US2913384A - Aluminum anodes - Google Patents

Description

ALUMINUM ANODES John Staley, Louisville, Ky., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Application November 20, 1957 I Serial No. 697,521

9 Claims. (Cl. 204-148) This invention relates generally to an improved system utilizing a new type of sacrificial aluminum anode in conjunction with cathode.

A specific usage of the invention is for a source of electric current accomplished by incorporating the new type of aluminum sacrificial anode with a suitable electrolyte and cathode in a similar arrangement as found .in the ordinary'dry cell.

Another and more important specific usage of the invention relates to an improved system for the cathodic protection of metal structures exposed to the action of tcorrosive media by employing the new type aluminous sacrificial anode, and to the method of utilizing such anode. More particularly, this embodiment of the invention relates to the cathodic protection of ferrous metal structures exposed to the corrosive action of aqueous saline media or other corrosive electrolyte by means of aluminous sacrificial anodes which contain a substantial percentage of an auxiliary metal, such as iron, and others which form an analogous relationship with aluminum. The aluminous anodes which are especially within the contemplation of this invention are those which comprise an" alloy of aluminum with zinc, and which con- Ptain in addition a significantly high percentage of iron. An adjunct to the cathodic protection of ferrous metal structures is the coating of the aluminous sacrificial anode material directly onto the structure.

Cathodic protection has become well established as one of the most effective methods of corrosion control. It involves supplying a polarizing current from some source other than the corroding metal which is to be protected. In one type of cathodic protection system the external source of electrical energy maybe a generator, rectifier, or battery, furnishing current through a relatively inert or insoluble anode such as graphite.- However, in cathodic protection systems of the type with which the present invention is concerned, the requisite protecting current is supplied by the corrosion of a corroding type of anode made of a metal which is more active than the metal in the structure to be protected.

Foremost among the factors to be considered in the choice of ,an anode metal are its potential and its current 'supplying ability with respect to the metal to be protected; this potential or current may tend to diminish from its. initial value so as to give rise to less active corrosion of the anode, accompanied and possibly complicated by passivity phenomena.

Corrosion reactions may be of two types; (1) the hydrogen type, in which the metal reacts with water to form a metal hydroxide,with evolution of hydrogen; and (2) the oxygen type in which oxygen enters into the reaction -of the metal with water. In both cases the driving force of the reaction is the free energy change, although other rate-controlling factors may be present. On the basis; of-solution potential and of free energy change, aluminurn'would appear to be the metal of choiceioranodeuse in cathodic protection systems. These theoretical indications have nevertheless not proved 1 rarest Patented Nov. 17, 1959 a suflicient basis for assuming that aluminum is the ideal cathodic protection metal, since there have been physical reasons which have prevented the corrosion reaction from proceeding at the required rate. For this and other reasons, aluminum and aluminum alloys have not been as widely adopted for this purpose as zinc an magnesium.

Other forms of aluminum alloys have been proposed for use in cathodic protection systems, in which the aluminum may be alloyed with small percentages of mercury or other metals. Such alloys tend to be expensive in terms of anode consumption per square foot of metal surface being protected.

The position of magnesium and zinc in the galvanic series, particularly when arranged in order of relative reactivity in aqueous sodium chloride solutions, is higher than that of aluminum, the standard oxidation potential at 25 C. being +2.34 for Mg and +0.762 for Zn, as compared with +1.67 for aluminum, and +0.44l for iron. .Thus, either magnesium, zinc or aluminum may be used to protect iron, because each corrodes preferentially. Because of this high potential, magnesium is used in low and high resistivity locations, including sea water, fresh water, and damp earth. However, since magnesium undergoes the hydrogen type of corrosion reaction, its use in salt water with its low resistivity is rendered objectionable' because enough hydrogen may be evolved at thecathode to constitute a fire hazard and to cause stripping of protective" coatings. This is of special significance in connectionwith protection of sea Water ballast tanks in ships, for which purpose magnesium is inadequate.

. Another factor to be considered in the choice of an anode metal is its electrochemical equivalent.

= In terms of electrochemical equivalents, alinninum possesses more favorable indications than the other two metals:

amperehour per 1b l 1, 352

but these figures are seldom attained in practical applications owing to side reactions. .7

In cathodic protection applications of aluminum heretofore made it has been consistently assumed that, aside from the presence of specific alloying elements, such as zinc or mercury, the aluminum base itself must be of high purity, and as free as possible from iron 101 other alloyingelements, in order to minimize local cell action. This assumption was based upon theoretical considerations with respect to potential differences at the surface of corroding metals. In accordance with prevailing theory',jmetals and alloys may be divided into three structural classes (1) pure metals; (2) alloys of two or more metals forming an analogous solid solution; and (3) alloys of two or more metals forming a solid solution and excess phases. In the case of solid solution alloys of the latter types, there may be present concentration gradients of alloying components from point to point resulting from the original casting or subsequent precipitation. Since the potential of a solid solution is a function of the solute concentration and the kind of solute, there exists a potential difference between areas of different alloy concentration. There may also be potential differences between excess phases and the solid solution. Hence-corrosion will progress in the appropriate electrolyte with the more anodic areas going into the solution. *The protective oxide film that is associated with not completely impervious to all electrolytes and corrosion of the analogous pure metal sometimes occurs at pores where the exposed metal acts as an anode, and the large area of surrounding film acts as a cathode. The rate of corrosion of aluminum in very dilute salt solutions is sharply reduced by the presence of oxygen. There are presently on the market aluminum anode alloys with about 5 percent zinc, but in which the remainder of alloying elements has been purposefully kept as low as possible. Such alloys are made from a high purity, premium grade of aluminum, of at least 99.7% pure Al, as a base, with subsequent addition of the desired amount of zinc.

In accordance with the present invention, it has been found, entirely unexpectedly, and contrary to indications based upon theoretical conceptions, that greatly improved cathodic protection systems may be achieved by means of employing as anodes therein, alloys of aluminum and zinc in which there are present, in addition, appreciable percentages of other auxiliary metals. Metals of this class, each having an analogous relationship to alumi num, include, for example, iron, chromium, nickel, and manganese. These metals form such intermetallic compounds with the aluminum as, for example, CrAl NiAl and the like.

While any of the aforementioned metals may be present in the aluminous anode alloy with beneficial results, it has been found that excellent performance is obtained when iron is present as the auxiliary metal. This discovery is all the more remarkable since the art has here tofore been unanimously of the opinion that iron was a contaminant'which had to be kept to a minimum, preferably not'cxceeding a few tenths of a percent.

In accordance with the present invention, aluminous alloy sacrificial anodes are employed in cathodic protection systems which comprise an alloy of aluminum with from about 2 to about 8 percent of zinc, and which contain additions of an auxiliary metal. Alloys which contain about 2 to 8 percent of zinc, preferably about 3 to 6 percent of zinc, and from 0.5 to 3.5 percent of iron have been found particularly well suited for this purpose. A preferred embodiment is an aluminous alloy contain ing about 5 percent of zinc and about 1 percent of iron.

The discovery of the significance of the presence of the auxiliary metal, such as, for example, iron, has made it possible to employ successfully, and for the first time, commercial grades rather than premium high purity (99.7% Al minimum) grades of aluminum. Anodes for the present invention can be made from regular grades of commercial purity aluminum, as well as from second ary ingot, commercial scrap, and even from aircraft scrap, which may contain such elements as zinc, copper, iron, silicon, manganese, nickel, or chromium, with substantial cost savings. Although it might be expected that such commercial grade aluminum anodes would be less etlicient and of shorter life than anode alloys made from high-purity or premium grades of aluminum be cause of local corrosion cells giving rise to parasitic currents and other side reactions, however, the contrary has been found. In actual tests, disclosed below, the cur rent elficiency of the high iron content anodes of this invention is of the order of 62%, in comparison with the 56% eificiency of high-purity aluminum base anodes.

The anodes of the present invention can be used in cathodic protection systems for underground structures such as pipe lines, foundations, and the like. They may be used in fresh water or in saline aqueous media. They are particularly well suited for use in sea water, and provide for the first time, cathodic protection systems for protection of iron, such as ships hulls, ballast tanks, and commercial fishing devices such as lobster pots, which are free from the shortcomings of previouslyused systems employing aluminum. This anode material may also be coated or laminated to other metal by any of the Well known coating or laminating techniques to afford cathodic protection to the base metal. Furthermore, this anode may be used in relationship with carbon or other electrode materials, such as carbons, in a suitable electrolyte in order to provide electric current.

Thus, among the advantages of the cathodic protection systems of the present invention may be mentioned greatly enhanced anode life, with attendant cost savings, dependability and continuity of protective action, a steady and sustained anodic current, increased current efiiciency, virtual elimination of self-exclusion or passivating effects, and exceptionally low anode weight consumption per square foot of surface protected.

In carrying out the present invention, the sacrificial anode of the type previously described, is attached to a metal structure to be protected, such as, for example, a ferrous metal structure, by means of a suitable electrical conductor, and then immersed or imbedded in the surrounding corrosive medium, in accordance with the customary practice. The alloy anode may be of any desired shape or size, such as for example, a cylindrical piece, or a trapezoidal shaped member.

The invention may be illustrated by means of the followingexamples, without, however, being stricted thereto:

EXAMPLE I Aluminum anodes were prepared having the following composition:

Inlaboratory tests using a standard saturated calomel cell in an aqueous solution containing 53 g. NaCl and 3 g. H O Vper liter, this alloy exhibited a solution potential' of 1.03 volts. In a 3.5% ,NaCl solution (simulated sea water), at room temperature, coupled to a piece of steel plate, the potential was +0.38 volt with a steady current being 0.50 milliampere. In comparison with anAl anode containing about 4% Zn, but only 0.20% Fe, the current in the latter when similarly coupled was only 0.12 milliampere. In comparison with a magnesium anode, the projected life of the aluminum anode was 19 months for this composition, and only 2.3 months for the magnesium anode. I

7 EXAMPLE II An anode alloy was prepared mainly from aluminum aircraft scrap metal and had the following composition:

. Percent Zn 4.97 Fe 0.85 Si 0.27

Cu 0.62 Al Balance Start 0.55 volt, 16 milliamperes. Steady (after 72 hrs.) 0.39 volt, milliamperes.

In the course of a 9-day test, the output of the anodes was determined to average 836 ampere-hours per lb. at a current density of milliamperes per square foot,

in simulated sea water. This corresponds to a current efficiency of about 62%. l I

The improved properties of the anode alloy of Example II are readily seen from a comparison with commercial types of anodes now on'the market,as shown in Table I: 5 to other metal by any It will be seen from Table 2 that the low iroii content,

high purity aluminum base alloys were inferior in performance to the much higher iron content anodes.

This anode material may also be coated or laminated of the well-known coating or 1 Table 1 1 PROPERTIES OF COMMERCIAL SACRIFICIAL ANODES 1 Example 11- Magnesium Zinc Aluminum Commercial De io'n ttinn AZ 63 MIL-A-l8001 13-605 Zn, 4.91; Al, 5.0-1.0 Trac 'znsa.

Composition, p n

' .62 Impurities s1, 6.10 max. Solution potential (Ag-AgCl) 0.91 (HgCl) 1.5 1.0 v.

- Actual current output, amp. hours per lb 725 at 56% ed.

The influence of various grades of aluminum purity, laminating techniques to aiford cathodic protection to commercial and premium, in aluminum anode material make-up was earlier studied. In a series of practical tests with aluminum anodes in the wing tanks of a seagoing ore carrier, the anodes were placed in the tanks,

coupled to the steelwork, and subjected to alternate wet and dry periods of about four days each, i.e. sea water ballast and loaded conditions, respectively. Fifteen anodes of various compositions were installed and by critical inspection at appropriate intervalsof time of the steelwork in the vicinity of the anodes, the wastage, physical nature of the corrosion products, and the like, the relative performances of high purity aluminum base anodes and those of the present invention were determined. Arbitrary designations A, B and C were assigned to these performance ratings, A being adjudged most desirable and C least desirable, based 'on condition of steelwork, wastage, and the like. The results are summarized in Table 2:

Table 2 I The average wastage of anode material in lbs. per sq. ft. of metal being .protected was determined as of about the following order of magnitude:

A rating 0.212 B rating 0.128 C rating 0.0450

The wastage for the A rating was adequate for effective protection of the steelwork whereas that lower values of B and C ratings indicated a measure of selfexclusion resulting in less effective protection of the steelwork.

Further practical tests were made as earlier described in the sea-going ore carrier and four anodes of composition shown in Example II were not coupled to the steelwork being electrically insulated therefrom. Negligible wastage of these anodes over a years time indicated that the iron, copper and silicon contents of said anodes do not in themselves cause increased wastage due to parasitic effects, as might have been expected from general corrosion theory.

the base metal. In the prior art there exist numerous techniques of coating metals with nominally pure or alloyed aluminum. By way of example two may be quoted. I

For steel parts, pole line hardware, bolts, fasteners and the like, the technique consists essentially of removing grease, dirt and the like and modifying orre-' for several minutes at or about a temperature of 1300 F. Subsequent treatments may be used to improve uniformity of coating, appearance, adherence or the like.

In another technique steel strip is coated continuously by passing through appropriate pro-treatments to prepare the surface for coating, one such pretreatment being passage through a controlled atmosphere furnace, and then immersion into a bath of molten aluminum or aluminum alloy held at a temperature of about 1300 F. or any other convenient temperature.

In these examples quoted, which are by no means the' only methodsin the art, thealuminum bath may be variously nominally pure aluminum or alternatively alloyed with, for example, afew percent'lof silicon. Serv: ice life and test evaluations have demonstrated the eifectiveness of these aluminiun or aluminum alloy coatings 'in combating atmospheric'or other'corrosion and protecting the steel core material.

Such protection is both mechanical in that the aluminum or alloy surface is inherently corrosion resistant and electrochemical in that the aluminum or alloy surface is anodic to the steel core and thusprotects the steel at holidays such as pits, scratches and the like by galvanic means.

Our alloy for anodic purposes has been shown to be superior to the aluminum or aluminum alloys used previously and consequently the use of our alloy for said coating or lamination purposes will show even more enhanced protective properties than now available with nominally pure or alloyed aluminum of the present art. Its freedom from self-exclusion extends the protective property over larger holidays in the coating and damaged areas.

The freedom from self-exclusion shown by our alloy furthermore offers distinct advantage in the construction of primary electric cells. The commonly used dry cell consists essentially of a carbon or other inert cathode coupled to a zinc anode in a paste electrolyte of ammonium chloride with, for example, manganese dioxide for depolarizing purposes. The zinc usually forms the container for the electrolyte. though other arrangements can be made. The galvanic cell so formed provides electricalnium chloride paste, with or without inhibitor such as aluminum chloride to restrain sidefefiects on standing'and to prolong shelf life and contained in a can of our aluminum alloy, though any'other convenient form may be used. a

While I have illustrated and described the preferred embodiments of the invention, it will be recognized that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

'I claim:

l. A cathodic protection system comprising a cathodic metal structure and at least one aluminous sacrificial anode electrically connected thereto, both the metal structure and the anode being in contact with a medium corrosive to said metal structure, said anode comprising an aluminum base alloy whose remaining constituents consist essentially of from about 2 to about 8 percent of zinc, and from 0.5 to 3.5 percent of an auxiliary metal selected from the group consisting of iron, chromium, nickel and manganese.

- 2. A cathodic protection system comprising a cathodic ferrous-metal structure and at least one aluminous sacrificial anode electrically connected thereto, both the metal structure and the anode being in contact with a medium corrosive to "said metal structure, said anode comprising an aluminum base alloy Whose remaining constituents consist essentially of from about 2 to about 8 percent of zinc, and from 0.5 to 3.5 percent of iron.

3. A cathodicprotection system comprising a cathodic ferrous metal structure and at least one aluminous sacri ficial anode electrically connected thereto, both'the metal structure and the anode being in contact with a medium corrosive to saidrnetal structure, said anode comprising an aluminum base alloy whose remaining constituents consist essentially of from about 3 to about 6 percent of zinc and from 0.5 to 3.5 percent of iron.

4. A cathodic protection system comprising a cathodic ferrous metal structure and at least one aluminous sacrificial anode electrically connected thereto, both the metal structure and the anode being in contact with an aqueoussaline medium corrosive to said metal structure, said anode comprising an aluminum base alloy-whose remaining constituents consist essentially of about percent zinc and about 1 percent of iron.

5. A cathodic protection system comprising a cathodic ferrous metal structure and at least one aluminous sacrificial anode electrically connected thereto, both the metal:

structure and the anode being in contact with an aqueous saline medium corrosive to said metal structure, said anode comprising an aluminum base alloy whose remaining constituents consist essentially of zinc and iron and prepared by adding from 2 to 8 percent of zinc to aluminum of commercial grade, and containing from 0.5 to 3.5 percent of iron.

6. The method of cathodically protecting a ferrous metal structure in contact with a medium corrosive there to which comprises connecting to said metal structure an aluminous sacrificial anode and immersing said anode in said corrosive medium, said anode comprising an aluminum base alloy whose remaining constituents consist essentially of from about 2 to 8 percent of zinc and from 0.5 to 3.5 percent'of iron.

7. The method of cathodically protecting a ferrous metal structure in contact with an aqueous saline medium corrosive thereto which comprises electrically connecting to said ferrouswmetal structure an aluminous sacrificial anode and immersing said anode'in said saline medium, said anode comprising an aluminum base alloy whose remaining constituents consist essentially of about 5 percent zinc and about 1 percent of iron.

8. A cathodic protection system comprising a cathodic metal structure and ananode electrically connected thereto, said anode comprising an aluminum base alloy whose remaining constituents consist esentially of from about 2 to about 8 percent of zinc, and from about 0.5 to 3.5 percent of an auxiliary metal selected from the group consist ing of iron, chromium, nickel and manganese.

9. The method of cathodically protecting a ferrous metal structure in contact with a medium corrosive thereto which comprises connecting to said metal structure an aluminous sacrificial anode and immersing said anode in said'corrosive medium, said anode comprising an aluminum base alloy whose remaining constituents consist essentially of from about 2 to 8 percent of zinc and from.

about 0.5 to 3.5 percent of an auxiliary metal selected from the group consisting of iron, chromium, nickel and manganese.

References Cited in the file of this patent FOREIGN PATENTS Switzerland Mar. 1, 1944 OTHER REFERENCES

Claims (1)

1. 9. THE METHOD OF CATHODICALLY PROTECTING A FERROUS METAL STRUCTURE IN CONTACT WITH A MEDIUM CORROSIVE THERETO WHICH COMPRISES CONNECTING TO SAID METAL STRUCTURE AN ALUMINOUS SACRIFICIAL ANODE AND IMMERSING SAID ANODE IN SAID CORROSIVE MEDIUM, SAID ANODE COMPRISING ALUMINUM BASE ALLOY WHOSE REMAINING CONSITTUENTS CONSIST ESSENTIALLY OF FROM THE 2 TO 8 PERCENT OF ZINC AND FROM ABOUT 0.5 TO 3.5 PERCENT OF AN AUXIALLY METAL SELECTED FROM THE GROUP CONSISTING OF IRON, CHROMIUM, NICKEL AND MANGANESE.