GB2096639A

GB2096639A - Automobile battery grid

Info

Publication number: GB2096639A
Application number: GB8206109A
Authority: GB
Original assignee: CHORIDE INDIA Ltd
Current assignee: CHORIDE INDIA Ltd
Priority date: 1981-03-03
Filing date: 1982-03-02
Publication date: 1982-10-20
Also published as: IN153954B; GB2096639B

Abstract

An alloy is disclosed which comprises a calcium in an amount of at least 0.03%, sodium in an amount up to 0.05%, optionally tin, and optionally aluminium, balance lead. A preferred method of making such an alloy comprises melting pure lead, adding flakes of pure sodium hydroxide into the molten metal at 360 to 380 DEG C, while stirring, adding between 0.001% to 0.05% pure sodium and raising the temperature of the bath to 460 DEG C before adding calcium to the molten bath as a calcium master alloy while maintaining the temperature of the melt at around 420 DEG C.

Description

SPECIFICATION Automobile battery grid The present invention relates to a lead-calcium alloy and to a method of manufacturing the alloy.

The invention particularly relates to an alloy containing lead, calcium and sodium and battery plate grids made therefrom. Such battery plate grids are used in lead acid batteries. Lead acid batteries are particularly, but not exclusively, used in automobiles and batteries appropriate for such use are referred to as automotive batteries.

The use of lead-calcium or lead-calcium-tin alloys for the grid of automotive batteries substantially improves the electrochemical properties, particularly in respect of gassing rates and shelf life. Such batteries are generally referred to as low maintenance or maintenance-free batteries, and in certain applications, such as automotive, these batteries can be supplied in virtually sealed form and require no topping up in service. In addition, an extended service life is expected of such batteries. However, the battery industry, in general, has shown considerable reluctance to adopt Pb-Ca or Pb-Ca-Sn alloys for automotive battery grids, thus conventional Pb-Ca alloys are relatively difficult to cast as grids and are liable not to have the strength required for further processing.Adoption of Pb-Ca technology, therefore, would require major changes in the manufacturing set up involving a sizable capital expenditure.

If, however, the Pb-Ca alloy technology could be improved in a manner to obviate most of the production problems, lead-calcium alloys would become considerably more attractive to the battery industry.

An object of the present invention is to produce batteries which would combine electrochemical advantages of conventional lead-calcium batteries with other desirable features, such as high strength and good castability of grids used in such batteries from such alloys. The present invention also aims to provide a lead-calcium battery grid having one or more of the following improved properties to mitigate the drawbacks experienced in the battery industry.~ 1. improved compositional stability of the Pb-Ca melt; 2. improved castability; 3. improved strength, both as cast and after ageing; 4. increased age hardening rate; 5. reduced corrosion rate in service; 6. lower plate growth in service; 7. improved paste adhesion to the grid surface; and 8. improved cohesive strength of the paste.

The Pb-Ca type grids of the present invention result in significant improvements in most of the above areas except for the cohesive strength of the paste. Although sodium is very prone to oxidation in the melt, we have found that sodium in small quantities is quite stable in molten lead. Higher sodium contents will lead to brittleness of the alloys, and also will give rise to the formation of Na2O in the melt which has strong oxidising properties and is detrimental to the stability of the melt.

We have found that by the addition of around 50 ppm of sodium to a lead-calcium melt, the strength of the grid can be substantially improved.

The invention provides an alloy comprising calcium in an amount of at least 0.03% preferably 0.03 to 0.13 e.g. 0.09 to 0.13%, sodium in an amount up to 0.05%, optionally tin and optionally aluminium, balance lead.

As a further improvement of this technology, we have also found that if a certain proportion of the free sodium present in the melt can be converted into sodium hydride, it leads to a significant improvement in the castability and stability of the alloy and grids made therefrom. Furthermore, addition of sodium to lead-calcium alloys enhances the as-cast strength of the castings and thereby enables the grids to be trimmed sooner after being cast, thus shortening the time taken for production. Further, we have found that the addition of tin in an amount of not more than 0.3%, preferably not more than 0.1%, in the aforesaid alloy containing lead, calcium and sodium increases the strength and the castability of such an alloy.

The present invention also-extends to a method of manufacturing an alloy containing lead, calcium and sodium which comprises melting pure lead, adding between 0.001% and'0.05% by weight sodium, stirring the melt, and raising the temperature of the melt before adding calcium to the melt.

The sodium is preferably added when the melt is at a temperature between 360 and 3800 C.

Preferably the melt is at a temperature of 420 to 4600C whilst the calcium is added.

The calcium is preferably added as a calcium lead master alloy, for example containing about 1.5% calcium balance lead.

The calcium is preferably added to the melt in an amount of at least 0.03% e.g. in the range 0.03 to 0.13% based on the total weight of lead in the melt and the master alloy added.

After the calcium has been added tin may be added to the melt.

The tin may be added as a tin lead master alloy, e.g. containing about 10% tin balance lead.

The pure sodium is preferably added to the melt when wrapped in aluminium foil and enclosed in a perforated steel cage which is removed once the sodium has melted.

Prior to the addition of the sodium, solid sodium hydroxide may be added to the melt. The solid sodium hydroxide is preferably added in an amount less than the amount of sodium. The solid sodium hydroxide may be added as flakes of sodium hydroxide. The amount of solid sodium hydroxide is preferably between 60 and 75% of the amount of sodium.

The method of manufacturing the alloy preferably comprises melting pure lead, adding between 0.001% and 0.05% by weight sodium to the melt at 360 to 2800C, stirring the melt and raising the temperature of the melt to 4600C before adding calcium to the melt as a calcium master alloy while maintaining the temperature of the melt at around 4200 C.

According to the present invention there is also provided a method of manufacturing an alloy containing lead, calcium and sodium which process comprises the steps of melting pure lead at 360 to 3800C adding flakes of pure sodium hydroxide into the molten metal while stirring, adding pure sodium and raising the temperature of the bath to around 4600C before adding to the molten bath calcium preferably as a master alloy of lead and calcium while maintaining the temperature around 4200C and, if desired, adding tin preferably as a master alloy of lead and tin.

In the aforesaid method the amount of sodium hydroxide added to the melt is between 65 and 70% by weight of sodium (the weight of pure sodium which is added). The amount of pure sodium in the melt is between 0.001% and 0.05%, preferably between 0.003 or 0.005% and 0.02% by weight.

A lead-calcium alloy according to the present invention comprising predominantly lead preferably contains calcium in an amount of at least 0.03% by weight preferably between 0.09% and 0.13% by weight. The amount of tin (preferably added in the form of a tin master alloy), when tin is used in the method of manufacturing the alloy of the invention is not more than 0.3% by weight preferably not more than 0.1% by weight. The method of manufacturing the alloy has been carried out successfully using a calcium master alloy containing 1.5% by weight of calcium. A trace amount of aluminium, e.g. up to 0.010% e.g. about 0.005% may be added to the melt prior to the addition of calcium master alloy.

The present invention further provides a method of preparing a battery plate grid which comprises the steps of forming a lead-calcium alloy e.g. prepared by the process according to the invention, if necessary, melting the alloy and casting it to the desired grid shape, or a shape from which the grid, e.g.

an expanded mesh grid, is formed by subsequent processing. The formation of a sodium hydride in the system is believed to be achieved by reaction of the sodium with the sodium hydroxide in the melt at around 3600 to 4000C according to the following reaction: 2Na + NaOH e NazO + NaH In the sodium containing alloys of the present invention, the presence of relatively high amounts of calcium has a less detrimental effect on the corrosion properties than in equivalent alloys not containing sodium. In fact, tin is more undesirable from the corrosion resistance point of view.

Tin is generally added to conventional lead-calcium alloys to give additional strength but this is no longer required in the improved lead-calcium alloys of the present invention. Whilst recovery from deep discharges is generally good with the sodium containing alloys of the present invention, a small amount of tin can still be used for additional improvement in this property. It should be noted that tin containing metals require higher casting temperatures. If tin is added to the lead-calcium alloy of the present invention, the amount should be kept at a low level, namely below 0.3% by weight, and preferably not more than 0.10% by weight The compositional stability and recyclability (i.e. ability for scrap alloys to be recovered and reused) of lead-calcium alloys are related to the calcium content.An alloy according to the present invention containing around 0.12% by weight calcium and a trace of sodium, preferably with a substantial proportion as sodium hydride is more stable than the conventional Pb-Ca-Sn melts.

The invention may be put into practice in various ways and certain specific embodiments will be described to illustrate the invention with reference to the accompanying examples.

EXAMPLE 1 Pure lead was melted and the temperature of the melt was held at 3600C to 3800C. The dross was skimmed off and pure sodium 0.005% by weight of the melt, wrapped in aluminium foil and enclosed in a perforated steel cage was dropped into the melt. The steel cage was removed after a predetermined length of time and the melt was stirred thoroughly for 1 to 2 minutes. The brownish dross containing sodium oxide (NazO) was removed from the system and the temperature of the melt was allowed to rise to 4600 C. The dross was once again completely removed at about 450 to 4600C and a predetermined amount of calcium master alloy (affording 0.12% calcium based on total alloy weight) added maintaining the temperature above 4200 C. The melt was again thoroughly stirred to obtain a homogenous mixture and finally the melt was allowed to cool.

The cast alloy had the following composition: Ca: 0.10%; Na: 0.00#%; balance lead.

EXAMPLE 2 Pure lead was melted and the temperature of the melt was held at around 3600 to 3800 C. The dross was skimmed off from the melt and the mixture stirred thoroughly for 2 to 3 minutes. Sodium hydroxide was added to the melt in an amount of 36% by weight of the total amount of pure sodium required for the formation of the alloy, which was 0.01%. Pure sodium wrapped in aluminium foil and enclosed in a perforated steel cage was dropped into the melt at about 3700 C. The aluminium amounted to % by weight of the lead. The steel cage was removed after a predetermined length of time and the melt was stirred thoroughly for 1 to 2 minutes. The brownish dross containing sodium oxide (Na > O) was removed from the system and the temperature of the melt was allowed to rise to 4600C.The dross was once again completely removed at about 450 to 4600C and a predetermined amount of calcium master alloy (0.125% calcium based on total alloy weight) was added maintaining the temperature above 4200 C. The melt was again thoroughly stirred to obtain a homogenous mixture and finally the melt was allowed to cool. The cast alloy had the following composition: Ca: 0.105%; Na: 0.004%; balance lead.

EXAMPLE 3 Lead calcium alloy containing a trace amount of sodium prepared according to the process described above in Example 1 or Example 2 was maintained at a temperature around 4200C and tin master alloy (affording 0.2% tin based on total alloy weight), the master alloy containing 10% by weight of tin balance lead, was added. The melt was then allowed to cool.

EXAMPLE 4 Pure lead was melted and the temperature was held at around 360 to 3800 C. The dross was skimmed off from the melt and pure sodium hydroxide flakes were added to the melt. The mixture was stirred thoroughly for 2 to 3 minutes. The sodium hydroxide was added to this melt in an amount of 68% by weight of the total amount of pure sodium required for the formation of the alloy, which was 0.015%. Pure sodium wrapped in aluminium foil and enclosed in a perforated steel cage was dropped into the melt at about 3700 C. The steel cage was subsequently removed after a predetermined length of time.

The brownish dross containing sodium oxide (Na2O) was removed from the system and the temperature of the melt was allowed to rise to 4600 C. The dross was once again completely removed and a predetermined amount of calcium master alloy (0.13% calcium based on total alloy weight) was added, maintaining the temperature at about 4200 C. The melt was again thoroughly stirred to obtain a homogenous mixture before the melt was allowed to cool.

The cast alloy had the following composition: Ca: 0.105%; Na: 0.007%; balance lead.

EXAMPLE 5 Pure lead was melted at about 360 to 3800C and the dross was skimmed off from the top before pure sodium hydroxide flakes were added to the melt with thorough stirring for 2 to 3 minutes. The sodium hydroxide was added to the melt in an amount of 70% by weight of the total amount of pure sodium required for the formation of the alloy, which was 0.02%. Pure sodium wrapped in aluminium foil and enclosed in a perforated steel cage was dropped into the melt at about 3700 C. The steel cage was removed after a predetermined length of time and the melt was stirred thoroughly for 1 to 2 minutes. The brownish dross containing sodium oxide (NazO) was removed and the dross was once again removed at about 450 to 4600C.A predetermined amount of calcium master alloy (0.11% calcium based on total alloy weight) was added while maintaining the temperature of the system above 4200 C. The melt was again thoroughly stirred to obtain a homogenous mixture and finally the melt was allowed to cool.

The cast alloy had the following composition: Ca: 0.09%; Na: 0.009%; balance lead.

EXAMPLE 6 Pure lead was melted at a temperature between 3600 and 3800C and pure sodium hydroxide flakes were added to the melt with stirring in an amount of 60% of the amount of pure sodium. Pure sodium 0.01% wrapped in aluminium foil and enclosed in a perforated steel cage was dropped into the melt at about 3700 C.

The steel cage was subsequently removed from the melt while stirring. The brownish dross containing sodium oxide (Na2O) was removed from the system and the temperature was allowed to rise to 4600 C. The dross was once again removed from the system and 0.005% by weight of aluminium was added prior to the addition of calcium master alloy (0.15% calcium based on total alloy weight) to the melt which was thoroughly stirred and then allowed to cool.

The cast alloy had the following composition: Ca: 0.125%; Na: 0.004%; Al: 0.003%; balance lead.

In all the examples the composition of the calcium master alloy was 1.5% calcium balance lead.

Some typical experimental data e.g. tensile strength and hardness values obtained from the sodium containing Pb-Ca alloy grids of this invention are given in Tables 1 and 2 below. The corresponding results of a conventional 2.5% antimony-lead alloy are also shown for comparison.

TABLE I Tensile Strength, psi Alloy Code Pb-Ca (A) Pb8Ca (B) Pb-Ca (C) Pb-2.5% Sb (D) After 1 hour 5740 5560 5600 4900 After 1 day 6150 5700 6875 5100 After 3 days 6320 5770 - 5200 After 7 days 6530 5840 7150 5200 After 28 days 6560 6100 7200 After 100 days 6800 6800 - TABLE II Hardness Profile, VHN Alloy Code Pb-Ca (A) Pb-Ca (B) Pb-Ca (C) Pb-2.5% Sb (D) After 1 hour 14.9 13.0 16.8 10.8 After 1 hour 17.1 15.1 18.3 12.5 After 3 days 17.7 16.1 - 14.4 After 7 days 17.7 16.1 19.2 16.9 In the above Tables I and II, alloy Pb-Ca (A) was made by the addition of 0.10% Ca, 0.01% Na, 0.0065% NaOH, Sn - Nil to the lead melt; Pb-Ca (B) by the addition of 0.10% Ca, 0.005% Na, Sn - Nil; Pb-Ca (C) by the addition of 0.12% Ca, 0.01% Na, 0.0065% NaOH, 0.05% Sn, and alloy Pb-2.5% Sb (D) comprises 2.5% Sb, 0.025% As, 0.05% Sn, 0.03% Cu, 0.015% Se.

Claims

-1. An alloy comprising calcium in an amount of at least 0.03%, sodium in an amount up to 0.05%, optionally tin, and optionally aluminium, balance lead.
2. An alloy as claimed in Claim 1 comprising 0.001 to 0.05% sodium.
3. An alloy as claimed in Claim 1 or Claim 2 comprising 0.003 to 0.02% sodium.
4. An alloy as claimed in Claim 1,2 or 3 containing about 0.005% sodium.
5. An alloy as claimed in Claim 1,2,3 or 4 containing 0.03% to 0.13% calcium.
6. An alloy as claimed in Claim 5 containing 0.09 to 0.13% calcium.
7. An alloy as claimed in any one of Claims 1 to 6 containing tin in an amount of not more than 0.3%.
8. An alloy as claimed in Claim 7 containing tin in an amount of not more than 0.1%.
9. An alloy as claimed in any one of Claims 1 to 8 containing aluminium in an amount of up to 0.005%.
10. An alloy as claimed in any one of Claims 1 to 9 in which at least part of the sodium is present as sodium hydride.
11. An alloy as claimed in Claim 1 substantially as specifically described herein with reference to any one of the examples.
12. A method of manufacturing an alloy containing lead, calcium and sodium which comprises melting pure lead, adding between 0.001% and 0.05% by weight sodium, stirring the melt, and raising the temperature of the melt before adding calcium to the melt.
13. A method as claimed in Claim 12 in which the sodium is added when the melt is at a temperature between 360 and 3800 C.
14. A method as claimed in Claim 12 2 or Claim 13 in which the melt is at a temperature of 420 to 4600C whilst the calcium is added.
15. A method as claimed in Claim 12, 13 or 14, in which the calcium is added as a calcium lead master alloy.
16. A method as claimed in Claim 15 in which the calcium lead master alloy contains about 1.5% calcium balance lead.
17. A method as claimed in any one of Claims 12 to 16 in which the calcium is added to the melt in an amount of at least 0.03% based on the total weight of lead in the melt and the master alloy added.
18. A method as claimed in any one of Claims 12 to 17 in which after the calcium has been added tin is added to the melt.
19. A method as claimed in Claim 18 in which the tin is added as a tin lead master alloy.
20. A method as claimed in Claim 19 in which the tin lead master alloy contains about 10% tin balance lead.
21. A method as claimed in any one of Claims 18 to 20 in which the tin is added to the melt in an amount of not more than 0.3% based on the total weight of lead in the melt and the master alloy added.
22. A method as claimed in any one of Claims 12 to 21 in which aluminium is added to the melt prior to the addition of the calcium.
23. A method as claimed in Claim 22 in which the aluminium is added in an amount of about 0.005% based on the lead.
24. A method as claimed in any one of Claims 12 to 23 in which the pure sodium is added to the melt when wrapped in aluminium foil and enclosed in a perforated steel cage which is removed once the sodium has melted.
25. A method as claimed in any one of Claims 12 to 24 in which prior to the addition of the sodium, solid sodium hydroxide is added to the melt.
26. A method as claimed in Claim 25 in which the solid sodium hydroxide is added in an amount less than the amount of sodium.
27. A method as claimed in Claim 25 or Claim 26 in which the solid sodium hydroxide is added as flakes of sodium hydroxide.
28. A method as claimed in Claims 25, 26 or 27 in which the amount of solid sodium hydroxide is between 60 and 75% of the amount of sodium.
29. A method of manufacturing an alloy containing lead, calcium and sodium which comprises melting pure lead, adding between 0.001% and 0.05% by weight sodium to the melt at 360 to 3800C, stirring the melt and raising the temperature of the melt to 4600C before adding calcium to the melt as a calcium master alloy while maintaining the temperature of the melt at around 4200 C.
30. A method of manufacturing an alloy containing lead, calcium and sodium which comprises melting pure lead, adding flakes of pure sodium hydroxide into the molten metal at 360 to 3800C, while stirring, adding between 0.001% to 0.05% pure sodium and raising the temperature of the bath to 4600C before adding calcium to the molten bath as a calcium master alloy while maintaining the temperature of the melt at around 4200C.
31. A method as claimed in Claim 12 substantially as specifically described herein with reference to any one of the examples.
32. An alloy as claimed in any one of Claims 1 to 11 when made by a method as claimed in any one of Claims 12 to 31.
33. A method of preparing a battery plate grid which comprises providing a molten alloy as claimed in any one of Claims 1 to 11 or 32, casting the alloy to the desired grid form or to a form from which the grid is made by subsequent processing.
34. A battery plate grid whenever prepared by a method as claimed in Claim 33.