurality of pins or wire inserts extending into the anode body at spaced intervals over the outer surface thereof. These pins or inserts are made of tantalum, titanium, niobium, zirconium, vanadium or an alloy containing one or more of these metals as the principal constituent and are coated with a thin outer coating of platinum or a similar noble metal from Group VIII of the Periodic Table. It has been found that these noble metals have surprisingly low deterioration or attrition rates when used on coated pins or inserts in lead metal anodes and that a plurality of such inserts can be used to reduce the deterioration rates of such anodes to a value of one-tenth or less that normally obtained with anodes containing conventional platinum pins or inserts. A conventional anode containing three platinum inserts per square foot, for example, may have a useful life of about 6 years; whereas an anode containing 15 of the coated pins of this invention per square foot may have a useful life of 50 years. This makes possible significant improvements in impressed current cathodic protection systems and other electrolytic processes without substantial increases in cost.

The pins or inserts used in the improved anodes of the invention will normally have diameters of from about 50 to about 250 mils and will be provided with coatings of platinum or a similar noble metal between 1 and about 10,000 microinches in thickness. This use of relatively large coated pins or inserts facilitates driving of the inserts into the anode bodies, permits the use of longer inserts, and makes the coated inserts more difficult to dislodge than the smaller diameter platinum inserts generally employed heretofore. It is preferred that the inserts extend through the anode body so that fresh platinum is continually exposed as the lead metal deteriorates and that the unexposed end of each insert be secured to increase the resistance to forces generated by the formation of lead peroxide which tend to extract the inserts from the anode body. These and other features of the improved anodes responsible for their improved performance will be described in detail hereafter.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 in the drawing depicts one embodiment of the improved anode of the invention which is particularly adapted for use on offshore drilling and production platforms and other marine installations;

FIG. 2 is an enlarged cross section of the anode of FIG. 1 taken about the line 2--2;

FIG. 3 illustrates an alternate embodiment of the invention which may be used on ships, underwater storage tanks, and similar marine structures;

FIG. 4 is a cross sectional view of the anode of FIG. 3 taken about the line 4--4;

FIG. 5 illustrates still another embodiment of the invention which may be employed in impressed current cathodic protection systems and other electrolytic processes; and,

FIG. 6 is a cross sectional view of the anode of FIG. 5 taken about the line 6--6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The anode depicted in FIG. 1 of the drawing comprises a cylindrical lead metal anode body 11 which is suspended by means of an insulated terminal member 12 and an insulated electrical conductor 13. The anode body may be made of metallic lead but will normally be composed of a lead alloy containing small amounts of silver, antimony, tellurium, bismuth and other metals. Experience has shown that alloys of this type generally less prone than metallic lead to form lead chloride in the presence of chloride ions and are therefore somewhat more effective for use as anodes in electrolytic processes. The preferred alloys will normally contain up to about 5 percent silver and up to about 10 percent antimony. Typical alloy compositions which may be used include the following: (a) lead-98 percent, silver-2percent; (b) lead-99.5 percent, antimony-0.5 percent; (c) lead-93 percent, silver-1 percent, antimony-6 percent; (d) lead-97.5 percent, silver-2 percent, tellurium-0.3 percent, bismuth-0.2 percent; (e) lead-96 percent, sliver-2 percent, antimony-1.5 percent, copper-0.4 percent, tellurium-0.1 percent; and (f) lead-99.8 percent, tellurium-0.1 percent, bismuth 0.1 percent. All of these compositions may include other materials present in small amounts as impurities. A variety of other lead alloys suitable for use as anodes after treatment to provide a surface coating of lead peroxide have been described in the literature and will be familiar to those skilled in the art.

The anode body 11 is provided with a plurality of pins or wire inserts 14 which are spaced at regular intervals over the outer surface of the body. Each of these pins or inserts is made of tantalum, titanium, niobium, zirconium, vanadium or an alloy containing one or more of these metals as the principal constituent and is coated with a thin outer coating of platinum or a similar noble metal from Group VIII of the Periodic Table. The use of titanium inserts coated with a layer of platinum or a platinum-iridium alloy between about 1 and about 10,000 microinches in thickness is generally preferred. These inserts will normally be between about 50 and about 250 mils in diameter and will be spaced to provide from about 5 to about 50 or more inserts per square foot of anode surface area. As shown more clearly in FIG. 2, each insert preferably extends through the wall of the lead body and if desired may be provided with circumferential, longitudinal, or spiral ridges 15 to aid in holding it in place.

The inserts used in the anode body are positioned by drilling holes in the body at the desired locations and then driving the inserts into place. In general, these holes should extend normal to the outer surface of the body and be small enough to insure a very tight fit. Otherwise, the formation of a thick layer of lead peroxide on the surface of the body may result in the inserts being pulled out of electrical contact with the lead alloy anode so that no further nucleation of lead peroxide can take place. This will in turn accelerate deterioration of the anode body, in some cases perhaps by a factor of as much as 100 times.

The relatively large coated pins or inserts used for purposes of the invention can be driven to much greater depths and are considerably more difficult to dislodge from the anode body than the platinum inserts used in the past. This is due in part to the greater rigidity of the relatively large coated inserts. The flexural rigidity of such an insert is a function of the modulus of elasticity of the metal employed and the moment of inertia of the cross sectional area about the transverse axis. Although the modulus of elasticity of platinum is slightly higher than that of titanium for example, the cost of platinum is such that the use of platinum inserts greater than about 50 mils in diameter is prohibitively expensive. Titanium inserts coated with 1 microinch of platinum cost about 1/100 as much as platinum inserts of the same size and hence the use of coated inserts with considerably greater diameters than those of the platinum inserts employed heretofore is feasible. The moment of inertia of an insert increases as the fourth power of the diameter, so that doubling the diameter will increase the flexural rigidity by a factor of 16. The flexural rigidities of conventional platinum inserts and typical coated inserts employed for purposes of the invention are set forth below.

______________________________________ Modulus of Flexural Type of Insert Elasticity Rigidity ______________________________________ Hard Drawn Platinum-50 mils Diameter 22.6 .times. 10.sup.6 6.95 Annealed Tantalum-100 mils Diameter 27.0 .times. 10.sup.6 133 Wrought Niobium-100 mils Diameter 15.0 .times. 10.sup.6 74 Titanium-100 mils Diameter 16.8 .times. 10.sup.6 83 ______________________________________

It can be seen from the above table that the coated tantalum, niobium and titanium inserts of 100 mils diameter have flexural rigidities of from about 10 to 20 times those of the 50 mil diameter platinum inserts. This greater rigidity permits driving of the coated inserts much deeper than the 1/2 inch generally considered a maximum for platinum inserts. For a given coefficient of friction between an insert and the lead anode, the force required to withdraw the insert is directly proportional to the area of contact. The area of contact is a linear function of the wire insert diameter and of its length. The force exerted on the insert due to the formation of lead peroxide on the lead anode surface is also proportional to the insert diameter and thus the net resistance to withdrawal of the insert from the anode depends upon the distance to which it is driven. The use of the coated inserts of the invention makes possible useful anode lives many times those that can be obtained with conventional lead anodes containing relatively short platinum inserts.

As pointed out earlier, it is preferred that each of the coated inserts be driven completely through the lead anode. Where the inner surface of the lead is not exposed, the inner end of each insert can be bent at a right angle to aid in holding the insert in place. The increased flexural rigidity of the relatively large coated inserts makes these inserts much more difficult to dislodge from the anode than conventional platinum inserts. Before an insert which is bent at the inner end as described above can be pulled out of the lead anode, the inner end must be straightened out by bending it through an angle of about 90.degree.. The maximum bending or deflection which takes place under a given force is an inverse function of the flexural rigidity of the insert and hence doubling the insert diameter will reduce by a factor of 16 the amount of bending or deflection that takes place. By using relatively large coated inserts, the danger of losing the inserts before substantially all of the lead is consumed can be minimized.

The entire length of each of the wire inserts is substantially covered with platinum or a similar noble metal. Tests in both the laboratory and field have shown that platinum and platinum alloys generally have deterioration rates of about 6 milligrams per ampere year when used as anodes in the conventional fashion but that such metals have negligible attrition rates of less than 1 milligram per ampere year when employed on coated inserts in accordance with the invention. Because there is thus virtually no loss of platinum or other noble metal from the inserts, coatings of as little as 1 microinch in thickness are feasible. As the lead surrounding each of the inserts deteriorates, new platinum is constantly being uncovered and exposed to the electrolyte in the system. In the event that the anode falls into the mud bottom where the rate of anode deterioration will be much higher than in a circulating aqueous environment, the exposed platinum may be destroyed. The exposure of fresh platinum after the anode is returned to the aqueous environment, however, will result in a return of the anode to the former low deterioration rate.

The dimensions and configuration of the anode will depend primarily upon the structure to be protected and the period over which effective protection is required. As pointed out earlier, the improved anodes of the invention are considerably less expensive than conventional lead type anodes containing platinum inserts or microanodes and permit the use of a much greater number of inserts than has generally been considered feasible heretofore. This use of more inserts results in a longer lasting anode, permits operation of the anode at higher current densities, and for a given life expectancy makes possible the use of a smaller diameter anode. The ability of the improved anodes to operate at higher current densities is particularly important because of the high initial current densities required for the effective protection of steel in sea water. The use of a smaller diameter anode for a given life expectancy results in savings in anode costs, simplifies suspension techniques, and makes the anodes more readily retrievable.

FIGS. 3 and 4 in the drawing illustrate an alternate embodiment of the improved anode of the invention which is intended for use on ships, underwater storage tanks and similar structures. The anode shown in FIGS. 3 and 4 comprises a lead or lead alloy anode body 20 which is mounted upon a dielectric backing member 21 and held in place by a gasket 22 and an apertured fixture or bracket 23, both made of polyvinylchloride, polychloroprene, fiberglass reinforced polyester and epoxy mastics or other suitable dielectric material. The portion of the anode body exposed by the apertured fixture is provided with a plurality of coated wire inserts 24. Each insert includes an inner core of tantalum, titanium, niobium, zirconium, vanadium or an alloy containing one of these metals as the principal constituent and a thin outer coating of platinum or a similar noble metal from Group VIII of the Periodic Table. The inserts are preferably between about 100 and about 250 mils in diameter and are spaced to provide between about 5 and about 50 or more inserts per square foot of anode surface. As indicated in FIG. 4, each insert extends through the lead metal anode body and is bent over parallel to the back of the body as indicated by reference numeral 25. This aids in resisting forces generated by the formation of lead peroxide on the exposed surface of the anode which may otherwise tend to pull the insert from the anode. The anode assembly will normally be mounted on a dielectric shield on the surface of the member to be protected. It is generally preferred that this shield extend for a distance of 10 feet or more beyond the anode in all directions. The dielectric shield may comprise a coating of coal tar epoxy resin, phenolic epoxy resin, epoxy mastics fiberglass reinforced polyester material, polyurethane, polyvinylchloride, neoprene rubber or other dielectric material and may be applied by spraying, baking, wrapping, or other conventional technique. The anode assembly may be mechanically mounted on the surface to be protected by means of countersunk brass screws which extend through holes 26 in the assembly and are covered with polychloroprene putty or similar dielectric material. Other mounting techniques may also be used. Current will normally be supplied to the backside of the anode in the conventional manner by means of a conductor which extends through an opening in the hull or other surface on which the assembly is mounted and is connected to the anode body.

Still another embodiment of the invention is shown in FIGS. 5 and 6 of the drawing. This embodiment comprises an elongated hollow anode body 30 of lead or a lead alloy which is filled with an inner core of epoxy resin or similar material 31. An insulated cap or terminal member 32 provided with insulated electrical conductor 33 is attached to the upper end of the anode body. Coated inserts 34 of platinum coated titanium, tantalum, or niobium are positioned in the anode body at regularly spaced intervals over its surface. These inserts extend into the opening in the lead or lead alloy body and are bent over before the opening is filled with the epoxy resin. This aids in resisting forces due to the formation of lead peroxide which tend to dislodge the inserts. In lieu of using an epoxy resin core, a close fitting rod of titanium, tantalum, niobium or similar metal can be driven into the opening in the body after the inserts have been placed in order to bend them over and provide the body with greater strength. The outer ends of the inserts are substantially flush with the outer surface of the anode body. Anodes of this type are useful in a variety of electrolytic processes using lead type anodes.

It will be apparent from the foregoing that the improved anodes of the invention can be constructed in a variety of different configurations. Regardless of the particular configuration used, the coated pins or inserts provide significantly longer anode life, permit operation at higher current densities, and for a given anode life make feasible the use of smaller anodes than do the platinum inserts or microelectrodes employed heretofore.

Claims

1. An anode for a cathodic protection system or similar electrolytic process which comprises a lead metal anode body; a plurality of pins extending into said body from the outer surface thereof, said pins being made of a base metal selected from the group consisting of tantalum, titanium, niobium, zirconium, vanadium and alloys thereof and being coated with a noble metal from Group VIII of the Periodic Table; and means for

3. An anode as defined by claim 1 wherein said lead metal is a lead-silver

4. An anode as defined by claim 1 wherein said lead metal is a lead-antimony alloy containing up to about 10 percent antimony by weight.

5. An anode as defined by claim 1 wherein said pins have diameters between

6. An anode as defined by claim 1 wherein the thickness of the noble metal

7. An anode as defined by claim 1 containing from about five to about 50

8. An anode as defined by claim 1 wherein each of said pins is provided

9. An anode for a cathodic protection system or similar electrolytic process which comprises a lead metal anode body; a plurality of pins extending from the outer surface of said anode body completely through said anode body; said pins being made of a base metal selected from the group consisting of tantalum, titanium, niobium, zirconium, vanadium and alloys thereof and being coated with a noble metal from Group VIII of the Periodic Table; and means for applying an electric current to said anode

10. An anode as defined by claim 9 wherein the inner ends of said pins extend beyond the inner surface of said anode body and are bent over adjacent said surface.