US3274085A

US3274085A - Galvanic anode

Info

Publication number: US3274085A
Application number: US275497A
Authority: US
Inventors: Herbert C Rutemiller; Allen M Montgomery
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1963-04-25
Filing date: 1963-04-25
Publication date: 1966-09-20
Anticipated expiration: 1983-09-20

Description

United States Patent 3,274,085 GALVANIC ANODE Herbert C. Rutemiller and Allen M. Montgomery, Cleveland, Ghio, assignors to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Apr. 25, 1963, Ser. No. 275,497 7 Claims. (Cl. 204-148) This invention relates to aluminous metal galvanic anodes for the cathodic protection of metals and relates particularly to consumable anodes of an aluminum base alloy for the cathodic protection of non-aluminous metal structures exposed to the corrosive action of aqueous media and particularly aqueous saline media.

Cathodic protection systems are well known in which a metal article immersed in an electrolyte is protected from corrosion by means of a sacrificial or consumable anode which is also immersed in the electrolyte and is electrically connected to the metal structure (cathode) which is to be protected. Protection against corrosion is particularly important when the metal article is exposed to the corrosive action of aqueous saline media. Sacrificial anodes are employed to provide cathodic protection for such structures as steel pipe lines, ship hulls, ship ballast tanks, meta-1 sea walls, and drilling rigs.

Sacrificial or consumable anodes are generally made in any desired shape or size to suit the structure to be protected and must be composed of a metal which is anodic to the metal body to be protected. The anodes may be in wrought or cast form but the latter has generally been preferred. Some convenient means for at taching the anode to the article to be protected is usually necessary such as an embedded metal core strap, rod or cable.

For many applications the expense of replacing consumed anodes represents a substantial part of the cost of the protective system. For this reason it has been recognized that a long life accompanied by adequate current output is highly desirable for reducing the cost of cathodic protection. This characteristic is referred to as high current efficiency and is generally expressed in terms of ampere hours of current delivered to the cathode per pound of anode metal consumed. The difference in potential between the anode and cathode must be great enough during the life of the anode to maintain an adequate flow of current and to polarize the cathode to a sufficiently high potential to avoid local corrosion cells on the cathode.

It is also desirable that anodes have a high electronegative solution potential in open circuit, which is expressed in volts relative to some standard electrode, without sacrificing current efiiciency. An aluminum base alloy anode having a higher or more electronegative potential than another aluminum base alloy of the same size will protect a larger cathode area than the aluminum base alloy anode with less electronegative potential.

Two very desirable features of an aluminum base alloy anode, therefore, are high current efficiency and high electronegativc potential in open circuit.

An object of this invention, therefore, is to provide an aluminum base alloy galvanic anode having a high current efficiency and capable of maintaining a high electronegative solution potential.

Another object is to provide an improved aluminum base alloy galvanic anode which has a more electronegative solution potential than heretofore attainable in an anode having substantially the same high current efficiency.

We have found that a very small amount of boron in aluminum-zinc-tin alloy type anodes which have been solution heat treated in accordance with the disclosure in copending patent application Serial No. 143,042, filed October 5, 1961, now US. Patent No. 3,227,644, of which one of us is the inventor, produces an anode having a more electronegative solution potential than the same anode without boron. The presence of boron, it has been discovered, increases the electronegative potential of the anodes without any significant change in their current efiiciency. From a practical standpoint this means that these anodes protect a larger cathode area than the same anode without boron. This, of course, is an important economical factor to be considered when designing cathodic protection systems. The alloy used for such anodes should consist essentially of aluminum, from 3.5% to 9.0% by weight of zinc, from 0.05% to 0.20% by weight of tin and from 0.02% to 0.0 8% boron. To obtain the best results, we prefer to use from 6.0% to 8.0% by weigh-t of zinc. All impurities in the aluminum base alloy, such as for example iron, silicon and copper, should not exceed a total of 0.50% and more specifically the alloy should contain less than 0.20% iron, 0.20% silicon and 0.02% copper since in greater amounts they reduce the current efficiency of the treated anodes. All other impurities should not be over 0.05% each.

The zinc component of the alloy is necessary to provide the desired basic electrode potential for the heat treated anode. Smaller amounts than 3.5% do not supply the desired characteristics in the anode while more than 9.0% does not produce any added improvement in performance. The element tin also favorably affects the behavior of the treated anodes. It has been found than tin serves to maintain a high level of solution potential over the life of the anode. Smaller amounts than the stated minimum do not produce a significant increase upon the solution potential whereas larger quantities produce no added improvement and increases the cost of production. With respect to boron, less than 0.02% does not produce a significant increase in solution potential while more than 0.08% does not produce any additional increase.

As described in copending application Serial No. 143,042, referred to hereinabove, the solution heat treatment of the alloy required to establish the desired condi tion for high anode performance consists of heating the anodes to a temperature between 800 F. and 925 F., and holding within this range for a sufi'lcient length of time to effect substantially complete solution of the soluble alloying elements and thereby establish a homogeneous internal structure. Generally the period of soaking within the foregoing temperature range should extend over a period of from 1 to 12 hours, the length of time being dependent upon the temperature and mass of the anodes being treated. Holding the anodes within the aforementioned temperature for about 2 hours has been found in many instances to be sufficient and can be considered to be a practical minimum for commercial heat treatment. Heating to a temperature in the lower portion of the temperature range usually requires a longer time to bring about a solution of the soluble elements than heating within the upper portion of the temperature range. Once the alloying elements are in substantially complete solution and a homogeneous condition is created there does not appear to be any advantage to continue the thermal treatment.

After the anodes have been held at the elevated temperature for a sufficient length of time, they should be rapidly cooled to room temperature. This can be accomplished in a known manner as by quenching in an air blast, by water spray, by immersion in a water bath, or by other means. The particular cooling means employed will in general be determined by the facilities at hand. In order to reduce warpage of the anodes, we

3 have found a quench in hot water at a temperature of about 180 to 212 F. to be quite satisfactory. No further thermal treatment is necessary or desirable after the drastic cooling operation.

The anodes can be made in either cast or wrought form but generally it is most convenient to produce them in the form of castings since the supporting rod or cable can be cast in place. The sand or permanent mold casting procedures are generally most convenient to employ. The size and shape of the anodes will vary with the type of installation, and generally weigh between and 50 pounds.

An open circuit potential difference of 0.2 to 0.4 volt between the treated aluminum alloy anode and the steel structure insures adequate protection on the one hand while on the other hand avoiding what is known as over protection. The presence of boron does not produce over protection, we have found, even though that element serves to raise the solution potential.

The improvement in anode solution potential resulting from the addition of boron to the anode alloy and solution heat treated under the same conditions is illustrated in the following example:

Three sample anodes were tested. Anode No. 1 was composed of an aluminum base alloy without boron and consisting essentially of aluminum, 7.18% zinc and 0.12% tin, with an impurity content of 0.002% copper, 0.03% iron, and 0.05% silicon. Anode No. 2 was composed of an aluminum base alloy consisting essentially of aluminum, 7.32% zinc, 0.11% tin, and 0.03% boron, with an impurity content of 0.002% copper, 0.03% iron, and 0.05% silicon. Anode No. 3 was composed of an aluminum base alloy consisting essentially of aluminum, 6.78% Zinc, 0.11% tin, and 0.07% boron, with an impurity content of 0.003% copper, 0.03% iron, and 0.05% silicon. All three of the anodes were given a solution heat treatment consisting of heating for 8 hours at 875 F. followed by a quench in boiling water before being tested. Each of the anodes were weighed and immersed in synthetic sea water in separate steel drums whose interior surfaces had been sand blasted prior to the test to remove all rust and scale. The anodes were lectrically connected with the drums through a 0.05 ohm resistance. The exposure period of the anodes was 21 days. Each day during the test period the circuit was broken and the electronegative potential of the anodes measured with a Leeds and Northrup Speedomax recording potentiometer with respect to a 0.1 N calomel electrode. After the initial reading there was no significant deviation in the readings for each of the anodes throughout the rest of the exposure period. As determined from the readings anode No. 1 had an initial electronegative potential of 1.190 v. and a final potential of 1.180 v., whereas under the same conditions anode N0. 2 had an initial electronegative potential of -1.190 v. and a final potential of 1.240 v. and anode No. 3 had an initial electronegative potential of --1.190 v. and a final potential of 1.230 v.

At the end of the 21-day test period the anodes were 4 removed, cleaned and weighed to determine the loss of metal. The current efficiency of both anode No. 2 and No. 3 was at least equal to that of anode No. 1.

Having thus described our invention and certain embodiments thereof we claim:

1. A thermally treated galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc, from 0.05% to 0.20% tin, and from 0.02% to 0.08% boron, the alloy containing a total of not over 0.50% of all impurities, said anode having a homogeneous internal structure resulting from a solution heat treatment and rapid cooling to room temperature and characterized by 'a more electronegative solution potential than the same anode without boron.

2. A thermally treated galvanic anode in accordance with claim 1, wherein the amount of zinc is from 6.0% to 8.0%.

3. A thermally treated galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, 3.5% to 9.0% zinc, from 0.05% to 0.20% tin, and from 0.02% to 0.08% boron, the alloy containing as impurities up to 0.20% iron, up to 0.20% silicon, up to 0.02% copper and all others not exceeding 0.05% each, the total of all impurities not being over 0.50%, said anode having a homogeneous internal structure resulting from a solution heat treatment and rapid cooling to room temperature and characterized by a more electronegative solution potential than the same anode without boron.

4. A thermally treated galvanic anode according to claim 3 wherein the zinc content is 6.0% to 8.0%.

5. A thermally treated cast galvanic anode composed of an aluminum base alloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc, 0.05% to 0.20% tin, and from 0.02% to 0.08% boron, the alloy containing a total of not over 0.50% of all impurities, said anode having a homogeneous internal structure resulting from a solution heat treatment and being characterized by having a more electronegative solution potential than the same anode without boron.

6. A thermally treated cast galvanic anode in accordance with claim 5, wherein the amount of zinc is from 6.0% to 8.0%.

7. A thermally treated cast galvanic anode composed of an aluminum base alloy consisting of aluminum, from 3.5% to 9.0% zinc, 0.05% to 0.20% tin, and from 0.02% to 0.08% boron, the alloy containing as impurities up to 0.20% iron, up to 0.20% silicon, up to 0.02% copper and all others not exceeding 0.05% each, the total of all impurities not being over 0.50%, said anode having a homogeneous internal structure resulting from a solution heat treatment and being characterized by having a more electronegative solution potential than the same anode without boron.

No references cited.

JOHN H. MACK, Primary Examiner.

T. TUNG, Examiner.

Claims

1. A THERMALLY TREATED GALVANIC ANODE COMPOSED OF AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF ALUMINUM, FROM 3.5% TO 9.0% OF ZINC, FROM 0.5% TO 0.20% TIN, AND FROM 0.02% TO 0.08% BORON, THE ALLOY CONTAINING A TOTAL OF NOT OVER 0.50% OF ALL IMPURITIES, SAID ANODE HAVING A HOMOGENEOUS INTERNAL STRUCTURE RESULTING FROM A SOLUTION HEAT TREATMENT AND RAPID COOLING TO ROOM TEMPERATURE AND CHARACTERIZED BY A MORE ELECTRONEGATIVE SOLUTION POTENTIAL THAN THE SAME ANODE WITHOUT BORON.