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
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment
  • C25D5/505—After-treatment of electroplated surfaces by heat-treatment of electroplated tin coatings, e.g. by melting
• • 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
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
• • 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
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel

Definitions

• This invention relates to electroplating metals and more particularly to a method of producing bright, smooth coatings of electroplated tin, cadmium, zinc and their alloys.
• Electroplated coatings of tin, cadmium, zinc and alloys of these metals when taken directly from the electroplating bath ordinarily have a dull finish and are very susceptible to fingerstaining and to tarnishing. For most purposes in a bright, non-tarnishing finish is desirable. Or-
• Coatings of these metals having a bright ap- 1; pearance, also may be obtained by dipping the metal articles to be coated in a molten bath of the coating metal.
• this method it is usually necessary to further treat the coated articles, e. g. by rolling, wiping or other treatment to remove excess metal and smooth the coatings.
• this hot dipping method. of coating metals is capable of producing a bright, smooth surface, it has a number of disadvantages. In the first place, when iron or steel articles are thus coated, careful and tedious preparation 1 of the metal surface to remove oxide and other impurities is necessary before the metal is hot dipped in order to obtain a smooth adherent coat.
• These preliminary steps include pickling operations and annealing.
• metals which are coated by hot dipping have a relatively thin layer of coated metal because of the fact that the molten metal has a relatively low viscosity and when the article is removed from the hot dipping bath excess metal drains ofi, leaving a relatively thin coating on the article. The thickness of the coating is further reduced when a wiping or rolling operation is used.
• Hot dipping methods are also in general not readily applicable to odd shaped or deeply recessed articles. Frequently great difliculty is encountered in hot dipping methods because the metal to be v coated tends to dissolve in the coating metal so 41-, that the coating metal bath becomes rapidly contaminated.
• an electroplating process is preferable for coating metals because less preparation of the metal prior to plating is required and ;o a thicker coating may be obtained.
• no entirely satisfactory process has been proposed for producing bright electroplated coatings of soft metals such as tin, cadmium or zinc. While bright electroplated coatings may be 35 obtained by plating under certain carefully controlled conditions or by buffing and polishing, such plates are very easily tarnished and flngerstained.
• An object of the present invention is to produce smooth, bright, electroplated coatings of fusible metals such as tin, zinc, cadmium or their alloys Without the use of mechanical polishing operations, bath addition agents, etc.
• a further object is to produce such coatings on iron or steel.
• a still further object is to produce electroplated coatings of tin, cadmium, zinc or their alloys on iron or steel which coatings have superior adherence to the base metal and increased corrosion resistance.
• the above objects are accomplished by electroplating metallic articles with metals fusible below the melting point of the base and subjecting the electroplated articles to a heat treatment under substantially non-oxidizing conditions at a temperature above the melting point of the coated metal.
• the coated metal is heated to a temperature slightly above its melting point for a short time, for example 10 to seconds.
• My invention also comprises a novel method of pickling iron or steel articles prior to electroplating as hereinafter described which results in a superior electroplated coating.
• base metals especially iron or steel may be plated with improved results if prior to plating the metal is treated with astrong acid to such extent that a distinct and uniform etching corrosion of the surface is obtained, followed by an alkaline treatment. With such pretreatment, the adherence of the plated coating is improved and the porosity of the plate is materially decreased.
• pretreated metal is plated with tin, cadmium, zinc or their alloys and the plated coating is heat treated to a temperature above its melting point as described above, the resulting plate is smoother and more uniform than is the case when the base metal is not so etched and alkali treated prior to plating.
• the above-mentioned etching prior to electroplating is especially useful in obtaining adherent, non-porous coatings of tin or the rust-resisting metals on cold-rolled sheet steel.
• Cold-rolled steel ordinarily has a bright, smooth surface.
• the plated metal has relatively poor adherence and corrosion resistance.
• the electroplated tin has good adherence to the base and the corrosion resistance is markedly improved, without increasing the depth of the electroplated layer.
• One method of practicing my invention will be illustrated by reference to the production of a smooth, bright coating of tin or tin alloy on steel.
• the surface of the steel after suitably cleaning to remove grease and dirt, is first treated by immersion in a strong acid solution preferably at an elevated temperature, e. g. 100-200 F., until the surface of the metal'is distinctly etched so that a uniformly etched or corroded surface is plainly visible to the unaided eye.
• the steel is then treated with an alkaline solution, preferably by making the steel the anode in an alkaline cyanide bath and applying a moderate electric current for a minute or two.
• the treated steel is electroplated with tin or tin alloy by any suitable method to produce a tin layer of the desired thickness. I prefer to electroplate from an alkaline'solution, especially by the methods described in U. S. Patents 1,841,978 and 1,919,000. These or similar methods may be used for plating tin or tin alloys on the treated
• the article is heated under substantially non-oxidizing conditions to a temperature above the melting point of the tin or tin alloy coating. If the coating is of substantially pure tin (which melts at about 450 F.), heating maybe to a temperature of about 460 to 500 F. for a period of ten to thirty seconds.
• Example 1 Samples of cold rolled sheet steel (automobile body stock) were first freed from grease and dirt by cleaning in an alkaline electrolytic cleaner containing OzJsal. Sodium cyanide 2 Caustic so 2 Trisodium phosphate 4 Temperature of solution-160-180 F. making it cathode and anode alternately; until entirely free from grease and dirt.
• the samples then were pickled in a sulfuric acid solution containing by volume of sulfuric acid (66 B.) at a temperature of 70-90 C. for a period of 4 minutes. Aside from the removal of oxide or scale, the pickling was continued until the rolled surface layer was removed and a distinct etching was visible to the naked eye.
• the samples then were water rinsed and further treated electrolytically in a sodium cyanide solution containing 4 oz./gal. of NaCN at 140-160 F., by making it the anode at 100 A/SF for 1 minute. After this treatment the samples again were water rinsed and then tin plated in an alkaline tin solution containing:
• samples were rinsed in cold and hot water and dried.
• the dry; samples were immersed for 10 to 20 seconds in a molten tallow bath at a temperature of 240-260 C.
• This treatment produced bright, smooth coatings of tin similar in appearance and hardness to that produced by hot tinning processes.
• Example 6 Pieces of cold-rolled sheet steel were degreased In place of the anodic cyanide treatment men- 2 32122 1 353% g -3:5 :6 2; 2 tioned in the foregoing examples, sheet steel 0 0 0 2 4 samples, after being etched in various acid solu- 32:23: 5 g gg fg z f 5225 :2: 3: $8, 22: ::gfigggiggggfiigf treated anodically in a sodium cyanide solution i oz lgal.
• Example 9 (6 oz./gal.) at 140 to 160 F., using a current Sodium hydroxide 2 density of 50 to 100 A/SF for 1 minute. After sodium carbonate v 4 water rinsing the pieces were then plated in the tin-cadmium bath described in Example 9. Upon The anodic treatment was continued for 1 to 2 t treatment in tanow as described in Exanb minutes at a c e density of 50 to 100 pie 9, adherent, bright coatings of cadmium-tin The samples were then plated and heat treated alloy were obtained. These coatings were smooth as in Example 1, with the same results. and entirely free from lumps.
• Example 11 Pieces of cold rolled sheet steel were degreased as described in Example 1. They were then acid etched as described in Example 13. Following this the same anodic treatment as in Example 13 was used.
• Emample 12 Pieces of cold rolled steel were prepared and plated with 0.0001 to 0.0003" of tin as described in Example 11. Following the tin plating, the samples were zinc plated in a solution containing:
• the tin plated pieces were zinc plated with 0.0001" to 0.0002" of zinc.
• Example 13 Pieces of hot rolled sheet steel were degreased in an alkaline electrolytic cleaner and than acidpiokled to remove rust and fire scale until a clean surface was obtained. Following the pickling, the pieces were immersed in a NaCN solution (6 oz./ gal.) and cleaned anodically for 1 minute using 50 to 100 A/SF. The samples were then plated as described in Example 11 with 0.0002" to 0.0003" of cadmium. The cadmium plated pieces were then plated in a zinc solution as described in Example 12 with 0.0001" to 0.0002" of zinc. The double plated pieces were then treated in tallow at 260 to 300 C. Adherent bright coatings of cadmiumzinc alloy were obtained free from lumps and other imperfections.
• this preliminary etching and alkaline treatment assists in producing a smooth coating of the final product, especially when the electroplated coating is relatively thick. If such relatively thick coatings are heat treated subsequent to plating without this preliminary treatment, the final coating sometimes has certain irregularities or lumpiness, apparently caused by an uneven flow of the molten electroplated coating during the heat treatment and/or a failure of the plated metal to adhere well to the base metal. Also, if the metal is thus etched but the alkaline treatment is omitted, the desired satisfactory results will not be obtained. I have found that when my preliminary treatment is used, such irregularities may be substantially entirely eliminated.
• the herein described preliminary etching and alkaline treatment is suitable for treating various heavy metals that are to be electroplated, e. g. steel, cast iron, brass and copper. It is especially advantageous when it is desired to produce bright coatings on cold rolled steel or cast iron by electroplating followed by heat treatment, as herein described.
• the tendency for the plated metal to fail to adhere properly to the base metal and form an irregular or lumpy coating during the heat treatment is most marked when the base metal is cold-rolled steel. For this reason, my preliminary acid etching and alkali treatments usually are necessary in order to produce a good coating on such steel by my invention.
• the etching and alkali treatments often may be omitted or replaced by the usual cleaning operations with more or less satisfactory resiflts.
• the extent to which the base metal should be acid etched during the preliminary treatment may vary, depending on the nature of the base metal That is, in some cases, the minimum etching may be suflicient for the best results.
• the proper minimum time for treatment in the etching acid may be readily determined by simple trials, but usually the etching is carried on till a visible appearance of etching is noted.
• Various acidic materials capable of corroding iron, steel or other base metals are suitable for the preliminary etching step in accordance with my invention.
• sulfuric acid, hydrochloric acid, nitric acid or mixtures of these acids may be utilized.
• the acid may be used as electrolyte and the article to be etched made the anode in an electrolytic system.
• this electrolytic method of etching ordinarily offers little or no advantage over simple immersion in the acid solution.
• the time and temperature of the etching operation as well as the concentration of the acid in the solution may be varied over a wide range, provided that the operation-is continued over such length of time as is necessary to produce the required etching effect. I pre- 2 normal to 6 normal strength and carry out the etching operation at an elevated temperature,
• the type of acid or acid mixture required may vary depending on the nature of the metal.
• the best method of etching a given sample may easily be determined by simple trials with various acids or mixtures at various temperatures.
• the steel should not be etched too drastically.
• the heat treatment subsequent to plating produces a distinct brightening and a smooth, dense plate.
• alkaline aqueous solutions are suitable for the alkali treatment following the above described etching operation.
• I have found that in general the best results are obtained when the alkaline bath contains substantial amounts of an alkali metal cyanide; if desired, a solution of alkali metal cyanide alone may be used or sodium hydroxide or other alkaline materials may be added.
• the best results generally are secured by keeping the bath hot, e. g.. at a temperature of 140 to 160 F. and using an anode current density of 50 to 80 amps.
• the time of treatment may be varied widely; ordinarily to 3 minutes is suflicient when the electrolytic method is used- If the alkaline bath is used without the aid of the electric current, a somewhat longer time of treatment usually will be required, e. g. up to around 30 minutes.
• my process is suitable for coating metals with alloys of tin, cadmium, or zinc as well as the pure metals.
• Various other relatively fusible metals or alloys e. g. lead or lead alloys, may be utilized in my invention.
• the alloy coatings may be obtained by electroplating the alloy from electrolytes containing salts of alloy constituents.
• An alloy coating also may be obtained in accordance with my inventibn by plating out the metals to be alloyed in two or more separate layers and'then subjecting the electroplated article to the above described heat treatment under non-oxidizing conditions.
• an article may be plated with a layer of tin and then with a layer of cadmium or the tin may be deposited on the cadmium plate and the article thus double plated subjected to a heat treatment under non-oxidizing conditions at a temperature slightly above the melting point of the resulting tin-cadmium alloy.
• other metals may be alloyed by this method with tin, cadmium or zinc; for example,- a tin-copper alloy may be made by first plating the article with tin and then with a light copper plate and subjecting the electroplated articles to the above described heat treatment. In this case, in order to produce a tin-copper alloy at the surface, it
• An advantage of my herein described inven-- tion is that it results in a smooth, bright plated coating which has superior corrosion resistant properties.
• the coating has the density, hardness, low degree of porosity and bright appearance which are characteristic of hot dipped coatings and furthermore, my coatings may be made of greater thickness than by hot dipping methods.
• the step of heat treating subsequent to plating in accordance with my invention simplifies the electroplating operation, since it eliminates the need of originally obtaining plates having the best .color and general appearance.
• the heat treatment brightens the dullest plates substantially as well as the brighter ones. Hence it is necessary only to regulate the electroplating bath to obtain plating suitable for heat treatment.
• a process for electroplating a metal comprising treating said metal in an acid solution until the metal is visibly etched; ⁇ thereafter subjecting said metal to an anodic treatment in an alkaline solution, electroplating the treated metal with a metal or alloy having a melting point below that of said treated metal and subsequently heating the electroplated surface under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated coating.
• a process for electroplating steel comprising treating said steel in a 2 to 6 normal acid solution containing at least one of the acids of the group sulfuric acid, hydrochloric acid and nitric acid until the metal is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkali metal cyanide solution and electroplattially non-oxidizing conditions to a temperature of 460 to 500 F.
• a process for electroplating cold-worked steel comprising treating said steel in a 2 to 6 normal acid solution containing at least of sulfuric acid until the metal is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkali metal cyanide solution and electroplating the treated metal with tin and subsequently heating the electroplated surface under substantially non-oxidizing conditions to a temperature of 460 to 500 F. for 10 to seconds.
• a process for electroplating cold rolled steel comprising treating said steel in an acid solution containing at least one of the acids of the group sulfuric acid, hydrochloric acid and nitric acid until said steel is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkali metal cyanide solution and electroplating the treated metal with a tin coating and immersing the electroplated metal in a hot, substantially non-oxidizing liquid, whereby said tin coating is fused to form a smooth, bright coating.
• a process for electroplating steel comprising treating steel in an acid solution until the steelis visibly etched, thereafter subjecting said steel to an anodic treatment in an alkaline solu--' tion, electroplating the treated steel with a metal or alloy having a melting point lower than that of said steel and subsequently heating the electroplated surface under substantially non-oxidizing conditions to 'a temperature above the melting point of the electroplating coating.
• a process for electroplating steel comprising treating steel in an acid solution until the steel is visibly etched, thereafter subjecting said steel to an anodic treatment in an alkaline solution, electroplating the treated steel with tin and subsequently heating the electroplated surface 40 under substantially non-oxidizing conditionsto a temperature above the melting point of the electroplated coating.
• a process for electroplating cold-worked steel comprising treating said steel in an acid solution until the steel is visibly etched, thereafter subjecting said steel to an anodic treatment in an alkaline solution, electroplatingthe treated steel with tin and subsequently heating the electroplated surface under substantially nonoxidizing conditions to a temperature above the melting point of the electroplated coating.
• a process for electroplating cold-rolled steel comprising treating said steel in an acid solution until said steel is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkaline cyanide solution, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated tin.
• a process for electroplating steel comprising treating steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solution for a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplawd tin.
• a process for electroplating steel comprising treating steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solution containing alkali metal cyanide for a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated tin.
• a process for electroplating cold-worked I steel comprising treating said steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solutionfor a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated tin.
• a process for electroplating cold-worked steel comprising treating said steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solution containing alkali metal cyanide for a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidiz-' ing conditions to a temperature above the melting point of the electroplated tin.

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• 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)
• Coating With Molten Metal (AREA)

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Publications (1)

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Cited By (10)

Families Citing this family (1)

• 1934
  • 1934-01-31 US US709171A patent/US2078868A/en not_active Expired - Lifetime
• 1935
  • 1935-02-01 DE DEP70706D patent/DE661936C/de not_active Expired

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