US2841546A - Extruded magnesium anodes with aluminum-coated steel core wires - Google Patents

Description

mxmww A. ROBINSON STEEL CORE WIRES Filed Dec.

EXTRUDED MAGNESIUM ANODES WITH ALUMINUM-COATED July 1, 1958 A TTORNE Y6 2,841,546 1C6 V Patented July 1958.

EXTRUDED MAGNESIUM AN ODES WITH ALU- MINUM-COATED STEEL CORE WIRES Harold A. Robinson, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich.,a corporation of Delaware Application December 3, 1952, Serial No. 323,748

1 Claim. (Cl. 204-197) This invention relates to an improved method of producing a steel-cored magnesium metal anode rod. It also concerns the rod so produced. a

In the cathodic protection of domestic water heater tanks, a consumable rod of magnesium metal is installed inside the tank in electrical contact with the wall. There results a flow of galvanic current which maintains the tank cathodic with respect to the water in it and thus minimizes corrosion. Anode rods for this purpose are advantageously provided with a steel core to insure good electrical connection to all portions of the magnesium and to prevent the rod from breaking into pieces at it is consumed. As a practical matter, however, the manufacture of such cored rods has been quite troublesome.

Massive cored magnesium anodes for heavy-duty service, such as the protection of underground pipelines, are conveniently made by casting the magnesium metal about a steel or galvanized steel core. However, difliculties are encountered with this method when attempt is made to prepare the long, slender rod anodes needed for protecting domestic water heaters. Only short lengths can be cast, and even then it is hard to keep the core properly centered. The rod usually has the surface imperfections characteristic of casting. In addition, the mechanical and electrical bond between the core and the magnesium cannot be kept strictly uniform from one rod to another, with the result that rods show unpredictable variations in service.

Some of these problems are overcome by an extrusion process for making water-heater anodes. In this method, rods are produced by forcing magnesium metal at a hotworking temperature through a rod-forming die while feeding a bare steel core wire through a suitable guide into the extruding magnesium justbefore it enters the orifice. Anodes made in this way are adequate for cathodic protection service at ordinary temperatures and in water heater service where temperatures not much over 150 F. are normally encountered and an anode life of around five years is acceptable. However, at the higher water temperatures demanded by some modern home appliances, and with glass-lined water heater tanks requiring an anode lasting ten years or more, such extruded anodes may prove inadequate because of having a life expectancy far lower than that dictated by the amount of magnesium present. Under such service conditions, corrosion products gradually form and penetrate along the interface between the core and the magnesium. This action often weakens the anode and raises its electrical resistance to the point where the anode becomes ineffective before the magnesium is entirely consumed.

Efforts to correct this type of failure by using galvanized core wire in place of bare steel in the extrusion process have been worse than useless. Under the conditions of extrusion, the zinc galvanizing on the core wire attacks the tool-steel of the wire guide and causes very rapid deterioration. Other treatments of the core wire, such as knurling, sand-blasting, and the use of braided cable, have also been ineifective.

2 It is therefore the principal object of this invention to provide a steel-cored magnesium metal anode rod, and a method of producing it, which are not subject to the difficulties heretofore encountered. A related object is to provide a simple, effective method of producing a a strong bond between the core wire and the magnesium metal in an extruded cored anode which, in service at the high temperatures in a water heater, maintains its strength and low electrical resistance, and remains uncorroded, until the magnesium is virtually all consumed.

These objects are realized according to the invention by forming the cored magnesium metal anode by an extrusion process and providing a coating of metallic aluminum on the steel core wire employed. Optimum results are obtained by controlling the rates of advance of the aluminum-coated wire and the magnesium metal during extrusion to effect relative motion between them immediately before entry into the extrusion orifice. During extrusion, the magnesium and the steel each interact with the aluminum interlayer and are thus bonded together extremely tightly. This bond is not only of high,-

mechanical strength, but also is of unusually low electrical resistance. In addition, the metal of the bond is of such a nature as to resist destructive penetration of hot water along the core.

The invention may be explained with reference to the accompanying drawings, in which Fig. 1 is a transverse section through an anode rod produced according to the invention;

Fig. 2 is a longitudinal section through the outlet end of an extrusion press and rod-forming die assembly used in making the rod of Fig. 1, the section being along the lines 2-2 of Fig. 3; and

Fig. 3 is an elevation of the die of Fig. 2, viewed from the press side.

As shown in Fig. 1, each anode consists essentially of an extruded rod 4 of magnesium metal having co-extruded therewith a central core 5 of steel wire. The rod and core are bonded together tightly by a layer of aluminum 6 coated on the core wire. In practice, the rod may be formed either of magnesium or of any of the extrudable magnesium-base alloys, all such materials being herein termed magnesium metal. The core is steel, typically black annealed wire, and has a diameter only a small fraction of that of the rod, e. g. about 0.2 inch in the case of a one-inch rod. The coating on the core is aluminum, preferably the commercially pure metal, although any of the commercial aluminum-base alloys may be used. The thickness of the coating is likewise only a small fraction of the diameter of the core, usually only a few thousandths of an inch.

The equipment shown in Figs. 2 and 3 includes a conventional direct extrusion press including a cylinder 7, hydraulically-operated ram 8, and pressure block 9. Seated tightly in the outlet end of the cylinder is a porthole die 10. This die has a flat entrance face 11 fitting snugly into the cylinder 7, from which four portholes or metal passages 12 extend forwardly into a central annular mixing chamber 13 hollowed out of the other end of the die. A rod-forming die 14 fits flush into a socket in the forward face of the die 10 and is provided with a central circular rod-forming orifice 15. This orifice, at its inner end, flares out, as at 16, to the diameter of the mixing chamber 13. Those portions of the flared surface 16 which are opposite the ends of the portholes 12 are further grooved, as at 17, to the outside diameter of the portholes, to provide smooth fiow channels from the portholes into the mixing zone.

A wire guide nozzle 18 is held socketed centrallyin the mixing chamber in the face of the porthole die 10 by a capscrew 19. The core wire 5 to be extruded enters radially through a narrow passage 20 in the side of the 3. die leading to a registering passage 21 going into the hollow center of the guide nozzle 18. From there, the Wire passes axially out the nozzle and through the center of the die orifice 15. The nozzle 18, like the dies, is made of hardened tool steel. The diameter of the nozzle outlet is only a few thousandths of an inch larger than that of the wire, to prevent extrusion of metal into the nozzle. The nozzle 18 has a smoothly tapered exterior leading to a fairly sharp tip 22 where the wire issues.

An important feature of the die assembly is the provision of contours to control the rates of flow during extrusion, such that the extruding magnesium metal undergoes motion relative to the core wire in the zone where the two meet just before the die orifice. In the assembly of Fig. 2, this result is accomplished by making the taper 16 of the die throat sharper than the taper of the nozzle 18, and by terminating the tip 22 slightly before the end of the taper 16, i. e. before the die opening reaches its minimum diameter. In this way, the cross-sectional area of the annular space between the nozzle and the surrounding walls becomes progressively smaller from the back of the chamber 13 toward the die. Likewise because of the taper 16, the reduction in cross-sectional area of the extruding metal continues even beyond the end of the tip 22 right up to the point of minimum diameter of the orifice 15. It is this progressive area reduction which insures the desired relative motion of the magnesium metal and the core during extrusion.

In making cored anodes using the apparatus of Figs. 2 and 3, the steel core wire 5 is first cleaned carefully, by mechanical action or by pickling, and is then coated with aluminum. The coating may be accomplished by dipping the wire in molten aluminum. Alternatively, aluminum-coated steel wire may be purchased as such and then carefully cleaned and degreased. The magnesium or magnesium-base alloy is formed in conventional manner into extrusion billets to fit the cylinder 7 and these are preheated to a hot-working temperature.

The coated core wire 5 is then fed into the passage until it protrudes through the die orifice 15. At the same time a hot billet is charged into the cylinder 7 and the ram 8 is actuated to move the block 9 and thus force the billet against the entrance face 11 of the die 10. Under the resulting pressure, the magnesium metal flows through the portholes 12, fills the chamber 13 and the throat of the die 14, and issues from the die as a solid rod 4 with the core 5 firmly bonded in it. As the finished rod is extruded through the die, it pulls more of the wire 5 through the guide 18 into the die entrance to be surrounded by and bonded to more magnesium metal. Endless lengths of cored rod may be made by feeding billets one after another into the press.

In the finished moving rod leaving the die 14, the core wire and magnesium metal travel at the same speed and of course are immovable relative to one another. However, within the die, in the working zone where the wire and magnesium meet just before the die orifice, the contours of the die are made such that the cross-sectional area of flowing magnesium is larger in the chamber 13 than at the orifice, as already explained. In consequence, except right at the die orifice 15, the magnesium metal is moving at a slower rate than the core wire. As a resuit, in the region ahead of the die orifice the fastermoving coated core is pulled through the magnesium metal, developing extensive frictional forces and mechanical working in the layer of aluminum coating on the wire. It is this action which creates the exceptional bond between the core wire and the magnesium metal in the rod of the invention.

Extruded anodes may be identified and distinguished from east anodes by several structural differences. Many of the difierences between extruded and cast magnesium alloys are discussed in the paper entitled The Metallography of Commercial Magnesium Aiioys, by J. :B. Hess and P. F. George, presented before the 24th An- 4 nual Convention. of The American Society For Metals, at Cleveland, Ohio, October 12th to 16th, 1942. The above paper is published in Transactions of The American Society For Metals, vol. 31, pp. 423458 (1943).

The following characteristics are typical of the differences in structure which may be used to distinguish extrusions of magnesium-aluminum alloys from castings of the same alloy:

(1) Cast alloys have massive magnesium-aluminum compounds in their structure which are not in extrusions of the same alloy.

(2) Cast alloys have a heterogeneous solid solution whereas in extrusions the solid solution is substantially homogeneous.

(3) Cast alloys have considerably larger grain size than do extruded alloys.

The following example will further illustrate the invention but is not to be construed as limiting its scope.

Example Cored water-heater anode rods 0.84 inch in outside diameter were prepared according to the invention, the anode metal being a magnesium-base alloy containing approximately 3.0 percent by weight aluminum, 1.0 percent zinc, and 0.3 percent manganese, balance magnesium of high purity. The core wire, 0.192 inch in diameter, was an annealed steel wire which was purchased coated with commercially pure aluminum, and carefully cleaned before use. Billets of the magnesium-base alloy were preheated to a hot-working temperature of about 850 F. and extruded in an 8-inch commercial extrusion press around the core wire in accordance with the procedure described with reference to Figs. 2 and 3. Several hundred feet of anode rod was made. There was no observable wear on the tool-steel wire guide at the end of the operation.

The resulting rod was subjected to accelerated tests to determine its effectiveness as an anode in cathodic protection work. The tests were carried out in a conventional galvanized domestic water heater held at a temperature of 230 F. under a pressure of 50 pounds per square inch gauge, the heater being filled with city tap water from a Lake Huron source. In the tests, short lengths of the anode were installed in the tank and each was connected electrically to the wall through a currentmeasuring device. The anodes were observed daily to see Whetherthey were still delivering current. When current failure at any anode was observed, the anode was immediately removed from the tank and the cause of failure noted. In every case, the anode failed due to exhaustion of the magnesium alloy, and in no case was there a failure of the core-toanode bond. The lengths of the test specimens and the lives were: Ai -inch, 33 days; /z-inch, 56 days; l-inch, 89 days; Z-inches, days. From these data, it was concluded, by extrapolation based on experience with the test, that the life of a conventional anode rod 36 to 48 inches long, in ordinary water heater service at about B, would be at least 10 years.

For purpose of comparison, identical tests were also carried out on rods extruded in a way identical with that just described but using as the core clean black annealed steel wire 0.241 inch in diameter. Because of the absence of an aluminum coating on the core, such rods were not in accordance with the invention. In the accelerated test, the cessation of current flow from the anode was in every case the result of failure of the coreto-anode bond. In no instance was the magnesium-base alloy depleted. The lengths of the test specimens, and the lives, were: Ai-inch, 3 days; /z-inch, 5 days; l-inch, 11 .days; Z-inches, 14 days. A comparison of these results with those observed with the aluminum-coated cored anodes of the invention show that the latter have from invention, water-heater anode rods were made by casting 0.84 inch diameter rods from the same magnesium-base alloy identified above about core wires of the same sizes employed above. Some of these cast anodes were made with bare wire and some with aluminum-coated wire. All anodes were then subjected to the accelerated test described above. In every instance, with both uncoated and coated cores, the anode stopped delivering current dueto failure of the core-to-anode bond. In no case was the magnesium-base alloy depleted. The results of the life tests for the anodes of various lengths were: Bare steel core, A-inch, 2 days; /2-inch, 4 days; 1-inch, 7 days; 2-inches, 10 days. Aluminum-coated core, V4- inch, 4 days; /2-inch, 6 days; 1-inch, 12 days; Z-inches, 14 days. A comparison of these results with those ob- 6 served for extruded aluminum-cored anodes according to the invention shows that the latter have a life 7 to 13 times that of either type of the cast anodes.

What is claimed is:

A consumable anode comprising a rod of magnesium alloy having a central core of aluminum coated steel wire, said alloy being characterized by a generally homogeneous solid solution and the small grain size typical of extruded magnesium alloys.

References Cited in the file of this patent UNITED STATES PATENTS

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