US3827866A

US3827866A - Surface treated steel plate

Info

Publication number: US3827866A
Application number: US00109938A
Authority: US
Inventors: H Uchida; O Yanabu
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1967-03-23
Filing date: 1971-01-26
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06

Abstract

THIS INVENTION RELATES TO THE SURFACE TREATMENT OF STEEL PLATES OF SHEETS AND DISCLOSES A NOVEL STEEL SHEET STRUCTURE, WHEREIN THE STEEL SHEET IS FIRST ELECTROLYTICALLY SURFACE COATED WITH A METALLIC CHROMIUM LAYER OF A THICKNESS NOT EXCEEDING ABOUT 0.1 MICRON AND IS THEN ELECTROLYTICALLY PLATED WITH A CHROMATE FILM OF A THICKNESS NOT EXCEEDING 0.1 MG./DM.2. THE CHROMIUM LAYER THICKNESS SHOULD PREFERABLY NOT BE LESS THAN 0.0016 MICRON. THE INVENTIVE STEEL SHEET CONSTITUTES A SUPERIOR CAN STOCK AND MAY BE PROVIDED WITH A TOP COAT OF ORGANIC COMPOSITION.

Description

SURFACE TREATED STEEL PLATE Original Filed March 23; 1967 FIG] INVENTORS filRam 00H! uoumn BY aspnu Yn/vnau WWW United States Patent 3,827,866 SURFACE TREATED STEEL PLATE Hiromu Uchida and Osamu Yanabu, Himeji, Japan, assignors to Nippon Steel Corporation Continuation of abandoned application Ser. No. 625,421, Mar. 23, 1967. This application Jan. 26, 1971, Ser.

Int. Cl. B32b /00 US. Cl. 29-195 10 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation of application 625,- 421, filed Mar. 23, 1967, now abandoned.

The invention also discloses a two-step process for producing the sheet. Pursuant to this process the metallic chromium layer is first electrolytically deposited on the steel sheet surface, whereupon the chromate film is electrolytically deposited on the metallic chromium layer. An important feature of the invention is that the chromate treatment is carried out in a solution which is devoid of a sulphuric acid radical having a tendency to deposit metallic chromium. Particular process conditions in re spect to bath composition, current density and treatment time are disclosed.

SUMMARY OF THE INVENTION This invention generally relates to the surface coating of steel and is particularly directed to novel surfacetreated steel plates or sheets in which a metallic chromium layer is directly plated on the steel sheet surface, a chromate layer thereupon being electrolytically superimposed on the metallic chromium layer. The invention is also concerned with a procedure for obtaining the novel surface coated steel sheets and encompasses the subsequent coating of the surface-treated steel sheets with organic coating compositions.

Relatively thin-gauge steel sheets imparted with the inventive chromium-chromate layers have proved to be superior can stock.

Various proposals have been put forward in the art for the purpose of chromate-treating steel plates, to wit, directly coating the steel sheet surface with a chromate film. According to one prior art procedure, the steel plate to be chromate coated is subjected to cathodic treatment in an aqueous solution containing hexavalent chromium ions and trivalent chromium ions.

According to another suggestion, the cathodic treat ment of the steel plate is effected in an aqueous solution containing chromic acid anhydride, trivalent chromium ions, phosphoric acid and boric acid. It has also been proposed that the cathodic treatment of the steel plate can be effected in a liquid system prepared by adding chromic acid ions to a phosphoric acid solution, the solution also comprising acidic phosphates of zinc, magnesium, calcium and manganese. In a more recent method, the steel plate is subjected to treatment in an aqueous solution of chromic acid, the solution also containing certain reducing materials which have a tendency to reduce the chromic acid to form a film of reduced chromic acid on 3,827,866 Patented Aug. 6, 1974 the steel plate. The thus treated steel plate is then heated to about 250 F. It has also recently been suggested to effect the cathodic treatment of the steel plate in a liquid containing phosphoric acid and dichromate.

US. Pat. 2,769,744 teaches still another procedure, according to which the cathodic treatment is performed in a bath prepared by adding phosphoric acid to a chromic acid anhydride solution. Similar processes have been taught in US. Pats. 2,733,199 and 2,780,592, according to which the cathodic treatment is performed in an aqueous bath liquid containing boric acid or its salts and chromic acid anhydride in aqueous solution.

Finally, U.S. Pats. 2,768,103, 2,768,104 and 2,777,785 suggest a procedure in which the steel plate is immersed in a liquid prepared by adding a reducing agent such as, for example, cane sugar, to an aqueous solution of chromic acid anhydride, whereupon the plate is dried under the formation of a chromate film. It has also been suggested that the cathodic treatment be performed in a chromic acid containing bath enriched by certain compounds that have a tendency to accelerate the film formation such as, for example, catechol-disulfonic acid, phenol disulfonic acid and the like compounds. Other film inducing agents have been suggested for this purpose as, for example, selenic acid, zirconic acid, potassium fluoride, sodium-silico fluoride, titanium potassium fluoride, perchloric acid and ammonium molybdate.

It should be appreciated that in all the prior art procedures referred to, the chromate film or layer is directly formed on the steel surface. This layer or film of the prior art procedures is mainly composed of chromium oxide and chromium hydrates, in which trivalent chromium is the prevaling constituent together with hexavalent chromium salts. Although the chemical structure of these surface layers has not been exactly identified, due to the amorphism of the structure, it is generally accepted by those skilled in this art that trivalent chromium oxide and its hydrates forms a film which adheres to the steel plate surface in an irregular gel state or formation, this film being covered by hexavalent chromium salts. In this manner, the steel surface is imparted with corrosion resistance due to the passivation of the structure.

A variety of modifications have lately been proposed for the purpose of improving the surface coating characteristics. According to these modifications, the proportions of the trivalent and hexavalent chromium are changed with or without the presence of a phosphate film or layer. In any event, the main components of the surface films formed on the steel plate surface are of theindicated nature and are generally referred to in the art as chromate films. In this connection it should be appreciated that the term chromate as used in the field of metal surface treatment has a particular meaning and generally refers to trivalent chromium oxide and its hydrates, the trivalent chromium oxide sometimes being admixed with hexavalent chromium oxide and its hydrates. I

As previously set forth, the prior art procedures result in the formation of a chromate film directly on the surface of the steel plate, and no metallic chromium layer is interposed between the steel surface and the chromate film. This is due to the composition of the bath or treating liquid and the prevailing cathodic treatment condi- According to the second group, the steel plate is subjected to cathodic treatment in which the electro-plating is performed in a liquid of similar composition with the steel plate constituting the cathode.

'It is well known by those skilled in the art that, although the direct formation of a chromate film on the steel plate surface improves the corrosion resistance of the steel, the chromate film is not fully satisfactory or adequate for a variety of purposes to which the steel plate may be put. Thus, for example, the corrosion resistance of chromate treated steel plates is generally satisfactory as long as the plate remains uncoated with organic coatings, but once an organic coating has been applied onto the chromate layer and the plates have been formed, the chemical resistance of the resulting product is unsatisfactory as evidenced by chemical resistance tests. In this connection it should be appreciated that if the chromate treated steel plates are to be used, for example, for the manufacture of cans, an additional top coating with organic compositions is necessary and indeed required by the laws of most countries. The reason that the characteristics of chromate treated steel plates to which an organic coating has been applied are unsatisfactory, is due to the fact that, although relatively good adherence is obtained between the organic coating composition and the chromate film, the adherence between the steel surface and the chromate film is unsatisfactory. It has thus been found that when a chromate treated steel plate, subsequently coated with an organic lacquer or varnish composition, is immersed in a solution containing citric acid and sulphuric acid, the acidic solution penetrates through the organic coating film to corrode the chromate film and the steel surface progressively, until the organic film peels off.

US. Pat. 3,113,845 is concerned with surface coated steel sheets for can stock purposes, wherein the steel sheet is electrolytically coated with a metallic chromium layer of predetermined thickness, the chromium layer in turn being coated with an organic coating composition. This patent constitutes an important improvement in the art of surface treating steel plates and teaches the production of steel plates which are superior to those coated with a chromate film. The US. patent referred to contains specific disclosures concerning the thickness of the metallic chromium layer, the maximum thickness of the layer being indicated as 0.1 micron. This maximum thickness of the chromium layer is important from the viewpoint of quality. In respect to the lower limit of the chromium layer thickness, consideration has been given to the minimum thickness necessary to obtain sufficient corrosion resistance, which latter should be equal if not superior to that of ordinary tin plates used as can stock. Chromium plated can stock of the nature disclosed in US. Pat. 3,113,845 and having a chromium layer thickness of from 0005-01 micron has been favorably received by the trade and the industry and is now widely accepted as superior and low-priced can stock material. It has also found other commercial applications.

It is a primary object of the present invention still further to improve on the qualities and characteristics of steel plates which have been directly chromium plated such as disclosed in US. Pat. 3,113,845.

According to the US. patent referred to, the steel plate, after the metallic chromium layer has been electrolytically deposited thereon, may be chemically treated with a diluted chromic acid solution. This treatment, according to the prior patent, is effected in a tank into which the chromium plated steel plate is dipped. Extensive experiments have indicated that this dipping treatment does not yield significantly improved results and, in spite of the dipping into chromic acid solution, the final plate is susceptible to pit corrosion before the organic coating has been applied, when the plate is subjected to salt spray testing. Further, it has been ascertained that the plates do not exhibit improved corrosion resistance even after the organic coating is applied subsequent to the chromic acid dipping treatment. It has thus been established that if the final products manufactured from the plates and after they have been topcoated with the organic composition, are subjected to corrosion tests, no improved results are obtained. This is particularly so if the chromium layer thickness is less than 0.01 micron. With layer thicknesses in the range of 0.01-0.1 micron better results could be observed. Investigations have revealed that when a chromium plated steel plate is dipped into a chromic acid solution, as taught in the prior patent, a chromate film is merely formed at the pin hole areas of the steel plate, but no chromate film formation can be observed on the metallic chromium layer. In other words, only at the exposed areas where no chromium layer is formed on the steel surface, does a chromate film adhere to the steel proper, thus covering the pin holes.

Our studies were then directed towards finding etficient treatment conditions for forming a chromate film on the metallic chromium layer by means of cathodic treatment and as a result of such studies, an efficient and simple method was developed electrolytically to deposit a chr0- mate film on the metallic chromium layer previously electroplated onto the steel surface. In accordance with this procedure which thus forms a primary aspect of the present invention, the pin hole portions are also covered by a chromate film (chromium oxide and/or chromium hydrate film) so that the chromate layer uniformly covers the entire top surface of the steel plate and overlies both the metallic chromium film on the sheet and the pin hole areas. It was further established that subsequent coating of the chromate layer with organic compositions results in a superior product and that such product has excellent corrosion resistance and adherence characteristics of the various layers. The chromate film formed in accordance with this invention has, moreover, desirable density characteristics.

Steel sheets first imparted electrolytically with a metallic chromium layer and then with an electrolytically deposited chromate layer exhibit excellent results in salt spray tests before the sheets are top-coated with organic compositions. Further, the products show improved corrosion resistance, both with and without organic top coats, if the chromium layer thickness is between 0.01-0.1 micron. Moreover, the corrosion resistance of the inventive product is at least equal, if not better, than that of tin plates even if the thickness of the metallic chromium layer is as thin as 0.001 micron. From a practical point of view it has been established that the preferred thickness range for the metallic chromium layer is 0.00l6-0.l micron.

Accordingly, the present invention provides for chromium plated steel sheets which have electrolytically superimposed thereon chromate films which in turn may be covered with organic coatings. The inventive sheets have superior surface properties and are economically produced in a simple manner. Since the thickness of the various layers is extremely thin and the price of chromium is less than that of tin, the inventive sheets can be more economically produced than tin plates.

Briefly, therefore, the invention essentially resides in the provision of a surface treated steel plate having directly surface plated thereon a metallic chromium layer of a thickness of between about 0.00160.1 micron, the chro mium layer, in turn, being electrolytically coated with a chromate film which overlies both the chromium layer and the pin hole portions.

In accordance with this invention it has been ascertained that the thickness of the chromate film superimposed onto the metallic chromium layer should not exceed 0.1 mg./ dm. The chromium-chromate plated steel sheet may thereafter be top coated with an organic composition as is known in the art and as disclosed, for example, in US. Pat. 3,113,845, previously referred to.

As stated, the thickness of the chromate film electrolytieally superimposed onto the chromium layer should not exceed 0.1 mg./dm. The reason for limiting the thickness of the chromate layer to the indicated upper value is primarily due to the fact that with thicker chromate films, the surface color tone of I products made from the surface treated sheets does not attain any significant metallic lustre or gloss. If the chromate layer is thicker, the surface color or hue has a tendency to be of a grayish or bluish tint, thus lowering the commercial value of the product. Furthermore, thicker chromate films may result in the ultimate peelingoff of the organic coating which is normally applied as top coat. This has been established in undercut film corrosion tests. Generally, the adherence characteristics suffer if the chromate layer exceeds the indicated limit and, moreover, discoloration may take place during heating of the product in an air atmosphere. This is demonstrated by the results of tests as tabulated in the following Table 1.

TABLE 1 Discolora- Thickness of Thickness Undercut; tion during chromium oi chromate film corheating in plating film Surface color rosron air (300 0., (micron) (micron) of product test 5 min.)

0.01 Metallic lustre. O No. 0.05 do O No. 0.1 do O No. 0.3 Bluish gray x To yellow.

O means normal, 1 means that some peeling of the organic lacquer coating as observed.

The undercut film corrosion test referred to in Table 1 was carried out by applying 30-40 mg./dm. of epoxy urea to the test piece, whereupon the thus coated test piece is baked at 200, C. for minutes. The coating was then cut or scratched with a needle in an X pattern formation and the thus cut portion-was extruded by 5 mm. with an Erichsen tester. The test piece was then immersed in an aqueous solution containing 1.5% of citric acid and 1.5% of sodium chloride with the addition of a few drops of formalinper liter of solution. The immersion was effected for 5 days at 80 F.

The various features of novelty which characterize the invention are pointed out with particularly in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

. 'BRIEF-DESCRIPTIO'N OF THE DRAWINGS Inthe drawingsi FIG. 1 shows on an enlarged scale and in diagrammatic manner a cross section of an inventive steel plate coated with a chromium layer and a chromate film, while FIG. 2 shows on an enlarged scale and diagrammatically a cross section of a steel plate which has been treated in accordance with the prior art method as, for example, disclosed in US. Pat. 3,113,845.

DETAILED DESCRIPTION 'Tu'r ning now to FIG. 1, a relatively thin-gauge steel *plate'is' indicated by reference numeral 1. A metallic chromium layer 2 is electrolytically plated onto the top and bottom surfaces of the plate, the subsequently electrolytically deposited chromate film being indicated by reference numeral 3. The chromate film 3 is firmly bonded to' the'chromium layer 2. It will be noted that the pin hole areas 4 are also covered by the chromate film 3.

Referring now to FIG'I'Z which illustrates a prior art steel sheet, it will be noted that the metallic chromium layer 2 is directly plated onto the steel sheet 1. The dipping of the chromium plated steel sheet into a chromic acid solution merely results in the formation of a chromate layer 5 at the pin hole areas, but no chromate film is formed on the metallic chromium film 2.

The inventive chromium-chromate coated steel sheet is advantageously produced according to the following method:

The surface of the steel plate is first degreased in any conventional manner. Electrolytic degreasing is thus feasible and indeed recommended. The plate is then washed and any rust is removed by acid pickling and additional washing. The steel plate is then chromium electroplated in a first stage in a chromium plating tank to obtain a chromium coating of 0.00160.1 micron thick ness. The thus chromium plated sheet is then washed in hot water whereupon, in a second separate stage, the chromium plated steel is subjected to an electrolytic treatment in which the chromium plated steel plate is the cathode. The electrolysis is performed in an aqueous solution containing hexavalent chromium ions such as formed by chromic acid or dichromate or containing hexavalent chromium ions mixed with trivalent chromium ions. Reducing agents or agents accelerating and inducing chromate film formation on the metallic chromium coating may be added to the bath.

Concerning the composition of the electrolytic bath for effecting the chromate film deposition, it should be noted that the bath initially may only contain hexavalent chromium ions, or trivalent chromium ions may be present in admixture with the hexavalent chromium ions. However, it should be appreciated that even if the bath initially contains hexavalent chromium ions only, trivalent chromium ions will always be produced due to the reducing action which takes place during the electrolysis. The presence of weak reducing agents, of course, further promotes the formation of trivalent chromium ions.

An important feature of the inventive procedure for forming the chromate film on the metallic chromium layer is that no sulfate ions or sulphuric acid radicals are permitted to be present in the bath which have a tendency to deposit metallic chromium on the cathode, to wit, the previously chromium coated steel sheet.

Another important feature of the inventive procedure is that the electrolytic deposition of the metallic chromium layer and the subsequent electrolytic deposition of the chromate film are carried out in two distinct and separate stages. Thus, as will be apparent from the above, the deposition of the chromium layer is carried out first and only after the chromium layer has been deposited is the thus surface treated steel sheet subjected to the electrolytic deposition of the chromate film in the absence of sulfuric acid radicals or the like catalysts which have a tendency to deposit metallic chromium.

As a general proposition, the conditions for depositing the two layers, to wit, the chromium layer and the chromate layer, may advantageously be, for example, as follows:

Conditions for formation of chromium layer Bath composition:

CrO 130-170 g./liter H2804: 0.2-0.4 g./litel Na SiF 3-7 g./liter Bath temperature: 50i2 C. Current density: 20-80 amperes Treating time: 1.25-5 seconds Time x density: 100.

Conditions for chromate film formation Bath composition:

CrO :5 g./liter Bath temperature: 55:5" C. Current density: 5 amperes Treating time: 0.5-2 seconds Comparative corrosion tests were carried out with three different products, to wit:

A. a steel plate on which a chromate film was directly formed in electrolytic manner;

B. a chromium plated steel plate which was dipped into chromic acid to form a chromate film; and

C. a steel plate manufactured in accordance with the present invention, to wit, having an electrolytically deposited chromium coating and superimposed thereon an electrolytically formed chromate film.

The tests were conducted before the products were coated with an organic composition. The results are shown in Table 2. The results of the salt spray corrosion test referred to in the Table are stated in terms of hours up to rust formation while the results of the ferroxyl corrosion tests are given in terms of comparative values in which numeral represents that substantially no rust is formed while numeral 4 represents that a few pit corrosion areas could be observed.

As is apparent from the results tabulated in Table 2, the inventive steel plate exhibits a significantly improved corrosion resistance as compared to that of the conventional surface treated steel plates, both in the salt spray test and the ferroxyl test.

As previously stated, the primary industrial application of the inventive steel plate is as can stock and since can making material is customarily lacquered or varnished by an organic coating prior to use, tests were conducted for the purpose of determining the chemical resistance of a can made from the inventive steel sheet and containing a top coat of organic material. Comparison tests with the prior art materials were also conducted. The results are tabulated in Table 3 and clearly indicate that the inventive steel plate has significantly improved characteristics as compared to those of the prior art materials.

TABLE 3 1. Lacquering with organic composition. Inside wall of can; phenol type varnish; baking at 200 0., minutes; amount 3540 up to pit rust formation). (4) 4% sulfuric acid immersion test hrs--. 3 hrs.

at room temp. (time up to pit rust formation). (5)- 0.4% caustic soda boiling test (time up to put rust formation) (6) 10% CuClzimmersion test, room First Second.

temp., 5 min. (7) Ferroxyl test, min- .do Do.

min 35 min.

filgorn 0.025 micron chromium plating and 0.005 mg./dm. chromate The above results are further discussed as follows: In a prior art steel plate which is directly subjected to electrolytic chromate treatment without prior interposition of a chromium layer, the adherence between the organic lacquer coating and the chromate film is satisfactory. By contrast, however, the adherence between the chromate film and the steel surface is poor. Particularly, when immersed in citric acid and sulphuric acid, these acidic liquids penetrate through the organic lacquer film into and between the chromate film and the steel surface, thereby causing corrosion. This corrosion progresses until the organic lacquer coating peels off.

By contrast, in the inventive steel plate a metallic chromium layer is positioned underneath the chromate film and this metallic chromium layer has excellent corrosion resistance. The metallic chromium layer therefore acts as a strong protecting barrier or wall against corrosion of the steel surface proper and thus prevents corrosion even if the acidic liquid should penetrate through the lacquer coating and the chromate film. The bondor adherence between the steel surface an the metallic chromium layer is very strong and, furthermore, the adherence or bond between the metallic chrominum layer and the superimposed chromate film is excellent. As a result, therefore, the metallic chromium film acts as a medium for enhancing the adherence between the chromate film and the steel surface, thus preventing penetration of acidic liquor onto the steel surface, thereby improving the total corrosion resistance of the product.

Comparative corrosion tests were also conducted in respect to cans which had been produced from the inventive steel plates and which were provided with a top coat of organic composition. Comparison tests with prior art steel plates which had been first chromium plated and thereafter chromate treated by dipping or spraying were also conducted. The results are shown in the following Table 4.

TABLE 4 Sulfuric acid Citric acid test, in 5% Thickness Adhesion test, in 1% H2804 sol. of chromium of organic citric acid at room layer lacquer sol. boiling temp., 48 (micron) coating for 5 hrs. hrs.

Directly chromate 0 a: C 0

treated steel plate. Steel plate, 0.05 a: A B chromium 0.03 z B B plated and 0.01 z B B chromate 0.005 a: C B tireated by 0. 001 .2: D C

ping. Inventive steel 05 z A B 0.03 z A B plate having 0 01 z A B 0.05 mg./dm. 0 6 z A B chromate film. 001 I A B Under 0.001 :2 B 0 Electric tin plate 1/ B 0 Exposure test, exposed Salt spray test, 48 hrs. Soy test, in soy on a rooftop for 10 at room temp. at 75 0.,

5 days days (3 rainy days incl.)

All of the test pieces were tested at a drawing rate of 5 mm. (Erichsen value) after they had been coated with an organic oil varnish of 10 micron thickness. Thevalues in Table 4 have the following meaning: A represents no damage, B represents pit corrosion in the convex portion of the test piece; C represents peeling off over substantially the entire convex portion of the test piece; D represents corrosion over the whole surface; x indicates that no peeling of the lacquer coating took place; y means slight peeling; and B indicates rust formation in the ring portion of the Erichsen test.

It will be notedthat in a chromium plated steel plate which, was subsequently chromatetreated by dipping, a lowering of the corrosion resistance was still observed if the, plating thickness was 0.005 micron-0001 micron. By cont astrin-the inventive steelplate the corrosion resistance is not affected down toa chromium layer thickness of 0,001. For this reason, the lower limit for the thickness of the metallic chromium layer. should not be below 0.001 micronwhilethe upper limit should not exceed 0.1 mieron. ,As previously stated, a lower limit of 0.0016 thickness .is, i howeyer,.preferred from a practical and processt'echnicalpointof' view. If the metallic chromium layer thickness ii eds J1- micron theprocessing of the can stock info cans is negatively/affected. and inferior prod- Pe are o tai ed... I f,lt lie rnetallic chromium layerh'a's a thickness range of 0115,30. f0.-l,..rn'icroh, the steelfplate, as is clear from Table 2, exhibits'a better corrosion resistance in nonlacquered statewthan. he, steel plate, corresponding to U.S.

ganic'compo'sition.

In the event that the chromium layer is thicker, cracks have a tendency to appear during the manufacturing process' into pans because, as is known, metallic chromium has a very significant hardnessrThis crack formation is counteracted in the inventive steel plate by the presence of theelectrolyticallydeposited chromate film which cov- 'e'r's"the entire surface of the chromium layer including the pin hole portions. Since the'chromate film is relatively soft, any crack formation is avoided during bending and processing in gener'al.

The inventive steelplates can be easily processed and wtirked, 'and,as"demonstrated above, have superior cor- "rosion' resistance characteristics. It" therefore provides a superior material, particularly for can stock, which is -produced at considerably lower cost than ordinary tin *platesJ'A wide variety "of cansand'can-like products such as food cans, oil cans, detergentcans, crown caps, screw "caps, etc: may'besuccessfully manufactured from the surface-treated:steelmaterial;

It2should. also be noted that -the inventive steel plates thave remarkable characteristics .in that they are not susceptible to sulphur stain. In this connection it will be appreciated that sulphur-stain yisyadreaded phenomenon taking place in tin plates. Sincethe inventive plates have better corrosion resistance non-lacquered state than ordinarychromium platedst eeljsheets, the inventive steel platesfean-be economically' produced even compared to the prior art chromium plated steels, as'a thinner layer ofchromiumis in most cases sufficient. In addition, of abana-me inventive plates have improved coating adher- 'ence '-and -corrosion -resistance, "particularly if compared with prior art .chromate treated steel.

-According to afurtherfeature of the .invention, the in- "ventivei procedure is.particularly suitable for surface treating=matte iinishedsteel sheets-of the nature disclosed in patent applicationSeri No. 610,025, filed Jan. 18,

1967. Thus, the inventive'procedure is applicable for ob- -;taining scratch-resistant. white silver'chromiumplated steel plates of the nature disclosed in said application wherein the steel plate has been matte finished. In accordance with this invention, the chromate film is then electrolytically superimposed on the chromium film. The surface roughness of the matte finished initial steel plate may be 10 EXAMPLE 1. Pretreatment A cold rolled steel sheet of 0.27 mm. thickness is subjected to electrolytic degreasing in a 5% caustic soda solution at C. and at a current density of 20 A./dm.'. The electrolytic degreasing treatment is carriedout for 2 seconds, whereupon the sheet is washed. The sheet is then subjected to acid pickling in a 10% sulphuric acid solution for 2 seconds whereupon further washing is effected.

2. Chromium plating The thus cleaned steel sheet is then employed as the cathode in an electrolytic circuit, the electrolytic bath containing 200 g./l. of chromic' acid and 20 g./l. of sulphuric acid. The electroplating is carried out at 50 C. and with a current density of 40 A./dm. for 1 second.

3. Chromate layer formation After washing, the chromium plated sheet is employed as the cathode in an electrolytic circuit, the electrolytic bath being a chromate treating liquor containing 50 g./l. of chromic acid and 2 g./l. of sodium silicofluoride. The temperature of the bath was 40 C. and the chromate deposition was effected under a current density of 5 A./ dm. for 2 seconds. The thus electroplated sheet was then washed, dried and oiled.

4. Organic coating The inventive steel sheet may be lacquered or varnished with an organic coating composition which may, for example, be phenol based, oil based, alkyd-based, vinyl resin based or epon or epoxy based. Varnishes and lacquers as disclosed, for example, in U.S. Pat. 3,113,845 may be used. It will be appreciated that generally any suitable coating composition used as top layer in cans may be employed for the inventive purposes, the various coating compositions being used either singly or in combination.

Synthetic resin coatings of the vinyl type, phenol type and polyethylene type may be used as lacquers with or without adhesives.

Recently, a one-step procedure has been proposed according to which a steel sheet is cathodically plated in an aqueous bath of chromic anhydride containing trivalent chromium ions and a sulphuric acid radical in specific amount. It is alleged that this procedure results in the formation of an intermediate metallic chromium layer and a non-metallic chromate top layer. This procedure has been disclosed in U.S. Pat. 3,296,100. Since from a practical point of view it is at the present state of analytical knowledge not possible to distinguish between a metallic chromium layer and a superimposed chromate layer, if the layers are extremely thin, the alleged result cannot be confirmed or vertified and is, in fact, questionable. In any event, the present procedure is quite different from that disclosed in U.S. Pat. 3,296,100.

As the composition of the treating liquids and "the process conditions are different and in accordance with the present procedure, the chromium layer and the chromate film are applied-in two separate steps while according to the process referred to both layers are allegedly formed in the same step, different results are obtained. Moreover, according to U.S. Pat. 3,296,100, the treating solution contains a sulphuric acid radical while according to the present procedure the chromate treatment must be effected in the absence of such radicals in order to avoid the deposition of metallic chromium.

As set forth, it is presently not possible to identify the chemical composition of the layers allegedly formed according to U.S. Pat. 3,296,100 since these layers are extremely thin and amorphous. However, since the bath compositions are different, it must be reasonably assumed that the chemical composition of the layers in the U.S. patent referred to and in the present steel sheet are dis- TABLE Steel late Steel plate as sur ace surface treated treated by according to present U.S. patent Immersion solution invention 3,296,100

Ion exchange water 0. 37 3% NazCOa 0.71 3% NaCl 0.15 50% ethyl alcohol Tr. 0.60

Tr. represents 0.01 p.p.m.

The significant ditferences in the amount of dissolved chromium as appearing in Table 5 indicate that there is an essential difference in the chemical composition of the top layers.

In this connection it should again be emphasized that the thickness of the chromate film as disclosed herein is critical and that the thickness of this film according to the present procedure is considerably greater than that of the U.S. patent. As is clear from this figure of the U.S. patent, when both a metallic chromium layer and a chromate top layer are simultaneously formed in a one-step procedure, as alleged in the U.S. patent, the thickness of the metallic chromium layer must have a certain ratio in respect to that of the chromate film. This ratio is 2 mg./dm. (0.0275 micron to 1.8 mg./dm. Although this ratio may vary slightly, it is not possible to produce a surface treated steel plate having a thickness ratio which is essentially different from the indicated ratio. Based on this ratio, if the thickness of the top layer is less than 0.1 mg./dm. the thickness of the metallic chromium layer is always less than 0.0015 micron.

Since the process of the present invention is a two-step process, it is thus feasible to apply a thicker metallic chromium layer (0.0016-0.1 micron) and a thinner top layer (0.1 mg./dm. Cr). Experiments have indicated that in the present procedure the best results are obtained if the electrolytic chromate deposition is carried out for 0.5-2 seconds only. Longer periods than 2 seconds should be avoided.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. Can stock consisting essentially of a thin-gauge steel sheet, said steel sheet having directly electro-plated thereon a metallic chromium layer of a thickness not exceeding about 0.1 micron, and a chromate film electrolytically deposited on said chromium layer, said chromate film having a thickness of not more than about 0.1 mg./dm. said can stock having the following characteristics:

(at) its plated surface exhibits metallic lustre, as distinguished from a bluish or'gray tint;'

(b) its plated surface does not discolorupon'heating in air at temperatures up to 300 C.; (c) its plated surface does not form rust'spots for'at least 20 hours if subjected to'thesalt spray corrosion test pursuant to JIS Z-237l; Y T

(d) the plating does not crack if the can stock is manufactured into square'cans,

(e) organic coatings adhere to it' without peeling;

(f) trace amounts of chromium of the order of'- 0.0l

p.p.m. dissolve when'3 dmfiof the can'stock'is immersed in an aqueous solution containing"3-%' -of Na CO 3% of NaCl and 50% of ethyl alco'holfor 24 hours at a temperatureof 50 C.; and (g) its plated surface exhibits lacquerability and paintability after boiling for 3 hours in watercontaining 1% of citric acid upon extrusion at an'Erichsen'value of5mm.

2. Can stock as claimed in claim 1 wherein the thickness of the chromium layer is at least about 0.0016 micron.

3. Can stock as claimed in claim 1, wherein said chromate film is deposited on said chromium layer from'a chromium containing treating liquid devoid of a sulphuric acid radical catalyst inducing metallic chromium deposition.

4. Can stock as claimed in claim 1, wherein said chromate film essentially consists of trivalent chromium oxide and its hydrates.

5. Can stock as claimed in claim 1, wherein said chromate film essentially consists of trivalent chromium oxide and its hydrates and of hexavalent chromium oxide and its hydrates.

6. Can stock as claimed in claim 1, wherein an organic coating is superimposed on said chromate film.

7. Can stock as claimed in claim 1, wherein said chromium layer is plated onto a steelplate whichhasbeen matte finished.

8. Can stock as claimed in claim 1, wherein said matte finished steel plate has a surface roughness of about between 0.8-3 microns in terms of Hr.m.s. v

9. Can stock as claimed in claim 1, wherein said chromate film is essentially free of metallic chromium.

10. Can stock as in claim 1, wherein said'chromate film covers and adheres to substantially. the entire.- chromium layer. Y r

References Cited 1 UNITED STATES PATENTS" 2,746,915 5/1956v Giesker 204+56 2,998,361 8/1961 Kitamura '204--56 3,113,845 10/1963 ,Uchida 204--.41 3,257,295 6/1966 Yonezaki .204-56 3,288,691 11/1966 Yonezaki, 2041-56 3,245,885 4/1966 Asano 2 204-756 3,296,100 1/ 1967 :Yonezaki 204--56 3,296,106 1/1967 Smith ...-....;...a 204-56 3,316,160 4/1967 Uchida 204-+41 3,479,260 11/1969 Rauch 204-166 3,519,542 7/ 1970 Inventofls) v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION August 6, 1974 Patent No. 3 827,866 Dated Hiromn Uchida and Osamu Yanabu,

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as "shown below:

In. the'I-Ieading of the Patent, insert:

--C1aims Priority applications Japan March 26, 1966, sho 41-18522 November ll 1966 sho 4l-74l47--.

Signed end sea led this 19th day of November 1974.

I 3 (SEAL) I Attest:

C. MARSHALL DANN McCOY M. GIBSON JR.

Commissioner of Patents Attesting Officer 1 FORM PO-IOSO (10-69) USCOMM-DC OOSIB-PGQ w u.s. GOVERNMENT PRINTING onlc: nu o-al-su,