US3341435A - Anodic oxidation of aluminum and of its alloys - Google Patents

Anodic oxidation of aluminum and of its alloys Download PDF

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Publication number
US3341435A
US3341435A US363782A US36378264A US3341435A US 3341435 A US3341435 A US 3341435A US 363782 A US363782 A US 363782A US 36378264 A US36378264 A US 36378264A US 3341435 A US3341435 A US 3341435A
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bath
chromic acid
aluminum
alloys
chromic
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US363782A
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Martinez Dionisio Rodriguez
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    • 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
    • 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
    • C25D11/08—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing inorganic acids

Definitions

  • FIG. 1 1 ANODIC OXIDATION OF ALUMINUM AND OF ITS ALLOYS Filed April 30, 1964 FIG. 1 1
  • the present invention relates to a process for the surface protection of aluminum and of its alloys by anodic oxidation in a bath of chromic acid.
  • the protective layer of alumina is thicker than in the chromic acid process (in general from 10 to 20 microns); it is less flexible and for equal thickness it is distinctly less resistant to corrosion.
  • the present invention has for its principal object to suppress entirely the deterioration of chromic acid baths caused by the reduction of hexavalent chromium to trivalent chromium.
  • the process which forms the object of the present invention relates to the protection of the surface of aluminum and of its alloys by oxidation in a chromic acid bath under constant voltage with direct current, the cathodic surface being chosen to be as small as possible,
  • the concentration in chromic acid is substantially between 15 and 19% by weight
  • the temperature of the bath is below 30 C.
  • the electrolysis voltage is below 25 volts, and preferably between substantially 21 and 23 volts;
  • the anodic surface is between substantially 1.7 B dm. and 2.3 B dm. B representing the volume of the bath expressed in liters;
  • the cathodic surface is below 0.6 B dm. and is preferably between substantially 0.03 B drn. and 0.05 B dm.
  • the electrolysis is preferably carried out without utilizing any agitation means
  • the ratio between the number of atom grams of chromium dissolved in the bath in the hexavalent state to the number of atom grams of chromium dissolve-d in the bath in the trivalent state remains constant, at a value which is always greater than 70 and may exceed 120.
  • the definition of the invention is in definite opposition to the state of the art given above.
  • the high concentration in chromic acid applied according to the invention namely 15 to 19% by weight, is discouraged and in particular the temperature below 30 is described as being definitely to be avoided.
  • the density of the anodic current must be considered to be a consequence of the bath temperature (Aluminum, loc. cit., page 173, 2nd column, and page 174, 1st column, paragraph 3.3) as well as of the concentration.
  • This density of anodic current is dependent upon the voltage (according to Joules law), upon the total intensity traversing the electrolysis cell and upon the anodic surface. This total intensity itself influences the density of the cathodic current. It is necessary to take account of this last density when one chooses the cathodic surface.
  • coatings with a thickness of 3 microns are obtained after 60 minutes of treatment; thicknesses of 4.5 microns after minutes; and of 6 microns after minutes. In practice, one rarely exceeds 90 minutes of treatment.
  • the periods practiced according to the invention are greater than those in use in the conventional processes of chromic anodization under constant voltage, which are generally of 40 minutes for a thickness of 3 microns instead of 60 minutes according to the new process.
  • this anodized surface is very much greater than those generally employed in industrial practice in the known processes of anodization in a chromic bath under constant voltage.
  • the coatings obtained according to the invention have a total opacity, which constitutes an unexpected result in complete contradiction to the prior art mentioned above regarding work at a temperature below 40.
  • the resistance to corrosion of coating obtained according to the new process is superior to that of the coatings obtained according to the known processes of anodization in a chromic acid bath. Based upon the state of the art already mentioned, one would have expected a resistance to corrosion which would have been inferior to that commonly obtained by the processes of anodization in a chromic bath, in view of the relatively low voltage used according to the invention (comparative corrosion tests have been made using the standardized brine mist test, the acetic brine mist test with copper chloride, also known as the Cass test, in addition in an industrial atmosphere, a sea water atmosphere and a sea-water industrial atmosphere).
  • Tests on resistance to abrasion, sealing tests, tests of bending capacity, and tests of coloring capacity give results which are highly comparable with those obtained according to the conventional processes of anodization in a chromic bath under a constant voltage.
  • the coating obtained according to the invention has an attractive appearance resembling enamel, at least as satisfactory as that obtained according to the previous processes with a chromic bath.
  • the surfaces treated according to the new process receive a coating which is very uniform and regular, even at points of the anode which are at a very considerable distance from the cathodic surface.
  • the bath is enriched neither in salts of The lnvention allows specific consumptions of elec- 0.8 kWh. per m. of anodized sur- Such a consumption of electrical energy is distinctly anodization in a chromic bath.
  • the weight of the object is sufiicient for ensuring a satisfactory electrical contact. Any fixing or strongly applying the said objects against the anode of the electrolytic cell is no longer necessary.
  • Such an improvement which is equally new against the already cited prior art, has a far-reaching economical effect owing to the important cost decrease involved. Large-sized pieces (objects), namely having the maximal size compatible with the size of the cell, may be treated.
  • FIGURES 1 and 2 are given by way of illustration, but not by way of limitation of the feature of the invention.
  • FIGURE 1 shows the anodization cell and the objectsholder in vertical and transverse section
  • FIGURE 2 shows the same in longitudinal section.
  • 1 is the cell made of polyvinylchloride
  • 2 is the bath
  • 3 the objects-holder connected with the anodic current supply
  • 4 one of the horizontal branches of the objects-holder
  • 5 a cross-bar joining the parts of the objects-holder
  • 6 are objects to be anodized, such as channel bars. It should be understood that other embodiments may be realized without departing from the field of that feature of the invention comprising electrically connecting the objects to be anodized solely by their own weight.
  • Example 1 Example 2 Example 3 New process Prior art; Prior art Thickness of the coating 4.5 microns 4.5 microns 15 microns.
  • Example 1 relates to the process according to the invention.
  • the bath contains 200 grams per liter of chromic acid, namely 17.5% by weight; the anodization is effected with 22 volts at 26 C., each operation lasting minutes.
  • the installation is provided with a device for the purging of aluminum salts by circulation of the bath over cation-exchange resins.
  • the ano dic parts to be treated in each operation are aluminum plates of 99.5%
  • the ratio of the concentration of dissolved hexavalent chromium to the trivalent chromium, in atom grams, remains constant from the start of operation, between 125 and 130.
  • the concentration in dissolved aluminum salts remains constant at the value of approximately 1 gram per liter.
  • the anodized surfaces obtained from the first to the 600th operation have the same satisfying appearance and the same advantageous characteristics as previously described. Since, after these four and a half months of working, the bath has not shown any indication of being used up, as shown by the analyses, the applicant considered the test to be sufficiently conclusive, and considered it useless to continue it longer. One can, however, consider it as certain that its life would exceed 500 days of working, which corresponds to more than 60 m? treated per liter of bath, and consequently to a consumption of the bath below 3.4 grams of chromic acid per m? of treated surface.
  • Example 2 relates to a known process of anodization in a chromic bath under a constant voltage of 40 volts at 40 C., each operation lasting 40 minutes.
  • the cathode is constituted by bars and plates of pure iron.
  • the surface treated per liter of bath, which is 0.22 dm. cannot be increased without increasing, at the same time, the proportion of trivalent chromium produced per unit time in the bath.
  • Example 3 relates to a known process of anodization in a sulphuric bath under a constant voltage of 15 v. at 20 C., each operation lasting 40 minutes.
  • Example 4 relates to the invention. It is not described in the above table. It constitutes a modification of Example 1 and differs therefrom in that it does not have recourse to any addition of hydrogen peroxide nor to purging (fixing) of the aluminum salts of the bath by a continuous passage thereof over cation-exchange resin.
  • the following operations last the same time as in Example 1, namely 90 minutes.
  • the ratio Cr6+/Cr3+ is maintained practically unchanged at its value given above until the end of the experiment, of 300 operations corresponding to 6 In. of treated surface per liter of bath. Beyond this limit, the bath ceases to be usable in that it is too highly enriched in aluminum salts. This bath may, however, be employed again in practice as if it were a new bath, provided that the aluminum salts are eliminated by passage over a cation-exchange resin.
  • a process for the surface protection of aluminum and of its alloys by anodic oxidation in an aqueous bath of chromic acid under a constant voltage with a direct current characterized by the following means employed in combination:
  • the chromic acid concentration is about 15 to 19% by weight; the bath temperature is below 30 C.; the electrolysis voltage is below 25 volts;
  • the anodic surface is between substantially 1.7 B dm. and 2.3 B dm. B representing volume of the bath expressed in liters;
  • the cathodic surface is below 0.06XB dm. whereby the ratio of the number of atom grams of chromium dissolved in the bath in the hexavalent state to the number of atom grams dissolved in the trivalent state remains constant with a value greater than and which may exceed 120.

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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)
  • Inorganic Chemistry (AREA)
  • Chemical Treatment Of Metals (AREA)
  • Electrolytic Production Of Metals (AREA)
  • Electroplating And Plating Baths Therefor (AREA)
US363782A 1963-05-04 1964-04-30 Anodic oxidation of aluminum and of its alloys Expired - Lifetime US3341435A (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
ES28769363 1963-05-04
ES29598464 1964-02-01

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US3341435A true US3341435A (en) 1967-09-12

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US (1) US3341435A (xx)
BE (1) BE647452A (xx)
CH (1) CH428371A (xx)
DE (1) DE1496709B2 (xx)
DK (1) DK128823B (xx)
FR (1) FR1399797A (xx)
GB (1) GB1067373A (xx)
LU (1) LU46011A1 (xx)
NL (1) NL142457B (xx)

Families Citing this family (1)

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Publication number Priority date Publication date Assignee Title
US4256547A (en) 1979-07-12 1981-03-17 General Dynamics Corporation Universal chromic acid anodizing method

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1771910A (en) * 1923-08-02 1930-07-29 Bengough Guy Dunstan Process of protecting surfaces of aluminum or aluminum alloys
US2066327A (en) * 1934-05-28 1937-01-05 Robert W Buzzard Bath for anodic treatment of aluminum
US2085002A (en) * 1934-05-22 1937-06-29 Robert W Buzzard Anodic treatment of aluminum
US2788317A (en) * 1954-02-25 1957-04-09 Koenig And Pope Aluminum and process applicable thereto

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1771910A (en) * 1923-08-02 1930-07-29 Bengough Guy Dunstan Process of protecting surfaces of aluminum or aluminum alloys
US2085002A (en) * 1934-05-22 1937-06-29 Robert W Buzzard Anodic treatment of aluminum
US2066327A (en) * 1934-05-28 1937-01-05 Robert W Buzzard Bath for anodic treatment of aluminum
US2788317A (en) * 1954-02-25 1957-04-09 Koenig And Pope Aluminum and process applicable thereto

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Publication number Publication date
DK128823B (da) 1974-07-08
NL142457B (nl) 1974-06-17
CH428371A (fr) 1967-01-15
LU46011A1 (xx) 1964-11-03
FR1399797A (fr) 1965-05-21
DE1496709A1 (de) 1969-06-04
BE647452A (xx) 1964-11-04
DE1496709B2 (de) 1971-04-15
GB1067373A (en) 1967-05-03
NL6404879A (xx) 1964-11-05

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