CA1170617A - Ac etching of aluminum capacitor foil - Google Patents
Ac etching of aluminum capacitor foilInfo
- Publication number
- CA1170617A CA1170617A CA000390233A CA390233A CA1170617A CA 1170617 A CA1170617 A CA 1170617A CA 000390233 A CA000390233 A CA 000390233A CA 390233 A CA390233 A CA 390233A CA 1170617 A CA1170617 A CA 1170617A
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- CA
- Canada
- Prior art keywords
- foil
- etching
- acid
- aluminum
- etched
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- ing And Chemical Polishing (AREA)
Abstract
AC ETCHING OF ALUMINUM CAPACITOR FOIL
Abstract of the Disclosure Aluminum electrolytic capacitor foil is etched in a bath containing 0.9 to 1.8 M hydrochloric acid, 0.2 to 0.5 M aluminum chloride, 0.03 to 0.3 M phosphoric acid, and 0.05 to 0.3 M nitric acid, while subjected to the action of alternating current at 28 to 45°C.
Abstract of the Disclosure Aluminum electrolytic capacitor foil is etched in a bath containing 0.9 to 1.8 M hydrochloric acid, 0.2 to 0.5 M aluminum chloride, 0.03 to 0.3 M phosphoric acid, and 0.05 to 0.3 M nitric acid, while subjected to the action of alternating current at 28 to 45°C.
Description
AC ETCHING OF ALUMINUM CAPACITOR FOIL
This invention relates to AC etching of alumi-num foil for electrolytic capacitors.
AC etching has been used to produce aluminum articles; such as lithographic plates and capacitor foil.
It also has been used to electropolish aluminum articles prior to metal plating.
The prior art has discussed the difficulties in obtaining an even or uniform etch structure and has overcome these difficulties in a variety of ways, e.g., interrupting the etch process to apply protective coat-ings, carrying out the etch process in stages of differ-ing degrees of aggressiveness, and using additives in the electrolyte bath to control pit size or to increase mechan-ical strength of the foil.
Another problem has been to prevent the precipi-tation of aIuminum hydroxide, formed during etching, on or into the etched surfaces.
The resolution of these problems has led to prior art processes in which the etch conditions are care-fully controlled to provide the desired increase in surfacearea, with little change in mechanical strength. Such a process is described by Arora and Randall in U.S. patent US 4,279,714 issued July 21, 1981, that utilizes a phos-phate in the hydrochloric etch bath. Another such process is described by Arora, Randall and Bernard in patent '7~ ~ 1'7 US 4,279,715 issued July 21, 1981, that utilizes a phos-phate and chromate in the hydrochloric acid etch bath.
Still another process is described by Arora in US 4,315,806 issued February 16, 1982, that utilizes an intermittent etch scheme.
The present invention overcomes the environmental concerns about the prior art use of the chromate, while improving on the results obtained from the prior art use of the phosphate only.
A feature of this invention is the provision of a process of etching aluminum capacitor foil which utilizes AC current to give a uniform etch structure while maintain-ing the strength of the foil. Another feature is the pro-vision of a process which also provides a greater etch pit density and size than prior art processes.
In accordance with this invention aluminum capaci-tor foil is etched by a process utilizing aLternating cur-rent and a chloride electrolyte bath containing phosphoric and nitric acids.
In a drawing which illustrates the process of this invention, The sole figure shows foil being etched in the present inventi.on.
In general, aluminum capacitor foil is etched in accordance with this invention by using an etchant solution containing hydrochloric acid, aluminum chloride, nitric acid and phosphoric acid at 28 to 45C, alternating current of 20 to 30 Hz frequency, and anodic current density of 1.5 to 3.0 A/in2.
The concentration of the hydrochloric acid in the etchant is 0.9 to 1.8 M to provide strongly acidic condi-tions that prevent the formation and precipitation of alu-minum hydroxide on the foil. The concentration of the alu-minu~ chloride is 0.2 to 0.5 M to initially catalyze the reaction, especially with fresh etchant solution. The con-centration of nitric acid is 0.05 to 0.3 M to give the desired uniformity and etch density (number of pits/units area of foil). It is postulated that the phosphoric acid .'7~
_ 3 _ in concentrations of 0.03 to 0.3 M serves to passivate sites that have already been started, so that etching will proceed at different sites, givîng the desired etch density, rather than concentrating at started sites. The phosphoric acid also appears to minimize the polishing effect and to maintain the etch structure. When phospho-ric acid is present, capacitance is increased by about 30%
over that o~tained with a hydrochloric-nitric acid etchant alone.
The current density is maintained at 1.5 to 3.0 A/in2 to provide the desired number of sites. If the cur-rent density is too low, i,e., below 1.5 A/in2, there will not be the desired number of sites; if too high, i.e., above 3.0 A/in2, the etch structure becomes fragile lead-ing to a smooth polished surface rather than an etched one and capacitance decreases. Foil is also thinned in the latter case.
The frequency of the alternating current is main-tained at 20 to 30 Hz, as this frequency with the electro-lyte of this invention gives etched foil with optimum ca-pacitance. The voltage, a function of current density and etch cell design, is +7V. ~ith a different cell design, the voltage will differ also.
The temperature of the etchant solution is main-tained at 28 to 45C to provide pores of suitable size and densit~. Lower temperatures result in fewer and larger pores, while higher temperatures give more pores but nar-rower pits and lower capacitance.
Aluminum foil 10 is passed over roll 20 into etch-ing tank 30 between insulated electrodes 31 and 32, under roll 40 and between electrodes 32 and 33, over roll 41 and between electrodes 33 and 34, under roll 42 and between electrodes 34 and 35, and out of tank 30 and over roll 50.
The electrodes are carried by insulated frames 43 and 44 that have openings for passage of foil 10. More electro-des and rolls may be used than shown. In fact, it is more efficient to use more electrodes, but enough have been shown to illustrate the invention.
l~ 7a~:l7 Because the electrodes 31, 32, 33, 34 and 35 are mounted in insulated frames 43 and 44, the alternating cur-rent passed through ~hem is forced to pass through the foil cmd not through the main body of etchant solution. In this way, the foil is electrochemically etched during the time l:he foil passes between a pair of electrodes and not elec-trochemically etched when outside the frame, e.g., between frame 44 and roll 40.
The etched foil obtained by the method of the pre-sent invention retains a solid metallic core that givesgood mechanical properties and low-temperature capacitance retention. There is an interrelationship among etchant composition and concentration, temperature, AC frequency, and current density, with the last being the least crucial, that gives the desired results, i.e. good capacitance and mechanical properties, not just etchant composition and concentration alone.
In the examples below, soft foil of 99.99% purity was used. Hard foil can be etched by this process, as can foil of different purity, but satisfactory capacitance is `
obtained without resorting to hard foil and the processing problem associated therewith.
The etch scheme used is the intermittent etch described by Arora in the above-identified US 4,315,806.
Since the foil is etched intermittently, the total charge passed to the foil is a more meaningful unit than etching time.
Example 1 Soft aluminum foil of 2.9 mil initial thickness was etched intermittently using alternating current and an etchant electrolyte of a mixture of hydrochloric acid, phos-phoric acid, aluminum chloride, and nitric acid.
In Table la, electrolyte composition and molar concentration, M (moles/liter), is given for each run.
Table la Run HCl AlCl3 H3PO4 HN03 1 1.2 0.35 0.11 0.116
This invention relates to AC etching of alumi-num foil for electrolytic capacitors.
AC etching has been used to produce aluminum articles; such as lithographic plates and capacitor foil.
It also has been used to electropolish aluminum articles prior to metal plating.
The prior art has discussed the difficulties in obtaining an even or uniform etch structure and has overcome these difficulties in a variety of ways, e.g., interrupting the etch process to apply protective coat-ings, carrying out the etch process in stages of differ-ing degrees of aggressiveness, and using additives in the electrolyte bath to control pit size or to increase mechan-ical strength of the foil.
Another problem has been to prevent the precipi-tation of aIuminum hydroxide, formed during etching, on or into the etched surfaces.
The resolution of these problems has led to prior art processes in which the etch conditions are care-fully controlled to provide the desired increase in surfacearea, with little change in mechanical strength. Such a process is described by Arora and Randall in U.S. patent US 4,279,714 issued July 21, 1981, that utilizes a phos-phate in the hydrochloric etch bath. Another such process is described by Arora, Randall and Bernard in patent '7~ ~ 1'7 US 4,279,715 issued July 21, 1981, that utilizes a phos-phate and chromate in the hydrochloric acid etch bath.
Still another process is described by Arora in US 4,315,806 issued February 16, 1982, that utilizes an intermittent etch scheme.
The present invention overcomes the environmental concerns about the prior art use of the chromate, while improving on the results obtained from the prior art use of the phosphate only.
A feature of this invention is the provision of a process of etching aluminum capacitor foil which utilizes AC current to give a uniform etch structure while maintain-ing the strength of the foil. Another feature is the pro-vision of a process which also provides a greater etch pit density and size than prior art processes.
In accordance with this invention aluminum capaci-tor foil is etched by a process utilizing aLternating cur-rent and a chloride electrolyte bath containing phosphoric and nitric acids.
In a drawing which illustrates the process of this invention, The sole figure shows foil being etched in the present inventi.on.
In general, aluminum capacitor foil is etched in accordance with this invention by using an etchant solution containing hydrochloric acid, aluminum chloride, nitric acid and phosphoric acid at 28 to 45C, alternating current of 20 to 30 Hz frequency, and anodic current density of 1.5 to 3.0 A/in2.
The concentration of the hydrochloric acid in the etchant is 0.9 to 1.8 M to provide strongly acidic condi-tions that prevent the formation and precipitation of alu-minum hydroxide on the foil. The concentration of the alu-minu~ chloride is 0.2 to 0.5 M to initially catalyze the reaction, especially with fresh etchant solution. The con-centration of nitric acid is 0.05 to 0.3 M to give the desired uniformity and etch density (number of pits/units area of foil). It is postulated that the phosphoric acid .'7~
_ 3 _ in concentrations of 0.03 to 0.3 M serves to passivate sites that have already been started, so that etching will proceed at different sites, givîng the desired etch density, rather than concentrating at started sites. The phosphoric acid also appears to minimize the polishing effect and to maintain the etch structure. When phospho-ric acid is present, capacitance is increased by about 30%
over that o~tained with a hydrochloric-nitric acid etchant alone.
The current density is maintained at 1.5 to 3.0 A/in2 to provide the desired number of sites. If the cur-rent density is too low, i,e., below 1.5 A/in2, there will not be the desired number of sites; if too high, i.e., above 3.0 A/in2, the etch structure becomes fragile lead-ing to a smooth polished surface rather than an etched one and capacitance decreases. Foil is also thinned in the latter case.
The frequency of the alternating current is main-tained at 20 to 30 Hz, as this frequency with the electro-lyte of this invention gives etched foil with optimum ca-pacitance. The voltage, a function of current density and etch cell design, is +7V. ~ith a different cell design, the voltage will differ also.
The temperature of the etchant solution is main-tained at 28 to 45C to provide pores of suitable size and densit~. Lower temperatures result in fewer and larger pores, while higher temperatures give more pores but nar-rower pits and lower capacitance.
Aluminum foil 10 is passed over roll 20 into etch-ing tank 30 between insulated electrodes 31 and 32, under roll 40 and between electrodes 32 and 33, over roll 41 and between electrodes 33 and 34, under roll 42 and between electrodes 34 and 35, and out of tank 30 and over roll 50.
The electrodes are carried by insulated frames 43 and 44 that have openings for passage of foil 10. More electro-des and rolls may be used than shown. In fact, it is more efficient to use more electrodes, but enough have been shown to illustrate the invention.
l~ 7a~:l7 Because the electrodes 31, 32, 33, 34 and 35 are mounted in insulated frames 43 and 44, the alternating cur-rent passed through ~hem is forced to pass through the foil cmd not through the main body of etchant solution. In this way, the foil is electrochemically etched during the time l:he foil passes between a pair of electrodes and not elec-trochemically etched when outside the frame, e.g., between frame 44 and roll 40.
The etched foil obtained by the method of the pre-sent invention retains a solid metallic core that givesgood mechanical properties and low-temperature capacitance retention. There is an interrelationship among etchant composition and concentration, temperature, AC frequency, and current density, with the last being the least crucial, that gives the desired results, i.e. good capacitance and mechanical properties, not just etchant composition and concentration alone.
In the examples below, soft foil of 99.99% purity was used. Hard foil can be etched by this process, as can foil of different purity, but satisfactory capacitance is `
obtained without resorting to hard foil and the processing problem associated therewith.
The etch scheme used is the intermittent etch described by Arora in the above-identified US 4,315,806.
Since the foil is etched intermittently, the total charge passed to the foil is a more meaningful unit than etching time.
Example 1 Soft aluminum foil of 2.9 mil initial thickness was etched intermittently using alternating current and an etchant electrolyte of a mixture of hydrochloric acid, phos-phoric acid, aluminum chloride, and nitric acid.
In Table la, electrolyte composition and molar concentration, M (moles/liter), is given for each run.
Table la Run HCl AlCl3 H3PO4 HN03 1 1.2 0.35 0.11 0.116
2 1.3 0.30 0.14 0.154
3 1.6 0.25 0.11 0.04
4 1.3 0.25 0.11 Q.31 1.3 0.30 0.08 0.154 6 1.4 0.40 0.20 0.195 7 1.5 0.35 0.03 0.154 8 1.5 0.35 0.11 0.154 9 1.5 0.35 0.11 0.154 lQ 1.6 0.35 0.11 0.154 In Table lb, etching conditions and results are presented. The temperature is degrees Celsius (~C), the total charge passed is in coulombs/in2 of foîl, alternating current frequency is in Hertz (Hz), anodic current density is in amperes/in2 of foil (A/in2), thickness is foil thick-ness in mils after etching, and lOV and 30V capacitance is capacitance/unit area (~F/in2).
Table lb Run Temp. Charge Freq. Current Thick- Wt.Loss Capacitance passed density ness % @ lOV @ 30V
1 32 500 25 2.6 2.80 33.3 27075.9 2 36 520 25 2.6 2.87 34.4 338 109 3 32 520 25 2.6 2.82 34.2 267 90 4 32 470 25 2.6 2.84 32.4 23365.2 36 520 25 2.6 2.87 33.3 333103.5 6 40 616 30 2.8 2.82 40.8 346102.2 7 28 465 25 1.94 2.85 3Q.0 30287.7 8 32 500 25 1.75 2.85 32.7 336 100 9 32 450 20 1.94 2.82 34.5 30777.5 32 500 25 1.55 2.83 30.0 32890.3 Example 2 Soft aluminum foil was etched in a pilot plant using as electrolyte: 1.15 M hydrochloric acid, 0.26 M alu-minum chloride, 0.11 M phosphoric acid; and 0.154 M nitric acid at 34C, AC current at 26 Hz, 2.9 A/in2 anodic current density, and 652 coulombs/in2 total charge passed. The weight loss was 30.1%, and 10 and 30V capacitance was 306 and 90.9 ~F/in2, respectively.
An added advantage of this electrolyte is that the concentration of the constituents can vary somewhat without adversely affecting results. The preferred concen-tration of the hydrochloric acid is 1.3 to 1.6 M, with 0.9 M as the lower limit below which not enough etching took place and 1.8 M the upper limit above which macro-thinning of the foil took place.
Some phosphoric acid is needed, with 0.03 M being the lower limit. However, if too much phosphoric acid is present, above about 0.3 M, capacitance decrea~es. The preferred amount is about 0.11 M.
As is known, aluminum chloride initially cat-alyzes the etching reaction, and for that reason at least 0.2 moles/liter must be present inîtially. Higher concen-trations of aluminum ion are preferred, as electrolyte re-cycle and recovery rate are a function of aluminum ion con-centration. However, above about 0.5 M aluminum ion concen-tration, the character of the etching changes to a uniform corrosion rather than preferential tunnel etching. There-fore, 0.5 M aluminum ion concentration is the upper limit with about 0.35 M preferred.
The presence of nîtric acid is critical. Without it, capacitance is about one-third lower and weight loss is 4-5~/O higher leading to poorer mechanical properties of the etched foil. The use of nitric acid in a nitric:hydrochlo-ric acid ratio of 1:8-10 permits~ etching at lower tempera-tures and permits a wider latitude in current density and concentration of electrolyte components, particularly hydro-chloric acid and aluminum ion, without adversely affecting capacitance, foil thickness, and weight loss. With lower 11 f~ 7 nitric acid concentrations, the temperature i5 increa$ed up to 45C to get the desired results, but below about 0.05 M nitric acid capacitance begins to fall off. Above 0.3 M nitric acid, capacitance also falls off. The pre-ferred amount is about 0.15 ~.
Frequency and current density parameters areinfluenced by machine design. In the laboratory, optimum current density was found to be between 1.5 A/in2 and 3.0 A/in2 , while pilot plant equipment was operated satis-factorily at up to 3.6 A~in2. The higher current densitiesare particularly useful for cathode foil etching. Optimum frequency was found in the laboratory to be 25 Hz with 20 and 30 Hz as the lower and upper limits, while the pilot plant machine was operated at 26 Hz.
Nevértheless, the presence of relatively small but definite amounts of both phosphoric and nitric acids in a hydrochloric acid etch solution gives an etched foil with higher capacitance and lower weight loss than are obtained with the hydrochloric acid with either alone.
Table lb Run Temp. Charge Freq. Current Thick- Wt.Loss Capacitance passed density ness % @ lOV @ 30V
1 32 500 25 2.6 2.80 33.3 27075.9 2 36 520 25 2.6 2.87 34.4 338 109 3 32 520 25 2.6 2.82 34.2 267 90 4 32 470 25 2.6 2.84 32.4 23365.2 36 520 25 2.6 2.87 33.3 333103.5 6 40 616 30 2.8 2.82 40.8 346102.2 7 28 465 25 1.94 2.85 3Q.0 30287.7 8 32 500 25 1.75 2.85 32.7 336 100 9 32 450 20 1.94 2.82 34.5 30777.5 32 500 25 1.55 2.83 30.0 32890.3 Example 2 Soft aluminum foil was etched in a pilot plant using as electrolyte: 1.15 M hydrochloric acid, 0.26 M alu-minum chloride, 0.11 M phosphoric acid; and 0.154 M nitric acid at 34C, AC current at 26 Hz, 2.9 A/in2 anodic current density, and 652 coulombs/in2 total charge passed. The weight loss was 30.1%, and 10 and 30V capacitance was 306 and 90.9 ~F/in2, respectively.
An added advantage of this electrolyte is that the concentration of the constituents can vary somewhat without adversely affecting results. The preferred concen-tration of the hydrochloric acid is 1.3 to 1.6 M, with 0.9 M as the lower limit below which not enough etching took place and 1.8 M the upper limit above which macro-thinning of the foil took place.
Some phosphoric acid is needed, with 0.03 M being the lower limit. However, if too much phosphoric acid is present, above about 0.3 M, capacitance decrea~es. The preferred amount is about 0.11 M.
As is known, aluminum chloride initially cat-alyzes the etching reaction, and for that reason at least 0.2 moles/liter must be present inîtially. Higher concen-trations of aluminum ion are preferred, as electrolyte re-cycle and recovery rate are a function of aluminum ion con-centration. However, above about 0.5 M aluminum ion concen-tration, the character of the etching changes to a uniform corrosion rather than preferential tunnel etching. There-fore, 0.5 M aluminum ion concentration is the upper limit with about 0.35 M preferred.
The presence of nîtric acid is critical. Without it, capacitance is about one-third lower and weight loss is 4-5~/O higher leading to poorer mechanical properties of the etched foil. The use of nitric acid in a nitric:hydrochlo-ric acid ratio of 1:8-10 permits~ etching at lower tempera-tures and permits a wider latitude in current density and concentration of electrolyte components, particularly hydro-chloric acid and aluminum ion, without adversely affecting capacitance, foil thickness, and weight loss. With lower 11 f~ 7 nitric acid concentrations, the temperature i5 increa$ed up to 45C to get the desired results, but below about 0.05 M nitric acid capacitance begins to fall off. Above 0.3 M nitric acid, capacitance also falls off. The pre-ferred amount is about 0.15 ~.
Frequency and current density parameters areinfluenced by machine design. In the laboratory, optimum current density was found to be between 1.5 A/in2 and 3.0 A/in2 , while pilot plant equipment was operated satis-factorily at up to 3.6 A~in2. The higher current densitiesare particularly useful for cathode foil etching. Optimum frequency was found in the laboratory to be 25 Hz with 20 and 30 Hz as the lower and upper limits, while the pilot plant machine was operated at 26 Hz.
Nevértheless, the presence of relatively small but definite amounts of both phosphoric and nitric acids in a hydrochloric acid etch solution gives an etched foil with higher capacitance and lower weight loss than are obtained with the hydrochloric acid with either alone.
Claims (3)
1. A process for AC electrolytic etching of alumi-num capacitor foil comprising passing the foil between electrodes supplied with alternating current at a frequen-cy of 20 to 30 Hz in a bath containing 0.9 to 1.8 M hydro-chloric acid, 0.05 to 0.3 M nitric acid, 0.03 to 0.3 M
phosphoric acid, and 0.2 to 0.5 M aluminum chloride, at a temperature of 28 to 45°C.
phosphoric acid, and 0.2 to 0.5 M aluminum chloride, at a temperature of 28 to 45°C.
2. A process according to claim 1 wherein the anodîc current density is 1.5 to 3.6 A/in2.
3. A process. according to claim 1 wherein the con-centration of the hydrochloric acid is 1.3 to 1.6 M, the concentration of the nitric acid is about 0.15 M, the con-centration of the phosphoric acid is about 0.11 M, the con-centration of the aluminum chloride is about 0.35 M, the temperature is 30 to 35°C, and the frequency of the alter-nating current is about 25 Hz.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US21094080A | 1980-11-28 | 1980-11-28 | |
| US210,940 | 1980-11-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1170617A true CA1170617A (en) | 1984-07-10 |
Family
ID=22784953
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000390233A Expired CA1170617A (en) | 1980-11-28 | 1981-11-17 | Ac etching of aluminum capacitor foil |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS57120700A (en) |
| CA (1) | CA1170617A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS61113799A (en) * | 1984-11-09 | 1986-05-31 | Hitachi Condenser Co Ltd | Etching method of metallic foil |
-
1981
- 1981-11-17 CA CA000390233A patent/CA1170617A/en not_active Expired
- 1981-11-27 JP JP19046881A patent/JPS57120700A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| JPS57120700A (en) | 1982-07-27 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |