Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/38—Chromatising

Definitions

• Untinned flat rolled steel stock when used for canmaking purposes, must be provided with a protective coating, as enamel, lacquer and the like, to prevent deleterious chemical reaction between corrosive can contents and the metal can body.
• a protective coating as enamel, lacquer and the like, to prevent deleterious chemical reaction between corrosive can contents and the metal can body.
• Such protective coatings are most frequently of a variety of compositions, being specific in nature and function to the intended application of the coated container, and are preferably applied to the flat metal container stock by the container manufacturer, rather than by the steel producer.
• the uncoated fiat-rolled steel is susceptible to rusting during extended periods of shipment and storage, and since rusting detracts from the desired clean, bright appearance of the steel and also deleteriously aifects adherence of subsequently applied protective coatings, it is essential that rusting be prevented and the steel delivered to the container manufacturer in a suitable condition for application and retention of the necessary protective coatings even after considerable periods of storage under humid conditions. Further, since the latter coatings are generally transparent, any treatment of the steel by the 3,532,608 Patented Oct. 6, 1970 steel producer must not detract from the desired bright metallic appearance of the finished fabricated article.
• the metal stock provided to the can manufacturer be of such a nature as to resist delamination of the subsequently applied enamel or lacquer coating during fabrication, and also that the stock resist undercutting of such overlying protective coating by corrosive action of the can contents at the sites of defects in the enamel or lacquer coating or breaks, cuts or other defects caused during container fabrication.
• tin-free steels have been made available for container manufacture and which provide, on the flatrolled steel surface, either a thin layer of electrolytically deposited metallic chromium (for example, Uchida et al. U.S. Pat. No. 3,113,845), a film of chromium-containing oxides (for example, Kitamura U.S. Pat. No. 2,998,361), or both (for example, Yonezaki et al. U.S. Pat. No. 3,296,100).
• a thin layer of electrolytically deposited metallic chromium for example, Uchida et al. U.S. Pat. No. 3,113,845
• a film of chromium-containing oxides for example, Kitamura U.S. Pat. No. 2,998,361
• Yonezaki et al. U.S. Pat. No. 3,296,100 for example, Yonezaki et al. U.S. Pat. No. 3,296,100.
• the concentration or" the bath chemicals must be low in order to avoid economic loss due to high dragout losses at the required high line speeds, e.g. 500 to 1500 feet per minute or greater;
• 3,113,- 845 teaches production of a thin metallic chromium layer on steel, such as can stock, by means of an electrolytic bath wherein the chronic acid content, as is usual in most prior art chromium plating baths, is relatively high 100 250 g./l.
• U.S. Pat. No. 2,998,361 teaches the production of hydrated chromium oxide films only, the formation of such films being promoted by the use of aromatic sulfonic acids and sulfonates, in a chromic acid-containing electrochemical bath.
• Chromium deposition in thin layers on can stock steel, has also been effected from electrochemical baths containing, as the metallic chromium-depositing promoter or catalyst, phosphoric acid, boric acid and oxalic acid (US. Pat. No. 3,032,487).
• a preferred embodiment of the invention comprises an aqueous chromium plating bath containing from about to about grams per liter (g./l.) of chromium trioxide (Cl O3), and
• CoCl -6H O cobaltous chloride
• H NSO H sulfamic acid
• a treated steel strip product having on the surface thereof a double-layered protective coating comprising a layer of metallic chromium adherent to the steel surface, and, overlying the chromium layer, a film of hydrated, chromium-containing oxide.
• the chromium layer ranges in weight from about 0.1 to about 2.0 mg. of metallic chromium per square foot of article surface, and the hydrated chromium oxide layer contains from about 0.1 to mg. of chromium values per square foot of article surface.
• the present invention provides an aqueous electrochemical bath and process by the use of which, in a sim- 4 ple, convenient, controllable, low cost operation, there is provided a surface-treated fiat-rolled steel product, suitable for can manufacture, and having deposited thereon both a metallic chromium layer and an overlying chromium-containing oxide film.
• the bath contains chromium trioxide (chromic acid), CrO as the readily available, low cost chromium-containing constituent, and in a relatively low concentration, i.e., most broadly from about 5 to about g./l., and preferably from about 15 to about 25 g./l.
• chromium trioxide chromic acid
• CrO chromium trioxide
• the electrochemical coating bath of the invention also comprises, as the only other essential constituent, as a combined metallic chromium deposition catalyst and a promoter of the rapid formation of a uniform chromiumcontaining oxide film, a material selected from the group consisting of cobaltous chloride, CoCl -6H O, and sulfamic acid, H NSO H. No pretreatment of such baths are required.
• cobaltous chloride or sulfamic acid in an amount of from about 0.25 to about 5.0 g./l., preferably from about 0.25 to about 1.0 g./l. of cobaltous chloride or from about 0.25 to about 2.0 g./l. of sulfamic acid, is provided in the electrochemical treating bath of the invention, such materials, in the course of performance of the inventive process, as hereinafter described, cause a vigorous formation of a doublelayered chromiumhydrated chromium oxide coating on the surface of the steel strip being treated.
• any surface treatment such as an electrolytic treatment, in order to provide an altered steel surface of the desired uniform properties, must be capable of fast and accurate response to such input variations.
• the action of the electrochemical baths of this invention is sufficiently fast in operation and consistent in result produced under a given set of operating conditions, e.g.
• the invention can be practiced under a wide range of conditions, to produce a usefully wide range of products, each of which is predictably consistent in properties.
• the operating conditions of the inventive process are sufficiently flexible that the invention can be practiced with existing continuous steel strip electroplating facilities with only minor changes. if any.
• steel strip e.g. plain carbon steel black plate
• pre-plating cleaning steps e.g. electrolytic alkaline treatment, rinse, cathodic acid treatment and rinse
• the strip is then introduced, as cathode, and in continuous fashion, between insoluble, e.g. lead, anodes into the inventive electrochemical bath.
• the acid pie-treatment may be omitted.
• Total treating time in the latter bath is most broadly between about 0.25 and 10 seconds, although bath residence times of 3 seconds and under are preferred for economic reasons and are sufiicient for the application of most coatings for can fabrication purposes.
• the current density applied to the cathodic strip in the treating bath may range from about 25 to about 300 a.s.f. since, below 25 a.s.f. insufiicient coating coverage is experienced, and best chromium plating efficiencies, bright metallic chromium surfaces, and optimum oxide production is not realized at current densities over about 300 a.s.f.
• Residence time of the strip in the bath and magnitude of the applied current density are selected within the above-provided ranges of each such that the total coulombic input to the strip in the bath is from about 50 to about 1000 coulombs/ft.
• the bath temperature is preferably maintained between about 70 and 190 F. in order to maintain best strip coating efficiency and to maintain a bright metallic appearance of the chromium layer.
• the steel to be electrochemically treated in the aforesaid bath may beneficially be given an anodic flash treatment prior to entry of the steel article into the coating bath. It has been found that adherence of the subsequently produced chromium oxide film is greatly improved by such treatment, thus in hibiting removal or disruption of such oxide film in the course of post-plating operations, or by rolls and the like.
• Such anodic pre-treatment is most conveniently accomplished by immersing the steel strip, as anode, in a first pass zone containing the same electrolyte as used for the following plating treatment. Such operation avoids contamination of the plating bath electrolyte and eliminates the necessity of providing a separate pre-treatment tank.
• anodic current density applied does not affect the quantity of metallic chromium or hydrated chromium oxide deposited in the electrochemical coating bath zone, and the magnitude and duration of such anodic current density application need be only sufficient to effect the aforesaid oxide film adherence enhancement.
• post-coating treating steps may also be used, if desired, to impart additional or enhanced product characteristics, such as further enhancement of rust resistance, e.g. when most dilute chromium trioxide solutions are used in the electrochemical coating bath.
• the process of the invention is productive of treated steel strip which meets all of the above-described requirements for use in the fabrication and use of food and beverage cans and the like.
• the double-layered coating applied in the electrochemical coating bath consists of a layer of metallic chromium deposited on the black plate surface and a film or layer of hydrated chromium oxides overlying the metallic chromium layer.
• the metallic chromium layer so produced ranges in weight from about 0.1 to about 2.0 mg. of chromium per square foot of article surface, and the overlying hydrated chromium oxide film ranges in weight from about 0.1 to about 30 mg. of chromium values (Cr,+++) per square foot of article surface.
• Preferred coatings consist of those wherein the metallic chromium layer is from about 0.20 to about 1.5 mg. of chromium per square foot and the hydrated chromium oxide layer contains from about 2.0 to about 15.0 mg. of chromium values per square foot.
• the most useful coatings are those wherein the hydrated chromium oxide film is so thin, generally under about mg. of chromium per square foot, that the film produces an interference color on the surface of the treated article, imparting to the article a most pleasing and beautiful appearance.
• oxide film thickness By varying the oxide film thickness, a wide range of colors can be produced. Color change is very sharp and sensitive to oxide film thickness, an oxide film thickness change of only about 2.0 mg. Cr'-]-l/ft. causing a change in color from deep violet to light blue.
• the same appearance can be imparted to each coil of steel treated, regardless of its original surface reactivity, by merely adjusting the coulombic treating level imparted to each coil until the desired color intensity, corresponding to a certain chromium oxide film thickness, is obtained.
• the desired color can be produced by detecting color changes in the treated strip either visually or by suitable optical instrumentation, and coulombic input to the strip in the electrochemical coating bath can then be varied manually or automatically, in accordance with the detected color changes.
• These colors being interference colors, disappear after application of the final lacquer coating by the can manufacturer and do not then alter the metallic appearance of the metallic chromium layer as seen through a ransparent lacquer coating.
• Example 1 Commercially produced, double cold-reduced plain carbon steel of 60 pound basis weight gauge (6.7 mils thickness), was given a preplating processing common to tinplating quality black plate (except for acid pickling), i.e., it was electrolytically cleaned in alkaline solution and water rinsed. It was then given an anodic flash, at a current density of 170 a.s.f. for 0.67 second (114 coulombs/ HP), in an electrochemical treating tank (containing the same electrolytic bath as given below). The cleaned strip was then treated in an aqueous electrochemical coating bath as follows:
• Process element Element specification Chromium trioxide g./l 20 Sulfamic acid g./l 1.0 Cathodic current density a.s.f 100 Cathodic treating time seconds 2.0 Solution temperature F Line speed f.p.m Coulombs per square foot (cathodic pass) 200
• Example 3 Steel strip similar to that of Examples 1 and 2 was given the same pretreatment, except that the anodic flash was for 1.0 second at a current density of 85 a.s.f. (85 coulombs/ft. and then catodically treated in an aqueous bath as follows:
• Process element Element specification Chromium trioxide g./l 5.0 Cobaltous chloride g./l 0.25 Cathodic current density a.s.f 127 Cathodic treating time seconds 3.0 Solution temperature F 120 Line speed f.p.m 100 Coulombs per square foot (cathodic pass) 381 procedures (as hereinafter described), are given in Table I below.
• ADHESIVE PEEL STRENGTH TEST This test is used to measure the effect of the steel stock surface on the strength of a cemented, lap-jointed side seam in tin-free steel cans.
• Such lacquered panels were air dried for 10 minutes at room temperature and then oven-cured at 415 F. for 30 minutes.
• the panels were then cut into 4 inch wide strips.
• Three nylon 11 pellets were heat-tacked to one of the lacquered strips for each of the treated steels to be tested, near one end thereof and equally spaced across the width of the strip.
• Another strip was positioned over the first and the two were wrapped in aluminum foil, clamped between heated (500 F.) platens (shimmed apart to maintain a 3 mil adhesive layer between the bonded test strips), the pressure raised to 8000 p.s.i.g. and held at that value for 3 seconds.
• the specimens were then removed and allowed to cool at room temperature.
• the strength of the resulting adhesive bond was determined by means of a standard tensile testing device provided with a special 180 peeling mandrel and operated at a speed of 2 inches per minute.
• ACCELERATED SALT WATER ENAMEL PROCESS ADHESION TEST This test is used to measure the degree of adhesion of can coating lacquers and enamels to the steel stock. Panel specimens, 4 x 6 inches, were provided with a dipcoating of a gold phenolic lacquer (No. 1457 Gold Lacquer, manufactured by Interchemical Corporation, of Clifton, N.J.), thinned to give a coating weight, after baking, of 1.82.2 mg./in. of panel surface area.
• a gold phenolic lacquer No. 1457 Gold Lacquer, manufactured by Interchemical Corporation, of Clifton, N.J.
• the panels were air dried for 10 minutes and then oven-cured for 10 minutes at 410 F.
• the panels were then placed in an aqueous solution consisting of sodium chloride, g./l., and 30% hydrogen peroxide, 10 ml./l., and maintained therein, at a immediately subjected to the Scotch Tape Test, wherein pressure-sensitive adhesive tape is applied to the sample and then quickly and forcibly removed.
• aqueous solution consisting of sodium chloride, g./l., and 30% hydrogen peroxide, 10 ml./l.
• CITRIC ACID TEST This test is used to measure the resistance of the treated steel stock to undercutting of an overlying can coating lacquer or enamel by corrosive can contents, and simulates undercutting conditions encountered at the site of scratches, breaks or holes in the lacquer on carbonated beverage cans.
• Panels were prepared and baked in the same manner as for the accelerated salt water test. Specimens 2 inches x 4 inches were taken from the panels and the lacquer cut through with a knife, making lines of 0.1 mm. Width and extending (1) in the steel rolling direction, (2) across the rolling direction, and (3) diagonally to the rolling direction. The specimens were then placed, with the out line face up, in an enameled tray containing a 3% citric acid solution, and stored for 96 hours at 75 to F. The specimens were then removed, blotted dry, and the total width of the cut lines was measured with a 7 magnifying glass having an 0.1 mm. scale on its field.
• HUMID STORAGE RUST RESISTANCE TEST This test is used to determine the resistance of steel can stock to rusting during storage in a humid atmosphere and is thus a measure of the usefulness of a surface treatment to inhibit rusting during shipment and storage prior to lacquer application.
• Unlacquered panels of the treated steels in the form of 4 X 6 inch panels, were placed, at an angle of 15, in slotted plastic racks with /2 inch separations between panels.
• the racks were placed in a sealed humidity cabinet at 100 F. and relative humidity for 30 days.
• STACK RUST RESISTANCE TEST This is a more severe rusting test and represents conditions that would rarely, if ever, be encountered under actual service conditions.
• the promoter is selected from the group consisting of from about 0.25 to about 1.0 g./l. of cobaltous chloride and from about 0.25 to about 2 g./l. of sulfamic acid.
• a method for the production of corrosion-resistant, tin-free carbon steel stock suitable for the manufacture of food and beverage containers comprising:
• a metho for the continuous production of corrosion-resistant, flat-rolled steel stock comprising continuously passing a flat-rolled steel-base stock through an electrochemical treating bath comprising an aqueous solution consisting essentially of from about 5 to about g./l. of chromium trioxide and from about 0.25 to about 5 g./l. of a promoter material selected from the group consisting of cobaltous chloride and sulfamic acid, while impressing upon the stock as cathode, an electrical current of about 25 to about 300 a.s.f. for a time of about 0.25 to about 10 seconds. in order to apply a total of about 50 to about 1000 coulombs/ft. of energy.
• An electrochemical treatment bath comprising an aqueous solution consisting essentially of from about 5 to about 80 g./l. of chromium trioxide and from about 0.25 to about 5.0 g./l. of cobaltous chloride.

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• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating Methods And Accessories (AREA)

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Citations (7)

• 1967
  • 1967-09-29 US US671577A patent/US3532608A/en not_active Expired - Lifetime
• 1968
  • 1968-09-25 ES ES358488A patent/ES358488A1/es not_active Expired
  • 1968-09-26 FR FR1604218D patent/FR1604218A/fr not_active Expired
  • 1968-09-27 BE BE721551D patent/BE721551A/xx unknown
  • 1968-09-27 DE DE19681796253 patent/DE1796253A1/de active Pending
  • 1968-09-30 NL NL6813967A patent/NL6813967A/xx unknown
  • 1968-09-30 GB GB46387/68A patent/GB1247881A/en not_active Expired
• 1973
  • 1973-12-30 MY MY185/73A patent/MY7300185A/xx unknown

Patent Citations (7)

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