US3634213A - Use of cationic permselective membranes in anodizing - Google Patents

Use of cationic permselective membranes in anodizing Download PDF

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Publication number
US3634213A
US3634213A US14849A US3634213DA US3634213A US 3634213 A US3634213 A US 3634213A US 14849 A US14849 A US 14849A US 3634213D A US3634213D A US 3634213DA US 3634213 A US3634213 A US 3634213A
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Prior art keywords
anodizing
electrolyte
acid
aluminum
cell
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US14849A
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English (en)
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Harold Jefferson Coates
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Reynolds Metals Co
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Reynolds Metals Co
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/04—Anodisation of aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/04—Anodisation of aluminium or alloys based thereon
    • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
    • C25D11/08—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing inorganic acids
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/02—Anodisation
    • C25D11/04—Anodisation of aluminium or alloys based thereon
    • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
    • C25D11/10—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing organic acids
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
    • C25D11/005—Apparatus specially adapted for electrolytic conversion coating

Definitions

  • a somewhat similar problem is found in the use of electroplating baths of reactive character, such as, for example, chromic acid baths, which tend to react with the metal of the anode, such as for example, lead, to form insoluble chromates.
  • This phenomenon is observed in the electroplating of conductive substrates generally, and particularly in the plating of aluminum and aluminum base alloys with chromium, employing lead anodes.
  • Lead chromate sludges tend to form during plating, with attendant losses of lead anodes, and depletion of the chromic acid content of the plating bath.
  • the formation of lead chromate results in a potential barrier or polarizing layer which increases the power consumption during plating.
  • aqueous anodizing electrolytes containing a mineral acid, an organic acid, and a metal salt of an organic acid.
  • Such baths are capable of yielding integral nonfading colors which show improved resistance to crazing, corrosion, and abrasion, on aluminum surfaces.
  • the baths are employed at ambient temperature, between about 50 and 80 F, and at a current density between about 12 and 60 amperes per square foot.
  • the organic acids are typically either aliphatic dicarboxylic acids, such as oxalic acid, or alphahydroxy monocarboxylic acids, such as glycolic acid.
  • the metals are typically the common metals of Groups I-B, VII-B and VIII of the periodic system, namely 1ron, nickel, cobalt, copper and manganese.
  • the ferric ions migrate to the cathode, where they are reduced to ferrous ions, combining with oxalic acid present in the bath, to form a sludge of insoluble ferrous oxalate, and causing depletion of the iron and oxalic acid content of the bath, requiring expensive and cumbersome regeneration procedures.
  • the method of the invention whether anodizing or electroplating, is performed either continuously, or semicontinuously or batchwise in an electrolytic cell comprising an anode and a cathode, the anode being immersed in an anodizing electrolyte, and the cathode being immersed in a mineral acid, the respective electrolytes being separated by at least one cationic permselective membrane.
  • cationic permselective membrane refers to a resinous membrane which permits the free passage of cations, while resisting the passage of anions. It customarily consists of a three-dimensional network of a waterinsoluble organic polymer which partakes of the nature of an ion exchange resin and of a filter. Such permselective membranes are well known and have been proposed in connection with the desalinization of water and the electrowinning of metals from solutions.
  • the permselective membrane serves both as a shield for the electrode with which it is associated, and at the same time it divides the cell into compartments, separating the mineral acid electrolyte from the anodizing or plating bath.
  • the mineral acid bath plays no particular role other than as an elec trochemically compatible ion carrier, and is maintained separated from the anodizing or plating bath by the membrane in such manner that the undesired ions cannot migrate into it.
  • a balance must be maintained between water concentration in the cathode cell and that in the anodizing electrolyte in order to prevent osmosis.
  • the mineral acid employed is advantageously the same as that contained in the anodizing or plating bath in order to prevent contamination in the event of leakage.
  • FIG. 1 is a view in cross-section of an anodizing cell in accordance with the invention
  • FIG. 2 is a plan view of the anodizing cell of FIG. 1;
  • FIG. 3 is a view in cross-section of an alternative embodiment of an anodizing cell.
  • FIG. 4 is a view in cross-section of an additional arrangement of the membrane-electrode system.
  • an anodizing cell 10 is provided with an anode compartment 11, and cathode compartment 12.
  • An article 13- to be anodized such as an aluminum article, serves as the anode, while a cathode 14 of acid-resistant material such as graphite or lead, is located in the cathode compartment.
  • the anode and cathode compartments are formed by a permselective membrane 15 which separates the two compartments.
  • the anode compartment thus formed is filled with a body of anodizing electrolyte 16, such as a hard coat anodizing bath of the type previously described.
  • the cathode compartment is provided with a body of a nonpolarizing electrolyte 17, such as a mineral acid, for example sulfuric acid.
  • a nonpolarizing electrolyte 17 such as a mineral acid, for example sulfuric acid.
  • the anode and cathode, respectively, are connected to a source of current, which is advantageously, but not necessarily, a direct current source.
  • FIG. 2 the anodizing cell of FIG. 1 is shown in plan view.
  • the cell is filled with the anodizing electrolyte 21, in which there is immersed the anode 22, which is positioned at the center of the cell.
  • a pair of cathode compartments 23 each of Which forms a separate permselective membrane cell.
  • These membrane cells are provided with a cathode 24 and are filled with a nonpolarizing electrolyte 25 such as sulfuric acid.
  • the membranes are substantially rectangular in cross-section, but may be of any desired shape and are made of a material inert to the electrolyte compositions, such as a synthetic resin, for example polyvinyl chloride. As shown in FIG.
  • the walls of the membrane cells are provided with membrane portions 26 which form windows sealed to the electrolytes, but with the membrane portion 26 serving to permit migration of selected ions into the membrane cell compartment.
  • membrane portions 26 which form windows sealed to the electrolytes, but with the membrane portion 26 serving to permit migration of selected ions into the membrane cell compartment.
  • the permselective membrane may be made of any resinous composition suitable for this purpose, which is permeable to cations, and impermeable to anions.
  • suitable membrane materials include: sulfonated cross-linked polymers of styrene, sulfonated phenolaldehyde condensation products, polystyrene-divinylbenzene copolymers, divinylbenzene-olefinic carboxylic acid forming copolymers, infusible condensation polymers of alkyl aryl ethers and aldehydes, e.g.
  • Mass. which is a sulfonated copolymer of styrene and.
  • the method and apparatus of the invention are especially suitable for anodizing the surface of aluminum and aluminum base alloys to form thereon a hard integrally colored oxide coating, in a single step process, employing a three-component aqueous anodizing electrolyte consisting essentially of (a) a mineral acid, such as sulfuric, boric or hydrofluoric acid, or sulfamic acid, the preferred acid being sulfuric acid in a concentration of from about 0.05% to about 4.5% by weight; (b) from about 0.5% by weight to a percentage represented by the limit of its solubility in the electrolyte of an organic acid selected from the group consisting of an aliphatic alpha-hydroxy monocarboxylic acid, and an aliphatic dicarboxylic acid; and (c) from about 0.1% by weight to a percentage represented by the limit of its solubility in the electrolyte of a metal salt of an organic acid selected from the group consisting of an aliphatic alpha-hydroxy monocarboxylic acid and an
  • the current density employed ranges from about 12 to about 60 amperes per square foot.
  • the anodizing temperature is between about 50 CF. and 80 F.
  • the cathode solution is preferably sulfuric acid, ranging from about 4% to about in H 80 concentration by weight.
  • Suitable aliphatic organic acids of the types mentioned above include glycolic acid (hydroxyacetic acid), lactic acid (alpha-hydroxypropionic acid), and malic acid (2-hydroxybutanedioic acid), oxalic, malonic, succinic and maleic acids.
  • Suitable metals include iron, copper, nickel, cobalt, and
  • the preferred organic acid is oxalic acid
  • the preferred metal salt is ferric oxalate
  • Example 1.IIard anodizing of aluminum Employing a conventional anodizing cell, an aluminum plate made of alloy 5252 was immersed as the anode in an anodizing electrolyte having the composition:
  • Example 2 Membrane anodizing of aluminum Using the apparatus of FIG. 1, with 30% sulfuric acid in the cathode compartment, and the bath of Example 1 in the anode compartment, an anode of alloy 5252, and a cation permselective membrane to separate the two electrolyte bodies, it was found that the presence of the membrane shielded the cathode and prevented ferric ion from reaching it and being reduced.
  • the cell was operated for 1000 ampere-minutes per liter with no formation of insoluble yellow ferrous oxalate taking place. Operation under conventional conditions without the membrane would have produced about 12 grams of ferrous oxalate.
  • Example 3 Membrane anodizing with water balance Utilizing the cell of Example 1, in order to reduce the amount of water transferred by osmosis from the anode cell to the cathode cell, an equimolar cathode-anode solution with respect to water content was prepared by calculating the molar water content of the anodizing electrolyte:
  • Example 4 Chromiurn plating of aluminum Using the cell of Example 1, with a sheet of aluminum alloy 5252 as the cathode, and a cation perm-selective membrane, a conventional chromic acid plating solution containing 250 g.p.l. chromic acid and 2.5 g.p.l. H SO in the cathode compartment, and 30% H 50 as the electrolyte in the anode compartment, and a lead anode, a direct current was applied initially at a voltage of 6-8 volts. No corrosion of the lead anode which normally would result in formation of lead chromate, took place.
  • the method of anodizing an anodizable metal which comprises passing electric current between the anodizable metal as anode immersed in an aqueous anodizing electrolyte and a conductive cathode immersed in a nonpolarizing electrolyte, said electrolytes being separated by at least one cationic permselective membrane.
  • Method of anodizing aluminum or an aluminum base alloy in an aqueous anodizing electrolyte containing a metal ion which comprises passing electric current between the aluminum as anode immersed in said anodizing electrolyte and a conductive cathode immersed in a nonpolarizing electrolyte, said electrolytes being separated by at least one cationic permselective membrane.
  • nonpolarizing electrolyte is an aqueous solution consisting essentially of a mineral acid.
  • Method of anodizing aluminum or an aluminum base alloy to form thereon a hard, integrally colored anodic coating which comprises passing electric current between the aluminum as anode immersed in an aqueous hard coat anodizing electrolyte including a soluble metal salt, and a conductive cathode immersed in an aqueous mineral acid electrolyte, said electrolytes being separated by at least one cationic permselective membrane.
  • Method of anodizing aluminum or an aluminum base alloy to form on the surface thereof a hard, integrally colored anodic coating which comprises passing electric current between the aluminum as anode immersed in an aqueous anodizing electrolyte consisting essentially of (a) from about 0.05% to about 4.5% by weight of sulfuric acid; (b) from about 0.5% by weight to a percentage represented by the limit of its solubility therein of an organic acid selected from the group consisting of aliphatic alpha-hydroxy monocarboxylic acids and aliphatic dicarboxylic acids; and (c) from about 0.1% by weight to a percentage represented by the limit of its solubility therein of a metal salt of an organic acid selected from the group consisting of aliphatic alpha-hydroxy monocarboxylic acids and aliphatic dicarboxylic acids; at a current density between about 12 and about amperes per square foot at an electrolyte temperature between about 50 and about F.; and a conductive cathode immersed in

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
  • Electroplating Methods And Accessories (AREA)
US14849A 1967-07-20 1970-02-16 Use of cationic permselective membranes in anodizing Expired - Lifetime US3634213A (en)

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3770600A (en) * 1969-10-08 1973-11-06 Vaw Ver Aluminium Werke Ag Process for coloring the surface of aluminum workpieces by anodization
US3775269A (en) * 1971-03-15 1973-11-27 Esercizio Dell Istituto Sperim Self-coloring anodic oxidizing process for aluminum and for alloys thereof
US4073708A (en) * 1976-06-18 1978-02-14 The Boeing Company Apparatus and method for regeneration of chromosulfuric acid etchants
US4219394A (en) * 1978-03-22 1980-08-26 Diamond Shamrock Corporation Membrane assembly for electrolytic cells
US4310400A (en) * 1980-02-26 1982-01-12 Mark Jr Harry B Thin layer electrode and cell
US4333810A (en) * 1980-04-11 1982-06-08 The Dow Chemical Company Analyzer for chemical oxidizing or reducing agents
US4891103A (en) * 1988-08-23 1990-01-02 Texas Instruments Incorporated Anadization system with remote voltage sensing and active feedback control capabilities

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2324805A (en) * 1997-04-30 1998-11-04 Platt Electromeck Limited Electroplating

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3770600A (en) * 1969-10-08 1973-11-06 Vaw Ver Aluminium Werke Ag Process for coloring the surface of aluminum workpieces by anodization
US3775269A (en) * 1971-03-15 1973-11-27 Esercizio Dell Istituto Sperim Self-coloring anodic oxidizing process for aluminum and for alloys thereof
US4073708A (en) * 1976-06-18 1978-02-14 The Boeing Company Apparatus and method for regeneration of chromosulfuric acid etchants
US4219394A (en) * 1978-03-22 1980-08-26 Diamond Shamrock Corporation Membrane assembly for electrolytic cells
US4310400A (en) * 1980-02-26 1982-01-12 Mark Jr Harry B Thin layer electrode and cell
US4333810A (en) * 1980-04-11 1982-06-08 The Dow Chemical Company Analyzer for chemical oxidizing or reducing agents
US4891103A (en) * 1988-08-23 1990-01-02 Texas Instruments Incorporated Anadization system with remote voltage sensing and active feedback control capabilities

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