GB2250856A

GB2250856A - Producing composite grid for lead accumulator by casting

Info

Publication number: GB2250856A
Application number: GB9123090A
Authority: GB
Inventors: Luciano Lemorini; Giuseppe Fossati
Original assignee: Industrie Magneti Marelli SRL; Magneti Marelli SpA
Current assignee: Industrie Magneti Marelli SRL; Marelli Europe SpA
Priority date: 1990-10-31
Filing date: 1991-10-31
Publication date: 1992-06-17
Anticipated expiration: 2011-10-31
Also published as: GB2250856A8; FR2668652A1; GB2250856B; ES2036938B1; IT1241001B; IT9067855A1; IT9067855A0; DE4135280A1; ES2036938A2; FR2668652B1; ES2036938R; GB9123090D0

Description

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1 DESCRIPTION

The present invention relates to a method of producing a grid for leadaccumulator electrodes by the cutting and stretching of a strip produced by the rolling of a bar cast from a lead-calcium base alloy.

There is an ever-increasing tendency to use grids made of a lead alloy including calcium for the production of lead accumulators because of the resulting improved performance in comparison with conventional leadantimony alloys. The expansion (stretching) of rolled strips of leadcalcium alloy is clearly advantageous since it enables electrodes to be produced as a result of the continuous coating of the grid thus obtained and avoids expensive, difficult and pollutive casting operations.

Positive' electrodes based on a lead-calcium alloy and produced by this stretching method, however, have d isadvantages, particularly in accumulators which are subject to very heavy discharge and successive periods of inactivity in the discharged state. In such cases, a passivating layer is created at the grid/active material interface and this makes it difficult to recharge the accumulator. In order to avoid this problem, it has been proposed to add a small percentage of tin (0.21% by weight) to the lead-calcium alloy and this considerably reduces the phenomenon. Larger percentages, which could further improve the situation, are not used because of the cost of the additive and because it would alter the mechanical characteristics of the substrate which must be such as to permit the expansion process.

1 2 According to a method proposed in patent application EP-A-0 252189, the substrate strip of lead-calcium alloy undergoes cold co-rolling with a thin sheet of tin-ricb lead alloy including small percentages of other metals such as antimony and silver, after which the co-rolled sheet produced is expanded to produce the grid.

According to another method proposed in Italian patent application No. IT 68005-A/88, a thin coating layer constituted by a lead alloy including small percentages of elements such as tin, antimony, silver, thallium, zinc and copper is deposited on the expanded grid by chemical or electrochemical deposition techniques in a galvanic bath or by sputtering or spraying with molten metal.

Both the methods mentioned above are expensive, however, since they require an additional operation such as rolling or coating after expansion.

In order to avoid the problems and high costs of the known methods, the present invention offers a method of the type indicated in the paragraph introducing the present invention, characterised in that it includes the step of feeding a flow of a liquid enriching alloy in a thin layer onto at least one surface of the f low of the liquid lead-calcium base alloy for casting so that the casting produces an integral bar with a core portion constituted substantially by the base alloy and an enriched surface portion haying the composition of the enriching alloy.

The bar thus produced is then rolled to the size and thickness desired for the production of a grid and is 3 then cut and stretched; this last operation being facilitated, in particular, by the presence of a core which is resistant to mechanical stresses.

The base alloy is a lead-calcium alloy without antimony but possibly including small percentages of tin, generally of the order of from 0.1 to 1% by weight.

The enriching alloy is a lead alloy including smal percentages of metals selected from the consisting of tin, antimony and silver.

group The enriching alloy may include a proportion of from 1 to 50% by weight, preferably from 2% to 8% by weight, of tin, from 0.1 to 10% by weight of antimony and from 0.05 to 0.5% by weight of silver (if it is used).

Advantages and characteristics of the method according to the invention will become clear from the description which follows with reference to the appended drawings, given by way of example, and in which:

Figure 1 is a schematic view showing the method of casting the base alloy and the enriching alloy, and Figure 2 is a diagram of a detail of Figure 1, on an enlarged scale.

In the drawings, the crucible of a continuous casting device of known type is indicated 1 and supplies a flow F of a base alloy into the space between a casting wheel 2 and a facing roller, schematically indicated 3.

The enrichinq alloy is kept in the molten state in a casting furnace 4 from which it is supplied as a liquid through a conduit 5 which is heated, for example, by 4 electrical resistances 6. A valve 7 is interposed in the conduit 5 and regulates the flow of enriching alloy in dependence on the speed of the cas-1--ing wheel 2. The enriching alloy is fed onto the casting flow of the base alloy, preferably onto both surfaces thereof, by spreader nozzles arranged in positions 8 and 9 adjacent the surface and the base of the mouth of the crucible 6 respectively.

A bar P is thus produced and has a core PA constituted by the base alloy and surface portions PS whose composition corresponds substantially to that of the enriching alloy.

The flow of the enriching alloy is regulated so as to produce two enriched surface portions whose thicknesses are in a ratio of about 1 to 10 to that of the core portion.

The bar thus produced is then cold rolled to produce a strip, typically about lmm thick, for use in the production of electrode grids _ by cutting and stretching.

Grids produced by the method of the invention and used as positive electrodes for lead accumulators provide results equivalent to those achieved by co-rolled strips as regards their recharging characteristics after heavy discharge and prolonged periods of inactivity in the discharged state.

Grids produced by the method of the invention and lead accumulators including the electrodes are intended to fall within the scope of the present invention.

1

Claims

1. A method of producing a grid for lead-accumulator electrodes by the cutting and stretching of a strip produced by the rolling of a bar cast from a lead-calcium base alloy, characterised in that it includes the step of feeding a flow of a liquid enriching alloy in a thin layer onto at least one surface of the flow of the liquid base alloy for casting so that the casting produces an integral bar with a core portion constituted substantially by the base alloy and an enriched surface portion having the composition of the enriching alloy.

2. A method according to Claim 1, characterised in that the base alloy is a lead-calcium alloy including from 0.05 to 0.1% by weight of calcium.

3. A method according to Claim 2, in which the base alloy also includes from 0.1 to 1% by weight of tin.

4. A method according to any one of Claims 1 to 3, characterised in that the enriching alloy is a lead alloy including from 1 to 50% by weight of tin.

5. A method according to any one of Claims 1 to 3, in which the enriching alloy is a lead-antimony alloy including from 0.1 to 10% by weight of antimony.

6. A method according to Claim 4 or Claim 5 in which the enriching alloy includes from 0.05 to 0.5% by weight of silver.

7. A method according to any one of Claims 1 to 6, characterised in that the enriching alloy is fed onto 6 both surfaces of the casting flow of the base alloy.

8. A method of producing a grid for lead accumulator electrodes substantially as described herein with reference to the accompanying drawings.

9. An electrode grid for accumulators produced by a method according to any preceding Claim.

10. A lead accumulator including positive electrodes including grids produced according to the method of any one of Claims 1 to 8.