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
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32

Definitions

• This invention relates-to improvements in tin plate and treatment of tin. plate containers.
• anodic protection is especially applicable to containers for foodstuffs with the object of preventing or reducing sulphide staining.
• suitable electrolytic treatment for example, in dilute ammonia, the tin plate forming the anode, films can be produced which, while being imperceptible, I have the effect of rendering the vtin plate resistant to sulphide staining by many foodstuffs which normally discolour tin, the containers appearing cleanandbright after retorting.
• lacquer e. g. with foodstuffs which produce sulphur heavily such as canned 35 shrimps.
• prolonged anodic treatment may be followed by electrodeposition of lacquer or by spraying or rolling lacquer, followed by stoving.
• this pre-existing film on the plate affects the anodic process, and owing to .the above-mentioned differences, between the films on plates from different sources, it cannot be ensured that-a given anodic treatment will ,produce the same result in all cases.
• This difliculty may be overcome by removing the preexisting film by electrolytic means, before subjecting the plate to an anodic treatment for protection against sulphide staining.
• pre-existing films are removed and an anodic film formed'on thetin plate surface by treatment in a single electrolyte, for example ammonia or [ammonium carbonate, in which the tin plate functions alternately as anode and as cathode, the last function being as anode.
• a single electrolyte for example ammonia or [ammonium carbonate, in which the tin plate functions alternately as anode and as cathode, the last function being as anode.
• the tin plate may be made first anode, then cathode, then'anode again.
• the complementary electrode is convenientlyof nickel or stainless steel.
• the preliminary anodic and cathodic treatments are preferably effected at such current densities and for such times that vigorous gas evolution occurs, with the object of disrupting the pro-existing film, but the final anodic treatment is preferably carried out under such conditions of current density and time that substantial gas evolution is avoided.
• the conditions are not critical, and the follow ing example is merely typical of many which have been found suitable.
• An important feature of the invention is that the whole of the operations may be efiected in the same electrolyte, thus avoiding the need for intermediate washing between the various stages.
• the sequence of operations may be carried out with a single 'charge of electrolyte in each container, but where tin-plate sheets are treated it may be advantageous to pass the sheets through a separate bath of electrolyte for each stage, in order to avoid short-circuiting of the current between successive sheets undergoing difierent stages of treatment.
• tin-plate cans may be treated on a conveyor or'on a circular rotating platform, an electrode being automatically brought inside each can.
• ammonia or ammonium carbonate as electrolyte
• the invention is not restricted to these substances. These are, however, preferred, singly or in conjunction, on account of their complete volatility on drying by heat. Thus no deleterious residue need be left on the plate through insufilcient rinsing after treatment. Alternatively, if the cans are rinsed but not dried, there is no possibility of harmful material entering the food. Many other solutions are suitable, not necessarily alkaline or oxidising, the essential requirement being that a compact film consisting of, or containing, tin
• sodium or potassium dihydrogen' phosphate (acid), sodium or potassium mono-hydrogen phosphate (alkaline), or an equimolecular mixture of these (approximately neutral) are all suitable.
• I may mention an electrolyte consisting of three parts of 0.880 ammonia and ninety-seven parts of distilled water.
• a principal object of the invention is to enable sulphur-containing foodstuffs of a neutral character to be packed in tin-plate containers without the necessity for using lacquer,
• the invention includes the preparatory de-- filming treatment whether followed by anodic protection, either in the same or in a different bath, or by application of lacquer.
• the acidity of the lacquer may be suflicient for an oxide film on the tin-plate surface, whether already existing or formed according to the present invention, to be dissolved in or disintegrated by the lacquer.
• tin-plate lacquered after treatment according to the invention so as to form an anodic oxide film does not show superior resistance to sulphur compounds, over plate lacquered without pretreatment, although the continuity of the lacquer film may be superior.
• Process of treating tin-plate which consists in subjecting the tin-plate to anodic oxidation in an aqueous solution of an electrolyte capable of producing a compact film containing tin oxide, then to cathodic treatment in a solution, the anodic and cathodic treatments being effected at such current densities and for such times that vigorous gas evolution occurs, and then coating with a thin film of tinoxide by a final anodic treatment carried out under such conditions of current density and time that substantial gas evolution is avoided.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating Methods And Accessories (AREA)
• Table Devices Or Equipment (AREA)
• Details Of Rigid Or Semi-Rigid Containers (AREA)
• Chemical Treatment Of Metals (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=10066919

Family Applications (1)

Country Status (5)

Cited By (19)

Families Citing this family (4)

• 0
  • BE BE421943D patent/BE421943A/xx unknown
• 1936
  • 1936-06-06 GB GB34196/37A patent/GB479746A/en not_active Expired
  • 1936-06-06 GB GB15865/36A patent/GB479681A/en not_active Expired
• 1937
  • 1937-06-05 FR FR822724D patent/FR822724A/fr not_active Expired
  • 1937-06-05 US US146691A patent/US2215165A/en not_active Expired - Lifetime
  • 1937-06-06 DE DEC52915D patent/DE682355C/de not_active Expired

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