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
  • C25D11/14—Producing integrally coloured layers

Definitions

• an inorganic acid such as sulfuric acid
• an additional acid which is either an aliphatic alphahydroxy monocarboxylic or an aliphatic dicarboxylic acid, or sulfamic acid
• a metal salt of either of the aforementioned organic acids such as a ferric salt.
• the process employs ordinary temperatures, and a current density between about 12 and 60 amperes per square foot.
• the hard anodizing of aluminum and aluminum base alloys has conveniently been performed in aqueous electrolytes containing mineral acids, or organic acids, or mixtures of such acids. Where it was desired to obtain resistance to abrasion it has been considered necessary to operate at low temperatures, in the range of to 32 F involving the use of expensive refrigeration.
• the known processes for hard coat anodizing also have the drawback of providing only a limited range of colors in the anodized product, namely dark gray to black shades, which are not suitable for decorative architectural applications where a market exists for a wider range of colors.
• hard coating or hard coat as used in the anodizing art, there is meant an oxide coating which, on 1 100 aluminum alloy (commercial purity aluminum), will have a Knoop hardness number, taken in the cross section of the coating, of 400 KHN or more; see also Wernick and Pinner, The Surface Treatment and Finishing of Aluminum and its Alloys," Chapter 8 (1959).
• the anodizing process of the invention provides a harder anodized surface which is less susceptible to crazing and which possesses greater corrosion resistance, and resistance to abrasion. Integral nonfading colors are imparted, ranging from light shades ofgold, and deeper shades of brasses and bronzes, to charcoal grays and blacks. This color range is obtainable with a smaller number of different aluminum alloys than is the case with existing methods, and scrap and rerun rates are considerably lower.
• a particular advantage lies in drastically reduced anodizing times and in operation at ordinary or ambient temperatures.
• the colored anodized products are applicable in such fields as architectural finishes and trim, coated cooking utensils, machine parts, automotive trim, or in any other areas where the decorative and special physical properties of hard coated aluminum are required.
• the novel process of the invention is applicable to pure aluminum as well as to various aluminum base alloys.
• the colors obtained range from light gold in the case of high-purity alloys, and shades of bronze-brown in intermediate alloys, to dark brown and black for high constituent alloys.
• a novel anodizing bath which contains three components: (l) a conductor component, (2) a voltage component; and (3) a color control component.
• the conductor component is a moderate concentration of an inorganic acid which will impart the necessary conductivity to the bath and which does not form undesirable byproducts during the electrolysis.
• suitable inorganic acids include sulfuric, sulfamic, hydrofluoric, and boric acids, or mixtures thereof. These acids are employed in amounts ranging from a lower limit below which the acid will not dissolve the oxide to an upper limit marked by excessive attack on the oxide or the coloring constituents of the aluminum.
• Sulfuric acid is the preferred conductor component meeting these requirements, and is employed within the approximate concentration range of 0.05 percent to 4.5 percent by weight.
• the hydrofluoric acid range is from about 0.00l percent to 0.5 percent; boric acid, about 0.5 percent to 2 percent; and sulfamic, about 0.5 percent to 5 percent.
• the voltage component of the bath is either sulfamic acid or an aliphatic organic acid which is soluble in the bath at the temperatures employed for anodizing and which possesses a dissociation constant facilitating the application of an increasing impressed voltage sufficient to maintain the necessary current density.
• the organic acid of the voltage component is selected so that any decomposition products which it forms on electrolysis will also behave as voltage components, and facilitate the electrolysis of the third, or color control component.
• the color control (dissolution inhibiting) component is a metal salt ofan organic acid, preferably the organic acid of the voltage component or a similar type of organic acid.
• the application of the increasing impressed voltage results in electrolysis of the metal salt, probably causing a complex metal oxide to form in the anodic film, and making the film harder. it is believed that the action is one of the metal salt inhibiting the dissolution attack of the acid components on the alloying constituents of the metal being anodized, and the oxide film.
• the inclusion of the undissolved or partially dissolved constituents in the oxide film produces light traps and colored particles which impart the principal coloring effect in the coating.
• the three component anodizing bath of the invention is operated at a current density advantageously in the range of about 12 to about 60 amperes per square foot, and preferably at about 48 amperes per square foot.
• the anodizing step involves a program for the gradual increase of the impressed voltage, for example, from about 20 volts direct current initially, to as high as 250 volts for heavy films on some aluminum alloys.
• the impressed voltage will range from about 20 to 35 volts.
• the control of impressed voltage makes it possible to apply that voltage necessary to decompose the metal salt and to obtain the desired color in the anodic film.
• the anodizing bath temperature may range from about 50 F. to about F., depending upon the solubility limits of the organic acids or their metal salts.
• the preferred operating temperature is between about 68 F. and about 72 F., and this constitutes an advantage over known methods employing low operating temperatures.
• the aliphatic organic acids which are employed as voltage components in the practice of the invention include saturated aliphatic alpha-hydroxy monocarboxylic acids, such as, for example, glycolic acid (hydroxyacetic acid), lactic acid (alphahydroxypropionic acid), and malic acid (2-hydroxybutanedioic acid), as well as also saturated and unsaturated aliphatic dicarboxylic acids, such as, for example, oxalic, malonic, succinic, and maleic acids.
• the foregoing acids are employed in the anodizing bath in concentrations ranging from about 0.5 percent by weight up to the limit of solubility.
• the preferred organic acid is oxalic acid.
• the color control component of the bath is a metal salt of one of the foregoing organic acids, and in general, is a salt of the same acid that constitutes the voltage component, but it may also be a salt of a specifically different acid.
• Metals which have been found to induce desirable colors to the anodic film are'the common metals of Groups 18, VHS and Vlll of the periodic system, namely iron, nickel, cobalt, copper and manganese.
• the preferred metal is iron, and the preferred metal salt is ferric oxalate.
• concentration of metal salt ranges from about 0.1 percent by weight up to the limit of solubility.
• the colors produced by the method of the invention are integral with the anodic coating.
• the color may arise from several sources.
• the oxalic acid reacts during anodizing, one of the reaction products will be colloidal carbon, some of which remains in the coating lending a yellow tint to the oxide film.
• iron salts employed, iron from the electrolyte is incorporated in the film, probable in the form of oxides of iron, or of combined iron and aluminum oxides.
• the color imparted by iron is reddish brown. Manganese constituents tend to turn the coating dark or black.
• the baths of the present invention produce opaque types of coatings. This may be due to entrapment of aluminum particles, and oxidation products or partly oxidized products, insoluble or slightly soluble, from other constituents of the alloy or the film.
• the method and electrolyte of the invention may be employed for the decorative hard coat anodization of aluminum metal, and of aluminum base alloys, for example in the form of sheet, extrusions, or honeycomb.
• sheet alloys which can be coated include Nos. 1100, 3003, 3004, 5005, 5052,5257, 5457, 5657, 5252, and 6061.
• extrusion alloys include 6061, 6063, 6463, and 6351.
• the application of the hard coat to aluminum honeycomb increases its crush strength greatly, by as much as 40 percent.
• the general procedure employed in the practice of the invention includes the preparatory steps of degreasing the article to be treated in a conventional bath of a nonsilicated alkaline cleaning agent, rinsing in tap water, etching for 5 minutes in sodium hydroxide-sodium gluconate solution at about 150 F., desmutting for 2 minutes in l-l nitric acid and rinsing in water. This is then followed by anodizing in the electrolyte ofthe invention, and subsequent sealing, if desired.
• the preferred anodizing electrolyte according to the invention is an aqueous solution of sulfuric acid, oxalic acid, and ferric oxalate, having the following composition range, by weight:
• the voltage range encountered will be dependent upon the resistance of the system to the passage of current.
• the current density applicable is of the order of 48 amperes per square foot. At this current density the required voltage will range from about 25 volts initially to as high as 250 volts for heavy films on some aluminum alloys. A 1 mil hard anodic film can be produced on most alloys in minutes.
• the color of the anodized part will depend upon the method of pretreatment,
• the alloy and temper the nature and concentration of the electrolyte, the anodizing time, the applied voltage, and the thickness of the coating.
• the preanodizing step may be carried out by anodizing for 3 minutes in lO-30 percent sulfuric acid at 12-20 amperes per square foot current density at 70 F., followed by a water rinse.
• the article is then anodized using the bath of the invention, as previously described. This results in a unique gloss finish in the colored article.
• the surface of aluminum and aluminum alloys is not homogeneous and conventional anodizing electrolytes tend to select spots of low resistance as nucleation sites for conversion of metal to oxide.
• the novel bath and process of the invention disregard the heterogeneity of the metal surface and result in uniform anodic coatings.
• This homogeneous response for a given alloy is improved still further by the aforementioned preanodizing step, whereby nucleation sites are established very homogeneously by forming only enough oxide by conventional sulfuric acid anodizing. These sites are then immediately utilized in further anodization with the electrolyte mixture of the invention.
• the preferred range of temperature in hard coat anodizing according to the invention is from about 50 F. to about F., and the preferred temperature is 70 F. t 2 F.
• the bath temperature affects conductivity, film-forming efficiency, and the color and reflectivity of the finished article.
• ferric oxalate Where ferric oxalate is employed, a certain amount of depletion of the electrolyte occurs during use as the result of the reduction of ferric iron to ferrous iron, forming a precipitate of ferrous oxalate which is insoluble in the electrolyte.
• the ferrous oxalate may be removed either by filtration or by oxidation to ferric oxalate by periodic additions to the bath of hydrogen peroxide.
• the anodizing bath of the invention exhibits good throwing power, permitting the formation of anodic coatings of uniform thickness in the crevices and recesses of irregularly shaped objects which are at different distances from the cathode.
• the anodic coatings produced according to the invention exhibit superior hardness qualities. Presently, there is no accepted method of measuring the hardness of anodic coatings.
• the hardness of coatings produced by the baths of the invention was tested using the Tukon Microhardness Tester. Samples having various coatings were mounted and polished in cross section. The hardness measurements were made with a gram load using a diamond indenter which had an included longitudinal angle of 17230' and an included transverse angle of C.
• the K (Knoop hardness, 100 grams load) values using the preferred bath are shown in table I. The hardness values depend upon the alloy treated and the thickness of the coating. As the purity of the metal increases, so does the hardness of the oxide formed.
• the hardness obtained in films produced by the method of the invention is superior to that produced by other known methods.
• the Knoop values range from about 400 to as high as 624 in the case of pure aluminum (99.99 percent).
• the apparent densities of coatings obtained with various alloys are also shown in table 1, They were measured by cutting specimens about 3 inches X 3 inches, measuring, cleaning in acetone, weighing, stripping with a boiling solution of 20g CrO and 35 ml. H PO in 965 ml. distilled water, weighing and calculating density. The apparent density increases as coating thickness increases.
• Tis the anodizing time in minutes C is the current density in amperes per square foot D is the thickness in mils.
• crazing may occur with conventional anodizing methods where there is a difference in thermal expansion between the oxide film and the metal.
• One type of crazing appears around certain grains, certain imperfections, scratches and microconstituents where there is preferential anodizing at these sites, with adjacent areas anodizing at different rates. The edges which are growing fastest tend to crack.
• Much of this type of crazing can be prevented, in accordance with the invention, either by preanodizing in sulfuric acid, which will start all the grains anodizing at a uniform rate, or by lowering the current density during hard coating.
• EXAMPLE 1 A 1 inch X 3 inch sample of 99.99 percent aluminum sheet was degreased in alkaline detergent, rinsed in tap water, etched with caustic soda 5 minutes at 150 F., rinsed for 2 minutes, and desmutted in l-l nitric acid, and rinsed in tap water. The sample was then anodized for minutes in a bath having the composition:
• the anodizing temperature was 70 F.
• the current density was 48 amperes per square foot.
• the voltage ranged from to 35 volts direct current.
• a reddish brown coating was obtained.
• the coating had a thickness of 1.20 mils, a density of 2.63, and a Knoop hardness of 5 l 2.
• EXAMPLE 2 To illustrate the effect of preanodizing, a sample similar to that of example 1 was degreased, bright dipped in a conventional phosphoric acid-nitric acid solution, and preanodized for 3 minutes in a 17 percent by weight sulfuric acid electrolyte at a current density of 15 amperes per square foot, at a temperature of 70 F. It was then anodized as in example 1. The product had a reddish brown color and exhibited a high degree of gloss.
• EXAMPLE 3 An extrusion of 6063-16 alloy was hard coated following the preanodizing and hard coat anodizing procedures ofexample 2.
• the anodizing time was 22 minutes, the voltage range was 38 to 62 volts.
• the product had a glossy light amber shade, film thickness 1.7 mils.
• EXAMPLE 4 A preanodized sheet of alloy 5052 was further anodized for 15 minutes, at current density 48 amps. per sq. ft., voltage range 30 to 33 volts DC, electrolyte temperature 31 C. in a bath having the composition:
• the anodizing produced a dark brownish and very hard film on the aluminum surface.
• EXAMPLE 5 A preanodized extrusion of 5052 alloy was further anodized for 15 minutes, at current density 48 amps. per sq. ft., voltage range 33 to 36 volts DC, electrolyte temperature 21 C., in a bath having the composition:
• a deep brown-grey oxide film was formed showing exceptional hardness.
• EXAMPLE 6 A preanodized sheet of alloy 5052 was further anodized for 13 minutes, at current density 48 amps. per sq. ft., voltage range 36 to 40 volts DC, electrolyte temperature 205 C. in a bath having the composition:
• a hard brown-grey anodic coat was produced on the alloy.
• the electrolyte produced a dark grey anodic film which was found to be very hard.
• EXAMPLE 8 A sheet of alloy 1 100 was anodized for 20 minutes, at current density 48 amps. per sq. ft., electrolyte temperature 20-22 C., voltage range 34 to 45 volts, DC, in the following bath:
• EXAMPLE 9 A sheet of alloy 1100 was anodized for 15 minutes, at current density 48 amps. per sq. ft., electrolyte temperature 70 F voltage range 40 to 60 volts, DC, in the following bath:
• EXAMPLE 10 A sheet of alloy 1 100 was anodized for minutes at current density 48 amps. per sq. ft., electrolyte temperature 70 F., voltage range 35 to 50 volts, DC, in the following bath:
• cuprous oxalate was dissolved in Nl-l Ol-l and the solution slightly acidified with oxalic acid.
• EXAMPLE 1 l A sheet of alloy l 100 was anodized for 15 minutes, at current density 48 amps. per sq. ft., electrolyte temperature 70 F voltage range 35 to 45 volts, DC, in the following bath:
• the cobaltous oxalate was dissolved in NH Ol-l and the solution slightly acidified with oxalic acid. A coating 1 mil-thick was obtained.
• EXAMPLE 12 A sheet of alloy 1 100 was anodized for IS minutes, at current density 48 amps. per sq. ft., electrolyte temperature 70 F., voltage range 42 to 60 volts, DC, in the following bath:
• the manganous oxalate was dissolved in NH OH to a clear blue solution, which was slightly acidified with oxalic acid. A film 1 mil-thick was obtained.
• EXAMPLE 13 A sheet of alloy 1100 was anodized for 15 minutes, at current density 48 amps. per sq. ft., electrolyte temperature 70 F., voltage range 35 to 50 volts, DC, in the following bath:
• the nickelous oxalate was dissolved in moon to a clear solution and acidified slightly with oxalic acid.
• the coating formed was 1 mil-thick EXAMPLE 14
• An anodizing electrolyte was prepared having the composi- Water 1,000 mi. Ferric oxalate gJl. Oxalic acid 60 3.1L Sulfuric acid 12 g.ll.
• a panel of aluminum alloy 1100 was anodized in this electrolyte with alternating current, at 20-25 C., 10 volts, and 48 amperes per square foot current density for 25 minutes.
• the resulting oxide film had a light gold color.
• a composition adapted to form components of an aluminum anodizing bath when dissolved in water, and which bath can be utilized in the practice of the invention is also included within the contemplation of the present invention.
• Said composition is a dry mixture of an organic acid and the metal salt of an organic acid.
• a preferred embodiment of such a mixture is one which consists essentially of from about 0.5 to about 9.0 parts by weight of oxalic acid and from about 0.5 to about 8.0 parts by weight of ferric oxalate.
• EXAMPLE 16 A sample of 6063-T4 extrusion 0.125 inches thickness was anodized for 30 minutes at a current density of 48 amps. per square foot in a bath containing 30 g./l. sulfamic acid, g./l. oxalic acid and 80 g./l. ferric oxalate at a temperature of 27 C. A voltage range of 77-123 was observed. A uniform coating of medium gold resulted.
• EXAMPLE 1 A sample of 6063-T4 extrusion 0.125 inches thickness was anodized for 45 minutes at a current density of 48 amps. per square foot in a bath containing 6.1 g./l. sulfuric acid, 30 g./l. sulfamic acid and 80 g./l. ferric oxalate at a temperature of 20 C. A voltage range of 39 to 85 volts was observed. A uniform black color resulted.
• a uniform golden-colored oxide coating was formed, having a film thickness of0.80 mil.
• said inorganic acid is selected from the group consisting of sulfuric acid in an amount from about 0.05 percent to about 4.5 percent by weight, sulfamic acid in an amount from about 0.5 percent to about 5 percent, hydrofluric acid in an amount from 0.001 percent to about 0.5 percent, and boric acid in an amount from about 0.5 percent to about 2 percent;
• said organic acid is from about 0.5 percent by weight up to a percentage represented by the limit of its solubility therein of an acid selected from the group consisting of aliphatic alpha-hydroxy monocarboxylic acids and aliphatic dicarboxylic acids;
• said salt is from about 0.1 percent by weight up 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, said metal being selected from the group consisting of iron, nickel, cobalt, copper and manganese;
• Said metal salt of said organic acid being substantially the sole source of said metal and of the anion of said organic acid, whereby said oxide coating is uniformly colored and abrasion-resistant.
• said anodizing electrolyte consists essentially of approximately gm. ferric oxalate, 60 gm. oxalic acid and 12 gm. sulfuric acid for each 1,000 ml. of water.

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• 1965
  • 1965-02-02 GB GB4521/65A patent/GB1022423A/en not_active Expired
  • 1965-02-05 FR FR4548A patent/FR1424656A/fr not_active Expired
  • 1965-03-02 DE DE1965R0040014 patent/DE1496970A1/de active Pending
  • 1965-03-18 ES ES0310747A patent/ES310747A1/es not_active Expired
  • 1965-03-19 CH CH385565A patent/CH453841A/de unknown
• 1969
  • 1969-06-16 US US833792A patent/US3616311A/en not_active Expired - Lifetime

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