US3691055A - Method of coating steel sheet surfaces - Google Patents

Method of coating steel sheet surfaces Download PDF

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Publication number
US3691055A
US3691055A US858782A US3691055DA US3691055A US 3691055 A US3691055 A US 3691055A US 858782 A US858782 A US 858782A US 3691055D A US3691055D A US 3691055DA US 3691055 A US3691055 A US 3691055A
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Prior art keywords
chromium
film
aluminum
coating
coated
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US858782A
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English (en)
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Motoharu Hamada
Hiroshi Nakagawa
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JFE Steel Corp
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Kawasaki Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
    • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
    • C23C14/14—Metallic material, boron or silicon
    • C23C14/16—Metallic material, boron or silicon on metallic substrates or on substrates of boron or silicon
    • 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
    • C25D11/38—Chromatising

Definitions

  • Quantity of chromium coated by electrolytic treatment with chromic acid 7 Symbol Metallic chromium Chromium in chromium Thickness of aluminum 45o oxides film '2 7 /m I04 /m2) 0.3 ,u A 30 /n2 9
  • the magnitude of corrosive current therethrough is still large as compared with the corresponding magnitude of corrosive current through isolated aluminum itself.
  • the density of the corrosive current (or corrosive speed) through the aluminum film is as large as 10 -,u'a./cm. and it is considerably larger than the corresponding density of corrosive current through isolated aluminum per se, which is about 1 ,ua./cm. although it is much smaller than the corresponding density of corrosive current through bare steel sheets, which is generally said to be about 10 ,ua./cm.
  • red rusts are generated in a comparatively short duration of salt water spraying, i.e., in about 12 to 19 hours of spraying.
  • Such red rusts are generated because the steel surface is partially exposed to the air through pin holesin the coated aluminum film which are inevitably formed in the course of depositing the aluminum film, and the thus exposed portion of the steel and the coated aluminum film form local galvanic cells, so that the aluminum film quickly dissolves in the proximity of each such galvanic cell.
  • each steel sheet is pretreated by subjecting it to cathodic electrolysis in an electrolyte containing chromium ions having a valency of 6, so as to produce a first coating film on the sheet, which first coating film mainly consists of electrically insulating chromium oxides, and then aluminum film is superposed on the first coating film by vacuum evaporation.
  • first coating film mainly consists of electrically insulating chromium oxides
  • the first coating film produced by the electrolytic pretreatment in the method of the present invention comprises two films, i.e., a metallic chromium film and a hydrous chromium oxide film.
  • the thickness of such coating film from the standpoint of corrosion resistance, thick coating films are desirable, while from the standpoint of workability, thin coating films are desirable.
  • the suitable thickness of such coating film is generally said to be about 0.05 micron for chromium plating, and 0.1 to 0.2 micron for coating films mainly consisting of hydrous chromium oxides.
  • steel sheets are at first coated with a first film by electrolytic treatment with chromic acid in the aforesaid manner, and then the steel sheets are preferably preheated prior to vacuum evaporation of aluminum for depositing aluminum films thereon.
  • preheating is optional in the method of the invention, and can be dispensed with.
  • the properties of the first coating film formed by the electrolytic treatment with chromic acid is greatly changed after the preheating, as compared with the properties of the coating film in the state as deposited.
  • FIG. 1 is a graph illustrating the relation between the temperature for heating steel sheets having a first coating film deposited thereon by electrolytic treatment with chromic acid and the amount of chromium ions dissolving in an aqueous solution of sodium hydroxide NaOH added with hydrogen peroxide H 0 out of the first coating film during the heating;
  • FIG. 2 is a graph illustrating the eifects of the preheating temperature on the results of salt water spray tests
  • FIGS. 3 and 4 are graphs illustrating the effects of the composition of the first coating film on the results of salt water spray tests.
  • FIG. 5 is a diagram showing the adhesiveness of coated films.
  • FIG. 1 illustrating typical variations of the chromium ion content in the first coating film deposited by the electrolytic treatment with chromic acid for different preheating temperatures
  • the initial content of hydrous chromium oxides in the first coating film in the state as deposited thereon, varies considerably for different conditions of electrolytic treatment.
  • most chromium in the depoisted film is converted into insoluble crystalline chromium oxides.
  • Such conversion of chromium can be proved by the fact that if steel sheets thus heated are dipped in an aqueous solution of sodium hydroxide NaOH (or an aqueous solution of sodium hydroxide added with hydrogen peroxide H 0 chromium ions having a valency of 3 and 6 do not dissolve therein any more.
  • Table l shows the corrosion-resistance and the film adhesiveness of steel sheets with double coating films deposited thereon by the method according to the present invention.
  • FIG. 2 illustrates the relation between the corrosion-resistance of steel sheets and the heating temperature thereof prior to vacuum evaporation in the method according to the present invention.
  • the corrosionresistance decreases. For instance, with preheating at about 200 C., the duration of salt water spraying prior to red rust generation is about 150 hours; at about 300 C., about 100 hours; and at about 400 0., about 30 hours. However, even with such high temperature preheating, the corrosion-resistance of the steel sheets coated with double films is considerably superior to that of steel sheets having only single coated film.
  • Test samples were prepared under different treating conditions, so as to generate films mainly consisting of hydrous chromium oxides in different contents.
  • the samples thus prepared were all preheated at 200 C., coated with 0.1 micron thick and 0.3 micron thick aluminum films, and then subjected to salt water spray tests.
  • the results are shown in FIGS. 3 and 4.
  • the corrosion resistance of the steel sheets with the evaporated aluminum film' is improved.
  • the effect of chromium oxides on the improvement of the corrosion resistance of steel sheets with double coating films including an evaporated aluminum film is greater than those of metallic chromium.
  • hydrous chromium oxides When non-conductive hydrous chromium oxides are coated on'the surface of metallic chromium, such hydrous chromium oxides change themselves into anhydrous chro- 'mium oxides upon heating prior to the vacuum evaporation, while retaining their non-conductive nature, and accordingly, the chance of forming local galvanic cells by direct contact of two metallic elements, such as aluminum- 2 Room temperature.
  • the content of chromium in the first coating film on steel sheets at least 2 mg./m. is necessary to achieve the object of the invention, and outstanding improvement can be achieved with a content of metallic chromium in excess of 10 mg./m. It has been found that hydrous chromium oxides to coat the metallic chromium film will be efiectivc if the content of chromium in such hydrous chromium oxides is more than 4 mg./m. and the effects are especially good when the content of chromium in the hydrous chromium oxides film exceeds 10 mg./m.
  • the result of tests concerning the effect of the preheating temperature prior to the vacuum evaporation of aluminum on the adhesiveness of double coating films deposited on steel sheets shows that the adhesiveness of the two coating films, or the workability of the double-film coated steel sheets, increases with the preheating temperature.
  • the critical minimum preheating temperature for achieving useful adhesiveness of the coating film is about C., and if extensive machine work is expected (for instance deep drawing causing critical contraction close to fracture), the preheating temperature should be higher than about 200 C.
  • the reason for the material improvement of the adhesiveness of the double coating films of the steel sheets by preheating at 200 C. to 250 C. is considered to be due to the fact that the first coating film experiences qualitative change in this temperature range.
  • improvement of the adhesiveness seems to be related to the fact that the hydrous chromium oxides as generated by electrolytic treatment, which has a weak bonding strength, are converted into crystalline chromium oxides.
  • a suitable temperature for preheating steel sheets prior to vacuum evaporation of aluminum for producing aluminum films with excellent corrosion resistance and adhesiveness is in the range of 150 C. to 400 C., and especially temperatures in the range of 200 C. to 250 C. give the best results.
  • the amount of chromium in the hydrous chromium oxide layer of the first coating film and the amount of chromium in the metallic chromium layer of the first coating film should satisfy the following relation:
  • FORMULA 1 Amount of chromium in the chromium oxide layer (mg./m. 0.4 (amount of chromium in the metallic chromium layer (mg/m?) +40).
  • FIG. shows the test results of Table l, with the abscissa representing the amount of chromium in the metallic chromium layer of the first coating film and the ordinate representing the amount of chromium in the chromium oxide layer of the first coating film.
  • solid black dots represent specimens which were slightly peeled off, while non-solid circles represent specimens which were perfect without any peeling.
  • a line is drawn in FIG. 5 which represents the relation of the Formula 1. It is apparent from FIG. 5 that the relation (1) clearly defines the upper limit of the chromium content in the chromium oxide layer of the first coating film, in view of the chromium content in the metallic chromium layer of the first coating film.
  • the amount of chromium in chromium oxides is the sum of the amount of chromium ions having a valency of 6, as determined in the state as electrolytically treated by dissolving such ions in an aqueous solution of sodium hydroxide with a pH above 8, and the amount of those chromium ions having a valency of 6 which are prepared by oxidizing chromium ions having a valency of 3 by adding hydrogen peroxide and dissolving.
  • the corrosion resistance of steel sheets with such double films increases with those thicknesses.
  • the proper thickness of the first coating film deposited by electrolytic treatment with chromic acid corresponds to the content of metallic chromium in the range of less than 300 mg./m. especially in the range of 2 mg./m. to 70 mg./ 111. If a preheating temperature of 150 C. to 250 C. is used, the thickness, or the amount of the chromium oxides to be deposited together with the metallic chromium film should satisfy the conditions of the Equation 1.
  • the preheating temperature is higher than 250 C.
  • the economical thickness of the aluminum film is less than 2 microns, and it is preferably selected in the range of 0.05 micron to 0.5 micron.
  • EXAMPLE 1 Sample steel sheets of 0.32 mm. thick were degreased and washed with acid, and electrolytically treated by using an electrolyte containing 75 g./l. of chromic anhydride and 0.75 g./l. of sulfuric acid at 50 C. while passing an electric current at a rate of a./dm. for 2 seconds, and as a result of it, a first film consisting of 42 mg./m. of metallic chromium and hydrous chromium oxides with 31 rug/111. of chromium content.
  • a sample steel sheet thus treated was subjected to a salt water spray test, according to the stipulations of I18 Z2371, and red rusts appeared after five hours of spraying.
  • Another sample sheet was prepared by directly depositing a 0.1 micron thick aluminum film on a steel sheet by vacuum evaporation, and with such sample, red rusts appeared in two hours in the aforesaid salt water spray test.
  • the aforesaid samples having the first films were preheated at 200 C., and then 0.1 micron thick aluminum films were deposited thereon by vacuum evaporation. It was proved by the salt water spray test that the corrosion resistance of the samples thus coated with aluminum film on the first film was materially improved, and in fact, red rusts appeared only after continuous salt water spray for 126 hours.
  • the surface of the sample thus coated with double films was scratched in lattice fashion with 2 mm. spacings between adjacent scratching lines, and then subjected to a deep drawing to a depth of 7 mm. by an Erichsen testing machine, and finally an adhesive tape, known by the name of Cellotape (trademark), was adhered to the thus scratched surface and then removed 8 therefrom. It proved that the coated film did not peel off by such test, and the adhesiveness was excellent.
  • EXAMPLE 2 Sample steel sheets were electrolytically treated at 50 C. by using an electrolyte containing 50 g./l. of chromic anhydride and 0.5 g./l. of sulfuric acid, while passing an electric current at a density of 30 a./dm. for 2 seconds. As a result of it, first films consisting of 18 mg./m. of metallic chromium and chromium oxides having 30 mg./m. of chromium were deposited on the sample sheets.
  • a sample sheet as treated by such electrolysis was subjected to the salt water spray test, in the same manner as Example 1, and red rusts appeared in 2 hours.
  • Example 2 The adhesiveness of the coating films of these samples of Example 2 were tested by the same method as Example l, and good adhesiveness was demonstrated.
  • Paint (thermo-setting epoxy resins) for painting inside surfaces of cans was applied to the double films deposited on such samples to form a 10 micron thick paint film.
  • the thus painted surface was scratched by a record player needle with gr. loading, and then subjected to the aforesaid salt water spray test for 30 hours in the same manner as Example 1.
  • the corrosion resistance of the thus painted sample proved to be very good, and only slight local corrosions were found. In other words, the corrosion resistance was significantly improved, as compared with that of steel sheets with a single aluminum film evaporated thereon.
  • EXAMPLE 3 Sample steel sheets were prepared by electrolytically treating them at 50 C. by using an electrolyte containing 30 g./l. of chromic anhydride and 0.3 g./l. of sulfuric acid while passing an electric current therethrough for 2 seconds at a density of 30 a./dm. The thus treated samples were then coated with 0.36 micron thick aluminum films by vacuum evaporation after preheating them at 250 C. When the thus coated samples were subjected to the aforesaid salt water spray test in the same manner as Example 1, with salt water spraying at a rate of 2.4 cc./hour to 2.6 cc./hour, red rusts appeared after 57 hours of such spraying.
  • a sample sheet with such double films thus coated thereon was subjected to cold rolling at a reduction ratio of 20%, and then the same salt water spray test as Example l was applied thereto. Red rusts appeared after 25 hours of spraying. It is apparent that the corrosion resistance after such cold rolling was better than that of unworked conventional steel sheet with single coating film. In other words, the excellent corrosion resistance achieved by the cumulative effects of the double coating films was not lost by such cold rolling.
  • composition of the steel sheets used in the aforesaid Examples 1 to 3 was as follows.
  • a method of coating a surface of a steel element with films having a high corrosion resistance comprising steps of forming a composite chromium coating film on said surface of said steel element by electrolytic treatment with chromic acid wherein said steel element acts as the cathode, said chromium film consisting essentially of a metallic chromium layer formed directly on said surface and a hydrous chromium oxide layer formed on saidmetallic chromium layer and said chromium film containing 6 to 460 mg./m. of chromium in the form of said hydrous chromium oxide and said metallic chromium, heating the thus coated steel element at a temperature in the range of 150 C. to 400 C., and then depositing an aluminum film on said chromium film by vacuum evaporation.
  • said metallic chromium layer contains 2-300 mg./m. of metallic chromium.
  • said metallic chromium layer contains -70 mg./m. of metallic chromium.
  • hydrous chromium oxide layer contains 4-160 mg./m. of chromium in the form of hydrous chromium oxides.
  • a method of coating steel sheet surfaces with films having a high corrosion resistance comprising forming a composite chromium coating film on each surface by electrolytic treatment with chromic acid wherein the steel sheet acts as the cathode, said chromium film comprising a metallic chromium layer directly on said surface and a hydrous chromium oxide layer over said metallic chromium layer, wherein the total amount of chromium as chromium oxide and metallic chromium in said chromium film is 6 to 460 mg./m. and the amount of chromium in the hydrous chromium oxide layer, in
  • 10 rug/m. is less than heating the thus coated sheet at a temperature of C. to 400 C., and then depositing an aluminum film on said first film by vacuum deposition.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Electrochemistry (AREA)
  • Chemical Treatment Of Metals (AREA)
  • Other Surface Treatments For Metallic Materials (AREA)
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US858782A 1968-09-27 1969-09-17 Method of coating steel sheet surfaces Expired - Lifetime US3691055A (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3833430A (en) * 1972-12-26 1974-09-03 Varian Associates Treatment of stainless steel and similar alloys to reduce hydrogen outgassing
US4432845A (en) * 1982-07-20 1984-02-21 Kawasaki Steel Corporation Method of producing tin-free steel sheets having improved resistance to retorting treatment
EP0319908A2 (de) * 1987-12-10 1989-06-14 Nkk Corporation Aluminiumplattierter Stahlblock für Dosen
US20080083498A1 (en) * 2006-10-06 2008-04-10 Benteler Automobiltechnik Gmbh Method of making a motor vehicle part

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3833430A (en) * 1972-12-26 1974-09-03 Varian Associates Treatment of stainless steel and similar alloys to reduce hydrogen outgassing
US4432845A (en) * 1982-07-20 1984-02-21 Kawasaki Steel Corporation Method of producing tin-free steel sheets having improved resistance to retorting treatment
EP0319908A2 (de) * 1987-12-10 1989-06-14 Nkk Corporation Aluminiumplattierter Stahlblock für Dosen
EP0319908A3 (de) * 1987-12-10 1990-05-23 Nkk Corporation Aluminiumplattierter Stahlblock für Dosen
US20080083498A1 (en) * 2006-10-06 2008-04-10 Benteler Automobiltechnik Gmbh Method of making a motor vehicle part

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DE1948836B2 (de) 1974-08-29
GB1272118A (en) 1972-04-26
DE1948836C3 (de) 1975-05-07
DE1948836A1 (de) 1970-05-14

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