Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/08—Electrolytic coating other than with metals with inorganic materials by cathodic processes
  • C25D9/10—Electrolytic coating other than with metals with inorganic materials by cathodic processes on iron or steel
• • 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
• • 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

• the present invention is concerned with hardening oxidic films formed on iron-base, chromium-containing, corrosion-resistant alloys, e.g., stainless steels, by treatment of said alloys in aqueous sulfuric acid solutions containing a pitting inhibiting oxidizing agent (advantageously chromic acid).
• a known method of treating stainless steel and other chromium-containing alloys comprises two main steps, in the first of which an oxidic film is formed on the surface of the alloy by immersion of the alloy in an aqueous solution of chromic and sulfuric acids, with or without other constituents, and in the second of which the alloy bearing the film is subjected to electrolysis as the cathode in an electrolyte from which chromium can be deposited.
• This method is the subject of US. application Ser. No. 114,357, now US. Pat. 3,755,117 and 252,459, now US. Pat. 3,766,023, filed in the name of Anthony C. Hart, in which it is explained that the electrolytic treatment is short, lasting for a period of time adequate to harden the film, but not so long that any chromium becomes visible on the surface as a white deposit.
• chromium metal In practice of the Hart process, there is an increasing tendency for chromium metal to be deposited from. aqueous electrolytes containing chromic and sulfuric acids as the current density increases, and, at the comparatively low current density required to ensure that no chromium metal is deposited, it is difficult to harden the film completely.
• Our object in the present invention is to eliminate or at least to reduce the deposition of metallic chromium on cathodes during electrolysis of solutions containing hexavalent chromium.
• the present invention contemplates a process for hardening an oxidic film produced on an iron-base, chromium-containing, corrosion-resistant alloy by treatment in an aqueous sulfuric acid bath containing a pitting inhibiting oxidizing agent comprising treating as a cathode the surface of such an alloy bearing such an oxidic film in an aqueous solution containing hexavalent chromium (introduced advantageously as CrO and an agent capable of promoting the cathodic deposition of chromium oxide in preference to metallic chromium.
• aqueous chromic acid electrolytes e.g., baths containing about 25 to about 750 grams per liter (gpL) of CrO ions produced by addition of substances from theigroup of CrCl HF, HNO H PO and CH COOH and ions derived from reaction of these substances with water and/or hexavalent chromium are capable of promoting the cathodic deposition of oxidic deposits such as chromic oxide, hydrated chromic oxide or chromic hydrpxide in preference to chromium metal.
• iron-base, corrosion-resistant, chromium-containing alloy includes stainless steels and other iron-containing alloys which also contain greater than about 11% and up to about 30% by weight of chromium.
• Stainless steels can be ferritic or austenitic and usually contain about 13% to about 25% (by weight) chromium.
• the metal carrying the film to be hardened is made the cathode in a hardening solution as contemplated in the present invention and cathodic electrolysis carried out, the hexavalent chromium solution is cathodically reduced and an oxidic deposit such as oxide, hydrated oxide or hydroxide is formed in the pores of the film.
• an oxidic deposit such as oxide, hydrated oxide or hydroxide is formed in the pores of the film.
• Local pH changes brought about electrolytically in proximity to the film are considered to assist in the oxide deposition.
• the color of the oxide deposited has little or no effect on the color of the film, although the color of the film is usually slightly advanced in shade.
• Chro'mic acid plus chromic chloride in aqueous solution was employed as the hardening electrolyte.
• the solutiom was made up of 250 gpl. of CrO and 2.1 gpl. of CrCl '6I-I O to provide a 2.5M CrO and 0.025N chloride ion, i.e., a molar ratio of about 100 to 1 hexavalent chromium to chloride ion.
• the increased hardness does not result in unacceptable color advancement. Provided that the process is not operated for too long a time, no deposit is formed on top of the color 'fi1m. Raising the operating temperature to 40 C. or 60 C. is beneficial, enabling longer process times to be used without surface deposition.
• EXAMPLE 2 Chromic acid plus hydrofluoric acid in aqueous solution was employed as the hardening electrolyte.
• the electrolyte was made up with 250 gpl. of CrO and 2.5 gpl. of 40% hydrogen fluoride in water to provide a 50 to 1 molar ratio of CrO to fluoride ion. Test results are set forth in Table II.
• EXAMPLE 5 Chromic acid plus acetic acid in aqueous solution was employed as the hardening electrolyte.
• the resulting hardnesses were comparable to or in excess of those achieved with chromic acid plus sulfuric acid hardening solutions, for example, equal to or better than 100-150 rubs to failure of the film. There was some evidence that a freshly prepared solution was more effective than a solution that had been allowed to stand, possibly, because on standing acetic acid may have been oxidized by the chromic acid.
• a process for hardening an oxidic film on iron-base, chromium-co taining, corrosion-resistant alloy, aid xidic film having been formed by immersion of said alloy in an aqueous solution of sulfuric acid and a pitting-inhibiting oxidizing agent comprising making said alloy bearing said film a cathode in an aqueous solution containing hexavalent chromium and an agent capable of promoting the cathodic deposition of chromic oxide, hydrated chromic oxide or chromic hydroxide in preference to metallic chromium and passing sufiicient electric current across the solution cathode interface so as to deposit said chromic oxide, hydrated chromic oxide or chromic hydroxide in said oxidic film to harden said oxidic film.
• the agent capable of promoting the cathodic deposition of chromic oxide, chromic hydroxide or hydrated chromic oxide in preference to metallic chromium is selected from the group of chloride ion, fluoride ion, nitrate ion, phosphate ion and acetate ion and ionic species derived from reaction of said ions with water and hexavalent chromium.
• a process as in claim 2 wherein the mole ratio of hexavalent chromium to ion of said agent is about 20 to 1 to about 500 to 1.
• a process as in claim 1 wherein the agent added to the aqueous solution is a chloride ion donor.
• a process as in claim 1 wherein the agent added to the aqueous solution is a phosphate ion donor.
• aqueous solution containing hexavalent chromium is a solution of chromic acid containing about 25 to about 750 grams per liter of CI'O3.
• a process as in claim 1 wherein the agent added to the aqueous solution is a fluoride ion donor.
• a process as in claim 1 wherein the agent added to the aqueous solution is a nitrate ion donor.
• a process as in claim 1 wherein the agent added to the aqueous solution is an acetate ion donor.

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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)
• Inorganic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Chemical Treatment Of Metals (AREA)
• Electrochemical Coating By Surface Reaction (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=10443517

Family Applications (1)

Country Status (16)

Cited By (3)

Families Citing this family (4)

• 1972
  • 1972-10-12 GB GB4704872A patent/GB1435518A/en not_active Expired
  • 1972-10-30 CA CA155,185A patent/CA1012922A/en not_active Expired
  • 1972-10-30 US US00301810A patent/US3832292A/en not_active Expired - Lifetime
• 1973
  • 1973-10-03 IT IT52892/73A patent/IT1066695B/it active
  • 1973-10-11 ES ES419556A patent/ES419556A2/es not_active Expired
  • 1973-10-11 FR FR7336340A patent/FR2202956B2/fr not_active Expired
  • 1973-10-11 CH CH1450873A patent/CH557889A/xx not_active IP Right Cessation
  • 1973-10-11 DE DE2350994A patent/DE2350994C3/de not_active Expired
  • 1973-10-11 DK DK551873A patent/DK144338C/da not_active IP Right Cessation
  • 1973-10-11 BE BE136580A patent/BE805953R/xx active
  • 1973-10-11 ZA ZA00737932A patent/ZA737932B/xx unknown
  • 1973-10-11 NL NL7313995.A patent/NL165229C/xx not_active IP Right Cessation
  • 1973-10-11 SE SE7313810A patent/SE435939B/xx unknown
  • 1973-10-12 AT AT873773A patent/AT326441B/de not_active IP Right Cessation
  • 1973-10-12 JP JP11525573A patent/JPS5546476B2/ja not_active Expired
• 1976
  • 1976-11-18 HK HK722/76*UA patent/HK72276A/xx unknown

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