US3721618A

US3721618A - Aluminum sacrifical anode

Info

Publication number: US3721618A
Application number: US00123284A
Authority: US
Inventors: J Reding; D Barnett
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1971-03-11
Filing date: 1971-03-11
Publication date: 1973-03-20
Anticipated expiration: 1990-03-20

Abstract

An aluminum base alloy comprising about 0.01 to about 0.2 weight percent mercury, about 0.1 to about 20 weight percent zinc, and a heavy metal. The heavy metal can be about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver. Methods of producing the alloy and of using the alloy as a sacrificial anode are described.

Description

Umted States Patent 11 1 [111 3,721,618

Reding et a]. 1March 20, 1973 [54] ALUMINUM SACRIFICIAL ANODE [56] References Cited [75] Inventors: John T. Reding, Lake Jackson; UNITED STATES TE S David W. Barnett, Clute, both of Tex 3,415,305 12/1968 Schrieber et a1. ..75/146 1 Assigneei l Dow Chemical Company, Primary Examiner-Richard 0. Dean Mldland, Mlch- Attorney-Griswold & Burdick, L. S. Jowanovitz and 22 Filed: March 11, 1971 Grace [21] Appl. No.: 123,284 57 ABSTRACT An aluminum base alloy comprising about 0.01 to [52] US. Cl. ..204/ 197, 75/146, 204/148,

bout

0,2 weight percent mercury, about 0.1 to about 0 /293 20 weight percent zinc, and a heavy metal. The heavy [51] Int. Cl. ..C23f 13/00 m t l can be about 0.03 to about 2.0 weight percent Field of Search bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver. Methods of producing the alloy and of using the alloy as a sacrificial anode are'described.

14 Claims, 1 Drawing Figure CORROJ/ON M/ /00% HUMIDITY ENVIRONMENT 0F HLUM/NUM 5% ZINC ALLOY ALUMINUM SACRIFICIAL ANODE BACKGROUND OF THE INVENTION This invention relates to sacrificial galvanic anodes and more particularly to a novel aluminum base alloy for galvanic anodes having a high resistance to air oxidation, a high oxidation potential, and a useful high electrical output per unit mass of metal consumed, that is, a high electrochemical equivalent.

Aluminum sacrificial anodes, containing elements such as mercury and zinc, can be successfully employed to cathodically protect metal structures, especially steel and other ferrous base metals, from corrosion in corrosive environments. Generally, the effectiveness of aluminum-mercury-zinc alloys for cathodic protection of metals in low chloride containing environments is minimal. This is believed to be primarily a result of the voltage of an aluminum-mercury-zinc alloy being at least partially dependent upon the chloride concentration in the electrolyte. That is, as the percentage of chlorine ion present in the electrolyte is reduced the voltage of the anode diminishes.

A sacrificial anode having a voltage in excess of about 0.9 volts with respect to a saturated potassium chloridecalomel reference electrode is usually desired, since the anode area can be reduced as the voltage increases. The utility of commonly used commercial aluminum-mercury-zinc anode alloys is generally negligible when the chlorine ion concentration in the electrolyte is lower than approximately 0.25 percent because the voltage approaches 0.9 volts. It is well known that increasing the mercury content in this ternary alloy system will result in an increase in the voltage; however, the higher mercury content usually results in more rapid air oxidation of the alloy. In fact, sufficient oxidation to cause a significant loss in anode weight can occur before the metal is effectively utilized as a sacrificial anode.

It is an object of this invention to provide an oxidation resistant aluminum alloy suitable for use as a sacrificial anode.

It is another object of this invention to provide an oxidation resistant aluminum alloy having a voltage of at least 0.9 in electrolytes having a chlorine ion concentration less than about 0.25 percent.

These and other objects and advantages will become apparent during the course of the following description of the invention.

SUMMARY OF THE INVENTION Thepresent invention comprises a novel aluminum base alloy composition containing small amounts of mercury, zinc, and a heavy metal. The invention also pertains to galvanic anodes prepared from said alloy.

More particularly, the present aluminum alloy composition consists essentially of from about 0.01 to about 0.2 weight percent mercury, about 0.1 to about weight percent zinc, and the heavy metal of bismuth, cadmium, or silver. Even more specifically, the heavy metal can be about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, or about 0.001 to about 0.04 weight percent silver.

The alloy preferably consists essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal. Suitable heavy metals are about 0.l to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and about 0.005 to about 0.02 weight percent silver.

An aluminum alloy sacrificial anode can be produced by melting aluminum by means known to them skilled in the art. Preferably the aluminum has a purity of 99.5 weight percent; and more preferably it has a purity of 99.8 weight percent aluminum. Mercury, zinc, and a heavy metal can be added to the aluminum as elements and/or alloys before, simultaneously, or subsequently to melting the aluminum. Prior to casting the molten aluminum alloy into the desired final shape, the molten bath should be stirred sufficiently to distribute the aforementioned alloying additions within the pure aluminum. Sufficient stirring is characterized by a bath havinga substantially uniform composition throughout. Casting of the molten bath can be carried out in accordance with procedures known to those skilled in the art, for example, pouring the molten aluminum alloy into a mold of predetermined shape and/or by cladding a substrate with the alloy of this invention. The recited method for producing sacrificial anodes can effectively use the aforementioned alloys.

The claimed 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 steel ship hulls or boilers, to afford an'electrical contact between the anode and steel causes preferential corrosion of the anode in corrosive environments.

As can be seen in the following examples, table and appended figure, addition of bismuth, cadmium, or

silver to an aluminum-mercury-zinc alloy produces a sacrificial anode having an acceptable resistance to air oxidation, a high useful voltage and a high current capacity (amp-hour per pound output) in corrosive environments having low concentrations of chlorine ion.

EXAMPLES l-l0 Aluminum having a purity of 99.99 weight percent was melted in a graphite crucible using a resistance heater as the heat source. When the aluminum had melted and attained a temperature of 700C sufficient mercury and zinc were added to the liquid aluminum bath to achieve a final alloy composition of 0.12 weight percent mercury and 5.0-weight percent zinc. The mercury was added to the liquid aluminum as a master alloy containing 10 parts of zinc to each part of mercury. After the alloying constituents had melted the bath was thoroughly stirred to disperse the ingredients uniformly throughout the aluminum.

For comparative purposes, two castings were produced from the molten aluminum-mercury-zinc alloy by pouring portions of the metal into a X 3% X 4 inches steel mold and solidifying the alloy into anode castings. The remaining molten metal was reheated to 700C and 0.01 weight percent silver added to the metal. The molten metal was stirred until the alloy composition was substantially uniform throughout. Two castings in this alloy were then poured and solidified in a 54 X 3% X 4 inches mold.

Each solidified. and cooled casting or slab was weighed and positioned in a humidity cabinet for a period of about 20 hours. The humidity cabinet had a continuous stream of steam passing through to promote oxidation of the sacrificial aluminum anode samples. After the stipulated exposure time had elapsed each casting was removed from the cabinet, washed with water, dried, and weighed. The magnitude of the weight loss indicated the oxidation loss during the test periods, that is, as the weight loss increased the metal corrosion increased. The above procedure was substantially followed for producing and testing aluminum a1- loys having the chemical compositions shown in Table l and in the appended figure.

As can be seen in thefigure, addition of bismuth, cadmium, or silver improved the alloys resistance to air oxidation. The bismuth addition also increased the alloy brittleness.

EXAMPLES 1 1-29 Aluminum alloy specimens of the composition specified in Table 1 were tested under simulated commercial conditions for approximately 30 days. Each five-eights inch diameter by 5 inch long sample was connected in electrical series with a galvanized steel cloth cathode having a mesh size of approximately onefourth inch. The electrolyte shown in Table 1 and a glass container completed the electrolytic test cell. Voltage measurements in the direct current cell were obtained several times during each test. The total ampere hours utilized during each test was also measured. Upon completion of the testing cycle, each aluminum anode was cleaned and weighed to ascertain the weight loss. The current capacities (amp-hour per pound) shown in Table l were calculated from weight loss and ampere hour measurement.

What is claimed is:

1. An aluminum base alloy consisting essentially of about 0.01 to about 0.2 weight percent mercury, about 0.1 to about weight percent zinc, and a heavy metal selected from the group consisting of about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver, and balance aluminum.

2. The alloy of claim 1 wherein the heavy metal is silver.

3. The alloy of claim 1 consisting essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal selected from the group consisting of about 0.1 to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and about 0.005 to about 0.02 weight percent silver.

4. The alloy of claim 3 wherein the heavy metal is silver.

5. A method comprising:

a. melting an alloy having at least about 99.5 weight percent aluminum;

b. adding to the aluminum sufficient elements to produce a final composition consisting essentially of about 0.01 to about 0.2 weight percent mercury, about 0.1 to about 20 weight percent zinc, and a heavy metal selected from the group consisting of about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver;

. stirring the molten aluminum to distribute the additions of step (b) in the molten aluminum; and d. casting the molten mixture of step (c). 6. The method of claim 5 wherein silver is added to the aluminum.

7. The method of claim 5 wherein the alloy melted 40 contains at least about 99.8 weight percent aluminum.

8. The method of claim 5 wherein the elements added to the aluminum are sufficient to produce a final TABLE I.PE RFORMANCES OF ALUMINUM ALLOYS AS SACRIFICIAL ANODES IN VARIOUS ELECTROLYTES Saturated Synthetic Composition, percent Tap water CaSO4 seawater Amp.- Amp.- Amp.-

Example Hg Zn Ag Bi Volts hr./lb. Volts hr./lb. Volts hr./1b.

1 Current density in: Tap waterabout 125 miliiamps/itfl; Saturated CBSO4-flb0t1t 300 mllliumps/itfl; Seawnter-abont 125 milliamps/l'tJ.

Seawater-about 20 ohm-cm.

composition consisting essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal selected from the group consisting of about 0.1 to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and'labout 0.005 to about 0.02 weight percent silver.

9. The method of claim 8 wherein silver is added to the aluminum.

10. The method of claim 8 wherein the alloy melted contains at least about 99.8 weight percent aluminum.

1 l. A sacrificial anode consisting essentially of about 0.01 to about 0.2 weight percent mercury, about 0.1 to about weight percent zinc, and a heavy metal selected from the group consisting of about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001

to about 0.04 weight percent silver and balance aluminum with a voltage of at least 0.9 in electrolytes with a chlorine ion concentration less than about 0.25 percent.

12. The anode of claim 1 1 wherein the heavy metal is silver.

13. The anode of claim 11 consisting essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal selected from the group consisting of about 0.1 to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and about 0.005 to about 0.02 weight percent silver.

14. The anode of claim 13 wherein the heavy metal is silver.

Claims

2. The alloy of claim 1 wherein the heavy metal is silver.
3. The alloy of claim 1 consisting essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal selected from the group consisting of about 0.1 to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and about 0.005 to about 0.02 weight percent silver.
4. The alloy of claim 3 wherein the heavy metal is silver.
5. A method comprising: a. melting an alloy having at least about 99.5 weight percent aluminum; b. adding to the aluminum sufficient elements to produce a final composition consisting essentially of about 0.01 to about 0.2 weight percent mercury, about 0.1 to about 20 weight percent zinc, and a heavy metal selected from the group consisting of about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver; c. stirring the molten aluminum to distribute the additions of step (b) in the molten aluminum; and d. casting the molten mixture of step (c).
6. The method of claim 5 wherein silver is added to the aluminum.
7. The method of claim 5 wherein the alloy melted contains at least about 99.8 weight percent aluminum.
8. The method of claim 5 wherein the elements added to the aluminum are sufficient to produce a final composition consisting essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal selected from the group consisting of about 0.1 to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and about 0.005 to about 0.02 weight percent silver.
9. The method of claim 8 wherein silver is added to the aluminum.
10. The method of claim 8 wherein the alloy melted contains at least about 99.8 weight percent aluminum.
11. A sacrificial anode consisting essentially of about 0.01 to about 0.2 weight percent mercury, about 0.1 to about 20 weight percenT zinc, and a heavy metal selected from the group consisting of about 0.03 to about 2.0 weight percent bismuth, about 0.001 to about 0.05 weight percent cadmium, and about 0.001 to about 0.04 weight percent silver and balance aluminum with a voltage of at least 0.9 in electrolytes with a chlorine ion concentration less than about 0.25 percent.
12. The anode of claim 11 wherein the heavy metal is silver.
13. The anode of claim 11 consisting essentially of about 0.08 to about 0.15 weight percent mercury, about 0.4 to about 14 weight percent zinc, and a heavy metal selected from the group consisting of about 0.1 to about 0.5 weight percent bismuth, about 0.005 to about 0.02 weight percent cadmium, and about 0.005 to about 0.02 weight percent silver.
14. The anode of claim 13 wherein the heavy metal is silver.