US3755091A

US3755091A - Process for reducing discoloration of electrochemically treated chromium plated ferrous metal strip

Info

Publication number: US3755091A
Application number: US00834872A
Authority: US
Inventors: L Austin
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1969-06-19
Filing date: 1969-06-19
Publication date: 1973-08-28
Anticipated expiration: 1990-08-28
Also published as: CA933484A

Abstract

Discoloration of ferrous metal strip electroplated with metallic chromium in a continuous electroplating line at high line speeds and then electrochemically treated to deposit a nonmetallic chromium containing film thereover is reduced by intimately contacting the metallic chromium plated strip with an aqueous solution of a water soluble hexavalent chromium compound prior to the electrochemical treatment. The aqueous solution preferably has a composition which is substantially the same as the chromium electroplating electrolyte. The invention is especially useful in producing tin free ferrous metal container stock having a composite protective coating including an undercoating of 0.1-0.5 microinch of metallic chromium and an overcoating containing chromium oxide.

Description

United States Patent Austin PROCESS FOR REDUCING STRIP 75 Inventor: Lowell w. Austin, Weirton, w. Va.

[73] Assignee: National Steel Corporatio Pittsburgh, Pa. I

' 221 Filed: June 19,1969

[2]] Appl. No.: 834,872

[52] 1.1.8. Cl..... 204/35 N, 204/35 R, 204/51 [51] Int. Cl...... C32b 11/00, C23b 5/48, C23b 5/06 [58] Field of Search 204/35 R, 36, 41, 204/35, 51

[56] v References Cited UNITED STATES PATENTS 3,642,587 2/1972 Allen et a1. 204/35 N 2,438,013 3/1948 Tanner 204/35 X 2,746,915 5/1956 Giesker 204/56 3,113,845 12/1963 Uchida et al. 204/41 X 3,216,912 11/1965 Shoemaker 204/35 X 3,296,100 1/1967 Yonezaki et al 204/41 3,323,881 6/1967 Nelson et a1 29/1835 3,498,892 3/1970 Seyb, Jr. et al. 204/51 X 3,526,486 9/1970 Smith et al 204/41 3,316,160 /1967 Uchida et a1. 204/28 3,567,599 /l 971 Carter et a1 204/41 Belgium [11] 3,755,091 1 Aug. 28, 1973 OTHER PUBLICATIONS Smith, E; .l., Chromium Coated Steel For Container Applications, prepared for presentation at 75th GeneralMeeting of Amer. Iron and Steel Institute, in New York, May 25, 1967 Graham, A. K. Electroplating Engineering Handbook (1st ed.) 1955, Reinhold Pub. Corp. Baltimore, Md,

.1. of Electrochem. Soc., March 1969, Fukuda et a1. (presented Oct. 1967) Y Priniary Examiner-F. C. Emundson Attorneyl-Shanley and ONeil 57 ABSTRACT solution preferably has a composition which is substantially the same as the chromium electroplating electrolyte. The invention is especially useful in producing tin free ferrous metal container stock having a composite protective coating including an undercoating of 0.1-0.5 microinch of metallic chromium and an overcoating containing chromium oxide.

11 Claims, N0 Drawings BACKGROUND OF THE INVENTION This invention broadly relates to a process for reducing discoloration of ferrous metal strip electroplated with metallic chromium in' a continuous high speed chromium electroplating line. In one of its more specific variants, theinvention is concerned with the preparation of thin gauge tin free ferrous metal strip useful as'a container stock.

Tinplate is widely used in manufacturing containers for storing and preserving foods and beverages. However, in view of the high cost of tin and the lack of a dependable source of tin in this country, it is desirable to provide tin free corrosion resistant ferrous metal strip or sheet for use as a container stock.

inasmuch as a ferrous metal surface is subject to rapid corrosion when exposed to the atmosphere, and especially in the presence of moisture or at elevated temperature, some type of an initial protective coating must be applied at the time of manufacture. The initially protected'surface should be receptive to the usual organic coatings that are applied to substrates such as blackplate and tinplate, includingpaints, varnishes, lacquers and enamels, to thereby provide relatively permanent protection against corrosion. The ferrous metal surface should also be receptive to organic adhesives used in joining the side seams of containers. in view of these requirements, a satisfactory tin free container stock should have a surface with a combination of desirable characteristics, including the following:

1. The ferrous metal surface should have aninitial protective film or coating which retards corrosion from the time of manufacture of the strip or sheet up until it is organically coated and is ready to be used for preparing containers.

2. The initially protected ferrous metal surface should have a pleasing bright silver to light grey appearance, as distinguished from a discolored or stained appearance, as often clear organic protective coatings are applied thereover and the substrate surface is visible.

3. The initially protected ferrous metal surface must be receptive to organic protective coatings of the type usually applied to tinplate and blackplate.

4. The organically coated ferrous metal surface should have good underfilm corrosion resistance and pass the peel and sideseam adhesion tests.

5. The initially protected ferrous metal strip or sheet must also be capable of competing in price with other low cost materials as this is a highly competitive field.

One method proposed heretofore for initially protecting the ferrous metal substrate involves-electrodepositing thereon a substantially nonporous layer of metallic chromium. This method produces a corrosion re-. sistant substrate surface which is also receptive to organic finishes. However, relatively thick metallic chromium coatings are required of the order of l to 2 microinches or thicker, in' order to obtain sufficient corrosion protection prior to application of the organic coating. Chromium electroplating is very inefficient as only a small fraction of the plating current is effective in plating out metallic chromium, and thus this method has the disadvantage of relatively high plating costs.

In accordance with an improved prior art process for .the manufacture of tin free container stock, a composite coating is applied to the ferrous metal substrate which includes an extremely thin metallic chromium undercoating and a chromium oxide-containing overcoating applied by electrochemical treatment in a chromate bath. The undercoating of metallic chromium needbe only 0.l-0.5 microinch in thickness, and a chromium coating of this thickness may be electrodeposited and thereafter electrochemically treated in a chromate bath to deposit the chromium oxidecontaining overcoating in a continuous electroplating. line operating at a high strip speedylt is necessary to operate-the continuous chromium electroplating line at a high strip speed in order to further reduce the cost of applying the metallic chromium undercoating. When efforts were made to increase the strip speed above about 500-600 feet per minute, it was observed that the nonmetallic chromium-containing film which is normally present in small amounts on metallic chromium plated, ferrous metal substrates increases in thickness at a rate which varies directly with increasing line speed. It was further observed that at strip speeds of approximately 600 feetper minute and higher, i.e., at levels of nonmetallic chromium in the film of about 0.45-0.5 milligram per square foot per side and higher, the surface of the ferrous metal substrate has a brownish discoloration which becomes more pronounced with further increases in line speed. As a result, the appearance of the electrochemically treated chromium plated ferrous metal strip is less pleasing. Heavy nonmetallic chromium containing films applied during the electrodeposition of the metallic chromium coating also adversely affects the. stability-of the lubricant applied. during oiling of the final product, luster, and ana-- lytical results in control tests for determining the amount of nonmetallic chromium deposited during the electrochemical treatment step. Prior to the present invention, it was not possible to operate a chromium electroplating line at high speeds above about 600 feet per minute and produce an entirely satisfactory electrochemically treated chromium plated ferrous metal strip which is satisfactory in appearance and free of the other disadvantages mentioned above.

It is an object of the present invention to provide a novel process for reducing discoloration of ferrous metal strip electroplated with metallic chromium in a continuous plating line at high line speeds.

It is a further object to provide a novel process for reducing discoloration when electroplating ferrous metal strip with metallic chromiumin a continuous plating line, and thereafter electrochemically treating the chromium plated strip to deposit a chromium oxidecontaining film thereon.

It is still a further object to provide a novel process for reducing discoloration when preparing tin free ferrous metal strip useful as a container stock in a high speed continuous electroplating line wherein a composite coating is applied which includes an undercoating of 0.1-0.5 microinch of metallic chromium and an overcoating containing chromium oxide.

Still other objects and advantages of the invention will be apparent upon consideration of the following detailed description and the examples.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTS THEREOF In accordance with the present invention, ferrous metal strip is electroplated with chromium in a continuous chromium plating line at a high line speed which results in staining or discoloration of the metallic chromium plated surface. The discolored chromium plated ferrous metal strip is intimately contacted with an aqueous solution of a water soluble hexavalent chromium compound immediately after plating and prior to washing in fresh water or drying of the chromium electroplating electrolyte thereon. The plated strip is contacted with the solution in the absence of electrolytic treatment and for a sufficient period of time to reduce the intensity of the discoloration. Thereafter, the metallic chromium plated strip may be electrochemically treated in an aqueous electrolyte containing a water soluble hexavalent chromium compound to deposit thereon a' corrosion resistant film which contains chromium oxide. As will be discussed in greater detail hereinafter, there are certain preferred materials, procedures and conditions which may be employed to produce superior results in the above steps of the invention.

Suitable prior art chromium plating baths may be used for electrodepositing the metallic chromium coating which is initially deposited on the ferrous metal strip. Prior art chromium plating baths usually contain chromic acid, alkali metal dichromates, or other water soluble hexavalent chromium compounds in an amount of about 50-450 grams per liter and preferably about 100-250 grams per liter, and a catalyst such as sulfate ion, fluoride ion, silicofluoride ion, or combinations thereof. The catalyst may be added as a free acid producing the desired ion, such as sulfuric acid, or as a water soluble salt of the free acid such as the alkali metal compounds thereof. The molar ratio of the chromium' compound to the catalyst is usually between about 40:1 and 200:1, and is preferably about 100:1, but any suitable prior art catalyst concentration may be employed. One suitable electrolyte for electrodepositing metallic chromium in high speed lines contains per liter 130-170 grams and preferably 150 grams of chromic acid, 1.0-1.4 grams and preferably 1.2 grams of sulfate ion, and 1.6-5 grams and preferably 1.8 grams of silicofluoride ion. The current density also may be the same as is used in prior art chromium plating, and may be, for example, about 300-2,000 amperes per square foot, and I is preferably about 500-l,000 amperes per square foot. The temperature of the chromium plating bath may be in accordance with prior art practice and may be about 100-150F., and is preferably about l-130F. A plating bath temperature of approximately 120F. usually gives the best results.

Insoluble anodes may be used, and preferably a composite steel anode having a lead coating on the side facing the ferrous metal substrate to be plated, and polyvinyl chloride or other inert insulating substances on the opposite side.

The surface of the ferrous metal strip may be scrubbed free of oil, grease, dirt and other surface contaminants by a prior art process prior to electroplating with the metallic chromium. For example, the ferrous metal strip may be given a wet pretreatment by electrolytically treating in an alkaline electrolyte, washing in water, pickling in sulfuric acid, washing in water, and drymg.

While the above plating baths and conditions are satisfactory, it is understood that still other prior art chromium plating baths may be used. Examples of other chromium plating baths and conditions are disclosed in United States Pat. Nos. 1,942,469, 2,177,392 and 2,415,724 and in the text Modern Electroplating, edited by Fredrick A. Lowenheim, Second Edition, John Wiley & Sons, Inc., New York, New York, 1963. Chapter 5, pages -140 and the references cited on pages 128-140 are especially pertinent as this portion of the test is directed to chromium plating. The teachings of I this text and the cited references are incorporated herein by reference.

The ferrous metal strip is electroplated with metallic chromium in a continuous prior art electroplating line which operates at high line speeds. Suitable continuous high speed electroplating lines are well known and may be used in practicing the-present invention when employing an electrolyte in the plater as described above for the electrodeposition of the metallic chromium coating.

The electroplating line is operated at a sufficiently high line speed to cause visible discoloration of the metallic chromium plated strip surface due to simultaneous deposition thereon of a nonmetallic chromiumcontaining film. Usually, this occurs at line speeds of about 600 feet per minute and higher, and his especially pronounced at line speeds of about 1,000 to 2,000 feet per minute. As a general rule, a line speed which results in a nonmetallic chromium content in the film of at least 0.5 milligram per square foot per side of strip surface will give discoloration, and when the nonmetallic chromium content reaches about 0.6-0.7 milligram per square foot per side and higher,- the discoloration is very noticable. 1n instances where the nometallic chromium content in the film is about 0.9-1.0 milligram per square foot per side and higher, which usually occurs at line speeds of 1,000-2,000 feet per minute, then the product is often considered to be unsatisfactory for use as a container stock in many commercial applications.

The thickness of the metallic chromium coating may be in accordance with prior art practice. Usually metallic chromium coatings of 10 microinches and less are electrodeposited at high strip speeds which give staining, and preferably less than 4 microinches. Most container stocks have metallic chromium coatings of l-3 microinches or less.

The invention is especially useful in preparing ferrous metal container stock which has a composite coating including an extremely thin undercoating of metallic chromium, such as 0.1-0.5 microinch. Unsatisfactory product performance is achieved due to insufficient metallic chromium at coating thicknesses less than about 0.1 microinch, and metallic chromium coatings above 0.5 microinch are more expensive to apply and add appreciably to the cost of manufacturing tin free steel for container stock due to the inefficiency of the chromium plating process. The best results are usually achieved by maintaining the metallic chromium coating at a thickness of about 0.1-0.3 microinch. A metallic chromium coating having a thickness of about 0.1 microinch is satisfactory for container stock to be used in the preparation of containers for the storage and preservation of beer and the less corrosive beverages and food stuffs, whereas metallic chromium coatings of 0.3 microinch and above give better results when storing and preserving the more corrosive food stuffs and beverages.

Thin metallic chromium coatings having thicknesses of 0.1-0.5 microinch do not provide satisfactory initial protection against corrosion of the ferrous metal substrate in the absence of further treatment. As a result, it is necessary to electrochemically treat the thin coatings to render them satisfactory for container stock and thereby increase the corrosion resistance prior to application of a permanent coating such as an organic coating. The corrosion resistance of heavier chromium coatings also may be improved by electrochemical treatment or other desirable properties may be imparted thereto. However, the discoloration on the metallic chromium coated strip surface must be reduced to an acceptable level prior to the electrochemical treatment as otherwise an unsatisfactory final product will be produced. The term discolored film or stain as used herein is intended to embrace all undesirable films deposited in the chromium electroplating zone simultaneously with the metallic chromium coating.

It has been discovered that the discoloration'or stain on the metallic chromium plated ferrous metal strip may be reduced markedly by intimately contacting it with an aqueous solution containing a water soluble hexavalent chromium compound. Preferably the solution' is contacted with the discolored or'stained strip at a level of electrolytic treatment below the threshold of metallic chromium deposition. The chromium compound may be chromic acid, alkali metal chromates and dichromates, or other suitable water soluble hexavalent chromium compounds. The solution preferably contains the hexavalent chromium compound ina concentration which approximates the ranges givenherein for the chromium plating electrolyte, and has a pH value of about 2 or less and about 0.5-1 for better results. In one imporatant variant, the solution has substantially the same ingredients and/or concentrations of ingredients as exist in the chromium plating electrolyte. In another important variant, the metallic chromium plated strip is treated in the chromium electroplating electrolyte subsequent to the metallic chromium electrodeposition step. In this latter variant, the electrolyte is preferably contacted with the discolored plated strip in the absence of electrolytic treatment and prior to withdrawing the plated strip from the chromium plating electrolyte.

The temperature of the solutionof hexavalent chromium compound to be contacted with the. strip may have approximately the same temperature as the electrolyte, but higher temperatures up to the boiling point may be used to reduce the period of treatment. For instance, the temperature may be about l00-l90F.,

' and often is preferably about l00-l30l-".

The aqueous solution of the hexavalent chromium compound is contacted with the discolored strip for a sufficient period of time to reduce the intensity of the discoloration and to lower the nonmetallic chromium content in the film deposited simultaneously with the metallic chromium to a desirable level. This may vary reduced to approximately that which normally eiiists' at plating speeds below 500. feet per minute, such as 0.3-0.4 milligram or less per square foot per side.

The period of treatment necessary to achieve the above mentioned levels of nonmetallic chromium in the film will vary somewhat. However, treatment for at least 0.3 second, and preferably for at least 0.5 second usually gives satisfactory results. The upper limit on the period of treatment is practical in nature as the solution does not'attack the metallic chromium coating and treatment periods of several minutes in duration do not adversely affect the product. However, treatment for longer than about 1 or 2 seconds is usually not preferred in high speed plating lines. I

The discolored strip is treated with the solution of the hexavalent chromium compound immediately after it has been electroplated with the metallic chromium, and prior to washing with fresh water or drying of a film of the metallic chromium plating electrolyte thereon; Otherwise, satisfactory results usually are not achieved. Agitation is not necessary, but may be used if desired. Satisfactory'results are obtained bypassing the discolored strip through a bath of the treating solution at the existing line speed. The rapidly moving strip seems to induce sufficient agitati'onin the vicinity of the strip surface to cause the nonmetallic chromium film to be somewhat from product to product, but as a general I rapidly dissolved. v

After reducing the discoloration of the strip, it may be given an electrochemical treatment in an aqueous solution of a water soluble hexavalent chromium compound such as chromic acid, alkali metal dichromates including sodium or potassium dichromate, and other water soluble hexavalent chromium-containing compounds. Chromic acid is usually preferred, and satisfactory baths may contain about 20-50 grams per liter and preferably about 30 grams per liter of chromic acid, or about 0.5-2 ounces, and preferably about l ounce per gallon of sodium or potassium dichromate. If desired the bath may alsocontain a small amount of sulfate ion (80,"), fluoride ion (F'), silicofluoride ion (SiF,' and mixtures thereof, such as up to about 0.5 grams per liter and preferably about 0.1-0.4 grams per liter. One very satisfactory electrochemical treatment bath contains per liter 30-40 grams and preferably about 35 grams of chromic acid, 0.05-0.2 gram and preferably about 0.1 gram of sulfate ion (80,"), and 0.3-0.6 gram and preferably 0.4 gram of silicofluoride ion (SiF- The electrochemical treatment bath may be operated often ata rate of about 200 amperes per square foot for- 0.5 second to thereby provide about coulombs per square foot of current, using a bath temperature of about F.

As a general r ule, about 0.6-5.0 milligrams per square foot per side of total chromium should be present in the film that is deposited in'the electrochemical treatment step. The cathodic electrochemical treatment results in the deposition of a chromium oxidecontaining film which also contains metallic chromium in instances where a small amount of a chromium plating catalyst is present. The chromium content of the chromium oxide seems to be largely in the plus 3 valence state, and may include hydrated Cr,O Preferably, the film should contain about 1.0-1.5 milligrams of nonmetallic chromiumper square foot per side in the chromium oxide content of the film.

The electrochemical treatment is especially useful as a post treatment for metallic chromium coatings having a thickness of about 0.1-0.5 microinch as it markedly improves the corrosion resistance. It is thought that for some unexplained reason the metallic chromium content of the electrochemically deposited film tends to seek out the pores in the initially deposited metallic chromium film, and thereby renders it less porous. In addition to this, the chromium oxide content of the electrochemically deposited film tends to cover and further protect the initial metallic chromium coating. A surface film or coating is also produced on the ferrous metal strip which renders the surface highly receptive to prior art organic coatings.

Organic coatings of the type, usually applied to tin free steel container stock or tinplate to further improve the corrosion resistance and to provide relatively permanent protection against corrosion may be applied over the electrochemically treated surface. While the invention is not limited thereto, examples of suitable organic coatings including phenolic, modified phenolic, epoxy, modified epoxy, vinyl resin, teflon, and drying oil based paints, varnishes, lacquers and enamels. The electrochemically treated surface also forms an excellent base for side seam adhesives which are sometimes used in the manufacture of containers.

The present invention is especially useful for produc- EXAMPLE Coils of 55 pound double reduced blackplate are cleaned by a prior art procedure to remove grease, dirt and other foreign matter from the surface. The treatment includes cathodic and anodic electrolytic treatment in a caustic cleaner (Orthosil), a rinsing with water, pickling in dilute (2-3percent) aqueous sulfuric acid, scrubbing and rinsing with water and drying. The clean strip is then passed continuously through a series of vessels in a prior art high speed electroplating line wherein it is electroplated with metallic chromium, passed through a body of a solution of a hexavalent chromium compound to reduce discoloration, electrochemically treated to deposit a chromium oxidecontaining film, rinsed with water, dried, oiled and coiled.

The electrolyte for the chromium plating bath contains 150 grams per liter of chromic acid, 1.2 grams per liter of sodium sulfate calculated as sulfate ion (804), and 1.8 grams per liter of sodium silicofluoride calculated as silicofluoride ion (SiFJ). The temperature of the electrolyte is maintained at 120F., and the current density at about 800-l,000 amperes per square foot. The anodes in the chromium plater are lead coated steel on the surfaces facing the strip, and are coated with polyvinyl chloride on the opposite side thereof. The amount of metallic chromium deposited on the strip is 0.3 microinch.

The metallic chromium plated strip is passed through a body of a solution of a hexavalent chromium compound immediately after completing the electroplating step, and intimately contacted therewith for a period of 1 second. The solution contains the same ingredients and concentrations of ingredients as the chromium plating electrolyte to avoid introducing other substances into the system, and the temperature is 120F. The path of the strip through the body of solution is lengthened or shortened as necessary to provide the one second treatment period when making runs at varying speeds.

The metallic chromium plated strip is withdrawn from the solution of the hexavalent chromium compound and is passed into an electrochemical treating vessel filled with an electrolyte containing 35 grams per liter of chromic acid, 0.1 gram per liter of sodium sulfate calculated as sulfate ion ($0 and 0.43 gram per liter of sodium silicofluoride calculated as silicofluoride ion (SiFf). The electrolyte temperature is maintained at 120F., and the strip surface is treated cathodically for'0.4 second at 250 amperes per square foot to thereby provide a surface treatment of coulombs per square foot. The strip is passed between steel anodes coated with lead on the surface facing the strip. This treatment results in about 1.4 milligrams per square foot of chromium oxide in the film deposited on the strip. i

The electrochemically treated strip is withdrawn from the electrochemical treating vessel, rinsed in water, dried, oiled and coiled. Samples of strip are cut from the various runs and tested.

Upon making runs at strip speeds of 300, 600, 800, 1,000 and 1,300 feet per minute and observing the samples, no discoloration is noted. The sample for the run made at 1,300 feet per minute is as lustrous and as free from discoloration as the sample from the run made at 300 feet per minute.

EXAMPLE ll The general procedure of Example I is repeated with the exception of omitting the 1 second treatment of thechromium plated strip in the body of the solution of the hexavalent chromium compound. The samples are observed for discoloration as in Example I.

The sample from the run made at 300 feet per minute is lustrous and free of discoloration. However, the samples from the remaining runs are discolored with the intensity of the discoloration varying directly with the strip speed.

EXAMPLE III The general procedure of Example I is repeated with the exception of omitting the 1 second treatment of the stained metallic chromium plated strip in the solution of the hexavalent chromium compound, and subsituting therefore a one second treatment in the chromium electroplating electrolyte. The stained strip was contacted with the plating electrolyte subsequent to the chromium eletrodepo'sition step and in the absence of electrolytic treatment. The results obtained in this Example are comparable with the results reported above for Example I.

I claim:

1. In a process for preparing electrochemically treated chromium plated ferrous metal strip in which ferrous metal strip is passed through a continous elecgrams per square foot per side of chromium, the aqueous electrolyte for said electrochemical treatment con-.

- taining about 30-35 g/ l of chromic acid and a catalytic fluoride ion and silicofluoride ion, the molar ratio of electroplated strip is electrochemically treated as a cathode in an aqueous electrolyte containing about 30-35 g/ l of chromic acid to deposit thereon a nonmetallic chromium-containing film including chromium oxide, the surface of the resulting electrochemically treated metallic chromium electroplated strip having a composite nonmetallic chromium-containing film thereon containing a total of about 0.6-5 milligrams per square foot per side of chromium and being discolored, the improvement in combination therewith comprising reducing the discoloration of the electrochemically treated chromium plated ferrous metal strip by intimately contacting the surface of the metallic chromium electroplated stripwith an aqueous solution of chroinic acid, said aqueous solution of chromic acid having substantially the same ingredients and about the same concentrations of ingredients as the chromium electroplating electrolyte, the metallic chromium electroplated strip being submersed in said aqueous solution of chromic acid and being intimately contacted therewith in, the absence of substantial electrolytic treatment until the nonmetallic chromium content in said initial nonmetallic chromium-containing film is reduced to not more than 0.4 milligram per square foot per side, the metallic chromium electroplated strip being intimately contacted with said aqueous solution of chromic acid after electroplating said coating of metallic chromium thereon and prior to said electrochemical treatment, the metallic chromium electroplated strip being intimately contacted with said aqueous'solution .of chromic acid without drying said chromium electroplating electrolyte thereon and before contacting the metallic chromium electroplated strip with amount of a catalyst selected from the group consisting of sulfate ion and admixtures thereof with at least one additional substance selected from the group consisting of fluoride ion and silicofluoride ion.

2. The process of claim 1 wherein the ferrous metal strip is electroplated with a coating of metallic chromium having a thickness between about 0.1 microinch and- 0.5 microinch, the said aqueous solution of chromic acid is chromium electroplating electrolyte and the metallic chromium electroplated strip is contacted therewith over a period of at least 0.3 second, the nonmetallic chromium content in said initial nonmetallic chromium-containing film is reduced to about 0.3-0.4 milligrams per square foot per side, and said composite nonmetallic chromium-containing film' contains not more than 1.5 milligrams of chromium per square foot per side.

3. The process of claim 2 wherein the metallic chromium electroplated strip isintimately contacted with the said aqueous solution of chromic acid over a period between=about 0.3 second and 2 seconds.

4.'Theprocess of claim '3 wherein the strip speed is at least 1,000 feet per minute, said initial nonmetallic chromium-containing film contains at least 0.6 milligram'of chromium per square foot per side, and said composite nonmetallic chromium-containing film contains about 1.0-1.5 milligrams of chromium per square foot per side. H

5. The process of claim 1: wherein said chromium electroplating electrolyte contains about l ;g/l of chromic acid. e

6. The process of claim 1 wherein the electrolyte for said electrochemical treatment contains about 30 g/l of chromic acid. r

7. The process of claim 1 wherein said chromium electroplatingelectrolyte contains about l50 g/l of chromic acid and the electrolyte for said electrochemical treatment contains about 30 g/l of chromic acid.

8. The process of claim 1 wherein the ferrous metal strip is electroplated with a coating of metallic chromium having a thickness between about-0.1 microinch mium electroplated strip is intimately contacted with said aqueous solution of chromic acid until the nonmetallic chromium content of said initial nonmetallic chromium-containing film is reduced to about 0.3-0.4 milligram per square foot per side. I

'11. The process of claim 1 wherein said aqueous solution of chromic acid is chromium electroplating electrolyte.

- 'aar: a a

Claims

2. The process of claim 1 wherein the ferrous metal strip is electroplated with a coating of metallic chromium having a thickness between about 0.1 microinch and 0.5 microinch, the said aqueous solution of chromic acid is chromium electroplating electrolyte and the metallic chromium electroplated strip is contacted therewith over a period of at least 0.3 second, the nonmetallic chromium content in said initial nonmetallic chromium-containing film is reduced to about 0.3-0.4 milligrams per square foot per side, and said composite nonmetallic chromium-containing film contains not more than 1.5 milligrams of chromium per square foot per side.
3. The process of claim 2 wherein the metallic chromium electroplated strip is intimately contacted with the said aqueous solution of chromic acid over a period between about 0.3 second and 2 seconds.
4. The process of claim 3 wherein the strip speed is at least 1, 000 feet per minute, said initial nonmetallic chromium-containing film contains at least 0.6 milligram of chromium per square foot per side, and said composite nonmetallic chromium-containing film contains about 1.0-1.5 milligrams of chromium per square foot per side.
5. The process of claim 1 wherein said chromium electroplating electrolyte contains about 150 g/1 of chromic acid.
6. The process of claim 1 wherein the electrolyte for said electrochemical treatment contains about 30 g/1 of chromic acid.
7. The process of claim 1 wherein said chromium electroplating electrolyte contains about 150 g/1 of chromic acid and the electrolyte for said electrochemical treatment contains about 30 g/1 of chromic acid.
8. The process of claim 1 wherein the ferrous metal strip is electroplated with a coating of metallic chromium having a thickness between about 0.1 microinch and 0.5 microinch.
9. The process of claim 1 wherein the ferrous metal strip is electroplated with a coating of metallic chromium having a thickness between about 0.1 microinch and 0.5 microinch, the strip speed is at least 1,000 feet per minute and said initial nonmetallic chromium-containing film contains at least 0.6 mIlligram of chromium per square foot per side.
10. The process of claim 1 wherein the metallic chromium electroplated strip is intimately contacted with said aqueous solution of chromic acid until the nonmetallic chromium content of said initial nonmetallic chromium-containing film is reduced to about 0.3-0.4 milligram per square foot per side.
11. The process of claim 1 wherein said aqueous solution of chromic acid is chromium electroplating electrolyte.