US3691055A

US3691055A - Method of coating steel sheet surfaces

Info

Publication number: US3691055A
Application number: US858782A
Authority: US
Inventors: Motoharu Hamada; Hiroshi Nakagawa
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1968-09-27
Filing date: 1969-09-17
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12
Also published as: DE1948836A1; GB1272118A; DE1948836B2; DE1948836C3

Abstract

A METHOD OF COATING STEEL SHEET SURFACES WITH DOUBLE COATING FILMS HAVING A HIGH CORROSION RESISTANCE. EACH STEEL SHEET IS AT FIRST COATED WITH A FIRST FILM MAINLY CONSISTING OF HYDROOUS CHROMIUM OXIDES BY ELECTROLYTIC TREATMENTS WITH CHROMIC ACID. AN ALUMINUM FILM IS COATED ON THE FIRST FILM BY VACUUM EVAPORATION.

Description

Sept. 12, 1972 MOTOHARU HAMADA ETA!- 3,691,055

METHOD OF COATING STEEL SHEET SURFACES Filed Sept. 17, 1969 5 Sheets-Sheet 1 Fig. l

o 5%m2X 24sec chromium ions having 3 valency o 5%g gxz4 chromium ions having 6 valency A lofim x l2sec chromium ions having 3 valency A l0 Am X lZsec chromium ions having 6 valency Amount of chromium lOllS dissolving in an aqueous solution of SOdlUlTl hydroxide NaOH added with hydrogen peroxide H202 N v 2023M 6sec chromium ions having 3 valency 50- v 20AM 6secchromium ions having 6 valency n BO /nFX 4sec chromium ions having 3 valency 30/drrFx 4sec chromium ions having 6 valency Nut-b 9e? 8 I50 200 25 300 350 200 A3 QIBCTI'OIWC Heating temperature of steel sheet having a first "emmem coating film deposited thereon by electrolytic treatment with chromic acid (C) (Keeping time 3min) p 1972 MOTOHARU HAMADA AL 3,691,055

, METHOD OF COATING STEEL SHEET SURFACES Filed Sept. 17. 1969 5 Sheets-Sheet 2 Fig. 2

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| 5 0. 3,u E] V 53 /n 42 /n O. 3 ,u 400- 0 7o /n2) 3e /n 0.3 ,u m flu Hours before red rust generation in salt water 5 o Preheating temperature of steel sheet'(c) p 1972 MOTOHARU HAMADA AL 3,691,055

METHOD OF COATING STEEL SHEET SURFACES Filed Sept. 17, 1969 5 Sheets-Sheet 5 Fig. 3

I60- Thickness of aluminum film 0.3,)

50 l Thickness of aluminum film 0. I ,u

Hours before red rust generation in salt water spray test of JIS 2237i (hr) 01 1 l l l 0 IO 203040 5060 0 IO 20 3O 4O Metallic chromium("%2) Chromium in chromium oxides Composition of the fist coating film p 1972 MQTQHARU HAMADA Em 3,691,055

METHOD OF COATING STEEL SHEET SURFACES Filed Sept. 17, 1969 5 Sheets-Sheet 4 A set F I g 4 woi 5 |6O 0 Thickness of aluminum film 0.3

N m A Thickness of aluminum film 01 Hours before red rust generation in salt water spray test of Jl m \l o o o O l l l 0 IO 20 0 I0 20 3O 4O 50 6070 Metallic chromium Chromium in chromium oxides("%,2)

Composition of the first coating film Sept. 12, 1912 Filed Chromium in chromium oxide layer /m Sept. 17, 1969 Mo'roHARu HAMADA AL 3,

METHOD OF COATING STEEL SHEET SURFACES 5 Sheets-Sheet 5 'Fig. 5v

0 To be considered good To be considered not'good I50 Chromium in metallic chromium Oyer /m IOO United States Patent Olfice Patented Sept. 12, 1972 3,691,055 METHOD OF COATING STEEL SHEET SURFACES Motoharu Hamada, Nishinomiya, and Hiroshi Nakagawa, Kobe, Japan, assignors to Kawasaki Steel Corporation, Kobe, Japan Filed Sept. 17, 1969, Ser. No. 858,782 Claims priority, applticatign Japan, Sept. 27, 1968,

Int. Cl. C23b 9/00, 11/00 US. Cl. 204-385 12 Claims ABSTRACT OF THE DISCLOSURE A method of coating steel sheet surfaces with double coating films having a high corrosion resistance. Each steel sheet is at first coated with a first film mainly consisting of hydrous chromium oxides by electrolytic treatment with chromic acid. An aluminum film is coated on the first film by vacuum evaporation.

evaporation, the magnitude of corrosive current therethrough is still large as compared with the corresponding magnitude of corrosive current through isolated aluminum itself. For instance, when steel sheets having a 0.5 micron thick aluminum film coated thereon by vacuum evaporation are dipped in a 5% aqueous solution of sodium chloride, 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.

In salt water spraying tests, as stipulated in the lapanese Industrial Standard (to be referred. to as JIS hereinafter) Z2371, for each micron thickness of the coated aluminum film, 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.

The inventors have made a series of studies on methods for completely preventing the formation of such local galvanic cells, and as a result, they have completed the method of the present invention. In other words, according to the method of the present invention, 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. With the method according to the present invention, the corrosion resistance of the aluminum coated steel sheet is greatly improved, as compared with that of conventional coated steel sheets having a single aluminum film applied thereon.

Generally speaking, it has been said that in electrolytic treatment with chromic acid, if an electrolyte having a chromic anhydride concentration exceeding g./l. is used, the coating film achieved thereby will mostly consist of metallic chromium, while with its concentration under 100 g./ 1., the coating film achieved will mostly consist of hydrous chromium oxides. In fact, however, the inventors have found that double films consisting of a film of metallic chromium and another film of hydrous chromium oxides are actually produced in either case. Therefore, strictly speaking, 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.

A number of propositions have heretofore been made concerning surface-treated steel sheets with chromate coating or chromium plating.

As regard-s 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. As a compromise of such incompatible standpoints, 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.

The corrosion resistance of such known surface treated steel sheets were tested by the aforesaid salt water spraying, and it was found that red rusts were generated in a few hours.

In the method according to the present invention, 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. However, such preheating is optional in the method of the invention, and can be dispensed with.

Therefore, 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.

For a better understanding of the invention, reference is made to the accompanying drawings, in which:

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; and

FIG. 5 is a diagram showing the adhesiveness of coated films.

Referring to 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, it is seen that 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. However, when the steel sheets are heated to 20 C. to 250 C., 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.

If steel sheets electrolytically treated with chromic acid, which have a coating film containing less than 300 mg./m. of metallic chromium and less than 190 mg./m. of chromium ions in hydrous chromium oxides, are subjected to salt water spray tests, red rusts appear in at most hours of spraying. If such steel sheets are heated to 200 C. to 300 C., their corrosion resistance or salt-resistance is greatly reduced possibly to lessthan one half of that of non-heated steel sheets.

When steel sheets haveing 0.1 micron to 0.5 micron thick aluminum coating films directly deposited thereon by vacuum evaporation are subjected to salt water spray tests, red rusts appear in only 2 hours to 8 hours of spray- 1ng.

On the other hand, if steel sheets are pretreated electrolytically with chromic acid so as to deposit a first film mainly consisting of hydrous chromium oxides and then coated with a second film of aluminum by vacuum evaporation, the salt-resistance of the thus coated steel sheets are remarkably improved. In fact, by selecting proper treating conditions, red rusts do not appear up to 100 hours to 200 hours.

Such remarkable improvement can be achieved only by a novel treatment, and it can be neither anticipated nor achieved only by simple combination of known arts of depositining a single metallic film on steel sheets.

Some results of tests made on steel sheets treated by the method according to the present invention are shown in Table l, and Table 2 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.

TABLE 1.CORROSIONRESISTANCE AND ADHESIVENESS OF ALUMINUM COATED STEEL SHEETS ACCORDING TO THE PRESENT INVENTION Duration of salt water spraying before red rust generation, in salt water spray test or J IS Z 2371 (hours) Double films, Quantity of chromium, coated deposited by by electrolytic treatment with Hours before red electrolytic treatchromic acid rust generation in ment with chromlc salt water spray acid and alumi- Adhesivcness, Chromium in test of J IS Z2371, Thickness oi Aluminum vacuum double films Metallic chromium as electrolytically aluminum num evaporation, of the chromium oxides treated with film according to the present (mg/m1) (mg/mi) chromic acid (micron) alone I present invention 1 invention l 3. 0 1O 1 0. 1 2 20 3. 3 20 l 0. 1 2 40 O 4. 4 15 1 0. 1 2 26 O 4. 4 56 2 0. 1 2 74 (B 10. 2 40 3 0. 1 2 72 0 14.3 33 3 0. 1 3 96 O 17. 7 2 0. 1 2 79 O 16. 9 2 0. 1 2 79 Q 15. 9 5O 4 0. 1 2 87 (B 17. 5 G0 7 0. 1 2 96 (9 37. 2 79 6 0. 1 2 77 G 41. 9 31 5 0. 1 3 126 O 63. 8 52 5 O. 1 2 79 77. 0 47 3 0. 1 2 10]. 0 89. 5 44 3 0.1 2 66 O 89. 5 44 3 0. 2 3 97 O 93. 5 3 0. 1 2 66 O 125. 0 54 8 0.1 2 O 115 102 3 0. 1 2 86 (9 115 102 3 0.3 4 20O G 115 102 3 0. 5 8 200 0 123 110 6 0. I 2 83 G 123 H0 6 0.3 '1 200 G 123 110 6 0. 5 8 (9 234 102 6 0. I I 102 O 234 102 6 0.3 5 117 O 234 102 6 0. 5 10 (192 O 296 38 I0 0. l 2 108 Q 300 160 10 0. 1 2 20O O 300 10 0. 1 2 200 (9 l The preheating temperature for vacuum evaporation is 200 0. Note.'O=perfcet; 6 =with slight peeling.

TABLE 2.EFFECTS OF PREHEATING TEMPERATURE PRIOR TO VACUUM EVAPORATION ON THE ADHESIVENESS F FILMS OF STEEL SHEETS HAVING DOUBLE COATING FILMS 'ILemppr- Thick}3 Erichsen drawing depth, 3 mm. Erichsen drawing depth, 5 mm. Erichsen drawing depth, 7 mm. a ure or ness 0 preheating aluminum Conditions for electrolytic treat- Conditions for electrolytic treat- Conditions for electrolytic treathstet film geament with chromic acid 1 ment with chromic acid l ment with chromic acid 1 s ee 5 pos1 e Sample No. 0.) (micron) 5 A./ 10 A./ A./ 30 A./ 5 A./ 10 A./ 20 A./ 30 A./ 5 A./ 10 A./ 20 A./ 30 AJ (1111. X dm. X dm. X dm. X dm. X dm. X dm. X dm. X dm. X drn. X drn. X dm. X 24 sec 12 sec. 6 sec. 4 sec. 24 sec. 12 sec. 6 sec. 4 sec. 24 sec. 12 sec. 6 sec. 4 sec 0.3 u 0!) iii to tilt (iii 09 4h ti: 0) tit 00 150 0.3 O O O O O O O O G (D G) (9 200 0.3 O O O O O O O O (B (9 (9 o 250 0.3 O O O O O O O O O O O O 300 0.3 O O O O O O O O O O O O 350 0- 3 O O O O O O O O O O O O 0 0.3 O O O O O O O O O O O O 1 The relation chromic acid and the composition of the deposited film is as follows:

between conditions of electrolytic treatment with When steel sheets electrolytically treated with chromic acid are coated with aluminum film by vacuum evaporation at room temperature, without heating, an excellent corrosion resistance is achieved. For instance, even when the aluminum film was very thin, e.g., 0.3 micron thick, red rusts did not appear in the saltwater spray test for 430 hours or more.

As the preheating temperature increases, 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.

The inventors studied how the metallic chromium and chromium oxides in the first coating film contribute to the improvement of the corrosion resistance of steel sheets with double coating films including an evaporated aluminum film. As a result of it, the inventors have found the following.

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. As seen from the figures, as the content of metallic chromium and the content of hydrous chromium oxides increase, 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. The reason for such improvement is considered to be in that if'the surface of steel sheets is completely coated with metallic chromium, there wil be no chance of direct contact "between the steel and aluminum, so that the speed of corrosion is significantly reduced, as compared with that of steel sheets having'an aluminum vfilm directly coated thereon. On the other hand, when chromium and aluminum films come into contact with each other, there are formed local galvanic cells between the two metals which cause corrosion on the coating film.

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.

NorE.-O=Perfect, O=Alm0st perfectto be considered good G=Slightly peeled, =Considerably peeled, =Completely peeledto be considered not good.

iron and aluminum-chromium, is completely eliminated. Thus, the corrosion resistance inherent to aluminum itself dominates in the thus coated steel sheets.

As regards 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. In other words, such 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. With such vacuum evaporation of aluminum preceded by the aforesaid heating step, in order to ensure stable adhesion of the coating films, 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. In FIG. 5, 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.

Here, 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.

As regards the thickness of the first coating film deposited by electrolytic treatment with chromic acid and the thickness of aluminum film deposited on the first film by vacuum evaporation, the corrosion resistance of steel sheets with such double films increases with those thicknesses. However, from the economic standard, 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. However, if the preheating temperature is higher than 250 C., there is no limitation in the quantity of the chromium oxides. 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.

The method according to the present invention will now be described in further detail, referring to examples.

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.

On the other hand, 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.

Furthermore, 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.

However, when a 0.1 micron thick aluminum film is deposited on such first film of the sample by vacuum evaporation after preheating it at 250 C., the corrosion resistance of the sample was materially improved. In fact, red rusts appeared only after 79 hours of salt water spraying.

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.

The adhesiveness of the samples of this Example 3 was tested by the same method as Example 1, and it proved to be very good.

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.

The composition of the steel sheets used in the aforesaid Examples 1 to 3 was as follows.

Carbon: no more than 0.12% Manganese: 0.25 to 0.50%

Silicon: no more than 0.1% Phosphorus: no more than 0.045% Sulfur: no more than 0.05%

Iron: remainder.

What is claimed is:

1. 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.

2. A method according to claim 1, wherein said first film contains 20-460 mg./m. of chromium.

3. A method according to claim 2, wherein said aluminum film is 0.05-0.5 micron thick.

4. A method according to claim 1, wherein said metallic chromium layer contains 2-300 mg./m. of metallic chromium.

5. A method according to claim 1, wherein said metallic chromium layer contains -70 mg./m. of metallic chromium.

6. A method according to claim 1, wherein said hydrous chromium oxide layer contains 4-160 mg./m. of chromium in the form of hydrous chromium oxides.

7. A method according to claim 1, wherein said hydrous chromium oxide layer contains 10-160 mg./rn. of chromium in the form of hydrous chromium oxides.

8. 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.

9. A method according to claim 8, wherein said composite chromium film is formed by electrolytic treatment with an electrolyte containing about 75 g./l. of chromic anhydride and 0.75 g./l. of sulfuric acid while passing an electric current at a rate of 5 to 30 a./dm. so as to deposit a metallic chromium layer on the steel surface while depositing a hydrous chromium oxide layer on the metallic chromium layer.

10. A method of coating steel sheet surfaces as set forth in claim 9. wherein said electrolytic treatment is carried out above a temperature of about 50 C.

11. A method of coating steel sheet surfaces as set forth in claim 8, wherein the heating step is performed in the range of 200 C. to 250 C.

12. A method of coating steel sheet surfaces as set forth in claim 8, wherein said metallic chromium layer contains from 2 to 300 mg./In. of chromium and said chromium oxide layer contains from 4 to mg./m. of chromium in the form of chromium oxide. i

References Cited UNITED STATES PATENTS 2,917,818 12/1959 Thomson 117-71 2,300,400 11/1942 Axline 117-71 3,438,754 4/1969 Shepard et al. 117-107 OTHER REFERENCES Chromium Plating, by Morisset et al., 1954, p. 195.

HOWARD S. WILLIAMS, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R.