US3496085A - Galvanic anode - Google Patents

Description

United States Patent 3,496,085 GALVANIC ANODE John T. Reding, Freeport, and John J. Newport III, Lake Jackson, Tex., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware N0 Drawing. Filed Apr. 15, 1966, Ser. No. 542,727

Int. Cl. C231? 13/00 US. Cl. 204-197 4 Claims ABSTRACT OF THE DISCLOSURE An aluminum based, iron impurity-containing, sacrificial galvanic alloy composition, particularly useful in the form of a sacrificial anode, having improved current efiiciency and good oxidation potential. The improved efficiency is attributed to controlled silicon addition to provide a silicon to iron ratio of between 0.5 and 5.

This invention relates to sacrificial galvanic anodes and more particularly is concerned with a novel aluminum based alloy exhibiting a high electrical output per unit mass of metal, i.e. a high current efiiciency and an oxidation potential in the range particularly suitable for use as a sacrificial anode in sea water applications and to anodes prepared therefrom.

At present, zinc is extensively used as a galvanic sacrificial anode in the cathodic protection of installations operated in or in contact with sea water. Zinc, which has a potential of about 1 volt (measured versus calomel reference), is satisfactory for this use and possesses the advantage that its working potential provides less possibility of damage to protective surface films such as protective coatings and paints than anode materials having ing higher working potentials, such as magnesium (-l.5 volts). Zinc, however, suffers from the disadvantage that it has a relatively low ampere-hour capacity of about 370 ampere-hours per pound.

Aluminum, which has a high theoretical electrical output per unit mass of metal consumed (about 1350 ampere-hours per pound), in actual practice has not proved to be useful as a sacrificial galvanic anode in that the presence of the normally passive oxide surface film on aluminum apparently presents a barrier to the oxidation of the metal thereby reducing the effective oxidation potential to about 0.7 volt (as measured in closed circuit at either about 250 or about 1000 milliamperes per square foot in a synthetic sea water electrolyte with a standard saturated KCl calomel cell as reference). At such a low operating voltage, no cathodic protection is given to ferrous based structures, for example; therefore, the anode exhibits no useful electrical output.

Aluminum alloys containing zinc and mercury have been found which provide oxidation potentials of from about 0.9 to about 1.2 volts and a high electrical output per pound. However, in general, these show a marked decrease in current efiiciency (ampere-hours per pound of anode consumed) as the purity of the base aluminum metal is lowered. This particularly is noted with respect to iron inclusion, a common impurity in primary aluminum.

It is a principal object of the present invention therefore to provide a novel aluminum based alloy composition which can use low purity aluminum as a base and which exhibits an oxidation potential of from about 0.9 to about 1.2 volts (versus calomel reference) and an electrical output per pound markedly increased over that shown by zinc.

It is another object of the present invention to provide a novel aluminum alloy composition exhibiting high 3,496,085 Patented Feb. 17, 1970 electrical output per unit mass of metal consumed (ampere-hours per pound) when used as a sacrificial galvanic anode in cathodic protection installations.

It is another object of the present invention to provide a novel aluminum alloy composition suitable for use as a sacrificial anode for cathodic protection of installation operated in or in contact with water, sea water or other waters having high salinity.

It is another object of the present invention to provide a novel aluminum alloy composition suitable for use as a galvanic protective coating for ferrous-base material.

These and other objects and advantages readily will become apparent from the detailed description of the invention presented hereinafter.

The present invention comprises a novel aluminum based alloy having from about 0.1 to about 20 weight percent zinc, from about 0.01 to about 0.1 weight percent mercury, a maximum of about 0.5 weight percent iron, a maximum of about 0.6 weight percent silicon and further characterized that the SiiFe ratio ranges from about 0.5 to about 5.

Preferably the alloy comprises aluminum having alloyed therewith from about 0.1 to about 15 weight percent Zinc, from about 0.02 to about 0.1 weight percent mercury, from about 0.08 to about 0.5 weight percent iron, and silicon in an amount to provide a Si:Fe ratio of from about 1 to about 3, the maximum silicon concentration being about 0.6 weight percent. All weight percents are based on total composition weight.

As the zinc concentration is extended beyond the range set forth herein, the corrosion pattern of the alloy becomes irregular and is accompanied by massive metal losses through segregation, i.e. spalling. This spalling loss becomes most pronounced during the last half of the life of the anode. This is objectionable as detrimental reduction in anode current efiiciency is realized.

Unexpectedly, the present novel alloy composition when employed as sacrificial galvanic anodes exhibits a satisfactory, relatively smooth corrosion pattern throughout the life of the anode, an operating oxidation potential of from about 0.9 to about 1.2 volts depending upon the current density of operation and electrical output in ampere-hour per pound of metal consumed which can closely approach theoretical.

Galvanic anodes can be prepared from the novel compositions by use of alloying and casting or fabricating techniques ordinarily employed in the aluminum art.

Aluminum for use in preparing the present novel alloy composition can be a low commercial grade (e.g. 99.5% or lower Al) metal having normal production introduced impurities associated therewith. The alloying metals also can be of commercial grade.

The following examples will serve to further illustrate the present invention but are not meant to limit it thereto.

EXAMPLE A number of anodes of the present invention were prepared by melting iron contaminated aluminum ingot in a graphite crucible positioned within an electric furnace. Predetermined amounts of mercury, zinc and silicon alloying ingredients were introduced into the molten aluminum and the resulting mixture stirred to effect dispersion and solution of the alloying ingredients throughout the melt. The resulting alloy was cast in a graphite mold into cylindrical specimens about 5 /2 inches long and either about /8 inch or 1 inch in diameter. The cooling and solidification rate of the castings were controlled such that these simulated the cooling rate experienced in production of commercial, field-sized cast anodes.

The performance of the alloys was evaluated by positioning a specimen of the cast cylinder (as anode) in a one-half gallon glass jar. A steel wire mesh cloth was placed adjacent to the inner Wall of the jar as a cathode. Synthetic sea water was used as an electrolyte with about 3 inches of each anode specimen being immersed. The cells were completed with respect to electrical circuitry, a rectifier being employed to maintain a constant direct current through a group of cells connected in series.

The results presented in Table I show the performance at a current density of about 1000 milliamperes per square foot over a 47 day test period for a number of the novel aluminum alloy anodes of the composition of the present invention. These results present data showing both the solution potential (vs. saturated calomel) and electrical output per unit mass of metal consumed for the anodes tested. The current efficiency was determined by weighing the test anodes before and after testing and comparing the actual weight loss with the theoretical calculated Weight loss.

The results obtained in the laboratory tests were confirmed by actual field tests in flowing sea water by using 3-inch diameter anodes.

TABLE I We claim:

1. An aluminum based sacrificial galvanic anode composition consisting essentially of: from about 0.1 to 20 weight percent zinc; from about 0.01 to about 0.2 weight percent mercury; iron in an amount up to 0.5 Weight percent; and silicon present in an amount in excess of normal impurity level, the silicon:iron ratio in the anode composition being within the range of from about 0.5 to about 5 and the maximum total silicon concentration being about 0.6 weight percent; the balance of the composition being aluminum.

2. The composition of claim 1 wherein the zinc is present in an amount of from about 0.1 to about percent, mercury from about 0.02 to about 0.1, iron from about 0.08 to about 0.5 percent and silicon in an amount to provide a silicontiron ratio within the range of from about 1 to about 3.

3. The composition of claim 1 in the form of a cast anode structure.

4. A method of improving an aluminum based, sacrificial galvanic anode; said aluminum base containing iron impurity which comprises adding silicon to said anode in an amount to obtain up to about 0.6 weight percent Alloy composition 1 weight percent Current Si: Fe, Potential, efficiency Zn Hg Si Fe ratio volts. percent Run number:

0.71 0.042 0.00 0.052 1.15 1. 04 90 0.40 0.000 0.47 0.12: 3.90 1.03 04 0.45 0. 0.37 0.32 1.15 1.00 as 0.47 0.052 0.02 0.10 3.87 0.95 80.5 0.45 0. 7 0. 0.32 0. 01 0. 0s 80 4.23 0.10 0.28 0.32 0.88 1.01 80 10.50 0.15 0.32 0.32 1.00 0.07 80 15.00 0.11 0.32 0.31 1.03 0.93 80 0.50 0.08 0.25 0.14 1.78 1.05 03.5 0.47 0.10 0.28 0.14 2.00 1.08 03 0.47 0. 070 0.20 0.14 1.42 1.02 89.5 0.47 0.069 0.08 0.10 0.42 1.00 70.5 0.44 0.07 0.13 0.30 0.43 1.01 73 4.30 0.14 0.12 0.32 0.37 1.02 78 10.50 0.10 0.13 0.32 0.41 0.97 70 15. 70 0.00 0.11 0.32 0.34 0.04 70 1 Balance aluminum. S1m1lar galvanic protection to ferrous-based substrates silicon 1n sald anode, and controlling said silicon content is realized by flame spraying the alloy of the present 1nto provide a srliconzrron rat1o 1n said anode within the vention onto the substrate, applying paint or binder sysrange of from about 0.5 to about 5 to enhance current tems thereto wherein the present alloy in powdered form is in high ratio to the carrying vehicle or binder, by spraying the alloy onto a heated ferrous surface, the temperature of the ferrous material being sufficient to melt the aluminum alloy and assure adherence between the alloy and substrate, and the like methods of application.

The novel alloy composition also is suitable for use as sacrificial anodes for applications such as galvanic pigments in paint films, galvanic anode materials for primary batteries and, as shown hereinbefore as sacrificial galvanic coatings for sheet steel and other metals cathodic to aluminum. Additionally this composition finds utility as an active ingredient in flares, for use in chemical reductions and in the preparation of aluminum alkyls.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

efficiency of said anode, said anode also containing zinc in an amount from about 0.1 to about 20 weight percent and mercury in an amount from about 0.01 to about 0.2 weight percent.

References Cited UNITED STATES PATENTS 2,758,082 8/1956 Rohrman 204197 2,985,530 5/1961 Fetzer et al -446 2,993,783 7/1961 Martin 75146 3,257,201 6/1966 Raclot 75-146 3,321,306 5/1967 Reding et a1 75146 3,343,948 9/1967 Raclot 75138 T. TUNG, Primary Examiner US. Cl. X.R. 75138, 146