DE1236898B - Process for the electrochemical production of protective coatings on metals - Google Patents

Description

GERMAN '// ZTTQSSP PATENT OFFICE

EDITORIAL

German class: 48 a - 11/00

Number: 1 236 898

File number: T 25031 VI b / 48 a

1 236 898 filing date: November 7, 1963

Open date: March 16, 1967

The invention relates to a method for the electrochemical production of protective coatings hydrated chromium oxide, which have high corrosion resistance and excellent Primer for paints, varnishes and other organic paints on the surface of iron, Form steel, aluminum and aluminum alloys.

There are already some processes for the electrochemical production of similar protective coatings known. For example, the following electrolytes have been proposed:

1. An aqueous solution of chromic acid with an addition of boric acid or one of its salts and an organic hydroxy compound (U.S. Patent 2,733,199, 2,780,592);

2. An aqueous solution of chromic acid with an addition of phosphoric acid (USA. Patent 2,769,774);

3. an aqueous solution of chromic acid, in which the quantitative ratio of chromium (VI) - to chromium (III) ions is kept in the range of 1 to 5: 1 (Japanese Patent Specification 272,741);

4. An aqueous solution of chromic acid with the addition of trivalent chromium ions, phosphoric acid and boric acid (U.S. Patent 3,032,487);

5. An aqueous solution of chromic acid with an addition of phenol or catechol derivative disulfonic acid or its salt (U.S. Patent 2,998,361);

6. an aqueous solution containing only chromic acid (US Pat. No. 3,081,238);

7. An aqueous solution which, in addition to chromic acid or chromates, contains sulfuric acid or hydrochloric acid (German patent 630 934).

The invention relates to a process for the electrochemical production of a protective coating of hydrated chromium oxide on metal surfaces, in which the metal to be coated is subjected to electrolysis as a cathode in an aqueous solution containing chromic acid, sulfuric acid and an organic hydroxy compound as a reducing agent or chromic acid and a phenolsulfonic acid , which is characterized in that inorganic fluorine compounds are added to the aqueous solution in an amount of at most 1.0 g per liter and at a bath temperature of 40 to 70 ° C, with a cathode current density of at least 12.5 Amp./ dm ² during 1 to 20 seconds is worked.

According to the method according to the invention, it is possible to use metal surfaces such as sheet metal, tires, Process for the electrochemical production of protective coatings on metals

Applicant:

Toyo Kohan Co., Ltd., Tokyo
Representative:

Dr. rer. nat. V. Vossius, patent attorney,
Munich 23, Siegesstr. 26th

Named as inventor:
Yoichi Kitamura, Yokohama;
Tsuneo Inui, Yamaguchi-ken;
Etsuro Shuto, Yokohama (Japan)

Claimed priority:

Japan November 10, 1962 (49 394)

Band material, wire, pipes and mechanically deformed metal objects, one in a very short time extremely thin protective coating with excellent corrosion resistance and adhesion for paints and make colors.

The metal surfaces treated according to the invention have a better primer for organic ones Coatings on than conventional tinplate tinplate, even after being mechanically deformed and subjected to various conditions after painting.

The electrolyte bath used results in a better scattering power and higher current yield, whereby a more stable operation within a wider concentration range is enabled in comparison to Baths of the aforementioned conventional methods.

The most suitable metals are to be treated by the process of the invention pure iron, wrought iron or fluoro iron, pure aluminum and its alloys with at least 90% aluminum. The anode metals are pure lead, lead-tin, lead-antimony and lead-silver alloys useful.

The metals to be treated must first be carried out in a conventional manner, e.g. B. by degreasing with Alkali and acid pickling for iron and steel

709 519/473

or by cleaning with alkalis for aluminum and aluminum alloys.

Electrolyte composition

The optimal concentration of chromic acid is in the range from 40 to 100 g / l. When the concentration of chromic acid is below 40 g / l, the coating produced has poor deformability, he forms a poor primer and is less resistant to chemicals. When the concentration of chromic acid is above 100 g / l, forms in place of the hydrated chromium oxide coating a metallic chrome coating with poor corrosion resistance.

The chromium ions present in the electrolyte are in the trivalent and hexavalent oxidation state before.

In general, the optimal weight ratio of hexavalent chromium ions to trivalent ones is Chromium ions in the range from 20 to 150: 1.

The concentration of trivalent chromium ions should not exceed a value of 2.5 g / l. if the ratio of Cr (VI) to Cr (III) is greater than 150: 1, the current efficiency decreases, the produced coating becomes uneven and uneven. On the other hand, when the amount ratio of Cr (VI) Cr (III) becomes smaller than 20: 1 or the concentration of trivalent chromium ions exceeds 2.5 g / 1, decreases the conductivity of the electrolyte, the bath voltage rises, the current yield falls, and so on the deformability of the produced coating becomes poor.

The electrolyte initially contains reducing agents, which serve to convert chromium ions into trivalent Maintain shape in solution, allowing continuous operation.

The reducing agent must be a water-soluble organic compound of simple structure with alcoholic or phenolic hydroxyl groups. These hydroxyl groups are easily oxidized in a hot aqueous solution of chromic acid and easily decomposed or evaporated without leaving a precipitate. By heating an aqueous solution of chromic acid, which has been added a reducing agent for about 30 minutes at 60 ⁰ C is generated the required amount of trivalent chromium ions.

Usable reducing agents are e.g. B. water-soluble alcohols, such as methyl alcohol, ethyl alcohol, Propyl alcohol and monoethanolamine, polyhydric alcohols such as ethylene glycol, propylene glycol, glycerine, Diethanolamine, triethanolamine, pentaerythritol, sorbitol and mannitol, phenol and phenol sulfonic acids, such as phenol-2,4-disulfonic acid, pyrocatechol-3,5-disulfonic acid, phenol-2,4,6-trisulfonic acid, ^ -naphthol-3,6-disulfonic acid and 1,8-dihydroxynaphthalene-3,6-disulfonic acid. Cresol, xylenol and polyphenols, such as Pyrocatechol, hydroquinone, etc., are undesirable because they are in the aqueous solutions of Chromic acid form precipitates. The reducing agent may be present in such an amount that the required Concentration of trivalent chromium ions in the aqueous solution of chromic acid is generated.

The sulfuric acid concentration should be in the range from 0.1 to 0.5 g / l. When the sulfuric acid concentration is below 0.1 g / l, it is difficult to make a uniform coating and none to produce yellow colorations on the coating,

which affect the appearance. The current yield increases with increasing sulfuric acid concentration gradually increases, however, the adhesion of the generated coating to the base metal as well the deformability and corrosion resistance of the coating are poor. For this reason, the Sulfuric acid concentration should not exceed a value of 0.5 g / 1. When using phenolsulfonic acids as a reducing agent, it is not necessary to additionally add sulfuric acid, since the required The concentration of sulfuric acid is adjusted by the phenolsulfonic acids.

The electrolyte according to the invention contains fluorides, acidic fluorides, fluosilicates and fluoborates. By the fluorine compounds will have a higher current efficiency and better throwing power and an excellent one obtained stabilized coating in a wide concentration range of the electrolyte bath. Typical examples of these fluorine compounds are hydrofluoric acid, fluorides such as sodium fluoride, potassium fluoride, Ammonium fluoride, acidic fluorides such as sodium hydrogen fluoride, potassium hydrogen fluoride and Ammonium hydrogen fluoride, as well as fluosilicic acid, fluosilicates such as sodium fluosilicate, potassium fluosilicate and ammonium fluosilicate and hydrofluoric acid, fluoborates such as sodium fluoborate, potassium fluoborate and ammonium fluorate.

These fluorine compounds should be added in an amount of not more than 1.0 g / l. Unless this Value is exceeded, the deformability, the adhesiveness of paints and paints and the Corrosion resistance of the coating produced is severely impaired.

The following conditions are used for electrolysis by the process according to the invention: bath temperature 40 to 70 ° C., current density at least 12.5 amps / dm ² , treatment time 1 to 20 seconds.

The current density is set to as high a value as possible, since this results in better results in terms of deformability and corrosion resistance of the coating in a shorter time. The current density should preferably not be higher than 40 Amp./dm ² . If the current density is below 12.5 amps / dm ² , the corrosion resistance cannot be improved even if a longer treatment time is used.

In general, a treatment of 1 to 20 seconds is sufficient. With a treatment of 6 to 9 seconds, a coating with a thickness of about 2 mg / dm ^{2 is obtained} .

When the method according to the invention is applied to iron and steel, the following color changes occur on: blue -> bluish purple - »bluish white with increasing chromic acid concentration; bluish yellow -> bluish purple -> bluish white with increasing concentration of trivalent Chrome; bluish purple - »bluish yellow -> yellow with increasing sulfuric acid concentration; blue-> bluish purple -> bluish white with increasing concentration of fluorine compound; yellow -> bluish yellow ~> blue -> bluish purple with increasing temperature; bluish yellow -> blue -> bluish purple with increasing current density and bluish purple -> blue -> bluish yellow yellow with increasing amount of electricity. To make a blue-tinged purple-colored coating with the desired properties is used a medium one Concentration of chromic acid and trivalent ones

Chromium ions, a lower concentration of sulfuric acid, a medium concentration of fluorine compound, a higher temperature and higher current density and a lower amount of electricity. In contrast to iron and steel, aluminum and aluminum alloys almost always form a colorless one to grayish white coating.

Iron and steel treated according to the process pass a monthly weathering test as well as a 24 hour salt spray test without any what signs of rust formation, even if the Layer thickness of the coating produced is only 0.05 microns. After applying one about 10 microns strong coating of a modified alkyd resin paint or a modified epoxy paint Treated by the inventive method steel sheet can be the painted sheet with a Draw ratio of 2.2 deep-drawing. If you put an adhesive strip firmly on the painted side of the deep-drawn hollow vessel obtained and then quickly peeled off, the adhesive tape does not take any Varnish with. Furthermore, the lacquer coating adheres firmly to the hollow vessel, even if it is with the adhesive tape was checked after the hollow vessel was immersed in boiling water for 1 hour. Even after the lacquer coating of the lacquered plate cross-hatched and the plate 8 hours in boiling If water is inserted, the cover cannot be peeled off with the adhesive tape. In the case of electroplated Tinplate, on the other hand, can be used for 70% of the paint coating under similar conditions after boiling for 1 hour.

Since the protective coating adheres very firmly to the base metals, it can be replaced by conventional mechanical Deformation such as drawing, folding and deep drawing cannot be removed.

Furthermore, the protective coating is very resistant to alkalis, salts, petroleum, fats and organic ones Solvents with the exception of strong acids. Sheet steel treated according to the process can instead of tinplate, which is sensitive to alkalis and salts, as a material for tins and containers used for surfactants, soaps, industrial chemicals, petroleum oils and paints will.

The metal objects treated by the method have excellent temperature resistance up to 300 ° C, even if the protective coating is extremely thin. Your weldability is as good as that of the base metals.

example 1

A 0.25 mm thick cold-rolled blackplate was purified 20 seconds at 80 ⁰ C and a current density of 4 Amp./dm ² in a 7 ° / ₀ sodium hydroxide solution cathodic, then rinsed with water. Pickled 10 seconds at room temperature in 7% sulfuric acid and rinsed again with water. A lead-antimony alloy (95: 5) was used as the anode. The bath contained 40 g / 1 chromic acid, 0.50 g / 1 ethyl alcohol, 0.62 g / 1 trivalent chromium ions, 0.10 g / 1 sulfuric acid, 0.05 g / 1 ammonium fluorosilicate, and the ratio of Cr (VI) : Cr (III) was 34: 1. The bath temperature was 50 ° C., the current density was 15 amps / dm ² and the treatment time was 10 seconds. The electrolyte was obtained by adding 0.5 g / l of ethyl alcohol as a reducing agent to an aqueous solution of 40 g / l of chromic acid, the mixture for 30 minutes

heated to 60 ⁰ C and thereby generated 0.62 g / 1 trivalent chromium ions. 0.1 g / l sulfuric acid and then 0.05 g / l ammonium fluosilicate were added to the solution.

No protection is claimed for the production of the electrolytes.

In Examples 2 to 8, the electrolyte was by adding the chemicals in the same manner as in Example 1 produced.

The coating produced was blue and clear and the treated black panel showed a few Rust spots after a salt spray test with a 5% sodium chloride solution at 35 ° C for 20 hours had been subjected. If you paint the treated black plate with a paint about 8 microns thick coated with a white melamine-modified alkyd enamel and made into a hollow vessel deep-drawn with a draw ratio of 2.0: 1, it was found that there was no loss of adhesion on the deep-drawn part of the hollow vessel of the baking varnish occurred.

In Examples 2 to 8, the same anode as in Example 1 was used in each case.

Example 2

The same type of cold-rolled black plate was subjected to the pretreatment described in Example 1. The bath contained 50 g / l chromic acid, 0.20 g / l monoethanolamine, 0.51 g / l trivalent chromium ions, 0.20 g / l sulfuric acid, 0.20 g / l ammonium fluorate; the ratio of Cr (VI): Cr (III) was 51: 1, the bath temperature 60 ° C., the current density 23 Amp./dm ² and the treatment time 10 seconds.

The coating produced was bluish purple in color and transparent, and on the treated black panel occurred after a 24 hour salt spray test at 35 ° C with a 5% sodium chloride solution only a few signs of rusting. The adhesion to paint after the mechanical Deformation was the same as in Example 1.

Example 3

The same type of cold-rolled black plate was subjected to the pretreatment described in Example 1. The bath contained 50 g / l chromic acid, 0.50 g / l phenol-2,4-disulfonic acid, 0.30 g / l trivalent chromium ions and 0.60 g / l hydrofluoric acid. The ratio of Cr (VI): Cr (III) was 87: 1, the bath temperature was 60 ° C., the current density was 20 amps / dm ² and the treatment time was 15 seconds.

The coating produced was bluish white and the treated black panel had similar corrosion resistance and paint adhesion like the black plate produced in Example 2.

Example 4

A 0.4 mm thick cold-rolled steel sheet was subjected to the same pretreatments as are described in Example 1. The bath contained 60 g / l chromic acid, 0.50 g / l phenol, 0.38 g / l trivalent chromium ions, 0.50 g / l sulfuric acid and 0.50 g / l sodium fluoride. The ratio of Cr (VI): Cr (III) was 82: 1, the bath temperature 45 ° C., the current density 12.5 Amp./dm ² and the treatment time 10 seconds.

Claims (1)

A bluish-tinged purple coating was formed on the steel sheet with properties similar to the coating described in Example 2. Example5 A 0.5 mm thick sheet of 99.0% sodium aluminum was rinsed for 10 seconds at room temperature in a 4 ° / ₀ Natriumbicarbonatlösunggereinigt strength, with water. The bath contained 50 g / l chromic acid, 0.15 g / l ethyl alcohol, 0.20 g / l trivalent chromium ions, 0.20 g / l sulfuric acid and 1.0 g / l ammonium nuosilicate. The ratio of Cr (VI): Cr (III) was 130: 1, the working temperature was 40 ° C., the current density was 12.5 Amp./dm ² and the treatment time was 20 seconds. A bright gray-tinged white protective coating was formed and the treated aluminum sheet showed no white rust formation at all after the 120-hour salt spray test according to Example 1. Exampleö A cold-rolled steel sheet was pretreated according to Example 4. The bath contained 50 g / l chromic acid, 0.20 g / l methyl alcohol, 0.40 g / l trivalent chromium ions, 0.20 g / l sulfuric acid and 0.25 g / l hydrofluoric acid. The ratio of Cr (VI): Cr (III) was 65: 1, the working temperature 55 ° C, the current density 20 Amp./dm ² and the treatment time 10 seconds. The coating produced was bluish purple in color and transparent, and the treated steel sheet showed only a few rust spots after the 24-hour salt spray test according to Example 1. If the steel sheet was painted in the same manner as in Example 1 and at a draw ratio of 2.2: 1 deep-drawn to a hollow vessel, it was found that on the deep-drawn part of the vessel there was no loss of adhesion of the baking varnish. Example 7 40 The same type of cold rolled steel sheet was subjected to the pretreatments described in Example 4 subject. The bath contained 50 g / 1 chromic acid, 0.20 g / 1 ethylene glycol, 0.37 g / 1 trivalent chromium ions, 0.10 g / 1 sulfuric acid and 0.20 g / 1 ammonium hydrogen fluoride. The ratio of Cr (VI): Cr (III) was 70: 1, the temperature is 60 ⁰ C, the current density 24 Amp./dm ² and the duration of treatment seconds. The resulting coating was bluish yellow in color and clear, and that treated Steel sheet had similar properties to the steel sheet obtained in Example 2. Example 8 A 0.8 mm thick cold-rolled steel sheet was subjected to the pretreatment described in Example 1. The bath contained 100 g / l chromic acid, 2.0 g / l ethyl alcohol, 2.5 g / l trivalent chromium ions, 0.20 g / l sulfuric acid and 0.10 g / l hydrofluoric acid. The ratio of Cr (VI): Cr (III) was: 1, the working temperature 70 ° C., the current density Amp./dm ² and the treatment time 6 seconds. The resulting coating was pale bluish white in color and the treated steel sheet was properties similar to those of the steel sheet obtained in Example 2. Claim: Process for the electrochemical production of a protective coating of hydrated chromium oxide on metal surfaces, in which the metal to be coated is subjected to electrolysis as a cathode in an aqueous solution which contains chromic acid and sulfuric acid and an organic hydroxy compound as reducing agent or chromic acid and a phenolsulfonic acid, characterized in that the aqueous solution of inorganic fluorine compounds in an amount of at most 1.0 g per liter is added and is carried out at a bath temperature of 40 to 70 ⁰ C with a cathode current density of at least 12.5 Amp./dm ² for 1 to 20 seconds. Considered publications:
German Patent No. 630 934;
U.S. Patent No. 2,998,361;
U.S. Patent 2,733,199, reported in Chem. Zentralblatt, 1957, p. 12,600. 709 519/473 3.67 ® BundesdruckereiBerlin