Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/04—Electroplating: Baths therefor from solutions of chromium

Definitions

• One object of the present invention is to produce a chromium coating which has a large number of microcracks, uniform color of deposit, good cathode efficiency and adequate throwing and covering power.
• Sulphates are often added to chromium electrolytes and a wide range has been suggested but not all proportions are suitable and suitable proportions vary with different electrolyte compositions.
• chromium plating baths containing a fluoride and sulphate a microcracked deposit is obtainable with a high proportion of fluoride in areas of an article where current density is high but on the same article microcracking is not produced on areas of low current density.
• a high fluoride content also will not tolerate the increasing oxidic impurities which occur over a period of use and involves a critical range of sulphate which is impractical to maintain.
• a lower proportion of fluoride does not produce a good deposit. If selenium is included in sufficiently large proportion with a view to facilitating the production of microcracks, a deposit is produced which has an undesirable blue color and the covering power of the deposit over areas of varying current density is reduced. If the proportion of fluoride is reduced and the solution is otherwise suitably adjusted, the deposit tends to have bands of different color across the article.
• the present invention provides a solution which gives a good microcracked deposit of good covering power over areas of varying shape and current density and good uniform color and which maintains these characteristics over a long period with little or no maintenance, using a low proportion of fluoride and an adequate range of sulphate.
• boric acid in chromiufn plating baths has been considered undesirable (e.g. page 179 of Electroplating Laboratory Manual" by R.C. Armet publishedby Robert Draper Limited of, Teddington which states that poor rinsing from a bright nickel solution will result in nickel, boric acid, sulphate and chloride entering the chromium bath, all troublesome impurities resulting in lowering of covering power").
• Our own experience confirms that boric acid is commonly used in nickel plating baths and this results in some boric acid being carried over with the chromium plating baths with detrimental effects, e.g., boric acid added to certain otherwise reasonably satisfactory electrolyte solutions causes loss of microcracks.
• boric acid has never been proposed as an addition to electrolytes for producing microcracked chromium'coating.
• the present invention is based on the discovery that the combination of a high content of boric acid or of a salt giving rise to borate ions (B0,), in combination with selenium ions produces a surprising combination of advantages.
• the boric acid or a salt giving rise to borate ions may be used in quantity from two grams per liter of electrolyte solution up to a saturated solution.
• boric acid or salt makes the electrolyte more tolerant to variation of the CrO /SO, ratios.
• a suitable salt is sodium or potassium perborate or borax.
• the electrolyte solution of the present invention gives a good bright color (less blue), is tolerant to a high proportion of oxides, has good throwing and covering power, good cathode efficiency, even distribution of microcracks, and tolerant to fluoride and wide range of sulphate.
• a solution for use as an electrolyte for electrolytic deposition of chromium comprising an aqueous solution containing chromic acid (CrO boric acid or salt giving borate ions, a fluorine-containing compound, a sulphate-ion-containing compound, and a selenium compound; the selenium compound containing from 0.003 to 0.012, e.g., 0.003 to 0.009 selenium dioxide gram per liter, preferably 0.005 to 0.009; the quantity of CrO being 180 to 250 grams per liter, and the ratio of CrO to fluoride ion is 110 to 320 to 1, preferably 190 to 220 to 1.
• the ratio of sulphate ion to fluoride ion may be 0.64 to 2.56 to l for best microcracked deposit best covering and throwing power.
• the ratio CrO to S0 may be to 250, for best microcracked coating.
• the boric acid may be used in any proportion up to maximum solubility but we normally prefer 10-30 grams per liter.
• the average operating temperature may be to F, preferably l05-115 F.
• the cathode current density may be -200 amps per square foot.
• the period of treatment is normally 6 to 10 minutes.
• a plating time of a minimum 7 minutes at 200 amps/sq.ft. will produce microcracked deposits having cracks of some 700-2000 per linear inch. Deposits of less than 0.00002 inch Cr. surprisingly showed a microporous structure of fine pit-like pores and small cracks that failed to join up as those at greater thickness.
• articles of complex shape can be plated by this process and even in low current density areas and/or low thicknesses of chromium improved corrosion resistance can be found by virtue ofhaving microcracks.
• the invention also provides a combination of additional advantages, viz. wide current density range, wide temperature range, low fluoride concentration, microcracks over the entire plating range at all thicknesses, and ability to operate satisfactorily with fluorinated hydrocarbons in order to suppress chromium fumes and spray.
• a particular example of an electrolyte solution in accordance with this invention consists of:
• Thickness of deposit .00003 inch.
• electrolyte solution of the present invention can be made up as follows:
• Sulphuric acid 0.7 to 2.0, e.g., 0.7-1.0 g/liter.
• CrO to S0 85 to 250 preferably 135-185 for best microcracking.
• CrO to S0 ion plus fluoride ion 45 to 140.
• Sulphate plus fluoride ion to Se ion is 212 to 1780.
• Sulphate to selenium ion is 120 to 1000.
• Fluoride ion to selenium ion is 90 to 320.
• the number of microcracks per inch in the final chromium deposit is 750-2500, preferably 1500-2000.
• the fluorine compound may be hydrofluoric acid and/or its salts; fluoboric acid and/or its salts; fluozirconate, fluoaluminate, fluotitanate, sodium and/or potassium silicofluoride.
• the selenium may be added as selenic acid, or its alkali metal salts, or selenious oxide.
• the solution may also include other agents such as surface active agents and foam depressants.
• the thickness of the microcracked chromium layer may in practice be from 0.00002 to 0.00004 inch.
• An acidic aqueous solution for electrodeposition of chromium in microcracked condition having at least v 700 microcracks per linear inch comprising (a) chromic acid (CrO (b) at least two grams per liter of solution ofa boron containing compound selected from the group consisting of boric acid and water soluble salts thereof giving rise to borate ions, 50) sul hate ions, (d) a fluorine containing compoun and e) a selenium compound.
• a solution as claimed in claim 1 wherein said selenium compound is selenium dioxide present in an amount of at least 0.003 grams/liter solution, the quantity of CrO is 180 to 450 grams per liter and the weight ratio of CrO to fluoride ion is :1 to 266:1.
• Selenium compound at least 0.003 g/liter selenium oxide Boric acid at least 10 g/liter.
• a process of preparing article having a microcracked chromium coating having 700 to 2000 microcracks per linear inch which comprises elec trolytically treating as the cathode an article selected from the group consisting of nickel articles, cobalt articles, nickel-cobalt articles and articles coated with nickel, cobalt or nickel-cobalt in an acidic aqueous solution consisting essentially of (a) 180 to 400 g/liter CrO at least 10 g/liter of a boron containing compound selected from the group consisting of boric acid or salts thereof, sulfate ions present in a weight ratio of CrO to S0,, 100:1 to 180:1, a fluorine containing compound selected from the group consisting of hydrofluoric acid, fluorboric acid, salts of fluorboric acid, fluozirconate, fluoaluminate, fluotitanate, sodium silicafluoride and potassium silicafluoride in an amount providing fluoride to CrO in a weight ratio

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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 And Plating Baths Therefor (AREA)
• Chemical Treatment Of Metals (AREA)

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• 1969
  • 1969-10-10 GB GB4980269A patent/GB1289117A/en not_active Expired
• 1970
  • 1970-10-05 ZA ZA706772A patent/ZA706772B/xx unknown
  • 1970-10-08 CA CA095159A patent/CA937531A/en not_active Expired
  • 1970-10-08 NL NL7014782A patent/NL7014782A/xx unknown
  • 1970-10-09 FR FR7036599A patent/FR2064241B1/fr not_active Expired
  • 1970-10-09 JP JP7091947A patent/JPS4948621B1/ja active Pending
  • 1970-10-12 US US00080220A patent/US3713999A/en not_active Expired - Lifetime

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