US3742588A

US3742588A - Consumable magnesium anode with a tin-coated, ferrous metal core wire

Info

Publication number: US3742588A
Application number: US00140672A
Authority: US
Inventors: P George
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1971-05-06
Filing date: 1971-05-16
Publication date: 1973-07-03
Anticipated expiration: 1990-07-03

Abstract

A consumable magnesium anode is provided which is particularly suitable for use in a household water heater. In a preferred embodiment, the magnesium anode is extruded over a steel core wire coated with tin metal. Prior to extrusion, the core wire is heated at a temperature which will convert the tin metal coating to a tin-iron alloy coating having a melting point high enough to permit successful extrusion of the anode rod around the core wire. Altering the composition of the tin metal coating by the heating step does not destroy the galvanic compatibility of the tin coating with the magnesium.

Description

United States Patent [1 1 George [111 3,742,588 (451 July 3, 1973 CONSUMABLE MAGNESIUM ANODE WITI-I A TIN-COATED, FERROUS METAL CORE WIRE [75] Inventor:

[73] Assignee: The Dow Chemical Company,

' Midland, Mich.

221- Filed! May 6, 1971 [21] Appl. No.: 140,672

Percy F. George, Lake Jackson, Tex.

[52] U.S. Cl... 29/458, 29/474.4, 29/480 [51] Int. Cl 823p 3/10 [58] Field of Search 29/458, 527.3, 527.2, 29/196.4, 474.4, 480; 204/197; 117/128, 114 B, 231

[56] l References Cited UNITED STATES PATENTS 2,735,l63 I 2/1956 Brooks et al 29/527.3 X 3,099,083 7/l963 De Long....... 29/458 3,558,463

1 1971- -Strobach et al 174/84 x OTHER PUBLICATIONS I Constitution of Binary Alloys, Hansen, 1958 pp. 718,

Primdry Examiner-Charles W. Lanham Assistant Examiner-D. C. Crane Attorney-Griswold & Burdick, V. Dean Clausen and Lloyd S. Jowanovitz 5 7 ABSTRACT A consumable magnesium anode isprovided which is particularly suitable for use in a household water heater. In a preferred embodiment, the magnesium anode is extruded over a steel core wire coated with tin -metal. Prior to extrusion, the core wire is heated at a temperature which will convert the tin metal coating to a tin-iron alloy coating having a melting point high.

enough to permit successful extrusion of the anode rod around the core wire. Altering the composition of the tin metal coating bythe heating step does not destroy the galvanic compatibilityof the tin coating with the magnesium;

5 Claims, 1 Drawing Figure Ex fruc/eo ma flea/um CONSUMABLE MAGNESIUM ANODE WITH A TIN-COATED, FERROUS METAL CORE WIRE BACKGROUND OF THE INVENTION This invention relates broadly to a consumable anode. More specifically, the invention covers a magnesium anode for a water heater, in which the anode is cast or extruded over a ferrous metal core wire coated with tin metal.

Consumable magnesium anodes are widely used for cathodic protection of iron or steel structures, such as underground pipelines, the hulls of ships and the inside wall surfaces of household water tanks. The anode structure used in a water heater usually consists of a magnesium rod extruded over a steel core wire. In operating position, the anode rod is usually suspended from the upper tank wall by connection to a hot water outlet fitting, with the core wire also being connected into the fitting. The core wire thus provides the required galvanic connection between the anode rod and the tank wall. Additionally, the core wire assures electrical connection to those parts of the anode which remain after some of the anode material has been completely consumed.

On the inside of, the water heater tank substantially more bare metal is exposed at the top and bottom of the tank than is exposed along the side walls. The extra bare metal area is created by the top and bottom heads of the tank, by the various inlet and outlet fittings and by flaws in the glass lining from welding of the top and bottom heads to the side walls. The additional current needed to protect the bare metal area from corrosion causes the ends of the anode to corrode first. Eventually, the ends of the anode will corrode down to the bare core wire and additional current flow is created between the anode and the core wire itself. The resulting accelerated corrosion of the anode shortens its life considerably.

Past attempts to construct an anode which is not subject to the problems mentioned above have not been entirely satisfactory. One attempt is an anode invention described in U.S. Pat. No. 2,841,546, to Robinson, in which the steel core wire is coated with aluminum prior to extrusion into the magnesium anode rod. An undesirable feature of this construction is that during extrusion the aluminum coating on the core wire forms an eutectic compound with the magnesium anode rod, which plugs the wire entrance hole into the extrusion die and causes the core wire to break. Another disadvantage is that the iron in the core wire will diffuse into the aluminum coating to give a composition which has very poor galvanic compatibility with the magnesium anode. Earlier attempts, as noted in the Robinson patent, included applying a zinc galvanized coating to the steel core wire prior'to extrusion into the magnesium coating, which is molten during the extrusion process, reacts with the metal of the extrusion die and causes 1 the die to crack from stress corrosion.

Although tin occupies a more noble location than magnesium inthe electromotive series, it has been structure which comprises a magnesium metal article and a ferrous metal article. The suppression is achieved by interposing a continuous layer of tin metal between the magnesium metal and the ferrous metal. The De- Long patent, however, does not teach the present in-' vention. The invention described hereincovers an improved method for extruding or casting a magnesium article over a tin-coated ferrous metal by heating the ferrous metal article prior to extrusion or casting to alter the composition of the tin coating.

SUMMARY OF THE INVENTION A broad object of the invention is to provide an improved method for fabricating a consumable magne sium anode having a ferrous metal core wire. 1

A more specific object is to provide a method for fabricating a magnesium anode in which the ferrous metal core wire is coated with tin and heated prior to extruding or casting the anode rod around the core wire.

Broadly, the invention provides a consumable anode fabricated of a ferrous metal core wire encased by' a magnesium metal sheath. A preferred method for fabricating the anode comprises applying a continuous coating of tin metal to the core wire. The tin-coated core wire is then heated in a nonoxidizing atmosphere at a temperature below the melting point of the tin. The heating period is for a time sufficient to convert the tin metal coating to a tin-iron alloy coating having a melting point not lower than about 800 F. After'obtaining the desired tin-iron alloy composition on the core wire, the magnesium metal sheath is shaped around the tincoated wire.

DESCRIPTION OF THE DRAWING The single FIGURE is a cross-section view of a con- I sumable magnesium anode fabricated according to the anode. The problem in this concept isthat the zinc,

known for some time that when the two metals are galvanically coupled the tin is suitably compatible with the magnesium. One reference which described this phemethod of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT The drawing illustrates a cross-section view of a consumable anode of this invention, as indicated generally by numeral 10. Basically, the anode 10 consists of a ferrous metal core wire 11, which has a continuous tin metal coating 12 thereon. The tin-coated core wire 11 is encased by a magnesium anode rod 13. The rod 13 may be shaped around the core wire by a conventional extrusion process or by casting. In practice, it is preferred to extrude the anode rod over the core wire.

In fabricating the anode structure, it is preferred to use commercially available anode core wire of the usual composition, such as black iron or steel. For the anode rod, the preferred magnesium alloy composition is AZ3lB, as designated by the American Society for Testing Materials (ASTM). For the continuous tin metal coating applied to the core wire, the preferred composition is a commercial tin "alloy composition comprising at least 40 percent tin. The tin metal coating may be applied to the core wire by any of various known commercial methods, such as spraying, hot dripping, casting or electroplating. A suitable thickness for the tin metal coating is from about 0.0002 to 0.0003

of an inch. It is especially preferred to coat the steel core wire with tin metal to a thickness of about 0.0002 I of an inch, using a commercial electroplating process.

Referring to extrusion of the anode rod'over the core enough to permit a successful extrusion without destroying the galvanic compatibility of the tin composition with the magnesium. Since tin melts at about 449 F. and the extrusion temperature of magnesium is about 800 F., a successful extrusion can be obtained only by altering the composition of the tin coating. In practice, this is achieved by heating the tin-coated core wire for a given period of time at a temperature below the melting point of tin, to thereby diffuse a small amount of iron into the tin coating. The resulting tiniron alloy coating has suitable galvanic compatibility with the magnesium anode rod and the melting point of the alloy is above the 800 F. required for extrusion. Specifically, the tin-coated core wire is heated in a nonoxidizing atmosphere at about 425 F. for from about 2 hours to 8 hours to obtain a tin-iron alloy coating having a melting point of between about 800 F. and l,000 F. For optimum extrusion conditions, it is preferred to heat the core wire for about 8 hours at 425 F., to obtain a tin-iron alloy coating having a melting point of about 1,000 F.

Several tests were conducted to determine the galvanic compatibility and other characteristics of a consumable anode structure fabricated according to the practice of this invention. The first test, which is described in Example I below, relates to galvanic compatibility of a tin-coated ferrous metal cathode with a magnesium anode.

EXAMPLE 1 Pieces of steel measuring about 1 inch wide, 3 inches long and 0.007 of an inch thick, were used as a cathode. Three (3) different types of'cathode pieces were used in the test. The first type was an uncoated steel piece, which was designated Cathode A. The second type, which was designated Cathode B, was a steel piece coated with tin metal about 0.0002 of an inch thick. The Cathode B pieces were not heat treated prior to the galvanic compatibility test. The third type, which was designated Cathode C, was a steel piece having a tin coating identical to Cathode B. The Cathode C pieces were heated at 500 F. (i.e., above the melting point of tin) for from about 1 hour to 6 hours prior to the galvanic compatibility tests.

Each cathode piece was galvanically connected by a metal fastener to a piece of magnesium (AZ3lB) of identical size, which had been precisely weighed. The metal coupled unit thus formed a galvanic cell, with the magnesium piece representing the anode. Each cell was placed in a water electrolyte, so that about 1 inch of the cell projected above the surface of the water. The cell was held in this position in the electrolyte solution for a period of about 24 hours. The water electrolyte had an electrical resistivity of about 450 ohm/cc and was maintained at a temperature of 150 F. to simulate average conditions found in a household water heater. At the end of the 24-hour period, each cell was disassembled and each magnesium anode piece was thoroughly cleaned in a chromic acid solution. Each anode piece was again precisely weighed to determine loss from galvanic corrosion during the period the cell was immersed in the water electrolyte. Results are shown in Table 1 below.

TABLE I Galvanic Compatibility of Mg Anode-Steel Cathode Unit of Example I Net Wt Cathode Heat Treatment of Cathode Loss of Type Temperature Time Mg Anode A 131 g. B 0.52 g C 500F. 1 hr. 0.33 g C 500F. 3 hrs. 0.46 g C 500F. 6 hrs. 0.74 g

From the data in Table I it will be apparent that the tin metal coating on the steel cathode piece has a defi nite corrosion-inhibiting effect on the magnesium anode. It will be noted, for example, that in the cell unit containing the uncoated steel cathode (Cathode A), the weight loss of the magnesium anode was more than double that of the magnesium anode in the cell units containing tin-coated cathodes.

EXAMPLE 2 The following test was to determine the galvanic compatibility of the magnesium anode with a tin-plated cathode in which the composition of the tin coating was altered by heating the cathode structure at a temperature below the melting point of tin. The anode and cathode pieces employed in this example were identical in size to those described in Example 1 and the galvanic cell units were made up according to the procedure of Example 1. In addition, the compatibility test of each cell unit was conducted under the same conditions described in Example 1. Prior to the compatibility test each of the tin-plated cathode pieces was heated at 425 F. for a periodof time sufficient to convert the tin coating to a tin-iron alloy coating. The melting point of each alloy coating was then checked by a conventional procedure to determine if it was high enough for successful extrusion of the cathode with a magnesium metal anode. Results are set out in Table ll below.

TABLE II Galvanic Compatibility of Mg Anode-Steel Cathode Unit of Example 2 Melting Point. Net Wt Heat Treatment of Tin-Iron Loss of of Cathode Alloy Coating on Mg Cathode Type Temp. Time. Cathode Anode Tin-Plated Steel 425F. l hr. 600F. 0.26 g. Tin-Plated Steel 425F. 2 hrs. 800F. 0.33 g. Tin-Plated Steel 425F. 8 hrs. 1000"F. 0.43 g.

EXAMPLE 3 The test of this example relates to current flow between the magnesium anode rod and the tin-coated steel core wire of an anode structure fabricated according to this invention, as compared to current flow between a magnesium anode and an uncoated steel core wire. Two types of anode structures were prepared by a conventional extrusion process. One of the anode structures, which was designated anode A, consisted of a magnesium anode extruded over a bare steel core wire, (i.e., the core wire was not coated with tin metal and it was not heat-treated). For this test the anode structure A served as a control blank. The second anode structure, which was designated as anode B, conabout 9 days. A standard condition for the current flow measurement was established by using a factor to correct the water resistivity to'an average of 450 ohms/cc and the water temperature to an average of 58 C. Th

sisted of a magnesium anode extruded over a steel core 5 results are Shown in Table m below:

('urrent Flow From Mg Anode Rod to (ore Wire in the Anode Structure of Example 3 Current Flow From Mg Anode to 'lank Wall +6 in. Exposed (Tore Wire (in milliamperes) TABLE 111 Difference in ('urrent Required from Anode (/\-Bl Current Anode Structure A Anode Structure B Measurement Period (Uncoated Steel Core (Tin-Plated Steel Core Percent (in Days) Wire) WireHeat Treated) Milliamperes (approximate) 1st 27.0 24.0 3.0 11.0 2nd 31.0 24.3 6.7 21.5 5th 28.0 23.0 5.0 18.0 7th 25.0 20.0 5.0 20.0 8th 26.5 23.0 3.5 13.2 9th 27.0 22.0 5.0 18.5

wire, which had been plated with a tin metal coating of about 0.0002 inch thickness. The core wire of anode B was heated in a nonoxidizing atmosphere at about 425 F. for about 8 hours prior to extrusion with the magnesium anode.

Each of the anode structures A and B were installed in a conventional glass-lined water heater and were set up to test for current flow between the anode rod and the core wire in the following manner. The anode structure was suspended from the upper tank wall of the heater by connection of one end to a fitting which was insulated from the tank wall itself. The fitting was insulated to prevent contact between the magnesium anode and the steel tank wall of the heater. The lower six (6) inches of the magnesium anode rod was carefully stripped away from the core wire. The exposed piece of core wire was cut off and then reconnected at its cut end to the core wire remaining in the anode rod. 1n the reconnection the cut end of the core wire was insulated from the anode rod and the main core wire remaining in the anode rod. The opposite end of the exposed core wire piece was connected to one end of a piece of heavily insulated copper wire. The opposite end of the copper wire was connected directly to the inner side wall of the heater tank.

The upper end of the main core wire in the magnesium anode was extended beyond the anode and was connected through a milliammeter to the upper tank wall of the heater. The milliammeter thus enabled measurement of the current flow from the anode rod to the tank wall, which also includes any stray current flow from the anode rod to the exposed core wire piece.

From the data of Table 111 it will be observed that the exposed core wire of anode structure B, which was tincoated and heat-treated according to the practice of the invention, demanded substantially less current from the anode than the exposed uncoated core wire of anode structure A. As indicated, in a comparison between the two anode structures, the average difference in current flow from anode to tank wall plus core wire is more than 15 percent. The substantially lesser amount of current demanded by the tin-coated core wire, therefore, contributes greatly to the life of the anode.

What is claimed is:

1. In the fabrication of a consumable anode consisting of a ferrous metal core wire-encased by a magnesium metal sheath, the method which comprises:

a. applying to the ferrous metal core wire a continuous coating of a tin metal,

b. heating the tin-coated core wire in a non-oxidizing atmosphere at a temperature below the melting point of tin for a period of time sufficient to convert the said tin metal coating to a tin-iron alloy coating having a melting point not lower than about 800 F., and

c. extruding the magnesium metal sheath around the said tin-coated core wire.

2. The method of claim 1 in which the core wire is a steel wire.

3. The method of claim 1 in which the tin-metal coat ing on the core wire has a thickness of from about 0.0002 of an inch to 0.0003 of an inch.

4. The method of claim 1 in which the tin-coated core wire is heated at about 425 F. for from about 2 hours to 8 hours.

5. The method of claim 1 in which the tin-iron alloy coating has a melting point of about 1,000 F.

Claims

2. The method of claim 1 in which the core wire is a steel wire.
3. The method of claim 1 in which the tin-metal coating on the core wire has a thickness of from about 0.0002 of an inch to 0.0003 of an inch.
4. The method of claim 1 in which the tin-coated core wire is heated at about 425* F. for from about 2 hours to 8 hours.
5. The method of claim 1 in which the tin-iron alloy coating has a melting point of about 1,000* F.