US2478478A - Potential gradient anode for galvanic protection - Google Patents

Description

Aug. 9, 1949. 4 J GREBE 2,478,478

POTENTIAL GRADIENT ANODE FOR GALVANIC PROTECTION Filed Feb. 3, 1947 INVENTOR. .Jo/u? J Gree ATTORNEYS tures.

using such anodes.

rent in order to protect the structure.

their life.

Patented Aug. 9, 1949 UNITED f STATES "PATENT. j OFFICE POTENTIAL GRADIENT ANODE FOR YGALVANIC PROTECTION [john .1. Grebe, Oak Ridge, Tenn., assignor to The Dow Chemical Company, a corporation of Delaware Application February 3, 1947, Serial No. 725,981

This invention relates to an improved construction for consumable metal anodes used in the galvanic protection of. corrodible metal struc- It also concerns metal protection systems In the galvanic protection of pipelines and other underground steel structures, consumable electrodes of a metal anodic to the steel are buried in the earth near the structure and are connected to it'by electrical conductors. The resulting flow .Of current maintains thestructure cathodic with respect to the soil and greatly minimizes its corrosion. The consumable anodes heretofore used have conventionally consisted of a body of 'a highly electronegative metal, such as zinc or magnesium, cast'abouta steel core. Unfortunately, such anodes, while effective, have, in general, exhibited lower overall energy efficiencies than is desirable.

Thus, the consumable anodes, when'first installedgmust develop a comparatively high cur- Then gradually, as the structure becomes polarized, much less current is needed. The conventional anodes, however, because of their construction,

develop a nearly constant potential, and hence roughly constant current, throughout most of They thus tend, over a long period, to supply considerably more current than is actually required by the structure. This excess current, of course, consumesa part of the active metal of the anodes. If attempt is made to limit the current after the initial stages by inserting resistances in the circuit, the anodes are forced to operate at low current densities, un-

der which conditions self-corrosionor pitting of the active metal is often severe. Inaddition, once the consumption of the active metal pro- 10 Claims. (Cl. 204-:197)

ceeds to a point where the steel core is partly provide an efiicient anode which is consumed uniformly, with but little pitting or self-corrosion even at low current densities. An additional object is to provide an anode in which the core tends to remain unexposed until the active metal is virtually all consumed.

These objects are realized according to the invention by a consumable anode, the bodynf which is formed ofan alloy of an anodically-active metal in which the proportion of suohnietal decreases .from the outside of the body to the core; With-such an electrode, the inner portions are less anodic than the surface. Consequently,

as the electrode is consumed, the potential developed, and the current output in a given installation, decline gradually. Further, with the outer portions of the electrode anodic to menuderlying metal, the electrode is of necessity consumed from the outside inwardly in a uniform and efiicient manner; pitting is very greatly minimized, and exposure of the core seldom occurs.

The invention may be illustrated with reference to the accompanying drawing, in which Fig. 1 is a transverse section through a consumable anode according to theinvention, with zonewise variation in alloy composition;

Fig. 2 is a longitudinal central section through the anode. of Fig. 1;

Fig. 3 is a transverse section through an alternative form of anode, with continuous variation in alloy composition;

Fig. 4 is a longitudinal central section through the anode of Fig. 3; and I Fig. 5 is adiagrammatic vertical section showing the manner of using a consumable anode in the galvanic protection of a buried pipeline.

Consumable anodes according to the invention :consist of a 'body of alloy of an anodically active metal with aless active metal, which body surrounds and is firmly bonded to an electrically conductive core of a material different from and cathodic to that of the body. The proportion of active metal in the alloy decreases, either zonewise or in continuous progression, from the outside of thebody to the core.

The active metal forming the alloy ,may be any of the morehighly .electronegative metals commonly used in consumable anodes for galvanic protection work, especially magnesium or zinc.

.The less active metal component of the alloy may, .so far as known, be any metal cathodic to and alloyable with the active metal, e.g. aluminum or zinc in the case of magnesium as the active metal.

The preferred anodesare formed of alloys of the magnesium-aluminum system. The core of the anode may, in conventional manner, be ofany electrically conductive and preferably also mechanically strong material different from and cathodic to that of the body. In the case of magnesium alloy anodes, a nonmagnesic metal core is preferred, mild steel rod or cable being convenient.

A preferred form of the consumable anodes of the invention, with zone-wise variation in alloy composition, comprises an elongated metal core surrounded by and bonded to an elongated body of magnesium-aluminum alloy in the form of two or'more-annularzones of differing magnesium content all-concentric with the core, the crosssectional area of each zone being a substantial 3 fraction of that of the entire body and the proportion of magnesium in the metal in an Zeiie being greater than that in the zone nearer the core.

In a particularly advantageous construction, using two zones, a steel rod core is surrounded by an annular body of an alloy'ontaiiiing from about 1 to about 92 percent by weight of magnesium, balance substantially aluminum,- this latter being in turn surrounded by another an-- nular body of a metal containing more than 95 percent by weight of magnesium, the cross; sectional area of each body being large in com parison to that of the core. In this anode, the alloy of the outside zone, being essentially magnesium, provides a high electrolytic potential and is virtually non polarizing in use. The central alloy zone, with 1 to 92 percent magnesium,

is of readily polarizable material. As the anode is consumed and this zone is exposed, the alloy, because of this polarization, as well as the fact that it is cathodic to the outer layer, r'en'iairis virtually unattached until the more magnesiumrieh layer is entirely gone. At that time, the central Zone then begins to deliver current at a lower potential and a reduced output.

It is highly desirable that the alley ebliipeition f the inner be controlled Within the lifni-ts f-lt6 92 fiercest magn sium, If the lower limit is passed, the alloy approachese pui 'e aluminum in behavior and is subiect to substantial eatliedic deriosi'or'i and pittingby the alkaline medium produced as the outer magnesium io'ne is cohsiim'ed; I-f'the upper limit is iieeded, the alloy behaves much like magnesium, does not polariie readily, and is not sufficiently cathedi'c with res eet to the outer metal; optimum construction calls for an anode having an inner Zone ofan alumini-ini bas' alloy eontai'ning frem 1 to 12 Ierfit by Weight of magnesium as the'major alloying element and an outer zone of a metal contain n moi-e than 95 percent of filagii'itlmJ A-r'isnode of this character is illustrated in Figs. 1; and- 2; inwhich a mild stee core-is bonded to an inner zone of acommercial alum u ase alloy c'o'ntaiiiingabout 4 percentmagnesium, balance substantially aluminum, the eiitside being eom'meieiany sure magnesium;

zone wise anodes as in Figs; 1 and 2 may be produced in any of several'ways. Iii oi'ie'rnthod,

a clean steel eere, preferably galvanized to assure good bonding,- is centered sins-11yin iicyli licast around it: The resulting body, after easiing, is centered in a larger cylindrical mold arid the metal of the outer zoiie cast around-it: 'Dif fusion and even intermelting of the metal of the two zones occur at the interface, providing solid bonding. Another method of making the anodes invention, with continuousrather th of zone.

w fchaiigef in alloy composition, is: illustrated jseheinatieallyinliigs. 3 and 4 Inthis anode,

4 the steel core is bonded to a body of a magnesiumalii'zhinuri alldyin whien the prepoftion of magnesium decreases in fairly u'hif'oini progression from the outside of the body to the core. In such anodes, the alloy nearest the core is preferably rich in aluminum while the outside metal V ture of 400 to 860 F. for a period of 24 hours or more. At thesev temperatures, the aluminum and magnesium of the various zones interdiffuse slowly, producing a body in which the magnesium content decreases progressivly from the outside The consumable anodes of the invention are used. if! galvanic flidte'tiohSYstlhs in conv ntien'al manner in place of the anodes heretofore used, and exhibit exceptional advantages, as already explained. In atypical installation, illustested in Fig.- 5; an anode of the preferred t'y'IJe shew'n in Figs: 1 and 2 is used in the galvanic protection or a steel pipeline buried i fthe earth. As shown, the anode is buried in a suitably-sized hole in the earth a short; (iistaneeirom the "pineii'ne'ane is surrounded'by a baekfill, such as a bentoiiite-g'y'psuin mixture, which serves to (iontroi the anode environment. An insulated wire eleetiica-lly connects the core or the anode to the pipeline. 7

While the anodes are partieularly useful in the protection of underground ferrous metal structures, they may be employed genefally. in the galvanic prot'eetion eiierrous' nietais immers'ed in natural aqueous electrolytes1'ineluding greund water, fresh water, and seawater. It is necessary only that one 'or more anodes be immersed in the same body of electrmyte, as the structure and be connected electricall to the latter. Typical uses are the protection of shins bottoms and the prevention/(if corrosion or domestic hot water heaters. The criteria for eho'osihg thesize and number of anodes for a given installation arewe ll kn wn in the art;

It i 'iiheejrstoe i that the is going destination is illustrative rather than li'mitative, aiicith t the invention is eo-exteiisivein sct iewith. the renew n claims;

Wha-tis laimed is:

7 1; A for, the in, galvanic protection of metallic st uctures comprisin "a mayor-e110 of an e oaitauy attire metal with a'iess active iii-etai, bath sueh metals being a oeie with respect to the metal of the structure to he i io'tected, such body having therein an eleetrically seed Ve care of a material different' item andeathoaie to'that i the teat,

" tal iii'tiie alia'nede the: prepoitioii t'ifmore active in loy forming the body decreasing from the outside of. thebody to the; ease.

2; A consumable anode for use in galvanic protection of ferrous metal structure's con-inns ing a body "of alloy of magnesium with a less anodic metal which is ahedie to the ferrous metaltithe structure to be protected; sueii hody having therein a core of non-magnesic metal cathodic tothat ofth'e body, the pro ortion of magnesium in the all'oy-formi'ng'the body decreasing from'jth'e sutsidscithe they to the core.

3. An anode accordingtoelaim 2 wherein the body is formed of a binary magnesiumalamialloy, i 1 r 4. anodeacdordlng to claiin Z WheI-eih the eorQ-ismadeof-steeli 1 i 5. A consumable anode for use in galvanic protection of metallic structures comprising a steel core surrounded by and bonded to a body of magnesium-aluminum alloy, the proportion of magnesium in the alloy forming the body decreasing in continuous progression from the outside of the body to the core.

6. A consumable anode for use in galvanic protection of ferrous metal structures comprising anelongated body of magnesium-aluminum alloy surroundin and bonded to an elongated core of non-magnesic metal cathodic to that of the body, the body being the form of a plurality of annular zones of difiering magnesium content concentric with the core, the cross-sectional area of each zone being a substantial fraction of that of the entire body and the proportion 'of' magnesium in the metal in any zone bein greater than that in zones nearer the core.

7. An anode according to claim 6 wherein the core is made of steel.

8. A consumable anode for use in galvanic protection of metallic structures comprising an elongated steel core surrounded by and bonded to an annular alloy containing from about 1 to about 92 percent by weight of magnesium, balance substantially aluminum, such body being surrounded by and bonded to an annular body of a metal containing more than 95 percent by weight of magnesium, the cross-sectional area ,6 of each body being large in comparison to that of the core.

9. A consumable anode for use in galvanic protection of metallic structures comprising an elongated steel core surrounded by and bonded to an annular body of an aluminum-base alloy containing from at least 1 percent to 12 percent by weight of magnesium as the major alloying element, such body being surrounded by and bonded to an annular body of a metal containing more than percent magnesium, the cross-sectional area of each body being large in comparison to that of the core.

10. In combination with a ferrous metal structure immersed in a natural aqueous electrolyte, galvanic protection means comprising an anode as defined in claim 1 immersed in the electrolyte near the structure and electrically connected to the same.

JOHN J. GREBE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,011,613 Brown et a1 Aug. 20, 1935 2,204,823 Rhodes June 18. 1940

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