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

• ABSTRACT v Aluminum is coloured by passing an alternating current between a previously anodized workpiece and a counter electrode in an aqueous acid tin salt.
• the invention relates to a process for electrolytically colouring of aluminium or aluminium alloys which have previously been anodically oxidized.
• the coatings obtained by the process have a uniform colour of good durability.
• aluminium and aluminium alloys which as a matter of ease will also be denoted as aluminium below.
• One of the mostly used methods consists in an immersion of anodically oxidized aluminium in a solution of organic dyes. By this immersion method some inorganic and semi-organic colouring solutions may also be used.
• An immersion of the previously oxidized aluminium into two successive inorganic baths in order to precipitate a coloured insoluble salt in the oxide layer is also known. In the first bath the oxidized aluminium is wetted with a reactant which is then reacted in another bath with a second reactant with formation of an insoluble coloured precipitate in the pores of the oxide layer.
• Processes have also been developed based on a so called integrated colour-anodizing whereby the colour is developed by the formation of compounds due to reaction of alloying elements in the aluminium with the electrolyte, the compounds being precipitated in the alumina layer as it is formed.
• British patent No. 1022927 relates to'a similar process wherein an alternating current is passed between a previously anodically oxidized aluminium article, in order to colour the aluminium article, and a counter electrode of carbon, carborundum or aluminium in an aqueous bath containing a salt yielding a coloured metal oxide or metal hydroxide.
• metal salts in the electrolyte it is suggested to use small amounts of a salt ofnickel, cobalt, chromium, cadmium, vanadium, gold, silver, iron or lead.
• the degree of colouring is on the one hand dependent upon the current density during the second process step and on the other hand upon the duration during the second process step of the treatment in the bath of the aluminium article which has been anodically oxidized in the first process step.
• the known conventional electrolytes e.g. the extended use today of nickel salts (cf. German patent No. 741753, Italian patent No. 338232, Canadian patents Nos. 762991 and 984928 and French patent No. 1477823)
• the current density must be kept as low as possible and it cannot'be increased beyond a certain value as puncture or the known concentric spalling off of the oxide layer will occur.
• the treatment time In order to obtain a sufficient degree of colouring the treatment time must, therefore, be correspondingly increased, and this involves an uneconomic treatment. Further, the duration of the treatment with the alternating current is limited because even at low current densities a too long treatment time will also increase the risk of puncture. It is, accordingly, not automatically possible to increase the degree of colouring as desired.
• the invention relates to a process for electrolytically colouring of aluminium by passing an alternating current, between a previously anodically oxidized aluminium article and a counterelectrode immersed in an acid, aqueous solution of a metal salt, and the process is characterized in that as metal salt a tin salt is used.
• the colours obtained by the present process have essentially bronze tones of varying degrees.
• the counter electrode used in the present process is not decisive, however, it should preferably be made of tin if it is desired continuously to replace the tin ions in the electrolyte which are consumed during the colouring. or of a material which is not attacked by the electrolyte, e.g., stainless steel or titanium, in order to prevent contaminations from being introduced into the electrolyte.
• a counter electrode of aluminium may also be used.
• the counter electrode used in the present process consists preferably of tin or titanium.
• the current density may be strongly increased compared with all other known processes without any risk of puncture arising.
• a doubling of the current density compared with the conventional values is possible without difficulties, and even by a 3 times increase of the current density compared with the permissible values for other known electrolytes for obtaining bronze tones there was not observed any concentric spalling off of the oxide layer.
• Because it is possible in the present process to use a substantially higher current density very dark colour tones are obtained in a relatively short time, and there may e.g. economically and without risk of puncture be obtained a black colour by the use of electrolytes of this invention which are also suited for the production of other colour tones which are acceptable in practice.
• tin salts it has been found particularly advantageous to use stannous chloride and stannous sulphate, and as acid in the electrolyte it has been found advantageous to use hydrochloric acid and sulphuric acid.
• hydrochloric acid and sulphuric acid it has been found advantageous to use hydrochloric acid and sulphuric acid.
• other soluble tin salts and acids may also be used.
• Electrolytic baths which specifically contain tin salts as the compounds to be determining for the properties of the electrolytic bath and the electrolytic colouring have hitherto not been suggested at all'for the second, coloring step of such two-step processes as stated above.
• this effect also occurs in connection with the electrolytic baths used in the present process and which essentially contain a tin salt as metal salt, whereby the electrolytic baths, in spite of their giving a very good colouring, are somewhat unstable.
• a weak organic acid is, therefore, added as a complexforming compound to the acid electrolytic bath containing tin salt and which is used according to the invention.
• the organic acids may e.g. belong to the group consisting of phe nolic acids and sulphonic acids, whereby aromatic radicals attached to the group preferably may from the benzene group.
• the solution becomes more stable, with the additional advantage that the permissible contamination of the bath by aluminium and sodium is strongly increased compared with the otherwise known maximum permissible limits and may be e.g. up to times the values for the hitherto known electrolytes.
• the electrolyte used in the present process is very little sensitive as regards the ratio between the surface of the counter electrode and the aluminium article to be treated and the distance and choice of material for the counter electrode. Due to the good scattering power of the electrolyte the geometrical shape of both the counter electrode and the treated aluminium article is essentially without significance.
• the present process can be carried out by the use of tin salts at a concentration of 0.5 to 20 percent by weight, preferably about 2.0 percent by weight, based on the electrolyte.
• This can preferably have a pH of about 1.5.
• the alternating current can be sinusoidal or have another curve shape and a frequency of 10 to 500 periods per second, preferably technical alternating current of 50 periods per second, and a voltage of 2 to 50 V, preferably 6 V, and there may be used a current density of 0.2 to L0 A/dm, based on the aluminium article. However, this upper limit may be exceeded.
• the electrolytic bath contains sulphuric acid it has been found particularly advantageous when renewing the bath to amend it to a certain degree either by the addition of a certain part of a previously used bath or by direct addition of an amount of aluminium sulphate, e.g. to provide about 50 g aluminium sulphate per liter.
• an amount of aluminium sulphate e.g. to provide about 50 g aluminium sulphate per liter.
• a lower concentration is generally sufficient to ensure a satisfactory colouring of the aluminium article from the beginning.
• the presence-of boric acid is also useful.
• EXAMPLE 2 An aluminium strip which had previously been anodically oxidized for 45 minutes in a 15 percent aqueous solution of sulphuric acid was electrolytically coloured with the use of an alternating current. a counter elec-' trode of stainless steel and an aqueous electrolyte containing 40 ml concentrated HCl per liter and 2.7 percent SnCl per liter. The alternating voltage supplied was at 3 volt and had a frequency of 50 periods per second giving a current density of 0.4 to 0.5 A/dm The colouring process lasted for 10 minutes. and a very attractive brownish black colour was obtained on the aluminium strip. The coating obtained again had a very uniform colour, and this shows that the electrolyte had a good scattering power.
• the aluminium articles were not damaged by the chlorine containing electrolyte for the periods .used, and the corrosion resistance of the coloured coatings was very good after rinsing the aluminium articles with water and sealing of the articles in known manner.
• EXAMPLE 3 There was used an electrolyte consisting of an aqueous solution containing g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter and l0 ml phenol sulphonic acidas complex-forming compound per liter.
• EXAMPLE 4 There was used an electrolyte consisting of an aqueous solution containing 20 g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter, 10 g boric acid per liter and 4 to 5 g of an aromatic carboxylic acid, eg sulphophthalic acid. per liter, the carboxylic acid acting as complex-forming compound together with the boric acid.
• an electrolyte consisting of an aqueous solution containing 20 g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter, 10 g boric acid per liter and 4 to 5 g of an aromatic carboxylic acid, eg sulphophthalic acid. per liter, the carboxylic acid acting as complex-forming compound together with the boric acid.
• EXAMPLE 5 There was used an electrolyte consisting of an aqueous solution containing 20 g stannous sulphate per liter, 10 ml concentrated sulphuric acid per liter, 2 g ammonium tartrate per liter, 10 g boric acid per liter and 4 g of an aromatic carboxylic acid per liter. The addition of the ammonium tartrate improved the conductivity of the electrolyte.
• a process for electrolytically coloring an aluminum article which comprises first, anodically oxidizing said article to form an alumina layer thereon; immersing said article in an aqueous bath comprising a strong acid and a solution of about 0.5 to 20 percent by weight of a divalent tin salt, based on said bath, 'said tin salt being the predominant source of metal ions in said bath capable of forming and determining the color of the colored compounds in said alumina layer, and passing an alternating current between said anodically oxidized article and a counter-electrode via said bath to develop said color on said article.
• said bath includes an aromatic sulfonic acid capable of complexing said tin in divalent form and selected from the group consisting of phenol sulfonic acids and sulfophthalic acid.
• An electrolytic bath for the aluminum coloring process according to claim 1 comprising asolution of a stannous salt in a concentration of 1 percent to 20,
• tin salt is selected from the group consisting of stannous chloride and stannous sulfate.

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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)
• Electroplating Methods And Accessories (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)
• Printing Plates And Materials Therefor (AREA)

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Citations (5)

• 1969
  • 1969-06-25 NO NO2638/69A patent/NO120248B/no unknown
• 1970
  • 1970-05-23 DE DE2025284A patent/DE2025284B2/de active Pending
  • 1970-06-05 NL NL707008252A patent/NL141580B/xx unknown
  • 1970-06-11 ES ES380638A patent/ES380638A1/es not_active Expired
  • 1970-06-18 AT AT547470A patent/AT298924B/de not_active IP Right Cessation
  • 1970-06-18 FR FR7022404A patent/FR2047917B1/fr not_active Expired
  • 1970-06-22 BE BE752359D patent/BE752359A/xx unknown
  • 1970-06-22 SE SE08588/70A patent/SE357392B/xx unknown
  • 1970-06-24 CH CH953570A patent/CH554945A/xx not_active IP Right Cessation
  • 1970-06-24 DK DK326970AA patent/DK131243B/da unknown
  • 1970-06-24 FI FI701764A patent/FI46987C/fi active
  • 1970-06-25 US US00049951A patent/US3849263A/en not_active Expired - Lifetime
  • 1970-06-25 GB GB3089070A patent/GB1311716A/en not_active Expired

Patent Citations (5)

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