Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/48—After-treatment of electroplated surfaces
• • 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

• Striker [57 ⁇ ABSTRACT A method for treating a material having a tincontaining surface to passivate and improve the corrosion resistance of the said surface which comprises making the material a cathode in an aqueous hexava lent chromium electrolyte consisting essentially of an aqueous solution of an alkali-metal dichromate in an amount of at least 20 and at most 25 grams per liter, an alkali-metal acetate in an amount of at least 5 and at most 10 grams per liter, and chromium trioxide in an amount of approximately 5 grams per liter, the said electrolyte having a hydrogen-ion concentration corresponding to a pH between 4.0 and 5.0 and an electrical conductivity of at least 28,000.
• the present invention pertains to the electrochemical passivation of tinplate and similar materials having a tin-containing surface to improve their resistance to corrosion so that they may be improved with respect to their suitability for use for the production of containers for use in the canning and packaging of foodstuffs. par ticularly protein-containing foodstuffs.
• Tinplate is made by coating iron or steel sheet materials with a film of tin, either by dipping the sheet into molten tin or by an electroplating operation.
• the thus coated sheet material has a bright glossy surface.
• tin unlike iron, acts as an anode in the presence of organic acids such as are present in foodstuffs. Because of its higher hydrogen overvoltage (1.22 volts compared to 0.82 volt for iron at 25C in 2-normal sulfuric acid at l amperes per square decimeter) the polarization of tin is changed. Because of this, tinplate is suitable for use as a packaging material for foodstuffs. It can be cold-worked, bent, and folded and sealed or welded together and meat and other foodstuffs do not readily adhere thereto.
• Tinplate however, has a tendency to stain by formation of tin sulfide when fish, meat, milk and certain vegetables containing proteins which include a sulfur-containing amino acid component are placed into contact therewith. Such staining is also referred to as marbling.
• the electric current density measured at the cathode that is applied is at least 3.76 and at most 21.5 amperes per square decimeter.
• the duration of the treatment that is, the period during which electrical charges are passed from the tinplate cathode to the electrolyte is between 1 and 3 seconds, during which period the desired amount of chromium and chromium oxide is deposited or plated on the tinplate.
• the term current density is to be understood to refer to the strength of the current per unit of cross-sectional area measured at the cathode and is expressed in terms of amperes per square decimeter.
• the total amount of electrical current that is consumed or the total electrical charge that is transferred to produce the specified chromium deposit on 2 the tinplate is specified herein in terms of coulombs (amperes per second) per square decimeter.
• the total amount of electrical current that is consumed is the product of the current density and the period in seconds during which the current was passed.
• the amount of chromium and chromium oxide that is deposited on the tin plate can be increased to between l5 and l8 milligrams of chromium per square meter.
• chromium and chromium oxide in an amount equivalent to about 20 milligrams of chromium per square meter will be deposited on the tinplate when the density of the current is maintained at about 9.5 amperes per square decimeter, which corresponds to a total amount of 14 coulombs per square decimeter of current consumed.
• the chromium coating deposited in accordance with this conventional method will have not only a reduced tendency to stain upon contact with sulfur compounds but will also be less susceptible to etching or corrosion when exposed to air or to acids or alkalies.
• a method is disclosed in US. Pat. No. 3,278,401 for improving the corrosion resistance of tin-containing surfaces and especially surfaces of tinplate on iron or steel substrates coated with iron-tin alloys by cathodically depositing chromium thereon.
• the electrolyte that is used in that method consists of an aqueous solution of a water-soluble chromate and sodium acetate or other water-soluble acetate.
• the sodium dichromate of that electrolyte was said to be replaceable by chromic acid (chromium trioxide CrO and other water-soluble chromates. Only conventional weights of the coatings of chromium and chromium oxide were applied in this manner to the tin-coated iron or steel substrate.
• a process is disclosed in German Democratic Republic Pat. No. 45,536 in which corrosion and staining by sulfur compounds is said to be inhibited by the use of an electrolyte consisting of an aqueous solution of an alkali-metal dichromate having a hydrogen-ion concentration corresponding to a pH between 4.0 and 6.0.
• deposits of chromium having only the normal weight or thickness which had been obtained in other processes and which have the same known disadvantages that other deposits having such weights could be obtained.
• the object of the present invention is to provide a method and an electrolyte for the production of passivated tinplate that does not have the foregoing disadvantages, from which tinplate only relatively insignificant amounts of the tin coating are dissolved and which coatings also have a high stability to heat.
• a further object of the present invention is to provide a method and electrolyte for passivating tinplate in which method the lining of the tank in which the electrochemical treatment is conducted is subjected to less destruction due to use.
• the electrolyte that is used in the method of the present invention is distinguished principally from those disclosed heretofore in containing at least 5 and at most grams per liter of an alkali-metal acetate together with 4 to 6, preferably 4.5 to 5.5, grams per liter of chromium trioxide (which is also referred to as chromic acid) the said electrolyte having a hydrogenion concentration corresponding to a pH between 4.0 and 5.0, and an electrical conductivity of at least 28,000, preferably at least 30,000, micromhos per centimeter at 50C.
• the electrolyte contains preferably at least and at most grams per liter of an alkali-metal dichromate.
• Sodium acetate is an especially suitable alkalimetal acetate for use in the electrolyte.
• the method of the present invention is distinguished principally from those disclosed heretofore in that the aftertreatment of the tinplate or sheets thereof that are to be used for the production of containers or cans, particularly those that are to be used for the packaging of foodstuffs, in the use of an electric current density measured at the cathode of at least 5.5 and at most 9.5 amperes per square decimeter, during a period of between l and 2 seconds, which corresponds to a consumption of electric current amounting to from 8 to 14 coulombs per square decimeter.
• the aqueous electrolyte bath is preferably prepared by use of distilled or deionized water and is maintained 4 during the electrochemical treatment at a temperature between 30 and C, and preferably between 40 and 50C.
• deposits of chromium and chromium oxides amounting to between 25 and 35 milligrams of chromium per square meter upon tinplate can be obtained by use of relatively low electric current densities and without maintaining the electrolyte at higher temperatures.
• Tinplate thus passivated in accordance with the method of the present invention has a higher resistance to staining by sulfur com pounds and its susceptibility to atmospheric corrosion and corrosion in acidic and alkaline media is substantially reduced.
• the method of the present invention can be adapted for use in existing electrolytic tinplate manufacturing plants without re quiring substantial alterations of the plant when the electrolyte of the present invention is used. A much smaller electric current density is required when the electrolyte of the present invention is used in place of the prior electrolyte that was used to deposit conventional coatings of normal thickness of chromium and chromium oxide to tinplate without changing the length of the period of treatment.
• the electrical conductivity of the electrolyte be increased from the values in the region of 20.000 micromhos per centimeter that were previously used to values of preferably at least 30,000 micromhos per centimeter and by the use of chromium trioxide and the specified additional amount of alkali-metal acetate.
• Tests extending for periods up to more than six months reveal that less than 60 milligrams of tin per kilogram of foodstuff dissolve or migrate in protein-containing foodstuffs that are packaged in containers made from tinplate passivated in accordance with the method of the present invention. Furthermore, such tinplate has a high resistance to corrosion, heat, and marbling.
• the passivating layers obtained consist of chromium, chromium trioxide and other chromium compounds, however, only the chromium content of the passivating layers is indicated.
• EXAMPLE 1 A solution was prepared by dissolving the following substances in distilled water in the amounts specified: 25 grams per liter of sodium dichromate (Na Cr O 5 grams per liter of chromium trioxide (CrO and 10 grams per liter of sodium acetate (CH COONa).
• the resulting solution had a hydrogen-ion concentration corresponding to a pH value of 4.6 and it had an electrical conductivity of 34,000 micromhos per centimeter at 50C.
• a continuous strip of tinplate both sides of which had been coated with tin having a width of 800 millimeters was connected to a source of electrical potential so that it could act as a cathode in an electrolytic reaction.
• This sheet was continuously passed between two pairs of anodes that were installed in a passivation tank containing the foregoing electrolyte, each anode having a length of l500 millimeters. These anodes were so arranged that one pair of anodes was above and the other pair of anodes was below the tinplate sheet as it passed through the tank so that the sheet passed between the two pairs of anodes for a total distance of 3000 millimeters.
• the sheet was passed between the two pairs of anodes at a linear speed of 2 meters per second so that the period during which electrical charges were actually transferred from the cathode to the electrolyte was l.5 seconds and the total surface area of the sheet, both sides of which were thus subjected to treatment, amounted to 480 square decimeters.
• the temperature at which the electrolyte was maintained during the electrolysis was 50C.
• the total amount of electrical current per unit of surface area that was consumed by the tinplate sheet in this passivating tank was varied as specified in the following table.
• the total amounts of chromium in milligrams per square meter (rounded off to the nearest integer) that were thus deposited on both sides of the sheet are listed opposite the amount of the total amount of electrical current that was consumed in coulombs per unit of surface area, expressed as coulombs per square decimeter, in this table.
• Example I 2.25 grams per liter of sodium acetate (CH COONa) dissolved in distilled water. It had a hydrogen-ion concentration corresponding to a pH of 5.3 and an electrical conductivity of 24,000 micromhos at 50C.
• CH COONa sodium acetate
• the continuous tinplate sheet that was used in Example I hereinbefore was then treated exactly as described in Example I while the temperature of the electrolyte was maintained at the same temperature that was used in Example I, with the following results:
• Example II When the continuous sheet of tinplate that was used in Example I was treated with this solution in the same manner as described in Example I with the electrolyte at the same temperature that was maintained in Example l, and with a current density of 0.21 ampere per square decimeter for a period of 1.5 seconds, corresponding to a total amount of electrical current consumed of 0.3l coulomb per square decimeter, the total amount of chromium deposited on both sides of the sheet was between 3 and 5 milligrams per square meter (conventional method).
• the total amount of chromium deposited on both sides of the sheet was between l4 and 16 milligrams per square meter.
• Example I together with comparative examples I to 3 demonstrate that the electrolyte that is used in the process of the present invention produces a substantially greater deposit of chromium in comparison with 7 8 known electrolytes at both low and high current densicominued ties.
• the passivating layers that are obtained with the Chromium Tin Iron electrolyte disclosed in U.S. Pat. No. 3,278,401 can 4 h :6 187 55 also be obtained with electrolytes containing only an 5 1:6 21:0 6:0 alkali-metal dichromate with no addition of sodium ac- 5 6 months 3 7.5
• Example 1 The same results that are described in Example 1 and the Comparative Examples 1 to 3 are obtained when this respect such highly passivated hhplate also deionized water is used instead of distilled water.
• hiblts high stability to marbling as as high resls' 1O tance to corrosion, even in sterilization experiments
• EXAMPLE 2 toward media that simulate foods which contain such To t bli h h amounts of ri chromium and iron substances as table salt, acetic acid, lactic acid, thioglythat migrate into canned meats from tinplate coated cohc acid and mixtures thereof at temperatures of with chromium passivating layers, pork and corned 121C fQT Periods between 30 and 60 mihutesy beef were packaged in cans formed from a commercial also have a greatly improved resistance to staining y tinplate coated on both sides known as tin plate No.
• the quantities of metals which migrated from cans made from tinplate in which evaporated milk having a butterfat content of 7.5% was packaged were also dely havlhg a hydrogeh'loh concentration Correspondtermined.
• the cans were made of the same tinplate as mg P f P between and and an electrical conspecified hereinbefore in connection with the canned duchvhy of at least 28,000 mlcromhos P Cehhmeter meats that had a chromium passivating layer equivalent at a emperahlre of and Passmg an electric to 25 illi f h i per Square meter Th rent through the material and the electrolyte until an amounts of the respective metals that migrated into the amouht of chromhlmjcohtalhmg h is deposited p evaporated milk packaged in such cans in milligrams the saldsurface that 1s sufiiclentto improve the corroper kilogram of the milk after storage at room temperareslsta
• An electrolyte for use in the method defined in claim 1 which consists essentially of an aqueous solution of an alkali-metal dichromate, an alkali-metal acetate in an amount of at least 5 and at most [0 grams per 10 liter, and chromium trio xide in an amount of at least 4 and at most 6 grams per liter, said electrolyte having a hydrogen-ion concentration corresponding to a pH between 40 and 5.0 and an electrical conductivity of at least 28,000 micromhos per centimeter at a temperature of 50C.
• the electrolyte consists essentially of an aqueous solution of an alkali-metal dichromate in an amount of at least 20 and at most 25 grams per liter, an alkali-metal acetate in an amount of at least 5 and at most 10 grams per liter, and chromium trioxide in an amount of at least 4.5 and at most 5.5 grams per liter, the said electrolyte having an electrical conductivity of at least 30,000 micromhos per centimeter at a temperature of 50C.

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• 1974
  • 1974-02-07 FR FR7404112A patent/FR2228857B1/fr not_active Expired
  • 1974-02-14 NL NL7402029A patent/NL7402029A/xx unknown
  • 1974-04-11 BE BE143073A patent/BE813574A/xx unknown
  • 1974-04-23 GB GB1764174A patent/GB1413940A/en not_active Expired
  • 1974-05-02 US US466247A patent/US3925171A/en not_active Expired - Lifetime
  • 1974-05-06 NO NO741620A patent/NO134756C/no unknown
  • 1974-05-08 JP JP49051098A patent/JPS5015742A/ja active Pending
  • 1974-05-08 SU SU742026747A patent/SU639461A3/ru active

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