Note: Descriptions are shown in the official language in which they were submitted.
~o66~'75
The invention resides in a method for forming
an improved aluminum alloy sacrificial anode and to an
anode as prepared by the method.
The aluminum alloy sacrificial anode of the pre-
sent invention is suitable for use as an anode in various
environments including water heating systems and under-
ground environments.
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.
Patents 3,240,688; 3,337,332; 3,337,333; 3,368,952 and
3,379,636.
The invention resides in a method for forming
an aluminum alloy sacrificial anode comprising hot working
a starting aluminum alloy containing from 0.02 to 2 weight
percent bismuth, from 0.005 to 0.05 weight percent gallium,
and from 0.005 to 0.5 weight percent indium, the balance
of the alloy being aluminum.
The invention also resides in an aluminum alloy
sacrificial anode as prepared by the method of this inven-
tion.
In the process of the invention, hot working of
the starting alloy causes a reduction in the cross-
sectional area of the starting alloy to the final worked
anode of a ratio of at least 9 to 1. Preferably the ratio
is at least 25 to 1. The hot working can be carried out by
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known processes such as, for example, drawing, forging, rolling, or
extruding. Preferably the work is imparted to the alloy by means of
extrusion. The aluminum alloy is worked sufficiently to provide the
desired oxidation potential. The tempera$ure of the starting aluminum
alloy during hot working is at least 200C, preferably from 400C to
600C.
A final worked alloy suitable for use as a sacrificial
anode has an oxidation potential from 1.0 to 1.3 volts at a current
density of approximately 108 milliamperes per square meter when measured
with reference to a standard saturated calomel half cell in an aqueous
electrolyte having a resistivity of 5000 ohm-centimeters. When measured
with reference to a standard saturated calomel half cell in a saturated
calcium sulfate aqueous electrolyte, sui~able sacrificial anodes have
an oxidation potential from 1.3 to 1.6 volts at a current density of
approximately 538 milliamperes per square meter.
The starting alloy is prepared by melting aluminum with a
purity of at least 95.0, preferably 99.5 weight percent aluminum and
then adding a sufficient amount of the elements bismuth, gallium and
indium to provide an aluminum alloy containing from 0.02 to 2 weight
percent bismuth, 0.005 to 0.05 weight percent gallium and 0.005 to 0.5
weight percent indium. Preferably the starting alloy contains from 0.03
to 0.3 weight percent bismuth, 0.005 to 0.04 weight percent gallium, and
0.02 to 0.3 weight percent indium. The starting alloy contains the normal
impurities present in the aluminum. To minimize th~ impurity level
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in the aluminum alloy, it is desirable to employ aluminum
having a purity of at least 99.7 weight percent, prefer-
ably at least 99.85 weight percent. The addition of
aluminum, bismuth, gallium, and/or indium alloys in
amounts sufficient to form an aluminum alloy within the
herein described composition ranges is contemplated and
included herein.
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 as
hereinbefore described. Preferably the temperature is
sufficient to afford extrusion into a shape adapted for
use as a sacrificial anode for the galvanic protection of
ferrous members in, for example, water heaters or other
aqueous environments having a resistivity of at least
200 ohm-centimeters.
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As can be seen in the following examples and
tables, hot working a starting aluminum alloy containing
bismuth, gallium and indium in the stipulated amounts
produces a sacrificial anode with a high useful voltage
and a high current capacity (ampere-hour per gram output)
in corrosive environments.
Examples 1-8
Aluminum with a purity of 99.9 weight percent
was melted and heated to a temperature of 750C. Suf-
~icient amounts of bismuth, gallium, and indium were
dissolved in the molten aluminum to provide alloys with
the compositions shown in Table I after mixing these
elements into the molten aluminum. The described alloys
were cast into 6.25 centimeter diameter by 15.2 centimeter
long ingots. The solidified ingots were removed from the
molds and heated to a temperature of 480C. prior to being
extruded into 1.27 centimeter diameter rod. The extruded
rod was cut into 17.8 centimeter long sections. The
individual sections were tested in an electrolyte com-
prising a mixture of tap water and deionized water. The
water had an electro-resistivity of 5000 ohm-centimeters
and a temperature of 70C. Each of the sections was
immersed in the aqueous electrolyte to a depth of 3.8
centimeters and electrically attached to a stainle~s
steel container, which acted as the cathode. The anode
current denisty was approximately 108 milliamperes per square
meter during testing. The voltage potentials shown in
Table I were measured with reference to a standard sat
urated calomel half cell.
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TABLE I
Measured Current
Analysis (wt.%) Potential Capacity
Example Bi Ga In (volts) (amp.hr~./q.)
1 0.06 0.01 0.02 1.021.18
2 0.07 0.01 0.09 1.060.97
3 0.07 0.02 0.08 1.151.06
4 0.07 0.04 0.02 1.031.06
0.22 0.05 0.04 1.8<0.22
6 0.23 0.04 0.01 1.050.75
7 0.26 0.01 0.02 1.180.90
8 0.31 0.01 0.12 1.150.89
ExamPles 9-20
Specimens obtained substantially a~ described
in Example 1 through 8 were tested in a saturated CaS04
aqueous electrolyte. Each specimen was immersed in the
aqueous electrolyte to a depth of 7.6 centimeters and
electrically connected through an 18,200 ohm resister to
the positive side of a rectifier. Stainless steel rods0
were connected to the negative side of the rectifier and
immersed in the electrolyte to act as cathodes. The anode
current density during testing was approximately 538 milli-
amperes per square meter. The voltage potentials shown
in Table II were measured with reference to a standard5
saturated calomel half cell.
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TABLE II
Current
Analysis (wt.%) Potential Capacity
Example Bi Ga In (volts)(amP.hrs./a.)
9 0.06 0.01 0.04 1.5 1.98
0.07 0.01 0.09 1.5 2.02
11 0.07 0.02 0.08 1.5 1.54
12 0.07 0.02 0.12 1.5 1.95
13 0.07 0.04 0.02 1.5 1.38
10 14 0.10 0.01 0.02 1.55 1.90
0.15 0.02 0.08 1.55 1.82
16 0.22 0.05 0.04 1.55 1.00
17 0.23 0.04 0.01 1.55 1.69
18 0.26 0.01 ~.02 1.50 1.82
20 19 0.45 0.01 0.02 1.44 1.36
0.51 0.01 0.07 1.37 1.46
Examples 21-25
Aluminum with a purity of 99.9 weight percent
was melted and heated to a temperature of 750C. Sufficient
amounts of bismuth, gallium, and indium were dissolved in
the molten aluminum to provide the alloy compositions
shown in Table III after mixing these elements into the
molten aluminum. The described alloys were cast into
6.35 centimeters in diameter by 15.2 centimeter long
ingots and 1.6 centimeters in diameter by 15.2 centimeter
long specimens. The solidified 6.35 by 15.2 centimeter
ingots were removed from the molds, heated to a temperature
of 480C. and extruded into 1.27 centimeter diameter rod.
The extruded rod was cut into about 17.8 centimeter long
specimens. The individual extruded and as-cast specimens
were tested in saturated CaSO4 aqueous electrolyte.
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The extruded rods were immersed in the electrolyte
to a depth of 7.6 centimeters and the as-cast specimens were
immersed to a depth of ~.35 centimeters. All extruded and
as-cast test samples were electrically connected through a
18,200 ohm resistor to the positive side of a rectifier.
Stainless steel rods were connected to the negative side
o the rectifier and immersed in the electrolyte to act
as cathodes. The anode current density was approximately
538 milliamperes per square meter. The voltage potential
as shown in Table III was measured with reference to a
standard saturated calomel half cell. It is readiIy
apparent that extrusion of the indicated alloys significantly
improved the anode characteristics of the alloys.
TABLE III
As-Cast Extruded _ _
Current
Analvsis (wt.%) Potential Potential Capacity
Example Bi Ga In (volts) (volts)(amP.hrs/q)
21 0.1 0.01 0.01 0.4 1.4 1.74
22 0.1 0.04 0.01 0.5 1.5 1.74
23 0.15 0.01 0.10 0.5 1.4 1.92
24 0.2 0.01 0.01 0.4 1.5 1.71
0.2 0.04 0.01 0.7 1.5 1.65
ExamPles 26-42
Aluminum alloy sacrificial anodes with a compo-
sition as shown in Table IV were prepared substantially as
described in Examples 21-25. The anode current density was
approximately 387 milliamperes per square meter. The data
contained in Table IV represents the anode characteristics
after about 30 days in the corrosive environmentO The anode
voltage potential and current capacity of the extruded anodes
are more uniform that and improved over the as~cast material.
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