Classifications

• • 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
  • C25D11/18—After-treatment, e.g. pore-sealing
  • C25D11/20—Electrolytic after-treatment
  • C25D11/22—Electrolytic after-treatment for colouring layers

Definitions

• the invention relates to a method for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys using alternating current or alternating current superimposed direct current, the electrolytic coloring being carried out with an electrolyte containing cationic organic dyes.
• the surface of the aluminum and its alloys can be changed mechanically or provided with metallic or non-metallic coatings.
• the enhancement of the natural protective oxide film by chemical or electrical processes is of great importance.
• an organic dye is introduced into the openings in the pores of the oxide layer, and this remains adsorbed in the surface region of the surface.
• the entire color spectrum can be obtained with great uniformity and reproducibility by the process mentioned.
• Various dyes that can be used for this are commercially available.
• the so-called color anodization has been in use for years.
• the finely divided inorganic color particles are not in the pores of the oxide layer, but remain as an alloy component in the aluminum oxide layer.
• special aluminum alloys are mostly anodized and colored using track voltages up to 150 V in only one process step, suitable organic acids, e.g. Maleic acid, oxalic acid, sulfosalicylic acid or sulfophthalic acid can be used.
• suitable organic acids e.g. Maleic acid, oxalic acid, sulfosalicylic acid or sulfophthalic acid can be used.
• the integral method is used less and less in practice.
• anodic oxidation using direct current in aqueous sulfuric acid and / or other electrolytic solutions first produces a colorless, transparent oxide layer, the coloring of which is then carried out in a second process step - in contrast to adsorptive coloring - by deposition of metal particles from metal salt solutions at the base of the oxide layer pores by means of alternating current.
• the color shades range from light bronze to dark bronze to black.
• the storage on the pore base gives completely lightfast stains (W. Sautter, Metall Structure, 32, 1978, pages 450 to 454).
• the electrolytic coloring processes are mainly used for coloring aluminum - which is to be used in architecture - due to its advantages such as higher light resistance and weather resistance.
• the electrolytic metal salt coloring clearly outweighs the integral coloring, with Sn (II), Co, Ni and Cu-containing electrolyte solutions preferably being used for this purpose.
• DE-OS 28 50 136 describes a process for the electrolytic metal salt coloring of aluminum, in which a defined oxide layer is first generated by means of direct current in acidic solution and this is then colored by means of alternating current using an acidic electrolyte containing tin (11) salts, whereby the electrolyte also contains stabilizers for the tin (II) salts.
• Such coloring electrolytes containing metal salts are, however, unsuitable for producing any color and brightness on the aluminum and aluminum alloy surfaces.
• DE-PS 32 48 472 describes a method for coloring anodically produced oxide layers on aluminum and aluminum alloys, in which a coloring electrolyte is used, with the colors of differentdazzlingness and brightness, in particular for use in profiles for windows, doors, facade elements and the like, can be produced on anodized aluminum surfaces.
• the coloring electrolyte contains an organic dye component in addition to a metal salt.
• a metal complex-containing azo dye is proposed as the organic dye component.
• DE-OS 32 48 472 thus describes a process for coloring anodically produced oxide layers in an electrolyte containing metal salts with simultaneous adsorptive coloring with a metal complex-containing azo dye.
• the dyeing processes described above are not completely satisfactory from an application point of view: the electrolytic coloring processes - both the integral process and the metal salt coloring - do not produce bright colors, but rather only gray or bronze to black shades.
• a wide range of bright colors can be achieved using adsorptive processes; however, the dyes are only adsorbed in the upper pore area.
• Such stains are therefore not resistant to abrasion: the surface is attacked by mechanical loads, i.e. the dyes are removed and the color is thus removed. Since such loads usually occur in a locally irregular manner, the scratches, stains, discolorations and the like produced in this way are particularly noticeable. The usability of such colored aluminum parts is therefore greatly impaired.
• Such surface coloring is also unsuitable for aluminum facades, since cleaning them with agents that usually contain abrasives leads to fading.
• the object of the present invention is achieved in that the electrolytic coloring is carried out using an electrolyte containing cationic organic dyes.
• the present invention accordingly relates to a process for the electrolytic coloring of anodized surfaces of aluminum or aluminum alloys using alternating current or alternating current superimposed on direct current, the electrolytic coloring being carried out with an aqueous electrolyte containing cationic organic dyes, which optionally also contains conductive salts.
• the advantage of the method for electrolytic coloring according to the invention compared to adsorptive coloring is that the cationic organic dyes penetrate to the bottom of the pores of the oxide layer during electrolytic coloring, whereby a better protection of the dyes against abrasion and corrosion is proven. As a result of this deep deposit in the base of the pores, it is possible to produce extremely abrasion-resistant, colorful shades on anodized aluminum in an economical manner.
• the electrosorptive coloring of organic dyes known in the prior art has so far only made it possible to obtain so-called achromatic colors, such as gray tones, on anodized aluminum.
• the method according to the invention enables the generation of a large variety of colors with a simultaneously high penetration depth.
• all cationic organic dyes can be used in the process according to the invention.
• these are dyes from the groups of triphenylmethane dyes, cyanine dyes, xanthene dyes (xanthene dyes of the rhodamine group), acridine dyes, azine dyes, thiazine dyes or pyrylium dyes.
• dyes from the groups of triphenylmethane dyes, xanthene dyes and azine dyes are particularly preferred for the purposes of the process according to the invention.
• representatives from these preferred groups of the cationic dyes are: crystal violet, malachite green, methyl violet, rhodamine 6G, methylene blue.
• Such dyes can be used both individually and in the form of mixtures in the process according to the invention.
• the cationic organic dyes can have all possible anions, provided that these do not have a disruptive influence on the electrolytic deposition of the cationic organic dyes.
• the anion when choosing the anion, it is of course important to note that the dye salt is soluble in water.
• the anions for the dye cations are the anions of the mineral and carboxylic acids, for example chloride, sulfate, perchlorate, acetate, tetrafluoroborate or oxalate.
• Preferred anions for the cationic organic dyes for the purposes of the invention are: chlorides, perchlorates and / or oxalates.
• the process according to the invention is carried out in the voltage and current density ranges customary in the prior art, which are usually used for electrolytic metal salt coloring.
• the method according to the invention is carried out at a voltage in the range from 8 to 30 V, which is dependent on the electrode spacing, and at the current densities which arise under these conditions.
• the frequency of the alternating current is usually 50 to 60 Hz.
• Stainless steel is usually used as the material for the counterelectrode, but other materials, for example graphite, can also be used for this purpose.
• the method is carried out at a voltage of 10 to 22 V and the resulting current density.
• Electrolytic coloring according to the invention is carried out in aqueous solutions.
• the upper limit of the concentration of the cationic dye in the aqueous electrolyte solution is determined by the upper solubility limit of the respective dye in water.
• the concentration of the cationic dyes in the electrolyte solution is therefore in the range from 0.01 g / l to the upper solubility limit of the respective dye.
• the aqueous electrolyte solutions in the process according to the invention contain cationic dyes in concentrations of 0.01 to 10 g / l; the concentrations are preferably in the range from 0.05 to 5 g / i.
• the electrolyte solution used in the process according to the invention can contain conductive salts in order to increase the conductivity of the solutions.
• conductive salts are known to the person skilled in the art from the relevant state of the art; for example, they can be selected from the group of the water-soluble alkali metal, ammonium and / or alkaline earth metal salts of those acids which also form the anion of the cationic dyes.
• sulfates preferably sodium sulfate or magnesium sulfate, are generally used as conductive salts.
• the concentration of the conductive salts in the aqueous electrolyte solutions is generally in the range from 1 to 50 g / l; a concentration range of 5 to 20 g / l is preferred.
• the addition of such conductive salts can, in individual cases, lead to a more intense color tint of the color obtained.
• the specialist is therefore in individual cases - i.e. depending on the dye used and the type and intensity of the desired color - decide whether such an addition is desired.
• pH and the temperature of the electrolyte solution are the pH and the temperature of the electrolyte solution and the residence time of the material to be colored in it.
• the pH of the electrolyte solution it is generally the case that the pH value that is optimal for the respective dye is that which occurs in the aqueous electrolyte solution when this dye is dissolved, in the concentration range indicated.
• the pH of the electrolyte solutions in the process according to the invention is generally in the range from 1 to 9; In view of what was said first, the pH range from 2 to 5 is preferred.
• acids or alkalis are used for this purpose which have no disruptive influence on the electrolytic deposition of the exert cationic dyes, for example dilute aqueous sulfuric acid or sodium hydroxide solution.
• the temperature of the electrolytic solution it is preferable to - at least in view of the associated energy saving - at room temperature, i.e. in a temperature range of approx. 15 to 25 C.
• the residence time of the material to be colored in the electrolyte solution depends primarily on the desired depth of color of the coloring. No generally applicable, binding guideline values can be given for this, rather the optimal dwell time must be tried out on a case-by-case basis. However, dwell times of approximately 15 to 30 minutes may be mentioned here as examples.
• the material to be colored that is, the anodized workpieces made of aluminum or aluminum alloys are first subjected to a treatment with direct current - in the same electrolyte - before the actual coloring treatment using alternating current or alternating current superimposed direct current.
• the workpiece or the workpieces is switched as an anode.
• the voltage of the direct current during this treatment is in the range mentioned above; the same applies to the other parameters what has been said above.
• the actual dyeing process does not yet take place during this pretreatment; rather, this pretreatment requires an increased uniformity of the subsequent coloring and a better depth dispersion of the same. More details on such a pretreatment by means of direct current are described in DE-OS 26 09 146.
• the most varied color shades of the aluminum oxide layers can be achieved by specifically coordinating the influencing variables of the individual treatments.
• the objects made from aluminum or its alloys are subjected to a customary pretreatment for producing the oxidic surface layer.
• the condition of the semi-finished products to be anodized i.e. the degree of gloss or mattness of the surfaces, as well as the electrolyte composition and the working conditions during the anodizing process are important influencing factors.
• Test sheets (dimension 50 mm x 40 mm x 1 mm) made of the material AI 99.5 (DIN material No. 3.0255) were used for the examples below.
• the sheets were degreased, pickled and pickled using conventional methods.
• Degreasing was carried out using an alkaline cleaner containing borates, carbonates, phosphates and nonionic surfactants (P3-almeco @ 18, Henkel KGaA, Düsseldorf); Bath concentration: 5% by weight, temperature: 70 ° C, immersion time: 15 minutes.
• a mixture (3: 1) of NaOH and a pickling agent containing alkali, alcohols and salts of inorganic acids (P3-almeco® 46, from Henkel KGaA, Duesseldorf) was used; Bath concentration: 8% by weight, temperature: 55 * C, immersion time: 10 minutes.
• the pickling was carried out using an acidic pickling agent containing salts of inorganic acids and inorganic acids (P3-almeco® 90, from Henkel KGaA, Düsseldorf), bath concentration: 15% by weight, temperature 20 ° C., immersion time: 10 minutes. After each process step, the sheets were thoroughly rinsed with deionized water.
• P3-almeco® 90 an acidic pickling agent containing salts of inorganic acids and inorganic acids
• the subsequent anodization was carried out using the direct current sulfuric acid method; Bath composition: 200 g / IH 2 SO 4 , 10 g / I Al; Air injection: 8 m 3 in the 2nd hour; Temperature: 18 ° C; DC voltage: 15 V.
• the anodizing times were about 3 minutes per um layer build-up; ie the total anodizing times for the oxide layer thicknesses of 15 to 25 ⁇ m given in the examples below were between 45 and 75 minutes.
• the electrolytic dyeing treatment according to the invention was carried out (details below).
• the sheets were then rinsed again and then compacted in hot water with the addition of a sealing deposit inhibitor based on salts of organic acids and nonionic surfactants (P3-almecoseal® SL, from Henkel KGaA, Düsseldorf); Bath temperature: 98 to 100 C, immersion time: 60 minutes, concentration of the sealing deposit inhibitor: 0.2% by weight.
• a sealing deposit inhibitor based on salts of organic acids and nonionic surfactants
• the cationic dyes used and the thickness of the oxide layers were varied.
• the dye concentration in the aqueous electrolyte was 5 gil, the temperature of the electrolyte was 20 ° C. and the treatment time (dyeing time) was 15 minutes.
• the pH values of the electrolyte were obtained in each case by dissolving the dye mentioned in the stated concentration. Only in the case of Example 1e was a lower pH set using H2S04. In each case an AC voltage of 15 V (50 Hz) - counter electrode made of stainless steel - was used.
• the thickness of the oxide layer was measured according to the eddy current principle in accordance with DIN 50984. Following the electrolytic coloring, the depth of penetration of the color was determined by rubbing the oxide layer until it began to lighten with an abrasion tester according to ISO / TC 79 / SC 2 N420E and then measuring the remaining layer thickness determined as indicated above. The values determined are summarized in Table 1 below:
• Example 1e e shows that the depth of penetration of the color can be influenced or controlled by varying the pH.
• Example 2i a conductive salt - 10 g / l MgSO 4 - was additionally added to the electrolyte.
• Example 2i The addition of the conductive salt in Example 2i likewise leads to a more intensive color tinting in comparison to Example 2f, but with the same dyeing time but less tension; however, the penetration depth is not significantly influenced by this.
• test sheets were used which were pretreated in the same way as in the examples according to the invention.
• Commercial anionic aluminum dyes were used to color the oxide layer. Work was carried out on the one hand in the conventional immersion process and on the other hand using AC - 15 V, 50 Hz. The temperatures of the aqueous bath and the electrolyte were 60 C; the dyeing times are 15 minutes. The pH of the baths corresponded to the values that resulted when the respective dye was dissolved in water.
• Dye type, concentration and thickness of the oxide layer as well as the coloration achieved and in particular the depth of penetration into the oxide layer - without and with the use of alternating current - are shown in Table 3.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Engineering & Computer Science (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Electrochromic Elements, Electrophoresis, Or Variable Reflection Or Absorption Elements (AREA)
• Electrochemical Coating By Surface Reaction (AREA)
• Solid Thermionic Cathode (AREA)
• Electrolytic Production Of Metals (AREA)
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• 1987
  • 1987-06-05 DE DE19873718849 patent/DE3718849A1/de not_active Withdrawn
• 1988
  • 1988-05-27 EP EP88108480A patent/EP0293774B1/fr not_active Expired - Lifetime
  • 1988-05-27 AT AT88108480T patent/ATE82596T1/de active
  • 1988-05-27 DE DE8888108480T patent/DE3876012D1/de not_active Expired - Fee Related
  • 1988-06-03 AU AU17344/88A patent/AU601047B2/en not_active Ceased
  • 1988-06-04 KR KR1019880006771A patent/KR890000698A/ko not_active Application Discontinuation
  • 1988-06-06 JP JP63140337A patent/JPS63312998A/ja active Pending
• 1989
  • 1989-02-02 US US07/306,287 patent/US4877495A/en not_active Expired - Fee Related

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