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
  • C25D11/06—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
  • C25D11/02—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 process for the surface treatment of aluminum or aluminum alloys by anodic oxidation of the aluminum or aluminum alloy (anodization) and to the use of an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys, an electrolyte composition for the anodic oxidation of aluminum or aluminum alloys and the use of workpieces based on aluminum or aluminum alloys and produced by the process of the present invention.
• the surface oxidation of the aluminum surface or the surface of aluminum alloys can be carried out by electrochemical means by dipping the workpieces into solutions of slightly aggressive agents or by chromating and phosphating.
• anodic oxidation by electrochemical means is generally more advantageous, since thicker oxide coatings can be obtained in this way than by chemical treatment
• anodic oxidation processes e.g. aluminum coil coating (for can manufacture) which is generally carried out by passing an aluminum strip through a sulfuric acid electrolyte.
• layer thicknesses of from 2 to 3 ⁇ m are desired.
• This process should be suitable both for anodization by dipping and for continuous anodization, e.g. of strip or wire by means of an electrolytic pull-through process.
• the process should, in hard anodization, make it possible to achieve a greater maximum layer thickness than is possible using the processes of the prior art, e.g. the DS process.
• the electrolyte preferably contains from 10 to 30% by weight, particularly preferably from 10 to 25% by weight, of an alkanesulfonic acid.
• the electrolyte may further comprise other acids, in particular acids selected from among sulfuric acid, phosphoric acid and oxalic acid.
• the electrolyte comprises sulfuric acid in addition to an alkanesulfonic acid.
• an electrolyte based exclusively on an alkanesulfonic acid is used.
• alkanesulfonic acids in the surface treatment of aluminum or aluminum alloys is already known from the prior art.
• these known processes concern essentially the use of alkanesulfonic acids in the electrolytic metal salt coloring of aluminum, where an alkanesulfonic acid is used as additive or basis of an acid electrolyte solution, and not the use of alkanesulfonic acid in anodic oxidation (anodization) of aluminum or an aluminum alloy.
• U.S. Pat. No. 4,128,460 relates to a process for coloring aluminum or aluminum alloys by electrolysis, comprising the anodization of aluminum or the aluminum alloys by customary methods and subsequent electrolysis in a bath comprising an aliphatic sulfonic acid and a metal salt, in particular a tin, copper, lead or silver salt, of the sulfonic acid.
• a bath comprising an aliphatic sulfonic acid and a metal salt, in particular a tin, copper, lead or silver salt, of the sulfonic acid.
• the stability of the electrolysis bath is increased by an increased oxidation stability of the metal salts used and a uniform coloration of the surface of the aluminum or the aluminum alloys achieved.
• the Brazilian patent applications BR 91001174, BR 9501255-9 and BR 9501280-0 also relate to processes for coloring the eloxidized aluminum by electrodipping, using electrolytes and metal salts which are mainly composed of pure methanesulfonic acid, methanesulfonates of tin or copper or methanesulfonates of nickels lead or other salts. According to these patent applications, an increase in the specific electrical conductivity of the solution, a reduction in the time for coloring in a simple manner and with reliable control, reproducibility of the color shade and low operating costs are achieved in this way.
• BR 9501255-9 discloses specific reaction conditions for anodization of the surface of aluminum, with the use of methanesulfonic acid as additive in an electrolyte based on sulfuric acid being mentioned.
• methanesulfonic acid is used in an amount of 10 parts by weight based on sulfuric acid, i.e. less than 2% by weight of the electrolyte.
• No further indication of the use of alkanesulfonic acids in the anodization step or advantages of such a use are disclosed in BR 9501255-9.
• the electrolysis time for achieving an aluminum oxide layer thickness optimum for a subsequent coloration step which is generally from 10 to 30 ⁇ m, preferably from 15 to 25 ⁇ m, is generally from 5 to 40 minutes, preferably from 10 to 30 minutes, with the precise time being dependent, inter alia, on the current density.
• alkanesulfonic acids have a significantly lower corrosive action on the aluminum oxide layer formed in the anodization than does, for example, the sulfuric acid customarily employed.
• the process of the present invention thus makes it possible, particularly in hard anodization, to achieve greater layer thicknesses in a shorter time than when using the processes of the prior art.
• a further great advantage of the process of the present invention is the significantly lower energy consumption during anodization, since a significantly lower voltage compared to the pure sulfuric acid electrolyte is established at the same current. As a consequence, the energy required for cooling the anodization bath is significantly lower.
• the process of the present invention is suitable both for anodization of aluminum or aluminum alloys by the electrodipping process and for continuous anodization, for example of strip, pipe or wire, by means of an electrolytic pull-through process, e.g. for producing aluminum sheets for can manufacture.
• the process of the present invention can be operated either using direct current or using alternating current; the process is preferably carried out using direct current.
• the electrolyte can farther comprise other acids, for example sulfuric acid, phosphoric acid or oxalic acid.
• the electrolyte comprises either an alkanesulfonic acid or a mixture of sulfuric acid and alkanesulfonic acid as only acid.
• the electrolyte preferably comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfuric acid, phosphoric acid and oxalic acid, where the sum of alkanesulfonic acid and sulfuric acid, phosphoric acid or oxalic acid is 100 parts by weight and makes up from 3 to 30% by weight of the electrolyte.
• the electrolyte particularly preferably comprises from 20 to 90 parts by weight of an alkanesulfonic acid and from 80 to 10 parts by weight of sulfuric acid.
• alkanesulfonic acid as sole acid in the electrolyte is, however, likewise possible.
• alkanesulfonic acids are aliphatic sulfonic acids.
• the aliphatic radical of these may, if desired, be substituted by functional groups or heteroatoms, e.g. hydroxy groups. Preference is given to using alkanesulfonic acids of the formulae.
• R is a hydrocarbon radical which may be branched or unbranched and has from 1 to 12 carbon atoms, preferably 1 to 6 carbon atoms, particularly preferably an unbranched hydrocarbon radical having from 1 to 3 carbon atoms, very particularly preferably 1 carbon atom, i.e. methanesulfonic acid.
• R′ is a hydrocarbon radical which may be branched or unbranched and ha from 2 to 12 carbon atoms, preferably 2 to 6 carbon atoms, particularly preferably an unbranched hydrocarbon radical having from 2 to 4 carbon atoms, where the hydroxy group and the sulfonic acid group can be bound to any carbon atoms, with the restriction that they are not bound to the same carbon atom.
• Aluminum and aluminum alloys can be anodically oxidized by the process of the present invention
• Particularly suitable aluminum alloys are alloys of aluminum with silicon, manganese, zinc, copper and/or magnesium.
• silicon, manganese, zinc, copper and/or magnesium can be present in the alloy in a proportion of 15% by weight (Si), 4% by weight (Mn), 5% by weight (Zn), 5% by weight (Cu) and 5% by weight (Mg), with casting alloys also being included.
• the present invention accordingly also provides a process in which the anodic oxidation is carried out in two stages, comprising:
• preanodization of the aluminum or the aluminum alloy in an electrolyte comprising sulfuric acid as sole acid or a mixture of sulfuric acid and oxalic acid;
• the process conditions of the preanodization preferably correspond to the conditions of the classical DS (direct current sulfuric acid) or DSX (direct current sulfuric acid-oxalic acid) electrolysis known from the prior art.
• the anodic oxidation is preferably carried out at from 0 to 30° C. If excessively high temperatures are employed, irregular deposition of the oxide layer occurs, which is undesirable.
• hard anodization in which thick oxide layers having a low porosity and thus a high hardness and high protection of the aluminum surface are sought is carried out at low temperatures of generally from 0 to 5° C., preferably from 0 to 3° C.
• high thicknesses of the oxide layer of >30 ⁇ m, preferably from 40 to 100 ⁇ m, particularly preferably from 50 to 80 ⁇ m, are possible by means of the process of the present invention in shorter times than when using pure sulfuric acid as basis of the electrolyte.
• These aluminum oxide surfaces obtained by hard anodization are generally not used for a subsequent step to color the surface.
• the anodization according to the present invention for obtaining a porous aluminum oxide surface which is particularly well-suited for subsequent coloration of the surface is generally carried out at from 17 to 30° C., preferably from 18 to 28° C.
• the process of the present invention differs from processes of the prior art in that it can be carried out at a higher temperature than the processes of the prior art. Usually, temperatures above about 24° C. give unusable, nonuniform oxide layers, while the process of the present invention allows the anodization to be carried out at up to 30° C. The ability of the process to be carried out at higher temperatures saves energy costs. In general, cooling of the electrolyte solution during anodization is necessary, since the anodization is exothermic.
• This embodiment of the process of the present invention at generally from 17 to 30° C. gives, depending on the current density and the electrolysis time, layer thicknesses of from 5 to 40 ⁇ m, preferably from 10 to 30 ⁇ m.
• the process of the present invention leads to aluminum oxide surfaces which are optimally suited to subsequent coloration, so that uniformly colored aluminum oxide layers can be obtained.
• the process of the present invention is generally carried out at a current density of from 0.5 to 5 A/dm 2 , preferably from 0.5 to 3 A/dm 2 , particularly preferably from 1 to 2.5 A/dm 2 .
• the voltage is generally from 1 to 30 V, preferably from 2 to 20 V.
• the electrolyte generally further comprises water and, if necessary, further additives such as aluminum sulfate.
• Apparatuses suitable for carrying out the process of the present invention are generally all known apparatuses which are suitable for electrodipping or for continuous anodic oxidation of aluminum or aluminum alloys, e.g. by means of an electrolytic pull-through process. Particular preference is given to using apparatuses made of metals which are resistant to alkanesulfonic acids or apparatuses which are lined with plastic, e.g. polyethylene or polyproylene.
• the present invention further provides a process for the surface treatment of aluminum or aluminum alloys, comprising the following steps:
• step e) if desired, recovery of the alkanesulfonic acid used and/or its salts, where step e) can follow or be carried out in parallel with any step in which an alkanesulfonic acid can be used, in particular the steps b) and/or, if employed, c).
• the pretreatment of the aluminum or the aluminum alloys is a critical step since it determines the optical quality of the end product. Since the oxide layer produced in anodization is transparent and this transparency is retained during the coloration process in step c), every surface defect on the metallic workpiece remains visible on the finished part.
• the pretreatment is generally carried out by customary methods such as mechanical polishing or electropolishing, dewaxing using neutral surfactants or organic solvents, brightening or pickling. This is generally followed by rinsing with water.
• solutions comprising alkanesulfonic acids are preferably also used in step a) (e.g. in the case of brightening and electropolishing).
• Preferred alkanesulfonic acids have already been mentioned above for use in the anodizing step (step b)). Particular preference is given to using methanesulfonic acid.
• Step b) is the anodization process according to the present invention which follows the pretreatment of the aluminum or the aluminum alloy. This process according to the present invention has been described in detail above.
• the aluminum oxide layer obtained in step b) can be colored.
• Coloration of the aluminum oxide layer occurs by uptake of organic or inorganic dyes into the capillary-shaped pores of the oxide layer obtained by anodization in step b).
• anodized aluminum or aluminum alloy is colored in the aqueous phase by means of suitable organic or inorganic compounds in the absence of an electric current.
• Organic dyes eloxal dyes, e.g. dyes from the alizarin series or indigo dyes
• Inorganic dyes can, in a chemical coloration step, be deposited in the pores by precipitation reactions ox by hydrolysis of heavy metal salts.
• the processes which occur here are difficult to control and there are frequently reproducibility problems, i.e. problems in obtaining constant color shades For this reason, electrolytic processes for coloring aluminum oxide layers have become increasingly established for some time.
• Step c) of the process of the present invention is therefore preferably carried out by an electrolytic method in an electrolyte comprising metal salts.
• the aluminum oxide layers obtained after step b) of the process of the present invention are colored in an electrolyte comprising metal salts by means of direct or alternating current, preferably by means of alternating current.
• metal is deposited in the bottom of the pores of the oxide layer from the metal salt solution.
• Suitable metal salts are generally salts selected from among tin, copper, silver, cobalt, nickel, bismuth, chromium, palladium and lead and mixtures of two or more of these metal salts. Preference is given to using tin, copper or silver salts or Yes thereof in the process of the present invention.
• the sulfates of the abovementioned metals are used and electrolyte solutions based on sulfuric acid are used.
• Additives can be additionally added to the electrolyte to improve the scatter and reduce oxidation of the metal ions used, e.g. the oxidation of tin(II) to the insoluble tin(IV).
• the electrolyte comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sulfuric acid, where the sum of alkanesulfonic acid and sulfuric acid is 100 parts by weight and makes up from 0.1 to 20% by weight, preferably from 0.1 to 15% by weight, of the electrolyte.
• the electrolyte very particularly preferably comprises 100 parts by weight of an alkanesulfonic acid
• Alkanesulfonic acids suitable for step c) of the process have been disclosed above for use in the anodization (step b)). Particular preference is given to methanesulfonic acid.
• electrolytes based on alkanesulfonic acids have a higher electrical conductivity, bring about more rapid coloration and display a reduced oxidation action, as a result of which the precipitation of, for example, tin(IV) salts from electrolytes comprising tin(II) salts is prevented and the addition of additives such as environmentally harmful phenolsulfonic or toluenesulfonic acid is not necessary.
• the metal salts are generally used in a concentration of from 0.1 to 50 g/l, preferably from 0.5 to 20 g/l, particularly preferably from 0.2 to 10 g/l, based on the metal used, in the electrolyte.
• the electrolyte In addition to the appropriate acid, preferably sulfuric acid or an alkanesulfonic acid or a mixture of the two acids, and the metal salt used or a mixture of a plurality of metal salts, the electrolyte generally firer comprises water and, if necessary, further additives such as scattering improvers. However, particularly when using electrolytes comprising alkanesulfonic acids, the addition of additives is generally not necessary.
• the electrolysis time in step c) is generally from 0.1 to 10 minutes, preferably from 0.5 to 8 minutes, particularly preferably from 0.5 to 5 minutes, with the electrolysis time depending on the metal salts used and the desired depth of color.
• the electrolytic coloration in step c) is usually carried out using alternating current.
• the current density is generally from 0.1 to 2 A/dm 2 , preferably from 0.2 to 1 A/dm 2 .
• the voltage is generally from 3 to 30 V, preferably from 5 to 20 V.
• Suitable electrodes are the electrodes which are usually suitable in a process for the electrolytic coloration of aluminum oxide layers, for example stainless steel or graphite electrodes. It is also possible to use one electrode made of the metal to be deposited, e.g. tin, silver or copper.
• a gold color of the oxidized surface of the aluminum or the aluminum alloys is achieved in an electrolyte comprising silver salts, if desired in admixture with tin salts and/or copper salts.
• Such gold-colored aluminum workpieces are of particular interest for producing decorative objects, since the demand for gold-colored aluminum objects is great.
• These gold-colored aluminum oxide surfaces are preferably obtained by carrying out the coloration process in step c) at a concentration of an alkanesulfonate of silver, calculated as Ag + , of from 2 to 50 g/l, preferably from 3 to 20 g/l, and a product of current density and voltage of from 0.5 to 10 AV/dm 2 , preferably from 1 to 5 AV/dm 2 , for a period of generally from 0.05 to 4 minutes, preferably from 0.3 to 3 minutes.
• a precise description of the production of gold-colored aluminum oxide layers may be found in the patent application DE-A . . . having the title “Production of gold-colored surfaces of aluminum or aluminum alloys by means of silver-con ng formulations”, which was filed at the same time.
• the workpieces are generally rinsed with water, in particular with running water. This rinsing step follows both step b) and step c) if this is carried out.
• step b) Subsequent to step b), if step c) is not carried out, or subsequent to step c) if this is carried out, the pores of the oxide layer produced are generally sealed to provide good corrosion protection, This sealing can be achieved by dipping the workpieces into boiling distilled water for from about 30 to 60 minutes. This causes swelling of the oxide layer, as a result of which the pores are closed.
• the water can also contain additives.
• the workpieces are after-treated in pressurized steam of from 4 to 6 bar instead of in boiling water.
• sealing is preferably carried out by means of water or steam.
• the alkanesulfonic acid used and/or its salts can be recovered.
• This recovery can follow or be carried out in parallel with any step in which an alkanesulfonic acid can be used, Recovery can be carried out, for example, in combination with the rinsing step (d1) following step b) and, if it is carried out, step c).
• Such a recovery can be carried out, for example, by means of electrolytic membrane cells, by cascade rinsing, or by simple concentration, for example, of the rinsing solutions.
• the present invention further provides for the use of an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys (anodization) for increasing the rate of the anodic oxidation
• an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys (anodization) for increasing the rate of the anodic oxidation
• This makes it possible to achieve more rapid aluminum oxide deposition than when using the processes of the prior art.
• thicker layers can be obtained in a shorter time when using alkanesulfonic acids as basis of the electrolyte than when using pure sulfuric acid as electrolyte basis.
• the energy consumption is significantly lower since a lower voltage is established and less cooling has to be employed.
• an electrolyte composition containing from 3 to 30% by weight of an alkanesulfonic acid for the anodic oxidation of alumina or aluminum alloys is claimed.
• Preference is given to an electrolyte composition comprising from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sumac acid, where the sun of alkanesulfonic acid and sulfuric acid is 100 parts by weight and makes up from 3 to 30% by weight of the electrolyte.
• Suitable alkanesulfonic acids have already been mentioned above.
• the alkanesulfonic acid used is particularly preferably methanesulfonic acid.
• electrolyte compositions are very suitable for use in a process for the anodic oxidation of aluminum or aluminum alloys and lead to more rapid aluminum oxide deposition than the processes of the prior art and to a thicker aluminum oxide layer in a shorter time, which is of particular interest in hard anodization, and to a reduced energy consumption.
• the workpieces based on aluminum or aluminum alloys produced according to the present invention can be used, for example, in building and construction in particular for producing window profiles or exterior wall components, in automobile or aircraft construction, both for producing body parts and for producing aluminum pressure castings, e.g. in engine construction, and in the packaging industry, in particular for producing cans, for example by a continuous electrolytic pull-through process, e.g. continuous coil anodization
• Anodization electrolytes comprising, in each case, 18% by weight of an acid or an acid mixture and 8 g/l of aluminum were used.
• the electrolytes were used for the anodization of pure aluminum sheets which had in each case been preanodied for 2 minutes by the classical DS method.
• Anodization was in each case carried out at a current density of 1.2 A/dm 2 for 30 minutes.
• the anodization bath was in each case thermostated at 20° C.
• the thickness of the aluminum oxide layer, the porosity or microstructure of the surface and the microhardness were determined on the anodized workpieces.
• Table 1 below shows the thicknesses of the oxide layer obtained as a function of the electrolyte used and the anodization voltage and any cooling necessary: TABLE 1 Thickness of the Anodization Cooling Electrolyte oxide layer in ⁇ m voltage in V necessary 1. 1) H 2 SO 4 12 ca. 12 Strong 2. 1) H 2 SO 4 /oxalic acid 11 ca. 11 Strong (90:10) 3. MSA 2) 16 ca. 2.5 Slight 4. MSA/H 2 SO 4 (50:50) 14 ca. 2.5 Slight
• the layers all displayed a significantly lower porosity and an increased hardness compared to Example 1.
• the aluminum sheets anodized in MSA methanesulfonic acid
• a coloring electrolyte was made up from 19 g/l of silver methanesulfonate (10 g/l of Ag + ) and 57 g/l of methanesulfonic acid. At a current density of 0.2 A/dm 2 and a voltage of about 8 V, the aluminum sheets anodized as indicated for No. 3 and 4 in Table 1 were colored for different periods of time. For both aluminum sheets, the colors indicated in Table 2 below were obtained, TABLE 2 Color at 0.2 Time [sec] A/dm 2 15 Pale gold 30 Light gold 60 Gold 120 Gold 180 Deep gold

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• 2000
  • 2000-07-10 DE DE10033435A patent/DE10033435A1/de not_active Withdrawn
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