CA1171696A - Magnesium alloys - Google Patents

Abstract

ABSTRACT

A magnesium alloy contains the following additives : A1 1-9%, Zn 0-4%, Sn 0.1 - 5%, Mn 0-1%.
It is useful as an anode in cells operating with a salt water electrolyte, especially in batteries powering equipment for deep-sea use in which a pulsed power source is required.

Description

Thi~ invention relates to magne~ium alloys and their use in electric cell~
Magne~ium alloy~ are commonly u~ed as the anode material in primary cell~ u~ing ~alt water a~ the electrolyte: ~uch cell~ find variou~ application~ to provide an underwater electric current ~upply for u~e at ~ea. It i~ de~irable to provide reliable cells which u~e sea-water a~ the eleotrolyte ~nd which are capable of operating reliably under widely differing di~charge conditions (e.g. at low and high currents, continuous or intermittent di~charge) at different temperature~ and at the con~iderable presYure~ encount~ered during deep-water u~e.
Known cells of this type normally use a cathode material such as silver chloride or lead chloride and the anode may be a magnesium alloy containing minor quantities of zinc, aluminlum, lead or thallium. Cells of this kind are described in United Kingdom Patent Specification ! :~
1,251,223 and United States Patent- 3,288,649.
A troubleso~e effect which appears during discharge of such cells lS that of ~sludgin$~ i.e. the formation of a solid deposit in the spaces between anode and cathode in ', which the electrolyte is located. Sludge interferes with the electrical behaviour of the cell, reducing the voltage delivered and reducing the coulombic efficiency of the cell.
i The nature of this deposit may vary from a fine, loose `~ powder;which accumulates mostly on the magnesium alloy plate and has only~a small~effect~on the cell to a spongy film whlch may fill the gap between the plates completely.

~ -2-J !~ 6 In the latter case the effectiveness of the cell is gross~
impaired.
It has be0n found that the tendency to form sludge increases as the temperature and pressure incre~se.
It has also been found that the ~oltage delivered - by the cells may be adversely affected by polarisation effects, even when severe build up of sludge is not visually evident. This is found for example in batteries subjectad to pulse loading, such as in sonobuoys, wher~ a high current drain is superimposed on a constant low current drain ~e.g. for 1 second every 10 seconds). When discharged at high hydrostatic pressure, the ~oltage obtained on pulsing a battery of cells using an alloy such as AZ61 (containing in weight percent Mg-6% Al_l~o Zn-0.20%
~) may decay rapidly, although no visibl~ evidence of thick sludge films l-~ seen. An example of such deterioration for AZ61 is shown in ~igure 3 of the ~
accompanying drawings. l -In batteries which are pulse loaded as described abo~e, the pulse of power is frequently required to acti~ate equipment such as a sonar signal generator. In ` ~ --such cases it is essential that the pulses generated are f sufficient power to activate the equipment. It is sometimes found that after the cell or battery in the inactive state has been fllled with seawat~r or operated only on Iow current drain for some time, initial pulsas are of insufficient power, and a slgnificant number of -~
pulses are generated before the po~er has reached a level sufficient to activate the equipment. Performance of .

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batteries in this respect deteriorate with increasing discharge time at low rates, and with increasing depth o~
operation (i.e. increasing pressure).
It has now been found that sludge formation in the cells may be reduced considerably by the use of magnesium alloys which contain minor amounts of tin. It has further been found that cells using these alloys show less tendency to voltage decay when discharged under pulsed load conditions and also give improved electrical performance over a wide range of pressures and mode of discharge.
It has been found that batteries using magnesium alloys containing additions of tin show faster`response to changes in discharge current density, and that maximum power is achieved in significantly fewer ,oulses than with the alloys currently used, such as AZ61.
According to one aspect of the invention, an electrode for a primary cell comprises the following constituents by weight (apart from normal impurities):
Al - 1 - 9%
Zn - O - 4%
Sn - 0.1 - 5%
Mn - O - 1%
With the balance being magnesium.
Preferred alloys of this type contain the following constituents:
Al - 4 ~ 7%
Zn - O - 1%
Sn - 0.25 - 3%
Mn :.
.

7~ 6 An especially preferred alloy contains the following constituents by weight:
Al - 5.5 6.50/o Zn _ 0.5 _ l.O~o Sn _ 0.5 - 1.5~o The invention also rel~tes to electric cells using such an alloy as the anode material, particularly of the type using a salt water electrolyte~ The cathode material may be lead chloride or silver chloride.
Embodiments of the invention will be described by way of illustration in the following Examples. Reference is made to *he accompanying drawings in which:
Figure 1 shows the voltage obtained from a p~imary cell plotted against tim~, Figure 2 shows oscilloscope traces of the voltage obtained from a battery plotted against time, Fig~re 3 shows the voltage obtained from a battery plotted against time.
Nine alloys having the compositions in weight percent given in Table 1 below were prepared by melting the pure constituents in graphite lined crucibles. The alloys were cast into plates 180 mm x 125 mm x 12.5 mm in a steel mould. The cast plates were homagenised at 400C, machined to remove casting skin and then rolled from 400 C to thickness in the range 0.28 mm - o.38 mm.
After rolling, plates were solution heat treatcd for a minimum of 3 hours at 400C, then rapidly cooled to room temperature to yreserve a single phase metallurgical structure in the alloy.

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l I , Alloy ¦ Al% Zn~/0 Pb% ¦ Tl% Sn% Mn%

AZ61 6.2 1.0 _ _ ~ 0.2 AP65 6.2 1.0 4.5 _ _ 0.2 MTA75 5.0 _ _ 7.0 _ _ AT65 6.o _ _ _ 5.0 _ AT62 6.o _ _ _ 2.0 _ AT61 6.o _ _ 1.0 _ ATM61~ 5 9 _ _ _ 1.0 o.26 I ., ATZ611 6.1 o . 6 _ _ 1.0 _ ~T6~ 5-9 _ _ 0.4 _ The electrochemical per~ormance of alloys AZ61, AT61 and AT65 were compared by forming a singla call from the alloy with a cathode of silver chloride sheet separatad from the alloy by glass beads to give an electrolyte gap of o-os6 mm wide. The cell was ssndwiched bstween silver plates which acted as anode and cathode current collectors within an enclosed acrylic case. Artificial seawater of electrical cond~ctiYity 0.053 mhos-cm was pumped through the cell at a flow rate of 120 n~s/min. The electrical output of the cell was connected to a variable carbon pile rheostat which was adjusted during the test discharge to maintain a constant current density from the cell of 387 mA/cm . Voltage/time cur~es were plotted for the three alloys and are shown ln Figure 1. It ma~ be seen that ~ _6-.
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all three alloys gav~ discharge curv~s of the same general shape but that AT61 and AT65 alloys gave higher Yoltage throughout, AT65 being the highest. On dismantlin~ of the c~lls after te~ting, all three alloyg showed clean metal surfaces, with no ind~cation of "sludging".
EXA~LE 2 .
Plates compo~ed of some of the alloys given in Table 1 were used to build bat*eries having an~des f~rmed by the plates and silver chloride cathode~, ~eparated by glass bead spacers to giva ~ space o.80 mm wide to allow slectrolyte to circulate between the plates~ The batteries each comprised 5 cells of this type and the plates were hald together in an apoxy resin mounting.
The performance of each battery was assessed by immersing it in a solution of sodium chloride in water, to ~imulate ~ea water, and connecting it to an external circuit having resistive loads to produce a constant current density of 5mA/cm2 with an intermittent pulse load equivalent to 150 mA~cm applied for 1 second every 10 second~ for 60-second periodsO Three pulse sequencas were applied, 3 minutes, 45 minutes and 75 minutes after activation of the battery, (pulses A, B and C respectively).
The voltage and current deli~ered by each battery at the start and end of each trial were measured by standard methods.
The voltage was also measured just bafore each pulse sequence, and the maximum v~ltage during the first pulse and during the sixth pulse of each sequence wa~ m~asured.
t Immediately after di~charge the batteri~s were dismantled and the type and degree of sludging was assessed visually .i7~.~9~;

on a scale from A (slight sludg~) to E (heavy sludge).
The anode plates wers then cleaned in chromic acid, washed, dried and weighed to estimate the apparent coulombic efficiency (i.e. the ratio of theoretical anode consumption, deriYed from the external charge supplied by the anode to ths total weight loss of anode material during discharge).
These trials were carried out at ambient pressure and also in a pressure vesYel at 60 bars pressure to simulate depth. The test conditions used were as follows:
(a) Pressure 1 bar, salinity 3.6%, temperature 30 C
(b) Pressure 60 "
Results of these tests in 3.6% NaCl at 30 C and 1 bar pressure are shown in Table 2 below.

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_ . _ U~ C~l U~
C~ O CO ~ 0~ O O ~O O 1~ C~lCJ~ C~l cO . ~
. . o . . ~ . . . . . . . . . ~ ¢
~ O O G~ CO 1~ ~ CO u~ U~ CO ~ U~ ~ ;r c~
_ __ U~ C~l . U~
u~ ~ ~Q Oo~ a~ ~ ~ a~ o ~ o~ co ~~ O ~D +
. . o. . ~ . . . , . . . . . ~ ¢
o o ~J~ I~ ~1 ,, ~ ~ ~ ;r C~l . . .
U~ ~ ~`, _1 c~l 0 c~ c~l ~ c~ 3 ~;r o o ~ . ~:
. . o. . ~;r . t . . . . . . . oo ~o o ~ Oo l~ ~ ,~ ~ r~co ~ ~ ~1 _ . ~ . .. _ .
~C`J
C~ C~ O C~ CO C~ O ~ O ~ l .
~D O. . U~ ~ . . . . ~ . ~
h¢ o o ct~~o co ,1 ~ co u~ u~co ~ u~0 ~ ~ ,~
.
. __ __ . . . ___.__ r~ ~ co N ~ ~1 Ir~ C~ O ~ 1 O ~I N ~--1 ~t . . O. . 11~ ~ .. . . . . .. . .
~ O o a~~oo _l ~ i cr~ o -- U\c~ _~ _ ~ ~ . __ __ __ C~ CO ~ C'l ~ . ~ o ~ ~ ~ D c~ I ~ 3 . o O. . U~ U~ . . .
~ O O O~cr~ a~ ~ a~ ~ ~ ~
-- Ir~ C`3 . . . . _._ c~ ~ ~r o co ~ u~ O ~ ~ O ~ ~1 ~ .
~ . . O. . U~ ~ .. . ~ . ~ CO
C~l P. O OC~ CO ~ _1 ~/ CO ~ t' 1 ~;t' CO t~ ~
_l ~ ~ _ .. ~ __~_ 0 I~:S' C`J _IIr~ O a:)~ CD ~:S' ~ ~ N ':
~; ~ . O 1~ . . . . . .. ~ ~ CO
~ o o ~ a~ co ~ ~1 0 ~ ~ cO c~;~ a~ _l _ . _ ~ C~l C~3 - -,1 C~ 0 cO oO c~ ~ a~ u~ ~ cO O ~ co c~ a~ .
~o . . o . . u~ . . . . . . . . . ;r ~ o o a~I~ I~ ~1 -I : 1~ I~ 3 1~ C~ ~ ~.~
_~i_ . ~ _ _ ___ .
. U~ C~l ~
~1 C~ 0 0 00 C`l N t~ l~ O t~ ~\ t~CO ~D ~ . ~¢
O ~ U~ ~ ~ . ~ ~ Il~
el O O Cl~ I~ rl ~1 . 1~ 0 1~ I~ C`~
V ~ C`~ _ . __ ~ .
~ c~l 00 0 0 C`~ CO ~ ~1 CO ~ U`,CO ~ U~ O ' '1 O O O~ ~1~ ~ 1~ [~ ~ . ¢ ~ ' _ . ___ . _ __~ _~ ~' :'' _~ _ ~ ~ ~ ~ ~ _,__ ~ '.
~7 ~ ~ ~ ~ Z;
~ _ ~ ~1 _I __~ ___ ___ E~ ~
_ ~rl ~ t~ H O
~4 E3 rl rl ~1 ~1 _t H ~ ,1 ~ `~D ~ rt ~D ~ H
u~ _ ~ t~ ~ 6~ t~ ~ ~5 H ~
V~ l ~ ~ . .,1 ~ q~ ~ ~rl /D ~J I:c. H
E~3 Z ~,1 ~rl ~ U~ ~q ~ D~ In V ~1 ~q t~ V~
Z ~ OH H E'l H ~ ~1 . ,.1 1_l ~_1 t~l ` q O --I V D ~ !~ ~ ~ 1~ t~ ~ ~ Z ~ -S ~1 Z ~ , D, 3, ~ ~ O r-l _9_ - ' .' ' ' ~ ' It can be seen from -the~e results that sludging performance of the alloys containing tin were at least as good as AZ61 and better than the other alloys tested.
Elactrical performance of tin containing alloys was also as good as, or better than other alloys. Thus, while in some instunces high vsltages were recorded for AP65 and M~A75, thesa alloys showed severe sludging and very low efficiency. Tin-containing alloys, on the other hand, consistently maintained high voltage level on pulsing even at the end of the tests, where erratic results were obtained from other alloys.
Similar results were obtained whe~ this trial was repeated at a pressure of 60 bars although the degree of sludging obtained was greater for all the alloys~ In this instance the benefit of the tin alloy~ in main~aining a uniform voltage on pulsing is illustrated by ths attached Figure 2, which ~how~ typical oscilloscope traces of voltage during pulsing with reference to the pre-pulse level. H~re it may be seen that although initially all the alloys showed ~imilar traces, as the trial proceeded, alloys AZ61, AP65 and M~A75 showed voltage curves falling at an increasing rate, while the two tin-containing alloys showed curves that remained essentially le~el, even up to 75 minuies.

The performance of batteries made in the same way as tho~e in Example 2 was assessed by immersing them, as before, in a solution of sodium chloride in water, but connecting the battery to resistive loads which were -10- 1.

, electronically controlled and timed so that the battery was discharged ~t a constant current density of 5 mA/cm2 for 75 minutes, but with a pulsed load to produce a nominal current de~sity of 150 mA/cm2 applied aftsr 30 minutas of discharge for 1 second every 10 seconds, for a total time of 30 minutes. Low load voltage and the pulse voltage at the start of pulsing, after 15 minutes pulsing and after 30 minutes pulsing were recorded. After discharge the battery wa~ stripped down and sludging assessed as in Example 2.
Results of te~t carried out in 3.6 NaCl at 30 C and 60 bar pressure are shown in Table 3. It will be seen that the tin-containing alloys showed better sludging behaviour and that the pulse voltage of the AT61 alloy9 containing 1% of tin was superior to that of the other alloys~ Pulse voltage for this alloy also showed the least variation from beginning to end of the pulse discharge (i.e. showed the flattest pulse voltage curve). In Table 3 the "initial"
~oltage is that immediately before the start of a pulsa, ~min pulse~ is the minimum voltage observed during that pulse and "max.return" i~ the maximum vvltage observed when that pulse has ceased, Figure 3 shows the variation of pulse voltage with time during the pulsing stage of the discharge for alloys AZ61 and AT61. It may be seen that although AZ61 achieved a satisfactory pulse voltage in the initial stages, thi~
rapidly decayed to lower valuas, whilst AT61 maintained a more constant level.

TA~LE 3 ALLO~' AZ61 AP65 MTA75 AT61 h~65 _ _ ~ _ AgCl THICKNESS (~tn) o. 25 0.25 0.25 0.25 0.25 ELECTROLYTE GAP lmm~ O.~2 O.82 0~82 O.82 O.82 RUN DURATION(Min~) 75 75 75 75 75 _ __ . .
VOLTAGE Initial (Y) 7.~0 8.50 9-15 8.15 7.88 Final (V) 7.73 6.85 8.28 7.78 7.00 _ CURRENT I~tial (mA) 151 153 152 152 151 Final (mA) 152 123 143 145 131 _ _ ~
_ __ _ .
lST PULSE Initial (V) 7~75 8.35 9.0 8.1 8.o Min.Pulse (V) 3-65 4.2 5~7 5.1 5~3 t Max.Return (V) 7.95 8.5 9~4 8.2 8.2 _ 15 MIN.PULSE Initial (V) 8.o 8.45 9-3 8.2 7.9 Min.Pul~e (V) 4.7 5.1 5-3 5.7 5.1 Max.Return (V) 8.2 8~5 9-3 8.25 8.1 _ _ ~_ . _ _ 30 MIN.PULSE Initial (V) 7.95 8.1 8.8 8.05 7-65 Min.Pul~e (V) 4.6 3.0 4.4 5.4 2.7 Max.Return (V) 8.2 8.1 8.8 8.o5 7.6 . _ _ COULO~IC_EFFICIENCY O~D 31.6 16.5 18.2 21.6 24.6 I -.~, . ._, , .
_ _ _ SLUDGE CLASSIFICATION* B D E+ A B+

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Further discharge tests were carried using the same technique as described in Example 3, using additional alloys containing tin, with ~mall additions o$ Mn or Zn.
The~e were evaluated in comparison with standard AZ61.
Results are shown in Table 4, for two different test conditions. Values shown are average for triplicate te~t~.

1 ~ , ' ` -13- 1 I .

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t, .,U~ ~ o~ ~o U~ ~ U~
~ ~DN o~) u~ O ~ .~ ,1 ~ ~ l î~
Z 1~ 1-~ . ~ ~ ~
O O 0 1~ O O u~ u~ u~ ~ N
~ _ _, OIr\ ~ ~_ ~ ~1 Il~ N I~ O~ Ir~ ~Il~ co ~ N
o E~ N 0 Ir~ O 1' .1 .~0 ;t~ N ;t~ o O ~:C O O 0 1`- O O ~ u~ ;t' N
L _ _ _ .~ ~Ir~ N a~ ) ~ C`l O~D 0 O 'C C~ 0 ~ 1~ t~ ~ 1 1~ ~ O ~ a~
c~ o o r~ ~ o o r~ r _ _ .1 ~U~
r1 ~D 11~ N t~ Q ~ r ~D N ~ o~
O NC`J 0 Ir~ ~ ~0 .~ ~rlN ~ N lo ~ ~~ t` . - .. .... .
~ ~ O O cO 1~, O O ~ 0~ . ~ ' '`D ~ Ir~ N
. .~ U~ C~ ~ .( U~ ~ O ~o ~ .4 ~r ~0 N o~ Ir~ O co .1 .~~0 co ~0 ~ O
o~ ~ O O ~ O O ~ ~ ;r ~
~ , C . u~ C~l co oO u~ ~ ~ CJ N N
S~ E~ c~ oo ~ O ~ ~ .~ co ~ O ~ l~
~ ~: O O ~0 1~ O O ~ u~ ~ N
C ~ u~u~
.~ u~ c~ u~ c~ u~ ~o~ ~ oo u~
~ c~ co I~ r~ r~ ~ ~ ~
¢0 0 ~ 0 0 ~U~ ~D ,, _ _ _ ~_ ~: _ ' __ __ _ _ : ~ ~ ~ ~ ~ ~ ~
Ei E _ __ __ _ _ .,.~ 0 ~ ~ ~ ~ ~ ., U~ P~ ~ r~ ~ ~ 1 ~Z; u~ ~ _~ ~ t3 ~ ~ ~ ~ 1 O ~t ~ ~ ~ ~ a~ a H ~D ~ ~ ~ o u~ ~
H ~~ ~ . H ~ H ~ ~ a . .

a or1~ ~ I ~ l l~ 3 Ei P'~
h L~ ~0 ~ r-~ ~ ~
¢ ~ 1 1 ~i 0.1 P~ O ..
1~3 c.~ O L,.~ ~ O
~ ~ li3 D ~ O ¢ O '~ ~
:
. _ _ ~ _ _ ~ _ ~.
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These results show that alloys AT61 and ATZ611, both containing l~o Sn, giv~ significant improvements in pulse voltaSe compared to AZ61. Addition of Mn to the tin-containing alloys reduced pulse voltage, but significantly increased coulombic effici ency of the alloy.
EXA~lE 5 In order to determine tha effect of alloy com~osition on the rate at which the battery achieved an adequate voltage level when pulsing commenced, the number of pulses requirod until the battery achi~ved 90% of its maximum pulse voltage wastnoted under several discharge conditions for battery tests as described in the previous Examples 2-4. Data is shown in Table 5. :

. _ _ , , . _ _ TEST CONDITIONS ALLOY Pulses to achieve 90 I :
. . . of Max Pulse Voltage 1 bar/30 C/3. 6% NaCl AT61 1 AT6 2 1 ~ ~ :
ATZ611 1 :

20 bar/20 C/3. 6~o NaCl AZ61 _ .~ .
60 bar~20C/3.6% NaCl AZ61 1 ATZ6 11 _ 1 bar~OC/1.5% NaCl AZ61 4 3 . I~Z61l_ 2 20 bar/O Cjl.5% NaCl AZ61 14 . ~ AT61 7 :: :
'~ ' ~.~'7~ 6 In all cases ~Icome_up~ time for alloys containing tin was less than that of AZ61.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION TO WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. An electrode for a primary cell comprising a magnesium alloy containing the following constituents by weight, apart from normal impurities:
Al - 1 - 9%
Zn - 0 - 4%
Sn - 0.1 - 5%
Mn - 0 - 1%
Mg - remainder.

2. An electrode according to Claim 1, containing the following constituents by weight:
Al - 4 - 7%
Zn - 0 - 1%
Sn - 0.25 - 3%
Mn - 0 - 0.3%.

3. An electrode according to Claim 2, containing the following constituents by weight:
Al - 5.5 - 6.5%
Zn - 0.5 - 1.0%
Sn - 0.5 - 1.5%.

4. An electric primary cell having an annode according to claim 1, 2 or 3.

5. An electric primary cell having an annode as claimed in claim 1, 2 or 3 and adapted to use salt water as the electrolyte.

6. An electric cell having an annode as claimed in claim 1,2 or 3 and a cathode comprising silver chloride or lead chloride and adapted to use salt water as the electrolyte.