Aluminum sacrificial anode
United States Patent [1 1 Reding et al.  
 ALUMINUM SACRIFICIAL ANODE Inventors: John T. Reding; Robert L. Riley,  
 Jr., both of Lake Jackson. Tex.  
 Assignee&#39;. The Dow Chemical Company,  
 Midland, Mich.  
 Filed: July 15, 1974 Appl. No.: 488,334  
 US. Cl. 204/197; 72/364; 72/365; 72/377; 75/138; 204/148; 204/293 Int. Cl. C231 13/00 Field of Search 75/138; 204/148, 197, 293; 148/32; 72/364, 365, 377  
 [451 Apr. 15, 1975 [56] References Cited UNITED STATES PATENTS 3,379,636 4/1968 Reding et a1 75/138 Primary Examiner-R. Dean Attorney. Agent, or FirmRobert W. Selby 23 Claims, No Drawings ALUMINUM SACRIFICIAL ANODE BACKGROUND OF THE INVENTION This invention pertains to sacrificial anodes and more in particular to a method of forming an improved aluminum alloy sacrificial anode.  
  Theoretically, aluminum should be expected to perform satisfactorily as a sacrificial anode because the element aluminum fulfills the two primary requirements for sacrificial anodes, that is, a high theoretical oxidation potential (1.90 volts v. saturated KCl calomel reference) and a high theoretical electrical output per unit mass of metal consumed (2.98 amp.-hrs. per gram). In actual practice, however, unalloyed aluminum has not proven to be satisfactory for use as a sacrificial anode since it does not exhibit these favorable theoretical properties when used as a sacrificial galvanic anode. The presence of the normally passive oxide surface film on the aluminum apparently presents a barrier to the oxidation of the aluminum metal, thereby reducing the effective oxidation potential to about 0.7 volt (as measured in a closed circuit at either 250-r 1,000 milliamperes per square foot (ma/ft in a synthetic seawater electrolyte). Unless otherwise specified, all voltages herein are with respect to a saturated potassium chloride (KCl)-calomel half cell as a reference.  
  The effective oxidation potential of the aluminum metal in fresh water is about 0.4 volt (as measured in a closed circuit at milliamperes per square foot in water having a resistivity of about 5000 ohm.cm). The effective oxidation potential of aluminum in a saturated calcium sulfate (CaSO electrolyte is about 0.4 volt (as measured in a closed circuit at a current density of 50 milliamperes per square foot). At such low operating voltages, no effective cathodic protection is given to, for example, ferrous based structures; therefore, the anode exhibits no useful electrical output.  
  It is known in the art to add certain elements such as bismuth, gallium, indium, lead, magnesium, tin and zirconium to aluminum in an attempt to provide an aluminum anode of commercial utility. Examples of such aluminum alloys are illustrated in, for example, U.S. Pat. Nos. 3,240,688; 3,337,332; 3,337,333; 3,368,952 and 3,379,636.  
  A method to produce an aluminum alloy suitable for use as an anode in various environments including water heating systems and underground environments is desired.  
 SUMMARY OF THE INVENTION The novel method of the present invention comprises first providing an aluminum alloy consisting essentially of about 0.02 to about 2 weight per cent bismuth, about 0.005 to about 0.05 weight per cent gallium, about 0.005 to about 0.5 weight per cent indium and the balance of the alloy being essentially aluminum. The aluminum alloy is then hot worked sufficiently to provide a worked alloy suitable for use as a galvanic anode with an oxidation potential of about 1.0 to about l.3 volts at a current density of 10 ma/ft in a fresh water electrolyte with a resistivity of 5,000 ohm.cin or about 1.3 to 1.6 volts at a current density of about 50 ma/ft in a saturated CaSO electrolyte. The sacrificial anode of the present invention is useful in low resistivity aqueous liquids and is especially useful for the galvanic protection of ferrous members in, for example, water heaters or other aqueous environments having a resistivity of at least about 200 ohms centimeter.  
  The method of forming the aluminum alloy galvanic anode comprises first hot working a preferred alloy consisting essentially of about 0.03 to about 0.3 weight per cent bismuth. about 0.005 to about 0.04 weight per cent gallium, about 0.02 to about 0.3 weight per cent indium with the balance of the alloy being essentially aluminum. This alloy contains the normal impurities present in aluminum. The aluminum alloy is worked sufficiently to provide the desired oxidation potential. The hot working can be carried out by the known processes of drawing, forging, rolling and the like; however, it is preferred that the work be imparted into the alloy by means of extrusion. Preferably the hot working is carried out in a manner to provide a reduction in crossectional area from the starting aluminum billet to the final worked anode of at least about 9 to l and more preferably at least about 25 to l. The temperature of the solid metal during hot working is at least about 200C. and preferably from about 400 to about 600C.  
  The described alloy is preferably prepared by melting aluminum with a purity of at least about 99.5 weight per cent aluminum and then adding a sufficient amount of the elements bismuth, gallium and indium to the molten aluminum to provide an alloy within the above defined composition ranges when the added elements are substantially uniformly dispersed within the aluminum. These elements can be readily dispersed in the aluminum by mixing equipment and methods commonly accepted in the art. Naturally the addition of aluminum, bismuth, gallium, and/or indium alloys in amounts sufficient to form an aluminu m alloyp vvithin the herein described composition ranges is contemplated and included herein. To minimize the impurity level in the aluminum alloy anode or to produce an aluminum base alloy consisting of bismuth, gallium and indium within the afore-specified ranges, it is desirable that the metal melted have an aluminum purity of at least about 99.7 weight per cent and preferably at least about 99.85 weight per cent.  
  After the bismuth, gallium and indium are admixed with the molten aluminum to provide the desired alloy, the molten metal is poured or cast into a suitable form or mold of a predetermined shape. The molten alloy is solidified and removed from the mold. The as-cast shape, such as an ingot, is heated to or maintained at a temperature sufficient for hot working of the metal. Preferably the temperature is sufficient to afford extrusion into a shape adapted for use as a galvanic anode in, for example, water heaters. The described worked alloy can be employed as a sacrificial anode using methods known to those skilled in the art. For example, attaching the anode to a more electropositive metal structure, such as a steel member contained in a water heater, to afford an electrical contact between the anode and the steel causes preferential corrosion of the anode in corrosive environments.  
  As can be seen in the following examples and tables addition of bismuth, gallium and indium in the stipulatedjamounts to aluminum produces a hot worked sacrificial anode with a high useful voltage and a high current-capacity (amp-hr. per pound output) in corrosive environments.  
  EXAMPLES 1-8 TABLE 1 Aluminum with a purity of 99.9 weight per cent was melted and heated to a temperature of 750C. Suffi- A .1 (M7) Pmemiul cient amounts of bismuth, gallium, and indium were 5 Ex &#34;a?&#34; M (mm) fir dissolved in the molten aluminum to provide alloys with the compositions shown in Table] after mixing if g 6 these elements 1nto the molten alummum. The de- H (H17 (101 m 9 scribed alloys were cast into 2 V2 inch diameter by 6 l2 (W2 700 13 0.07 0.02 0.12 1.5 885 inch long mgots. The sol1d1f1ed mgots were removed (m7 (m4 (m2 L5 625 from the molds and heated to a temperature of 480C. 15 0.10 0.01 0.02 1.55 860 prior to being extruded into /2 inch diameter rod. The H extruded rod was cut into about 7 inch long sections. 121 0.23 0.04 0.01 1.55 767 The individual sections were tested in an electrolyte {is 2%? comprising a mixture of tap water and deionized water. 15 21 0:51 0:01 0:07 1:37 662 The water had an electro-resistivity of 5.000 ohmcentimeters and a temperature of 70C. Each of the EXAMPLES 22-26 sections was immersed in the aqueous electrolyte to a Aluminum with a purity of 99.9 weight per cent was depth of l /2 inches and electrically attached to the melted and heated to a temperature of 750&#34;C. Sufflstainless steel container, which acted as the cathode. cient amounts of bismuth, gallium. and indium were The anode current density was approximately 10 ma/ft&#34; dissolved in the molten aluminum to provide the alloy during testing. The voltage potentials shown in Table I compositions shown in Table 111 after mixing these elewere measured with reference to a standard saturated ments into the molten aluminum. The described alloys potassium chloride-calomel half cell. were cast into 2 /z-inch in diameter by 6 inch long ingot 25 and /8 inch in diameter by 6 inch long specimens. The solidified 2 A: by 6 inch ingots were removed from the TABLE 1 molds. heated to a temperature of 480C. and extruded into /2 inch diameter rod. The extruded rod was cut Measured Current into about 7 inch long specimens. The individual ex- Analysis (wtfk) Potential p y truded and as-cast specimens were tested in saturated Ex B1 Ga in Al (volts) (t1mp.hrs/lh) caso1 aqueous electrolyte.  
  1 0.06 0.01 0.02 B111. 1.02 535 The extruded rods were immersed in the electrolyte 2 440 to a depth of 3 inches and the as-cast specimens were 3 0.07 0.02 0.08 1.15 480 4 x 7 4 2 I03 4 0 1mmersed to a depth Of ll&#39;lCheS. extruded and 3-S- 5 022 cast test samples were electrically connected through 3 1. 21 8 a 18,200 ohm resistor to the positive side of a rectifier. 8 0.31 0.01 0.12 1.15 405 Stainless steel rods were connected to the negative side of the rectifier and immersed in the electrolyte to act as cathodes. The anode current density was approxi- 40 mately 50 malft The voltage potential as shown in Table 111 was measured with reference to a standard EXAMPLES 941 saturated KCl calomel half cell. It is readily apparent Specimens obtained substantially as described in Exthat extrusion of the indicated alloys significantly imamples l through 8 were tested in a saturated CaSO proved the anode characteristics of the alloys. aqueous electrolyte. Each specimen was immersed in EXAMPLES 27-58 the aqueous electrolyte to a depth of 3 inches and elec- Aluminum base alloys with a composition as shown trically connected through an 18,200 ohm resistor to in Table IV were prepared substantially as described in the positive side of a rectifier. Stainless steel rods were Examples 22-26. The anode current density was about connected to the negative side of the rectifier and im- 36 ma/ft The data contained in Table IV represents mersed in the electrolyte to act as cathodes. The anode the anode characteristics after about 30 days in the cor-. current density during testing was approximately 50 rosive environment. It is apparent that the anode voltma/ft The voltage potentials shown in Table ll were age potential and current capacity of the extruded anmeasured with reference to a standard saturated KC] odes are more uniform than and improved over the ascalomel half cell. cast material.  
 TABLE III AsCast Extruded Current Analysis (MT/r) Potential Potential Capacity Example Bi Ga In Al (volts) (volts) (amp.hrs/lb) 22 0.1 0.01 0.01 Bal. 0.4 1.4 800 23 0.1 0.04 0.01 0.5 1.5 790 24 0.15 0.01 0.10 0.5 1.4 870 25 0.2 0.01 0.01 0.4 1.5 775 26 0.2 0.04 0.01 0.7 1.5 750 TABLE IV As-cast Extruded Current Current Analysis (wtf r 1 Pot. Capacity Pot. Capacity Examples Bi Ga ln (volts) (amp. hr/lb) (volts) (amp. hr/lh) 27 0.040 0.031 0.01 0.62 880 1.48 787 28 0.075 0.016 0.01 1.35 861 29 0.12 0.010 0.011 0.45 1ll l 1.45 800 30 0.12 0.023 0.015 1.34 794 1.50 647 31 0.12 0.011 0.12 1.51 430 1.57 693 32 0.13 0.011 0.020 0.50 1202 1.55 804 33 014 0.005 0.012 1.31 995 1.52 769 54 0.15 0.011 0.01 0.5 93 1 1 55 691 15 0.1 0.009 0.032 0.51 l 1 5 1.50 831 6 0.17 0.010 0.01 0.4 005 1.55 736 3&#39; 0.17 0.010 0.0116 0.55 1191 1.45 871 3h 0.17 0.018 0.010 0.40 95*) 1 56 778 39 0.18 0.012 0.032 1.36 923 1.55 765 40 0.20 0.009 0.01 0.50 l 195 1.51 794 41 0.20 0.024 0.01 0.46 94h 1.47 720 42 0.21 0.012 0.056 1.44 70,&#34;: 1.58 695 43 0.21 0.043 0.01 0.43 11 5 1.54 652 44 0.22 0.014 0.01 0.43 lll&#39;s&#39;l 1.56 637 45 0.2. 0.019 0.010 0.39 956 1.5 702 46 0.23 0.040 0.01 0.50 1 101 1.54 767 47 0.25 0.019 0.012 1.33 880 1.57 737 48 0.26 0.008 0.011 0.46 l 168 1.51 776 49 0.27 0.013 0.01 0.41 1219 1.50 654 50 0.29 0.022 0.053 1.34 742 1.59 696 51 0.31 0.012 0.01 1.2 860 1.54 602 52 0.32 0.011 0.01 0.7 927 1.53 747 53 0.46 0.032 0.01 0.48 1020 1.49 599 5-1 052 0012 0.01 0.4 961 1.56 522 55 0.52 0.040 0.01 1.0 735 1.55 502 56 0.55 0.039 0.01 0.95 694 1.53 572 57 0.58 0.020 0.012 1.3 7511 1.55 507 X 0.59 0.015 0.01 1.2 822 1.55 510 What is claimed is:  
  1. An extruded sacrificial aluminum anode consisting essentially of about 0.02 to about 2 weight per cent bismuth. about 0.005 to about 0.05 weight per cent gallium. about 0.005 to about 0.5 weight per cent indium and the balance being essentially aluminum.  
  2. The extruded anode of claim 1 with an oxidation potential of about 1.0 to about 1.3 volts at a current density of about 10 ma/ft in an aqueous electrolyte with a resistivity of about 5,000 ohm.cm.  
  3. The extruded anode of claim 1 with an oxidation potential of about 1.3 to about 1.6 volts at a current density of about 50 Ina/ft in a saturated calcium sulfate aqueous electrolyte.  
  4. The extruded anode of claim 1 consisting essentially of about 0.03 to about 0.3 weight per cent bismuth, about 0.005 to about 0.04 weight per cent gallium and about 0.02 to about 0.3 weight per cent indium.  
  5. The extruded anode of claim 4 with an oxidation potential of about 1.0 to about 1.3 volts at a current density of about 10 ma/ft in an aqueous electrolyte with a resistivity of about 5000 ohm.cm.  
  6. The extruded anode of claim 4 with an oxidation potential of about 1.3 to about 1.6 volts at a current density of about 50 ma/ft in a saturated calcium sulfate aqueous electrolyte.  
  7. A method to form a sacrificial anode comprising providing an alloy consisting essentially of about 0.02 to about 2 weight per cent bismuth, about 0.005 to about 0.05 weight per cent gallium, about 0.005 to about 0.5 weight per cent indium and the balance being essentially aluminum; and working the alloy sufficiently to provide a reduction ratio of the crosssectional areas of the starting alloy to the extruded anode of at least about 9:1.  
 8. The method of claim 7 wherein the reduction ratio is at least about 25 to l.  
  9. The method of claim 7 including working the alloy sufficiently to provide a sacrificial anode with an oxidation potential of about 1.0 to about 1.3 volts at a current density of about 10 ma/ft in an aqueous electrolyte with a resistivity of about 5,000 ohm.cm.  
  10. The method of claim 7 including working the alloy sufficiently to provide a sacrificial anode with an oxidation potential of about 1.3 to about 1.6 volts at a current density of about 50 ma/ft in a saturated calcium sulfate aqueous electrolyte.  
  11. The method of claim 9 wherein the working is carried out by extrusion.  
  12. The method of claim 10 wherein the working is carried out by extrusion.  
  13. The method of claim 7 wherein the working is carried out by extrusion.  
  14. The method of claim 13 including heating the alloy to provide an extrusion temperature of from about 400C. to about 600C.  
 15. The method of claim 13 wherein the reduction ratio is at least 25 to l.  
  16. The method of claim 7 including heating the alloy to provide a working temperature of at least about 200C.  
  17. The method of claim 7 wherein the alloy provided consists essentially of about 0.03 to about 0.3 weight per cent bismuth, about 0.005 to about 0.04 weight per cent gallium and about 0.02 to about 0.3 weight per cent indium.  
  18. The method of claim 17 wherein the reduction ratio is at least about 25 to 1.  
  19. The method of claim 17 wherein the working is carried out by extrusion.  
  20. The method of claim 19 including heating the alloy to provide an extrusion temperature of from about 400C. to about 600C.  
  21. The method of claim 20 wherein the reduction essentially aluminum. 7 ratio is at least about 25 to 23. The alloy of claim 22 consisting essentially of aluminum alloy consisting essentially of about about 0.03 to about 0.3 weight per cent bismuth, about to about 2 Weight per cent i about 0005 0.005 to about 0.04 weight per cent gallium and about to about 0.05 weight per cent gallium, about 0.005 to about 0.5 weight per cent indium and the balance being to about welght per Cent mdlum&#39; UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO. 5,878,081 Dated p il 15, 1975 l t r( John T. Reding; Robert L. Riley, Jr&#39;.  
  It &#39;is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 Column 1, line 66, change &#34;and&#34; to -but-;  
 Column 4, line 50 after &#34;the&#34; second occurrence, insert saturated CaSO Table III,  
 Column 4, the word &#34;Extruded&#34; should be centered above columns 7 &amp; 8.  
 Signed and Sealed this Arrest:  
 RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uflalenls and Trademarks