1. Field of the Invention
This invention relates to a low-antimony lead alloy for use in the manufacture of paste-holding grids for lead-acid storage batteries.
2. Discussion of the Prior Art
The grids of lead-acid storage batteries are usually made from a Pb-Sb alloy which contains 4 to 11 percent antimony. Because pure lead is inherently soft and cannot be used to make storage battery grids, antimony is added to ensure that the material used to make storage battery grids has the required strength and castability. But the antimony added to these alloy is expensive and appreciably adds to the cost of making the grids. Additionally, the self-discharge rate of the storage battery increases with the antimony content. Moreover, a higher antimony content results in an increase evolution of gas and the storage battery in operation results in a poisoning of said electrode.
For these reasons it has been endeavored to decrease the antimony content of the grids for leading acid accumulators as far as possible.
Hypoeutectic lead-antimony alloys have been used, which contains 1.5 to 3.5 percent antimony as well as 0.025 to 0.2 percent arsenic, 0.005 to 0.1 percent selenium and 0.01 to 0.05 percent tin (German Pat. No. 2,151,733). The grids made from such lead-antimony alloy and used in starter batteries require no maintenance, i.e., no topping up, during their lifetime. Besides, such grids have a useful cycle strength, which means that the changing and discharging cycles of the battery do not result between the grid and the positive paste in a formation of insulating cover layers by which a discharge of the plate would be rendered more difficult and may even be inhibited. It is known from German Pat. No. 2,439,729 that the cycle strength of the above-mentioned low-antimony lead alloys can be further improved by an addition of tin in an amount of, e.g., 0.05 to 0.5 percent. On the other hand, storage battery grids made from such tin-containing low-antimony lead alloys exhibit a relatively high self-discharge rate so that the storage battery must be re-charged when it has been out of operation for considerable time. This is particularly significant with new motor vehicles, which have been for sale for a relatively long time or are shipped over long distances.
It is an object of the present invention to provide a lead alloy which is intended for use in storage batteries and in which the requirements regarding freedom from need for maintenance, low self-discharge rate, high cycle strength and ease of processing are optimally combined.
This object is accomplished by the provision of a low-antimony lead alloy which contains 0.05 to 1.5 percent, preferably 0.4 to 0.5 percent antimony, balance lead and impurities which are due to the manufacture. Preferably, the antimony content is less than 1.5 percent.
If the lead alloy according to the invention additionally contains 0.005 to 0.1 percent selenium for grain refinement, the alloy can directly be cast to form grids without hot tearing or can be rolled into strip, which can be expanded to form grids.
The strength and hardness of the grid can be improved by an addition of 0.02 to 0.5 percent arsenic, if desired. The arsenic content is preferably 0.08 to 0.15 percent with cast grids and 0.04 to 0.06 percent with wrought grids or expanded metal grids.
In accordance with an additional preferred feature of the invention, selenium can be replaced by 0.002 to 0.012 percent preferably 0.002 to 0.006 percent sulfur. This results in a lower cost and in a lower toxicity than where selenium is used because the maximum allowable concentration of selenium at the work-place is only 1/10 that of sulfur. In addition, the quantity of sulfur required for the desired effect is lower for sulfur than for selenium. This is of high significance for the manufacture and processing as well as for the reprocessing of storage battery scrap. As to castability, sulfur in the presence of 0.01 to 0.1 percent, preferably 0.015 to 0.06 percent copper acts like selenium and is a fully satisfactory substitute for copper in this respect. Another advantage afforded by the use of copper is the fact that it opposes the drossing of the molten metal to be cast. It will also be advantageous to include 0.010 to 0.1 percent, preferably 0.03 to 0.06 percent silver in the alloy according to the invention.
Any difficulty which may arise regarding the castability of the lead alloy according to the invention can be eliminated by an addition of 0.002 to 0.5 percent tin.
As a result of the manufacture, the lead alloy contains up to 300 grams bismuth per metric ton; this addition increases the strength.