1. Technical Field
The field of invention is lead-acid storage batteries, and more particularly, a positive plate grid alloy composition composed of lead, at least 0.8% tin and minimum amounts of calcium and silver.
2. Background Art
General requirements for lead/acid batteries, alloys for use in grids, all technologies (bookmold, expanded, stamped, woven and composite) are as follows:
1. Mechanical strength sufficient for the technology, and specifically with sufficient hardening rate and hardness to allow battery production with grids of these alloys with state-of-the-art production technology.
2. Very good corrosion resistance, especially at the high under-the-hood temperature of modern cars.
3. Grids free from casting defects such as hot cracks.
4. Stability of the microstructure (averaging, softening).
5. Stability of grid material against releasing components in the electrolyte which degrades primary cell functioning.
6. Rechargeability.
7. Recycling capability.
Alloys of the Pb/Ca/Sn/Ag system can meet all these requirements, but not all alloy compositions can be used in practice. Amounts of Ca, Sn, Ag and Al need to be selected according to some rules. The principal impact of the basic alloy elements are:
Calcium: Primary hardening agent by different calcium based precipitation reactions. However, too much calcium will cause averaging and unacceptable corrosion rate. Calcium content has to be balanced to get sufficient hardening and tolerable corrosion rate. Without additional alloying agents, alloys without calcium or with very low calcium (&lt;0,06%) are very soft and only usable with special production technologies.
Tin: Adds new precipitation reactions leading to (Pb,Sn)3Ca or Sn3Ca. Homogeneous Sn3Ca precipitate is dominant intermetallic product when the Sn:Ca ratio is greater than 9:1 and provides better corrosion resistance than the discontinuous (Pb,Sn)3Ca precipitate which is dominant at lower Sn:Ca ratios. (References: German patent DE2758940 of Assmann 1979; Assmann and Borchers, Z. Metallkunde 69 (1978), pages 43-49; Bouirden, Hilger, and Hertz, J. Power Sources 33, (1991), pages 27-50; Prengaman 7th Int. Lead Conf. Pb '80; Power Sources 67, 1997 267-278). Also, improves rechargability by increasing the conductivity of the corrosion layer. (References: H. Giess in Proc. Symp. Advances in Lead Acid Batteries 84-14, Electrochem. Soc. 1984; Miraglio et. al., J. Power Sources 53, 1995, 53-60); and stabilizes a wrought microstructure. (Reference: Prengaman U.S. Pat. No. 3,953,244).
Aluminum: Reduce calcium loss in melt pots.
Silver:
1. Increasing mechanical strength, especially creep strength of grain boundaries (Prengaman). PA1 2. Increasing hardening rates (Assmann), which makes low or medium calcium alloys a possible option in terms of productivity. PA1 3. Increased hot cracking as described by Gene Valeriote, (1995) 6th Asian Battery Conference. PA1 4. Decreased oxygen overpotential. PA1 a. providing an alloy for a grid supporting structure in a battery cell wherein the grid has a rapid hardening for manufacturability; PA1 b. providing an alloy of the foregoing type which has excellent hardness; PA1 c. providing an alloy of the foregoing type which has low corrosion rate for extended service life; PA1 d. providing an alloy of the foregoing type which has improved casting quality with minimum hot cracking susceptibility; and PA1 e. providing an alloy of the foregoing type which is particularly suited for use in a lead acid battery.
Only some combinations Pb/Ca/Sn/Ag are used for the special needs of battery components:
1. PbCa binary: Ca 0.08-0.12%; fast hardening, but fast averaging and corresponding high corrosion rate. Used in the form of book mold grids, drum cast strip or wrought strip for Negatives.
2. High Ca, low Sn: Typical Ca 0.08% Sn 0.3%; In use world wide for book mold grids. Fast hardening, but averaging caused by discontinuous precipitation reaction resulting in (local) softening and pretty high corrosion rate. Easy to handle in production, but not meeting today's service life expectations under-high temperature, high stress conditions, especially for thin SLI battery grids.
3. High Ca, high Sn: Ca 0.06-0.10%, Sn 0.8-1.5%, Sn:Ca&gt;9:1 with the preferred homogenous Sn,Ca precipitation reactions. Microstructure with better stability compared to low Sn; Corrosion rates still significantly higher than those for Ca free lead alloys.
4. Low Ca, low to medium Sn: 0.025-0.06% Ca, 0.3-0.5% Sn. According to rule Sn:Ca&gt;9:1 with preferred Sn.sub.3 Ca precipitation. Improved corrosion resistance, but reduced hardness and stiffness of grids hinder use for thin SLI grids, but in use for thick industrial battery grids. Use in thin grids especially expanded metal only with the addition of greater than 150 ppm Ag (preferred greater than 200 ppm Ag) is required to improve handling for thin SLI grids. This is indicated in the subsequently referred to U.S. Pat. Nos. 5,298,350; 5,434,025; and 5,691,087.
5. PbSn binary: Calcium free PbSn alloys used for strap material. Generally too soft to be suitable for conventional SLI plate making technologies. Has been used in large industrial batteries and in spiral wound configurations.
In the present SLI application, the ultimate life of a lead acid battery is largely determined by the positive grid alloy. Several factors contribute to making the positive grid the life limiting component of the battery: 1) highly oxidizing potential created by the presence of the positive active material and sulfuric acid, 2) high temperatures accelerating the grid oxidation due to the battery being enclosed in a confined space in close proximity to the ICE engine, 3) relatively poor conductivity of the active material placing most of the current carrying burden on the Pb grid member, and 4) relatively poor match of the crystal structure of the active material compared to the Pb grid to which is must be in electrical contact. For this reason, the alloy of the positive grid has been the subject of a large body of literature and patents.
Today most SLI positive grids are made of Pb/Ca/Sn or Pb/Ca/Sn/Ag alloys. These alloys have won favor over the traditional Pb/Sb alloys in the market place due to their lower water loss and are often referred to as "maintenance free". The Pb/Ca/Sn ternary alloy has been studied extensively. At this point, it is clear that the overall corrosion rate for this ternary alloy is controlled by two key factors: 1)Ca concentration, and 2) the ratio of the Sn/Ca concentration in the alloy. Keeping the calcium concentration as low as possible significantly reduces the corrosion rate, while maintaining the Sn to Ca concentration ratio greater than 9:1 reduces the amount of Pb.sub.3 Ca intermetallic in favor of Sn3Ca and reduces the tendency for discontinuous precipitation. As a result, one of the most successful strategies for improving the positive grid alloy was to use as little Ca as necessary to produce an alloy which age hardened sufficiently fast to an adequate hardness to survive the manufacturing process without damage. Following this approach, the corrosion rate was reduced to the lowest practical limit while simultaneously reducing the amount of expensive Sn necessary to keep a favorable Sn/Ca ratio. It has been found that the composition of positive grid alloys typically used in SLI batteries prior to 1993, that the practical range for calcium was 0.07% to 0.10% Ca with associated tin ranges from 0.6% to 1.3% Sn.
A smaller body of work has explored some aspects of the Pb/Ca/Sn/Ag quaternary alloy system with implications for positive battery grid applications (German patent DE2758940 of Assmann, 1979). More recently, in U.S. Pat. Nos. 5,298,350 and 5,434,025 a lead alloy is described containing 0.3-0.7% tin and 0.015-0.045% silver. U.S. Pat. No. 5,691,087 also discloses a similar composition. However, the amount of tin is 0.3 to 0.9% with the amount of silver being the same as in the previously discussed patents, i.e. 0.015 to 0.045%.
The most important advantage gained from addition of silver to the Pb/Ca/Sn alloy has been the ability to increase the age hardening rate of the alloy when the Ca level is 0.06% or less such that thin SLI grids can be manufactured using conventional processing equipment after an acceptable heat treatment period. Unfortunately, most of the work reported to date, including in the file of U.S. Pat. No. 5,298,350 a declaration by Rao, has drawn conclusions from data on common ternary Pb/Ca/Sn alloys doped with various levels of silver. Based on the best data available at the time, the previously referred to patents concerning the Pb/Ca/Sn/Ag alloy disclosed that positive grid alloys having less than 0.015% silver would possess only marginal mechanical properties even after heat treatment and positive grid alloys having tin in excess of 0.7% would have unacceptable service life.
It is one of the important discoveries of the present work that by using tin at a relatively high level, as well as a high ratio of tin to calcium, the use of silver is not a major factor in such features as rapid hardening for manufacturability, hardness and low corrosion rate for extended service life. Silver is expected to improve creep resistance, but the level can and must be restricted to less than 0.02% by weight to obtain good quality grids. This finding represents an unexpected and unreported interaction of tin and silver in this alloy system. As a direct result of this interaction, the optimum tin and silver composition is placed precisely at levels previously reported to be unacceptable for good service life of a lead acid battery (see U.S. Pat. No. 5,298,350).