CA2140504C

CA2140504C - Process for converting lead and lead oxides to barium metaplumbate

Info

Publication number: CA2140504C
Application number: CA002140504A
Authority: CA
Inventors: Mary E. Fiorino
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1994-02-28
Filing date: 1995-01-18
Publication date: 1998-08-25
Anticipated expiration: 2015-01-18
Also published as: JPH07272727A; EP0669668A1; US5507842A; DE69511301D1; CA2140504A1; EP0669668B1

Abstract

The present invention provides improved processes for forming anti-corrosion layers, particularly barium metaplumbate, on lead, lead alloy-, and lead oxide-containing substrates. The processes of the invention are used to form corrosion-resistant current collectors which are assembled into lead-acid batteries.
The inventive methods used to form barium metaplumbate employ a salt solution which includes a barium compound and a solvent salt. In a first embodiment, a substrate material having at least a surface comprising elemental lead reacts with a salt solution to form barium metaplumbate. The salt solution includes a barium compound and an oxidizing agent. The solvent salt or barium compound may themselves be oxidizing agents, or an additional oxidant may be added to the solution. The molten salt solution is applied to the substrate in any known manner such as dipping, spraying, and brushing. Advantageously, a lead or lead alloy-containing substrate is dipped into a molten salt solution heated to a temperature at which a portion of the lead-containing substrate is directly converted to bariummetaplumbate. In a further embodiment of the invention, barium metaplumbate is formed on a lead oxide-containing substrate. This process employs a salt solution comprising a barium compound and a solvent salt to convert lead oxide to barium metaplumbate.

Description

1 4 ~ 5 ~ ~
PROCESS FOR CONVERTING LEAD
AND LEAD OXIDES TO BARIUM METAPLUMBATE
Back~round of the Invention 1. Technical Field The invention relates to the formation of barium metaplumbate BaPbO3 layers on substrates having at least a surface which includes elemental lead or lead oxides. More particularly, the invention relates to the formation of barium metaplumbate layers on lead, lead alloy, and lead oxide surfaces exposed to sulfuric acid envi~ nl~ such as surfaces of lead-acid battery com~on~ c 10 2. Description of the Related Art Lead-acid batteries are the most commonly-used batteries in the world today and represent approximately 60% of all battery sales. Lead-acid batteries find use in such diverse fields as automotive, lighting, power tools, and telephone systems. Lead-acid batteries typically employ t vo electrodes, a positive lead 15 dioxide electrode and a negative metallic lead electrode. A sulfuric acid solution is used as the electrolyte.
The charge/discharge mech~ni~m of lead-acid batteries is known as the "double-sulfate" reaction. During discharge, both the metallic lead of the negative electrode and the lead dioxide of the positive electrode are converted to lead sulfate.
20 The reverse process occurs during battery charging, namely, lead sulfate is converted to metallic lead at the negative electrode and to lead dioxide at the positive electrode.
Although lead-acid batteries have numerous designs, many batteries employ lead or lead alloys as current collectors for electrodes. The current collectors may take on a variety of configurations, however, all are designed to mech~nic~lly 25 hold the active material. The active material initially takes the form of a paste comprising active lead and lead oxides, water, and sulfuric acid. The paste is mech~ni~lly molded into the lead current collector to make a battery plate. The final battery is constructed by interleaving positive and negative battery plates using separators to m~int~in plate spacing. A more detailed discussion of lead-acid 30 baKeries may be found in Linden, Ed., Handbook of Batteries and Fuel Cells, (McGraw-Hill Book Company, New York), c. 1984.
A variety of battery components in numerous battery configurations are exposed to a sulfuric acid environment. In particular, lead-containing current collecting elements such as grids, lead spines in tubular batteries, Plante-type35 electrodes, thin film electrodes, made using lead sheets, and bipolar electrodes employ lead-containing surfaces which contact sulfuric acid.

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-2 -Lead-acid batteries are extremely reliable and can be constructed to have long service lives. However, due to the nature of the battery environment, particularly the potentials generated at the positive plate, one of the main battery failure modes is corrosion. During corrosion, the lead current collector reacts with the acidic electrolyte S and is converted into lead oxides. These reaction products are less dense than lead in the elemental form. As more reaction products form, the resultant stress in the oxide layers extrudes the current collector, a process termed "grid creep" when applied to grid extrusion. Grid creep is an irreversible mechanical distortion of a current collector such as a battery grid, resulting in separation of the active material and/or physical distortion 10 of the entire battery. In severe cases of grid creep, the battery housing may crack as the current collector is forced against the walls of the battery. To avoid this problem, battery designers must include extra space in the battery housing to accommodate future expansion due to grid creep.
Numerous solutions have been proposed to alleviate the problem of grid creep in lead-acid batteries. One technique involves alloying the lead current collectors with various elements to increase their rigidity, and hence their resistance to grid creep.
However, these alloying elements, which typically add strength through forrnation of precipitates within a lead matrix, increase the susceptibility of the material to corrosion.
The design of the current collector itself can also enhance battery life by configuring to promote uniform mechanical expansion during corrosion. Increasing the thickness of the battery current collectors also increases their resistance to mechanical distortion.
However, these prior approaches to current collector design merely minimize the effects of grid creep. They do not deal with the fundamental problem of current collector corrosion. In U.S. Patent No.5,143,806 to Bullock et al., the problem of lead-acid battery grid corrosion is addressed through the formation of a protective layer of barium metaplumbate, BaPbO3, on the lead battery grid. Barium metaplumbate is a conductive oxide having the perovskite crystal structure and is resistant to attack by sulfuric acid. Although use of barium metaplumbate alleviates grid creep, the method of the Bullock patent involves multiple processing steps. In the patent process, a layer of lead dioxide must first be formed upon the substrate, usually by electrochemical oxidation of the grid in a sulfuric acid bath. Following formation of the lead dioxide layer, the lead-dioxide-coated lead is covered with finely divided barium hydroxide which is heated for at least 8 hours in a stream of flowing oxygen to convert the lead dioxide layer to barium metaplumbate. The grid is then cooled to room temperature in 3 hours, soaked in a NH4CI solution, and rinsed. This process 2 11 ~ ~ 5 ~ 4 is a time-consuming technique for forming barium metaplumbate. Although the patent parenthetically suggests that a sample could be dipped into molten Ba(OH)2 8H20, it rejects this approach by warning that barium hydroxide is very sensitive to carbon 5 dioxide and will easily convert in air to barium carbonate and then precipitate.
Thus, there is need in the art for improved processes for forming corrosion-resistant layers on substrates, especially lead battery current collectors.
More particularly, there is a need in the art to rapidly and easily form barium metaplumbate layers on lead battery current collectors. Such processes would 10 facilitate the fabrication of batteries with longer lives as well as permitting the use of thinner battery current collectors by protecting the current collectors from corrosion.
Summary of the Invention The present invention provides improved processes for forming anti-corrosion layers, particularly barium metaplumbate, on lead, lead alloy-, and lead 15 oxide-cont~ining substrates. The processes of the invention form corrosion-resistant current collectors which are assembled as part of a lead-acid battery. The inventive methods are used to form barium metaplumbate through a salt solution which includes a barium compound and a solvent salt.
In a first embodiment, a substrate material having at least a surface 20 comprising elemental lead reacts with a salt solution in a single reaction step to form barium metaplumbate. The salt solution includes a barium compound and an oxidizing agent. The solvent salt or barium compound may themselves be oxidizingagents, or an additional oxidant may be added to the solution. The salt solution is applied to the substrate in any known manner such as dipping, spraying, and 25 brushing. Advantageously, a lead or lead alloy-cont~ining substrate is dipped into a salt solution heated to a temperature at which a portion of the lead-containing substrate is directly converted to barium metaplumbate.
In a further embodiment of the invention, barium metaplumbate is formed on a lead oxide-containing substrate. This process employs a salt solution comprising a 30 barium compound and a solvent salt to convert lead oxide to barium metaplumbate.

' ' - 3a-In accordance with one aspect of the present invention there is provided a method for forming a battery having a current collector coated with a layer including barium metaplumbate comprising: providing a current collector having at least one 5 surface comprising lead or a lead oxide; contacting the surface with a salt solution comprising at least one barium compound in a solvent salt in an amount sufficient to convert at least a portion of the surface to barium metaplumbate; converting a portion of the surface to barium metaplumbate; and assembling the current collector into a battery.
In accordance with another aspect of the present invention there is provided a method for forming a layer including barium metaplumbate comprising: providingan article having at least one surface including a lead oxide formed thereon;
contacting the lead oxide-including surface with a molten salt solution comprising at least one barium compound and a solvent salt; and converting at least a portion of the 15 lead oxide-including surface to barium metaplumbate.
Detailed Description According to the invention, methods are provided for forming anti-corrosion layers, particularly barium metaplumbate (BaPbO3), on substrates to be protectedfrom corrosion. These substrates include lead-cont:~ining battery components 20 such as current collectors. Current collectors include electrode configurations such as grids, sheets, tubes, bipolar electrodes, and the like. To form ~,~ ~ ,..

- - 214050q a barium metaplumbate layer on the substrate, a salt solution is used. As used herein, the term "salt" broadly denotes an ionic crystalline compound while the term "solution" denotes an essent~ y ul~ifollllly dispersed mixture, at the molecul~r or ionic level, of one or more substances (the solute) in one or more other subst~nces S (the solvent). The salt solution comprises a barium co...~u~d and a solvent salt.
In a first embodiment of the invendon, barium metaplumbate is directly formed from a reacdon between elel..enti~l lead and a salt solution without the need to first form a lead dioxide layer on the lead surface. The substrate may be anymaterial, pardcularly con~uc~ive metals and ceramics. Thc substrate inclu~l~s at10 least one exterior surface incol~lating elemental lead, e.g., pure elem~nt~l lead or a lead alloy. The ele ..ent~l lead may be an integral part of the substrate or formed as a layer on the substrate in any known manncr. Lead or lead alloy current collectors for use in a lead-acid battery are examples of such substrates.
An oxidi_ing salt solution comprising a barium compound and a solvent 15 salt is used to convert lead to barium metaplumbate. By employing a solvent salt, the barium cGI.lpound is diluted to retard reaction with at..,osl,he.ic carbon colllpoulids. The solvent thus minimi7~s the undesirable formation of barium c~l,onat." a solid which is not soluble at the reaction te.llp~,.dlul~. Formation of barium c~l~nate depletes the barium available for Ir~ction. The solvent salt 20 dissolves the barium co...pound and is molten at the reaction tempe.a~ , i.e., a hlllpe..llul~ on the order of the melting point of lead. Examples of suitable solvents include sodium chlorate (NaCl03) and a eut~l;c colllpo~;hQn of sodium nitrate (NaNO3) and pot~csi-lm nitrate (KNO3) in a 1:1 molar ratio.
Suitable barium coll.pounds for use in the salt solution include, but are 25 not limited to, barium oxide, BaO, barium peroxide, BaO2, and barium salts such as barium hydroxide, Ba(OH)2, and hydrated barium hydroxides, such as Ba(OH) 2 ~ 8H2 O. Mixtures of these compounds with each other or other barium colllpoullds are also suitably employed in a salt solution for forrnation of barium .--e larl.~.... .......bate.
The barium coll,po-lnd is diluted by the solvent salt in the salt solution.
Adv~nta~olJcly~ ution retards c~l,onate formation to a subs~nh~l extent. The e~p~ssion "call,onate formation to a subst~nh~l extent" denotes call,onate forrnation which depletes the initial conce~ tion of barium in the salt soluhion by more than app~A;...~tely 40%. In general, the barium compound should not conctinlt~ more 35 than approximately 20 mole percent of the solution and is preferably less than 10 mole percent with S mole percent being an exemplary concentration based on solubility considerations and minimj~tion of reaction with atmo~,h~-ic carbon _ 5 _ 2140504 dioxide. The solvent salt addi*onally dissolves any decomposition products or dissociated species, if any, of the barium compound employed in the salt solution.
By m~inl~ining such species in solu*on, the amount of barium available to form barium metaplumbate is m~;n~in~
S Formation of barium metaplumbate .~ui~es lead to be oxidized to its +4 valency. To render the salt solution sufficiently oxif1i7ing~ the solvent salt and/or barium co"~poulld can be an oxidant. An oxidizing additive or addi*ves can also be supplied to the salt solu*on. Sodium chlorate is a strong oxidant capable of oxi~li7ing lead to its +4 valency. ~d-li*on~lly, sodium chlorate has a mel*ng point of 255~C, substan*ally less than the 328~C melting point of lead. Recfl~l~e of these desirable l~u~s, sodium chlorate can be used as an oxidizing solvent or it can be added as an oxidant to other salt solutions.
The barium compound can itself be used as an oxidizing agent, such as barium peroxide. Because barium peroxide does not oxidize lead to its +4 valency, 15 an ~d-lition~l oxilli7ing agent is supplied to the reaction site. This a~l~1ition~1 oxygen can be supplied in the form of an oxygen-co~ ning gas, such as ~2 or ozone, bubbled into the salt solution.
Advantageous co",positio~s of salt solutions are given in Table I below ~s in~ te molar percent):

Table I
K,Na Nitrate O 2-containing NaClO3 Eutectic BaO BaO2 Ba(OH)2 8H2O gas A 20% 75% --- --- 5% ---B --- 80% --- 20% --- Yes C 80% ------ 20% ---- ---- ---D 90% --- 10% --- --- ---E 90% --- --- --- 10% ---- 6 ~ 21405D4 In use, the components of the salt solution are applied to the substrate in any conventional manner including, but not limited to, spraying, brushing, and dipping. The salt sol~l*Qn can be applied in a molten state, or alternatively delivered in a finely divided form as a powder, suspencion, paste, or slurry followed by heating S to form a molten solution. Dipping the substrate into a molten solution is a suitable applin~tion technique.
For application by dipping, the pulverized salt solution conctitl1entc are placed in a suitable cont~iner and heated to forrn a molten solu ion The solution is then heated to a le~ atul~_ lower than the 328~C melting point of lead, but high10 enough to perrnit the formation of barium metaplumbate at an ~ccept~ble rate. If the substrate is immersed in a molten salt solution for a short period of time, or the substrate has a higher melting point than that of lead, solution temp~,.alul~s higher than 328~C are useful. The substrate is immersed in a molten salt solution until a barium metaplumbate layer of desired thickness, e.g., thir~ness in the range of 5 to 15 20 microns, is obtained, usually for applu~ilnately 1-3 hours. During immersion, a~1-1itior-~1 oxidant, e.g., an oxygen-cont~ining gas, is optionally supplied in the forrn of, for example, ~2 or ozone bubbled to the reaction site. After removal from the salt soluti~ n the substrate is cooled followed by rinsing in cithcr watcr or a solution which removes the excess salt sohltion such as an aqueous NH4Cl solution.
In another embodiment of the present invention, barium metaplumbate is formed using a substrate having a pre-existing lead oxide surface. The lead oxide surface is a forrn of lead dioxide or a mixture of lead oxides. This embodiment uses a salt solution co,.",l;~ing a barium compound and a solvent salt. The lead oxide surface is part of a lead oxide-cont~inin~ article, such as a battery current collector, 25 or formed by con-e.ling a surface which inrludes ele...el.~l Iead into a lead oxide.
For the latter me-ho 1 a layer in~hl~ing ele~ ntal lead is deposite~ through known methn-ls, or a lead-co~ ining substrate is employe~ Oxidation to form the initial lead oxide is ~.Ç~JI...ed using any desired oxidation technique, examples of which include oxi~1~tion by sodium chlorate and el~ hc~ir~l oxid~tion Once a lead oxide surface has been provided, the substrate is i~l.. e.~ed in the salt solution which includes a barium compound dissolved in a solvent salt.
The barium co--.pounds and solvent salts described in the previous embodil--ent ar useful in the salt solutionc of this embo~liment- Examples of salt solution~ are given in the table below (pe.c~inn~s indic~tc molar percent):

~ 7 ~ 214050 Table Il K,Na Nitrate O2-con~ining NaCI03 Futectic BaO BaO2 Ba(OH)2 8H20 gas A 95% -- -- -- 5% Yes B 90% -- -- -- 10% Yes The solution consl;t~ent~ are mixed and melted as in the previous embod;..~en~ The lead oxidc-cont~ining substrate is i~ e.xl in the molten salt solution to convert at least a portion of the lead oxide to barium metaplumbatc.15 During irnmersion, it is possible to provide ~ddition~l oxygen in the forrn of oxygen or ozonc bubbled into the molten salt solution adj~cent the substrate. The additional oxygen prcvents lead dioxidc decomposition.
The following examplcs are illustrative of the methods for forming barium metaplumb~t~ according to the processes of the present invendon. While the 20 exarnples employ lead foils, it is unde.~lood that the nature of the inventive .loce sses permit substrates of complex shapes, such as battery current collectors, to be coated.

Example 1: Sin~le-Step Process Ground Illi~lu~S of 3.2 grams of BaO and 20.0 grams of NaC103 werc 25 placed in a 25 ml pyrex beaker. The bcaker was placed in a ~ inf~berg vertical tubc furnace and held at 300~C in air until thc mixturc was molten. Lead foils a~ tely 0.5 mil thick were }~lu luccd from 99~9999to purc lead. Foils werc dcgl~ascd for 2 minutes in acetonc in an ultrasonic cleaner beforc use.
Poils werc i".,.,e.~d in the molten salt solution for 3 hours. Following 30 removal, the foils were cooled in air to room te",pe,ature. Car~y-over salt films we~
removed from the foils by swirling them in water. Resist~nt salt films were removed by rinsing the foils in an ultrasonic field for 15 secon~l~ in water or for one minute in 10 wtJvoV~r~enl solution of NH4CI. The foils we~ then air~ried and sectioned for el~rnin~tion- X-ray dirr~ on pal~llls confirmed formation of BaPbO3.
3S Sc~nnin~ clechon mic~scol~y in~ tç-1 dense and continl~o~lc films.

219û50~

Example 2: Sin~le-Step Process The procedure of Example 1 was repeated with the salt mixture being 2.1 grams of NaClO3, 1.6 grams of Ba(OH)2 ~8H2O, and 7.0 grams of 1:1 NaNO3 :KNO3 .
As in example 1, X-ray diffraction confirmed the presence of BaPbO3.
Sc~nning electron microscop~ of these films in~1ic~teA some discontinuities on the substrate surfacc.

Example 3: Lead Oxide Formation Followed by Conversion to Barium Metaplumbate In this Example, lead foils were oxidized by immersion for 1 hour in 20 - grams of molten NaClO 3 to form a dense, adherent oxide surface. The oxidized lead foils were then iln,.lc.~d in a molten salt solution of 20 grams of NaCI03 and 6.3 grams of Ba(OH) 2 ~ 8H2 O at 300~C for 3 hours. Dense and continuous BaPbO3 films were formed.
Following formation of the barium metaplumbate layer, according to any of the processes described above, a further plut~,.,Live layer of lead and/or lead oxide may be d~posi~i thereon. These layers aid in prevendng attack of the barium metaplumbate layer in the battery envi.on-"cnt. The lead and/or lead oxide layers may be applied through any known coating process.
The processes of the present invendon find particular application in the p.ot~lion of battery current collector component~. Following formation of a barium metaplumbate layer on a lead or lead alloy battery collectQr component, such as a grid cle~ de (and, optionally, a lead or lead oxide layer on the barium metaplumbate), the battery current collectors are pasted with active material and 25 assembled to form interleaving positive and negative plates of a lead acid storage battery. Se,pdlalO~a are used to electrically insulate each plate from its nearest coullt~ lectrode neighbors. The plates are assembled in a battery container and filled with a sulfuric acid-based elect~lyte solution. The battery is sealed with a cover through which project the terminal posts.
While thc forcgoing invention has becn described with ~fe.~"1ce to the p~ d embodil"cr,ls, mo-1ific~ ion~ and changes are readily appal~,nt to those skilled in the art. For example, numerous othcr lead battery componer,ts such asposts, straps, lugs, etc., may be treated by the inventive ~ cesses. According, such moAifir~tions and changes such as those suggested above, but not limited thereto, are 35 conside~ed to be within the scope of the present invention.

Claims

Claims:

1. A method for forming a battery having a current collector coated with a layer including barium metaplumbate comprising:
providing a current collector having at least one surface comprising lead or a lead oxide;
contacting the surface with a salt solution comprising at least one barium compound in a solvent salt in an amount sufficient to convert at least a portion of the surface to barium metaplumbate;
converting a portion of the surface to barium metaplumbate; and assembling the current collector into a battery.

2. A method for forming a battery as recited in claim 1 wherein the current collector is a grid.

3. A method for forming a battery having a current collector coated with a layer including barium metaplumbate comprising:
providing a current collector having at least one surface comprising elemental lead;
contacting the surface with a salt solution which includes at least one barium compound and an oxidizing agent in an amount sufficient to convert at least a portion of the elemental lead to barium metaplumbate;
converting a portion of the elemental lead to barium metaplumbate; and assembling the current collector into a battery.

4. A method for forming a battery as recited in claim 3 wherein the current collector is a grid.

5. A method for forming a battery as recited in claim 3 wherein the at least one barium compound is a barium salt.

6. A method for forming a battery as recited in claim 5 wherein the barium salt is selected from barium hydroxides and hydrated barium hydroxides.

7. A method for forming a battery as recited in claim 4 wherein the grid is selected from lead and lead alloy battery grids.

8. A method for forming a battery as recited in claim 3 wherein the step of contacting is performed by dipping the surface into a molten salt solution.

9. A method for forming a battery as recited in claim 3 wherein the oxidizing agent is a salt.

10. A method for forming a battery as recited in claim 9 wherein the salt is sodium chlorate.

Il. A method for forming a battery as recited in claim 3 further comprising forming a layer of material selected from lead and lead oxide on the barium metaplumbate layer.

12. A method for forming a battery having a current collector coated with a layer including barium metaplumbate comprising:
providing a current collector having at least one surface including a lead oxide formed thereon;
contacting the lead oxide-including surface with a molten salt solution comprising a barium compound and a solvent salt;
converting at least a portion of the lead oxide-including surface to barium metaplumbate; and assembling the current collector into a battery.

13. A method for forming a battery as recited in claim 12 wherein the current collector is selected from lead, lead alloy, and lead oxide battery grids.

14. A method for forming a battery as recited in claim 12 wherein the step of contacting is performed by dipping the surface into a molten salt solution.

15. A method for forming a battery as recited in claim 10 further comprising forming a layer of material selected from lead and lead oxide on the barium metaplumbate.

16. A method for forming a layer of barium metaplumbate comprising:

providing an article having at least one surface comprising elemental lead;
contacting the at least one surface with a salt solution comprising at least one barium compound and an oxidizing agent in an amount sufficient to convert at least a portion of the elemental lead to barium metaplumbate; and converting a portion of the elemental lead to barium metaplumbate.

17. A method for forming a layer of barium metaplumbate as recited in claim 16 wherein the barium compound is selected from barium oxides, barium peroxides, and barium hydroxides.

18. A method for forming a layer of barium metaplumbate as recited in claim 16 wherein the oxidizing agent is a salt.

19. A method for forming a layer of barium metaplumbate as recited in claim 18 wherein the salt is sodium chlorate.

20. A method for forming a layer including barium metaplumbate comprising:
providing an article having at least one surface including a lead oxide formed thereon;
contacting the lead oxide-including surface with a molten salt solution comprising at least one barium compound and a solvent salt; and converting at least a portion of the lead oxide-including surface to barium metaplumbate.

21. A method for forming a layer including barium metaplumbate as recited in claim 20 wherein the step of contacting is performed by dipping the surface into a molten salt solution.

22. A method for forming a layer including barium metaplumbate as recited in claim 20 wherein the at least one barium compound is selected from barium oxides, barium peroxides, and barium hydroxides.