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

• oxide layers on Al and Al alloys manufactured by the DC-sulphuric acid process for example, can be subsequently coloured by an alternating current treatment in acidic solutions of metallic salts.
• the oxide layer has an effect like a valve on the electrical current with the result that the alternating current is to a large extent rectified to a direct current and in the phase in which the item being treated is cathodic precipitation of metal takes place in the oxide pores as a result of the reduction of the cations.
• the pH range of the metal salt solution is of decisive importance for the precipitation of the metal.
• the resultant free protons have little effect in a strongly acidic range in which the proton concentration is very high.
• a weakly acidic range in which the proton concentration is about an order of magnitude less the additional protons have an important effect.
• electrolytes of metal salts from which metals are precipitated in weakly acid solutions e.g. in the pH range between 4 and 6, a considerable shift in the pH is to be expected.
• the buffering with boric acid is not sufficient to prevent a pronounced reduction in the pH value.
• the increase in free acid during the colouring process takes place mainly when the other electrodes are inert e.g. of graphite.
• the object of the invention is to develop a process for colouring previously anodically-oxidised articles of aluminum or aluminum alloys by electrolytic treatment of the aluminum oxide layer with alternating current in an acidic aqueous electrolyte which contains metal salts for colouring, in which process a superior quality of colour can be achieved rapidly and reproducibly.
• the said process should also eliminate the above mentioned difficulties.
• an addition of an amino-alcohol is made to the electrolyte.
• This addition can be made to an electrolyte which is already in service, in order to correct the pH value during the colouring process.
• the amino-alcohols are added either continuously or periodically until the desired pH value is again reached.
• the colouring electrolyte can contain all known metal salts, preferably the ions of at least one of the metals Ni, Co, Cu, Sn, Ag, Fe.
• the amino-alcohol additions in particular, mono-, diand tri-ethanol-amine have the advantage that not only can the pH value be raised but that there are then more favourable conditions for the metal ions to be precipitated Thus for example the uniformity of colour in the oxide layer is improved because of the increased conductivity.
• a further advantage of using additions of amino-alcohols is that with the customary etching times, more intensive and darker colour tones are achieved. For a certain colour tone therefore, a shorter colouring time is needed and/or the voltage of the alternating current can be reduced.
• the electrolyte is less sensitive to the accidental addition of impurities such as alkaline, ammonium and alkaline-earth ions, which means that under normal conditions, also in the case of dark colours, no undesirable pit or dot shaped marks appear.
• the coloured aluminum items are preferably but not exclusively used for decorative purposes and used as exteriors in architectural applications.
• EXAMPLE 1 Shee samples efan' aluminum alloy of the following composition? 1.4 1.8% Mg, 0.4% Fe, 0.4% Si, 0.1 0.3% Mn, 0.1% Zn, 0.05% Cu with Al having the usual impurities as the rest, were anodised in sulphuric acid and then treated at 20 25C with alternating current in an aqueous colouring electrolyte of the following composition:
• a graphite rod 6 mm in diameter, positioned at a distance of 8 cm from the centre of the sample surface, was used as the other electrode.
• alternating current for 120 seconds a medium bronze co1ourtone was obtained on the sample.
• EXAMPLE 2 About pre-anodised samples, giving a total surface area of 1 m were successively coloured in an electrolyte, whereby the alloy from which the sample was made, the composition of the electrolyte, the geometrical arrangement and the voltage of the alternating current were as in Example 1.
• EXAMPLE 3 Pre-anodised samples of the same alloy composition as in example 1 were treated using alternating current in an aqueous colouring electrolyte containing 20 g/l SnSO, and 7 g/l H SO with a pH of 1.4 at 20 25C. When the pH fell to 1.3, 3 g/l of mono-ethanolamine was added. This way not only was the pH value raised to 1.4 but the colour uniformity was improved and the colouring time was shortened. Although, apart from the addition of the mono-ethanolamine, the same conditions as in the normal processes are employed, a particular colour tone from the available range, can be produced faster and in a better quality.
• amino-alcohol addition consists of mono-, dior triethanolamine or mixtures of these.

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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)
• Electroplating And Plating Baths Therefor (AREA)
• Non-Silver Salt Photosensitive Materials And Non-Silver Salt Photography (AREA)
• Cosmetics (AREA)
• Electrochemical Coating By Surface Reaction (AREA)
• Electroplating Methods And Accessories (AREA)

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Citations (6)

• 1973
  • 1973-11-09 CH CH1579973A patent/CH581706A5/xx not_active IP Right Cessation
• 1974
  • 1974-10-22 DE DE19742450175 patent/DE2450175A1/de active Pending
  • 1974-10-31 IT IT29091/74A patent/IT1025421B/it active
  • 1974-11-04 NL NL7414354A patent/NL7414354A/nl active Search and Examination
  • 1974-11-06 ES ES431704A patent/ES431704A1/es not_active Expired
  • 1974-11-06 BE BE150253A patent/BE821899A/xx not_active IP Right Cessation
  • 1974-11-07 DK DK580374A patent/DK150187C/da active
  • 1974-11-07 US US521664A patent/US3912602A/en not_active Expired - Lifetime
  • 1974-11-07 NO NO744012A patent/NO137241C/no unknown
  • 1974-11-07 SE SE7414014A patent/SE407084B/xx not_active IP Right Cessation
  • 1974-11-08 CA CA213,332A patent/CA1030483A/en not_active Expired
  • 1974-11-08 FR FR7437174A patent/FR2250835B1/fr not_active Expired
  • 1974-11-08 AT AT896274A patent/AT331607B/de not_active IP Right Cessation
  • 1974-11-08 GB GB4844374A patent/GB1466708A/en not_active Expired

Patent Citations (7)

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