US2478479A - Cored magnesium anode in galvanic protection - Google Patents

Description

Aug. 9, 1949.

J. J. GREBE El AL CORED MAGNESIUM ANODE IN GALVANIC PROTECTION 'Filed Feb. 3, 1947 A/ base 0//oy con/dining 4 A My.

C onsum a/e INVENTORS. I Jo/zn J Grebe BY f/qro/oCkRob/nson MMM A TTOR/VEYS Patented Aug. 9, 1949 UNITED STATES PATENT, OFFICE- 2,]

- CORED MAGNESIUM ANODE 'I'N GALVANIC rno'rEo'rIoN r J ohn J. Grebe, Oak Ridge, Tenn; and Harold 'Robinson, Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich.,'. a corporation of Delaware' mutation February 3, 1947-;sena1No; 725,982 g- L ,This invention relates to systems for the 9.1- i

'vanic protection of .corrodible metal structures .using improved cored ,m agnesium anodes, v

In the galvanic protection of pipelinesand 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 the structure cathodicwith respect to the soil and greatly minimizes its corrosion. The consumable anodes heretofore used have conventionally consisted of a body of a highly electroneg ative metal, such as magnesium or zinc, cast about a steel core. The core is required in order to maintain structural 'unity of the anode as the active metal is consumed. With magnesium anodes, however, steel cores are somewhat disadvantageous in that, as soon as any part of the steel is exposed byc'onsumption of the magnesium, a strong local electrolytic couple isset up betweenthe core and the outer metal which, under unfavorable conditions, results in appreciable consumption of the magnesium.

' It is therefore the principal object of the present invention to provide a galvanic protection system using an improved cored magnesium anode which is not subject to the disadvantages mentioned.

The improved consumable anode used in the present invention consists essentially of a core of an alloy containing from about 1 to about 92v percent by weight of magnesium, balance substantially aluminum, surrounded by and bonded to an annular body of a magnesium-base alloy containing a substantially higher proportion of magnesium than the core alloy. 7

In the galvanic consumption of such an anode, the electrolytic potential difierence between the alloy of the core and the outer metal is sufficient to maintain the core almost unattacked until the outer metal is virtually all consumed,

Fig :1 is a transverse section through a consumable anode according ,to the invention; and

.- Fig.2 is adiagrammatic vertical section showing the manner of using a consumable anode in the galvanic protection of a buried pipeline.

As stated, the core alloy should contain from about 1 to about 92 percent; by weight of magnesium,,the balance being substantially alumi num. If the lower limit is passed, the alloy approaches pure aluminum in behavior and is subof an aluminum-base alloy containing from about ltoabout l2percent by weight of magnesium 'as "themajor alloying element, surrounded ,by

an annular body of a metal containing at least .85'percent magnesium. A typical anode would be one in which the core consists of a commer cialaluminum-base alloy containing about} .4 per 'cent'ma'gnesium, balance substantially aluminum, and the annular body consists of commercially pure magnesium. An anode of this type is illustrated in Fig. 1.

In an alternative construction, the core consists of a magnesium-base alloy containing not over 92 percent by weight of magnesium and the outer body contains more than 95 percent magnesium. For example, a core of a commercial magnesium-base alloy containing 8.5 percent aluminum, 0.5 percent zinc, and 0.2 percent man- 7 ganese, balance substantially magnesium, is surrounded by a body of commercially pure magnesium.

Anodes of the types described are most conveniently made by a composite extrusion technique. For example, a cylindrical billet of the aluminum-base alloy is inserted in a central bore in a larger cylindrical magnesium billet and the resulting composite billet is forced under heat and pressure through an extrusion die, according to the procedure described in U. S. 2,023,498. In an alternative procedure, the central core is first made by casting and is then centered in at; larger mold, and the outer metal cast around 1 A typical installation of an improved anode of the invention is shown in Fig. 2, in which an anode of the type shown in Fig. 1 is used in the galvanic protection of a buried steel pipeline. As illustrated, the anode is buried in a suitablysized hole in the earth a short distance from the pipeline and is surrounded by a. backfill, such as a bentonite-gypsum mixture, which serves to control the anode environment. An insulated wire electrically connects the core of the anode to the pipeline... 7 a

While the new anodes are particularly useful in the protection of underground ferrous metal structures, they may be employed generally in the galvanic protection of ferrous metals immersed in natural aqueous electrolytes, including ground water, fresh water, and seawater. It is necessary only that one or more anodes be immersed in the same body of electrolyte as the structure and be connected electrically to the latter. Typical uses are the protection of ships bottoms and the prevention of corrosion of domestic hot water heaters. The criteria for choosing the size and number of anodes for a given installation are well known in the art.

It is to be understood that the foregoing deserlptlon is illustrative rather than limitatl've,

and that the invention is ctr-extensive in scope with the followmg, claims. 7

What is claimed is:

I. In combination with a ferrous metal structure immersed in an aqueouselectrolyte, galvanic protection means including a consumable anode immersed in the electrolyte and connected electricall y to the structure, such anode comprising a coreof an alloy containing from about 1- to about 9 2" per cent byweight of magnesium, balance substantially aluminum, surrounded by and bonded to: a body-of a magnesium-base alloy containing a substantially higher proportion of magnesiumthan the core alloy and containing at least 85 percent magnesium.

2. In combination with a ferrous metal structure immersed in: a natural aqueous electrolyte, galvanic protection means including a. consumableanodeimmersed in the electrolyte and connectedelectrically to the structure, such anode comprising a core of'an aluminum-base alloy contaming; from about i to. about 12' percent by weight of magnesium, balance substantially aluminum, surrounded by and bonded to an annular body of a metal containing at least percent magnesium.

3. In combination with a ferrous metal structure immersed in a natural aqueous electrolyte, galvanic protection means including a consumable anode immersed in the electrolyte and connected electrically to the structure, such anode 7 comprising a core of an aluminum-base alloy containing about 4 per cent magnesium, balance substantially aluminum, surrounded by and bonded to an annular body of magnesium.

4. In combination with a ferrous metal structure immersed in a natural aqueous electrolyte, galvanic protection means including aconsumable anode immersed in the electrolyte and connected electrically to the structure, such anode comprising a core of a magnesium-base alloy containing not over 92 per cent by weight of magnesium, balance substantially aluminum, surrounded by and bonded to an annular body of a metal containing more than 95 percent magnesium.

JOHN J. GREBE. HAROLD" A. ROBINSON.

' REFERENCES CITED The following references. are of recordin the file of this patent:

UNITED- STATES PATENTS OTHER REFERENCES 7 "Ghem-ical 8r Metallurgical- Engineering, June 1943; p; 66.

Corrosionfivol. 1, June 1945, pp. 59 through 69.

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