Aluminum Base Alloys And Anodes

Abstract

Aluminum base alloys which are suitable for use in the as cast state as galvanic anodes, the alloys comprising 1-15% zinc, 0.005-0.1% indium and 0.4-10% magnesium, the balance being aluminum of at least 99.8 percent purity with inconsequential impurities. The alloy may optionally include some tin, for example in the range of 0.1-0.5 percent, some gallium in the range of from 0.005 to 0.017 percent and a grain refiner such as titanium and zirconium.

Description

BACKGROUND OF THE INVENTION

The requirements for galvanic anodes are a high operating potential and a high efficiency measured as electrical output per unit mass of metal consumed.

Many conventional alloys used for forming anodes, for example aluminum-zinc-tin alloys in production at the present time require a heat treatment after being cast before they are suitable for use. There are considerable economic and technical advantages in being able to produce high capacity sacrificial anodes which will operate satisfactorily in the as-cast condition without the necessity for formal heat treatment.

BRIEF SUMMARY OF INVENTION

We have found that the incorporation of magnesium in an aluminium-zinc-indium alloy in the correct proportion provides a satisfactory as-cast product.

Accordingly the invention provides in one aspect an aluminum base alloy comprising 1-15 percent zinc, 0.005-0.1 percent indium and 0.4-10 percent magnesium, the balance being aluminum of at least 99.8 percent purity with inconsequential impurities.

The proportion as impurities of silicon and iron should each preferably be below 0.2 percent. Preferably the zinc content is between 2 and 10 percent and with advantage between 2.5 and 8 percent. Preferably the indium content is between 0.01-0.05 percent and with advantage between 0.03 and 0.04 percent. Preferably the magnesium content is between 0.4 and 1 percent and with advantage is between 0.6 and 0.8 percent particularly where restrictions to incendive sparkling apply. The alloy may also include some tin, for example in the range 0.1 to 0.5 percent.

It is preferable to include some gallium in the range 0.005 to 0.017 percent and preferably for example 0.01 percent.

In this specification all percentages are by weight.

It is further preferred to include a grain refiner of any suitable form (for example titanium and zirconium) to improve the cast product.

In another aspect the invention provides an anode in the as-cast state made from an alloy as set out above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some aluminum base anodes were made up and tested by way of example as follows:

Aluminum was melted and raised to a temperature of 710.degree. C. Zinc, indium, (and in some cases tin) were added as required and this operation was followed by degassing. Magnesium was then added as required and to minimize oxidation effects additions were made under a layer of Coverall 33F. The melt was then thoroughly stirred with the temperature controlled between 710.degree.-730.degree. C.

Casting was then carried out into dies held at 100.degree.-250.degree. C.

Half inch sections were cut from 1 -inch-diameter test bars of the material to be tested and electrical connections made through a threaded length of one-eighth-inch-diameter aluminum rod screwed into the upper machine surface of the specimen. Both machined surfaces were marked off using a stopping off medium and the weighed samples were then mounted concentrically in 9-inch diameter, 12-inch high, shot-blasted mild steel drums containing about 11 liters of natural sea water. Gentle agitation by stirring was used during the test and the electrolyte in the tank was changed regularly. The tests were carried out at laboratory temperatures.

The current supplied by the dissolving anode after being connected to the steel drum was restricted to an anode current density of 10 ma./in..sup.2 by means of a variable resistor in the external circuit. The potential drop across a further accurately calibrated resistor (also in the external circuit) was used to monitor the current flow and these values were recorded automatically every four hours during test. The total current supplied by the anode section could therefore be calculated.

The period of testing varied from 40-60 days by which time about 50 percent of the specimen had been consumed. After removing from the test environment specimens were cleaned in 1:1 nitric acid (to remove adherent corrosion product,) and were then reweighed after drying.

The theoretical output for the particular weight loss of the specimen was calculated using an electrochemical equivalent for the particular alloy on test (i.e. allowance made for the zinc content of the alloy) and the efficiency calculated as the percentage of the theoretical output actually supplied by the anode during testing.

Potentials were measured at regular intervals throughout the test using a saturated calomel electrode in contact with the dissolving aluminum anode or on the outside of any adherent corrosion present on the specimen surface. The value of potential quoted is that measured on the final day of the test.

Examples of specifications of and properties of anodes made from alloys in accordance with the invention are shown in table 1. ##SPC1##

It will be noticed that in the case of composition I where the magnesium content is below 0.4 percent the operating potential fell to -1,000 mv. and the resulting anode was less satisfactory than those of higher magnesium contents.

Further examples of specifications of and properties of anodes made from alloys in accordance with the invention are shown in table II. In these examples, the copper content was in each case found to be less than 0.005 percent and the silicon and iron contents are shown. ##SPC2##

The applicants have investigated the effect of variations in casting techniques on the electrochemical properties of alloys according to the invention. These investigations involved variations in the liquid metal temperature, variations in the mould temperature and variations in the cooling technique. Three cooling techniques were tried, one of these identified as the standard procedure involved casting the metal into a mold and, when it had become sufficiently solidified, removing it from the mold and allowing it to cool, the second technique identified as "water quench-cold water" involved casting the metal into a mold and, when it had become sufficiently solidified, removing it from the mold and quenching it in cold water, and the third technique identified as "very slow cool in molds" involved casting the metal into a mold and allowing it to cool in the mold to ambient temperature. This latter technique gave a retarded rate of cooling and a superior result as can be seen from table III below. The alloy used for the investigation of which the results are given in table III was an alloy containing 0.68 percent magnesium, 4.01 percent zinc, 0.038 percent indium, 0.012 percent gallium, 0.12 percent silicon, 0.07 percent iron and less than 0.005 percent copper. ##SPC3##

The addition of grain refining elements such as Zirconium and titanium to alloys according to the invention was investigated and the results are shown in table IV below. These results indicate that beneficial results are obtained with the addition of a grain refining element, preferably titanium in the range of from 0.005 to 0.07 percent and more specifically in the range of from 0.01 to 0.04 percent. In each of the alloys shown in table IV the copper content was less than 0.01 percent. ##SPC4##

Claims

I claim:

1. An aluminum base alloy for use as a sacrificial anode material consisting essentially of 1-15 percent zinc, 0.005-0.1 percent indium, 0.4-10 percent magnesium, 0-0.5 percent tin and 0-0.017 percent gallium the balance being aluminum of at least 99.8 percent purity with inconsequential impurities.

2. An aluminum base alloy according to claim 1 containing less than 0.2 percent of each of silicon and iron as impurities.

3. An aluminum base alloy according to claim 1 in which the zinc content is not less than 2 percent and not more than 10 percent.

4. An aluminum base alloy according to claim 3 in which the zinc content is not less than 2.5 percent and not more than 8 percent.

5. An aluminum base alloy according to claim 1 in which the indium content is not less than 0.01 percent and not more than 0.05 percent.

6. An aluminum base alloy according to claim 5 in which the indium content is not less than 0.03 percent and not more than 0.04 percent.

7. An aluminum base alloy according to claim 1 in which the magnesium content is not less than 0.4 percent and not more than 1 percent.

8. An aluminum base alloy according to claim 7 in which the magnesium content is not less than 0.6 percent and not more than 0.8 percent.

9. An aluminum base alloy according to claim 1 in which the tin content is in the range of from 0.1 to 0.5 percent.

10. An aluminum base alloy according to claim 1 in which the gallium content in the range of from 0.005 to 0.017 percent.

11. An aluminum base alloy according to claim 10 in which the gallium content is 0.01 percent.

12. An aluminum base alloy according to claim 1 including, as a grain refiner, titanium in the range of from 0.05 to 0.07 percent.

13. An aluminum base alloy according to claim 12 containing not less than 0.01 percent and not more than 0.04 percent titanium.

14. A method of producing a galvanic anode comprising forming an alluminium base alloy consisting essentially of 1-15 percent zinc, 0.005-0.1 percent indium, 0.4-10 percent magnesium, 0-0.5 percent tin and 0-0.017 percent gallium, the balance being aluminum of at least 99.8 percent purity with inconsequential impurities, casting the alloy into a mold and allowing it to cool in the mold to ambient temperature.

15. A method according to claim 14, in which the alloy includes, as a grain refiner, titanium in the range of from 0.005 to 0.07 percent.

16. A cast galvanic anode composed of an aluminum base alloy consisting essentially of 1-15 percent zinc, 0.005-0.1 percent indium, 0.4-10 percent magnesium, 0-0.5 percent tin and 0-0.017 percent gallium, the balance being aluminum of at least 99.8 percent purity with inconsequential impurities, said anode having been cast in a mold and allowed to cool therein to ambient temperature.