US3770600A - Process for coloring the surface of aluminum workpieces by anodization - Google Patents

Process for coloring the surface of aluminum workpieces by anodization Download PDF

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US3770600A
US3770600A US00079242A US3770600DA US3770600A US 3770600 A US3770600 A US 3770600A US 00079242 A US00079242 A US 00079242A US 3770600D A US3770600D A US 3770600DA US 3770600 A US3770600 A US 3770600A
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acid
percent
anodization
aluminum
weight
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US00079242A
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W Sautter
A Gilak
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Vereinigte Aluminium Werke AG
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Vereinigte Aluminium Werke AG
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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
    • C25D11/14—Producing integrally coloured layers
    • 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
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D17/00—Constructional parts, or assemblies thereof, of cells for electrolytic coating
    • C25D17/002—Cell separation, e.g. membranes, diaphragms

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  • ABSTRACT This invention relates to the coloring of the surface of aluminum workpieces by anodization process wherein the anolyte consists principally of maleic acid with a small proportion of sulfuric acid for conductivity, and the catholyte consists of an organic acid of good conductivity or a mixture of organic acid to which a small quantity of sulfuric acid is added to conductivity.
  • the anolyte and the catholyte are separated by a porous, ion-permeable diaphragm.
  • Colored oxide layers of aluminum can be prepared in a variety of known electrolytes consisting of aqueous solutions of sulfonic acids such as sulfosalicylic acid and sulfophthalic acid, or of dicarboxylic acids with small additions of sulfuric acid.
  • sulfonic acids such as sulfosalicylic acid and sulfophthalic acid
  • dicarboxylic acids with small additions of sulfuric acid.
  • Maleic acid is a member of the last group and is frequently used as the electrolyte with additions of oxalic and sulfuric acids because of its low cost and the uniformity of the obtainable colors at relatively low voltages. The colors range from light brown to dark gray and to black.
  • a specific disadvantage of using the unsaturated maleic acid as a major component of the electrolyte results from the fact that the hydrogen given off at the cathode reduces maleic acid to succinic acid.
  • This material has a low solubility and consequently it crystallizes out at the opening of the necessary compressed air circulator and on the equally necessary cooling coils.
  • the bath after throughput of to din of aluminum surface per liter of solution must be regenerated, this being carried out by cooling the entire bath or at least a substantial portion thereof to 10 to C in contrast with the usual operating temperature' which is 20 to 30C and removal of the crystallized out succinic acid.
  • the original concentration of the electrolyte must be maintained by addition of suitable quantities of maleic acid.
  • the formation of succinic acid from maleic acid can be avoided by separating the cathode space and the anode space. This is carried out, preferably, by enclosing the cathode in a boxed-shaped or tube-shaped enclosure, the wall of which consists of a fine-pored, ion-permeable material and which contains an electrolyte with a solute consisting of an organic acid with good electrical conductivity, or a mixture of an organic acid with a small quantity of sulfuric acid.
  • a suitable container for the cathode is a mixture based on fine aluminum silicates fired at high temperature where the pore diameters are in the neighborhood of 1 micron. At a wall thickness between preferably A; and 2/5 inch, such a container has adequate mechanical strength and at the same time a low electrical resistance.
  • the surface area of the container or the surface areas of a battery of parallel-connected containers is so regulated that a current density of about 3 A/dm is not exceeded.
  • the concentration of the electrolyte within the cathode container should preferably be so high that at worst only a small quantity of maleic acid can diffuse thereinto.
  • Suitable mixtures of electrolyte are for example, 3 to 8 wt. percent of oxalic acid and 0.2 to 1 weight percent sulfuric acid.
  • Another suitable combination is 5 to 30 wt. percent citric acid or sulfosalicylic acid with an addition of sulfuric acid in the same quantity.
  • Such electrolyte compositions have additional numerous advantages such as the fact that in the event of damage to the container, no essential change in the properties of the electrolyte in the anode chamber occurs.
  • the composition of the anolyte may be, for example, 20 weight percent of maleic acid, 1 weight percent of oxalic acid and 0.4 weight percent of sulfuric acid with the remainder being water.
  • a further advantage of the process according to the present invention lies in the fact that the aluminum which goes into solution as a result of the anodic oxidation, which according to the present state of the art is removed by ion exchange, in this case collects in the cathode container without any hindrance to the anodic oxidation process. The lifetime of such a bath is thereby enhanced.
  • FIGURE shows schematically the arrangement of an anodization cell according to the present invention.
  • the anode l which is the object to be supplied with a colored coating is placed in a vessel and is separated from a cathode 2 by means of a diaphragm 4.
  • the numeral 3 indicates the aqueous electrolyte which, for example, may consist of 20 weight percent of maleic acid, 1 weight percent of oxalic acid, and 0.4 weight percent of sulfuric acid with the remainder being water.
  • the electrolyte within the diaphragm is indicated by the numeral 5.
  • the cathode 2 is shown to be perforated as indicated at 6; the objective is to permit hydrogen to enter into the inside of the cathode and to escape at the top.
  • the following examples illustrate the advantages accruing from the operation according to the present invention.
  • the first example illustrates the results obtained when operating according to the prior art.
  • EXAMPLE I Anodization of an extruded shape consisting of AIM- gSi 0.5 composition was carried outin a solution consisting of 20 weight percent maleic acid, I weight percent oxalic acid, and 0.4 weight percent of sulfuric acid. The anodization was continued until a dark brown color resulted.
  • the cathode sheet was made of aluminum which was placed in direct contact with the electrolyte. It was found to be necessary to regenerate the solution after a throughput of about 6 dmll of bath content.
  • EXAMPLE 2 A cathode of the same type as used in Example 1 was placed in a separate container containing an electrolyte consisting of 20 weight percent citric acid, 0.4 weight percent of sulfuric acid; the anode was placed in a composition similar to that of Example 1. After a throughput of 100 dm ll of bath contents, only a small quantity of succinic acid has been formed; this amount was completely harmless with respect to continuation of the anodization process. In the cathode space, after the same period of time, the solution contained 8 percent of dissolved succinic acid, and percent of succinic acid had crystallized out. This formation of succinic acid was a result of the inward diffusion of maleic acid. This quantity of transformed maleic acid had no efi'ect whatsoever on the further color-anodization process.
  • a further effect was the increase of the aluminum content in the cathode space; this amounts to about 40 percent of the aluminum which had gone into the solution during the color anodization process.
  • the quantity of aluminum to be removed by ion exchange was greatly decreased.
  • renewal of the small quantity of solution contained in the cathode space made it possible to proceed with the color anodization as before.
  • the voltage required for operation was increased only by about 4 volts as a result of the presence of the cathode container.
  • a process for coloring the surface of an aluminum workpiece by anodization which comprises passing an electric current between the aluminum workpiece immersed as anode in an aqueous anodizing anolyte consisting essentially of maleic acid containing a minor proportion of sulfuric acid or oxalic acid or both, and a conductive cathode immersed in an aqueous catholyte which comprises an electrically conductive organic acid other than maleic acid or a mixture of the said organic acid together with a minor proportion of sulfuric acid, the said anolyte and catholyte being separated from each other by a porous ion-permeable diaphragm.
  • anolyte comprises approximately 20 percent by weight of maleic acid, approximately 1 percent by weight of oxalic acid, and approximately 0.4 percent by weight of oxalic acid.

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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)
  • Printing Plates And Materials Therefor (AREA)
  • ing And Chemical Polishing (AREA)
  • Cell Electrode Carriers And Collectors (AREA)
  • Joining Of Corner Units Of Frames Or Wings (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
  • Electrochemical Coating By Surface Reaction (AREA)
  • Hybrid Cells (AREA)
  • Cosmetics (AREA)
  • Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Abstract

This invention relates to the coloring of the surface of aluminum workpieces by anodization process wherein the anolyte consists principally of maleic acid with a small proportion of sulfuric acid for conductivity, and the catholyte consists of an organic acid of good conductivity or a mixture of organic acid to which a small quantity of sulfuric acid is added to conductivity. The anolyte and the catholyte are separated by a porous, ionpermeable diaphragm.

Description

United States Patent Sautter et al.
Nov. 6, 1973 l l l l PROCESS FOR COLORING THE SURFACE OF ALUMINUM WORKPIECES BY ANODIZATION Inventors: Werner Sautter, Bonn; Ahmad Gilak, Bonn-Beuel, both of Germany Vereinigte Aluminium-Werke Aktiengesellschaft, Bonn, Germany Filed: Oct. 8, 1970 Appl. No.: 79,242
Assignee:
Foreign Application Priority Data Oct. 8, [969 Germany P 19 50 716.5
US. Cl. 204/58 Int. Cl C23b 9/02 Field of Search 204/58 References Cited UNITED STATES PATENTS 1/1972 Coates 204/58 Primary Examiner-John H. Mack Assistant ExaminerR. L. Andrews Attorney-Michael S. Striker [57] ABSTRACT This invention relates to the coloring of the surface of aluminum workpieces by anodization process wherein the anolyte consists principally of maleic acid with a small proportion of sulfuric acid for conductivity, and the catholyte consists of an organic acid of good conductivity or a mixture of organic acid to which a small quantity of sulfuric acid is added to conductivity. The anolyte and the catholyte are separated by a porous, ion-permeable diaphragm.
4 Claims, 1 Drawing Figure PROCESS FOR COLORING THE SURFACE OF ALUMINUM WORKPIECES BY ANODIZATION BACKGROUND OF THE INVENTION Colored oxide layers of aluminum can be prepared in a variety of known electrolytes consisting of aqueous solutions of sulfonic acids such as sulfosalicylic acid and sulfophthalic acid, or of dicarboxylic acids with small additions of sulfuric acid. Maleic acid is a member of the last group and is frequently used as the electrolyte with additions of oxalic and sulfuric acids because of its low cost and the uniformity of the obtainable colors at relatively low voltages. The colors range from light brown to dark gray and to black.
A specific disadvantage of using the unsaturated maleic acid as a major component of the electrolyte results from the fact that the hydrogen given off at the cathode reduces maleic acid to succinic acid. This material has a low solubility and consequently it crystallizes out at the opening of the necessary compressed air circulator and on the equally necessary cooling coils. As a result of this difficulty, the bath after throughput of to din of aluminum surface per liter of solution must be regenerated, this being carried out by cooling the entire bath or at least a substantial portion thereof to 10 to C in contrast with the usual operating temperature' which is 20 to 30C and removal of the crystallized out succinic acid. Moreover, the original concentration of the electrolyte must be maintained by addition of suitable quantities of maleic acid.
SUMMARY OF THE INVENTION It is an object of the present invention to develop a process for coloring the surface of aluminum workpieces while avoiding the difficulties of loss of solute by reduction and crystallization at the cathode.
It is a further object of the present invention to develop a method of anodizing aluminum surface to produce colors ranging from light brown through dark gray to black.
According to the present invention, the formation of succinic acid from maleic acid can be avoided by separating the cathode space and the anode space. This is carried out, preferably, by enclosing the cathode in a boxed-shaped or tube-shaped enclosure, the wall of which consists of a fine-pored, ion-permeable material and which contains an electrolyte with a solute consisting of an organic acid with good electrical conductivity, or a mixture of an organic acid with a small quantity of sulfuric acid.
A suitable container for the cathode is a mixture based on fine aluminum silicates fired at high temperature where the pore diameters are in the neighborhood of 1 micron. At a wall thickness between preferably A; and 2/5 inch, such a container has adequate mechanical strength and at the same time a low electrical resistance. The surface area of the container or the surface areas of a battery of parallel-connected containers is so regulated that a current density of about 3 A/dm is not exceeded.
The concentration of the electrolyte within the cathode container should preferably be so high that at worst only a small quantity of maleic acid can diffuse thereinto. Suitable mixtures of electrolyte are for example, 3 to 8 wt. percent of oxalic acid and 0.2 to 1 weight percent sulfuric acid. Another suitable combination is 5 to 30 wt. percent citric acid or sulfosalicylic acid with an addition of sulfuric acid in the same quantity. Such electrolyte compositions have additional numerous advantages such as the fact that in the event of damage to the container, no essential change in the properties of the electrolyte in the anode chamber occurs. The composition of the anolyte may be, for example, 20 weight percent of maleic acid, 1 weight percent of oxalic acid and 0.4 weight percent of sulfuric acid with the remainder being water.
A further advantage of the process according to the present invention lies in the fact that the aluminum which goes into solution as a result of the anodic oxidation, which according to the present state of the art is removed by ion exchange, in this case collects in the cathode container without any hindrance to the anodic oxidation process. The lifetime of such a bath is thereby enhanced.
By operating according to the present invention, it becomes possible to color anodize parts made of pure aluminum as well as parts made of aluminum alloys specially adapted for color anodization, in electrolytes based on maleic acid, at substantially lower costs than was possible with previous techniques of anodization. The throughput without regeneration of the bath is from 10 to 50 times greater than was previously possible. The requirement for periodic additions of maleic acid now depends only on replacement of drag-out losses.
The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows schematically the arrangement of an anodization cell according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS An arrangement for carrying out the process in accordance with the present invention is shown in the drawing schematically. The anode l which is the object to be supplied with a colored coating is placed in a vessel and is separated from a cathode 2 by means of a diaphragm 4. The numeral 3 indicates the aqueous electrolyte which, for example, may consist of 20 weight percent of maleic acid, 1 weight percent of oxalic acid, and 0.4 weight percent of sulfuric acid with the remainder being water. The electrolyte within the diaphragm is indicated by the numeral 5. The cathode 2 is shown to be perforated as indicated at 6; the objective is to permit hydrogen to enter into the inside of the cathode and to escape at the top.
The following examples illustrate the advantages accruing from the operation according to the present invention. The first example illustrates the results obtained when operating according to the prior art.
EXAMPLE I Anodization of an extruded shape consisting of AIM- gSi 0.5 composition was carried outin a solution consisting of 20 weight percent maleic acid, I weight percent oxalic acid, and 0.4 weight percent of sulfuric acid. The anodization was continued until a dark brown color resulted. The cathode sheet was made of aluminum which was placed in direct contact with the electrolyte. It was found to be necessary to regenerate the solution after a throughput of about 6 dmll of bath content.
EXAMPLE 2 A cathode of the same type as used in Example 1 was placed in a separate container containing an electrolyte consisting of 20 weight percent citric acid, 0.4 weight percent of sulfuric acid; the anode was placed in a composition similar to that of Example 1. After a throughput of 100 dm ll of bath contents, only a small quantity of succinic acid has been formed; this amount was completely harmless with respect to continuation of the anodization process. In the cathode space, after the same period of time, the solution contained 8 percent of dissolved succinic acid, and percent of succinic acid had crystallized out. This formation of succinic acid was a result of the inward diffusion of maleic acid. This quantity of transformed maleic acid had no efi'ect whatsoever on the further color-anodization process.
A further effect was the increase of the aluminum content in the cathode space; this amounts to about 40 percent of the aluminum which had gone into the solution during the color anodization process. In contrast with the processes previously used, the quantity of aluminum to be removed by ion exchange was greatly decreased. Moreover, renewal of the small quantity of solution contained in the cathode space made it possible to proceed with the color anodization as before. Also, by keeping the current density at about 2 A/dm of surface area of the container, the voltage required for operation was increased only by about 4 volts as a result of the presence of the cathode container.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intendeded to be comprehended within the meaning and range of equivalence of the following claims.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:
1. A process for coloring the surface of an aluminum workpiece by anodization which comprises passing an electric current between the aluminum workpiece immersed as anode in an aqueous anodizing anolyte consisting essentially of maleic acid containing a minor proportion of sulfuric acid or oxalic acid or both, and a conductive cathode immersed in an aqueous catholyte which comprises an electrically conductive organic acid other than maleic acid or a mixture of the said organic acid together with a minor proportion of sulfuric acid, the said anolyte and catholyte being separated from each other by a porous ion-permeable diaphragm.
2. A process as defined in claim 1 in which the anolyte comprises approximately 20 percent by weight of maleic acid, approximately 1 percent by weight of oxalic acid, and approximately 0.4 percent by weight of oxalic acid.
3. A process as defined in claim 1 in which the catholyte comprises between 3 and 8 percent by weight of oxalic acid and between 0.2 and 1 percent by weight of sulfuric acid.
4. A process as defined in claim 1 in which the organic acid in the catholyte is oxalic, citric, sulfosalicylic, or sulfophthalic acid, in an amount equivalent to between 5 and 30 percent by weight.

Claims (3)

  1. 2. A process as defined in claim 1 in which the anolyte comprises approximately 20 percent by weight of maleic acid, approximately 1 percent by weight of oxalic acid, and approximately 0.4 percent by weight of oxalic acid.
  2. 3. A process as defined in claim 1 in which the catholyte comprises between 3 and 8 percent by weight of oxalic acid and between 0.2 and 1 percent by weight of sulfuric acid.
  3. 4. A process as defined in claim 1 in which the organic acid in the catholyte is oxalic, citric, sulfosalicylic, or sulfophthalic acid, in an amount equivalent to between 5 and 30 percent by weight.
US00079242A 1969-10-08 1970-10-08 Process for coloring the surface of aluminum workpieces by anodization Expired - Lifetime US3770600A (en)

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DE1950716A DE1950716C3 (en) 1969-10-08 1969-10-08 Process and device for color anodizing of aluminum materials

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5367196A (en) * 1992-09-17 1994-11-22 Olin Corporation Molded plastic semiconductor package including an aluminum alloy heat spreader
US5608267A (en) * 1992-09-17 1997-03-04 Olin Corporation Molded plastic semiconductor package including heat spreader

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3634213A (en) * 1967-07-20 1972-01-11 Reynolds Metals Co Use of cationic permselective membranes in anodizing

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR1430561A (en) * 1965-03-03 1966-03-04 Vaw Ver Aluminium Werke Ag Process for obtaining colored oxide layers on aluminum or aluminum alloy parts
NL6912194A (en) * 1968-08-14 1970-02-17

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3634213A (en) * 1967-07-20 1972-01-11 Reynolds Metals Co Use of cationic permselective membranes in anodizing

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5367196A (en) * 1992-09-17 1994-11-22 Olin Corporation Molded plastic semiconductor package including an aluminum alloy heat spreader
US5608267A (en) * 1992-09-17 1997-03-04 Olin Corporation Molded plastic semiconductor package including heat spreader

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DE1950716C3 (en) 1979-05-23
ZA706579B (en) 1971-05-27
NO126868B (en) 1973-04-02
SE367441B (en) 1974-05-27
LU61309A1 (en) 1970-09-10
DE1950716B2 (en) 1978-09-28
CA947230A (en) 1974-05-14
CH541628A (en) 1973-09-15
FR2064187A1 (en) 1971-07-16
FR2064187B1 (en) 1975-01-10
GB1319658A (en) 1973-06-06
DE1950716A1 (en) 1971-04-22
BE754994A (en) 1971-02-01
NL7014669A (en) 1971-04-14
JPS5010696B1 (en) 1975-04-23
AT292409B (en) 1971-08-25
GB1319659A (en) 1973-06-06

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