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
  • C25D11/38—Chromatising

Definitions

• a metal product is used as a cathode in an electrolyte prepared by adding a small amount of water-soluble organic compound containing a thio radical to an aqueous solution of a chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ion.
• the water-soluble organic compound containing a thio radical used in this process includes thioformamide, thiourea, thiosemicarbazide, thioglycol, thioacetate, and thiosalicylate.
• This invention relates to a method for the electrolytic surface treatment of metal product which comprises subjecting a surface of metal to electrolytic formation treatment. More particularly, it relates to the above-mentioned method wherein a treatment solution prepared by adding a water-soluble organic compound containing a thio radical to an aqueous solution of chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ions is used as an electrolyte, the metal product is electrolyzed as a cathode and the electrolytic rteatment is practiced.
• composition of such a chromate film electrolytically formed on the surface of metal is represented by the general formula (CrO (Cr O (H O)z, and is said to be a gluey complex compound consisting of a hydrate of trivalent chromium oxide and hexavalent chromium oxide.
• a metal product acts with an acidic treatment solution to generate hydrogen and is eluted as metal ion
• electron released from the metal acts on hexavalent chromium ion which is the principal component of the treatment solution on the ice surface of the metal to reduce it to trivalent chromium ion and, further as a result of increase in a pH of the solution thereabouts on account of decrease of a concentration of hydrogen ion attributed to the generation of hydrogen, a film of the aforesaid gluey complex compound is precipitated on the surface of the metal.
• Methods applicable to the treatment of iron and steel which are today known as chromate treatment method, include a method which comprises treating iron and steel by dipping the same in a treatment solution, as the electrolyte, prepared by adding a small amount of reducing agent to an aqueous solution comprising principally of hexavalent chromium ion (Japanese patent publication NO. 3,219/60, U.S. Patent Nos.
• an object of this invention is to provide surface treatment method of metal product, which affords better corrosion resistance to the treated metal product than produced by the conventional methods.
• Another object of this invention is to remarkably improve corrosion resistance of metal product, and at the same time is to form an electrolytically treated film on the surface of metal for the purpose of improving adhesitivity to an organic film of pigment and the like.
• a further object of this invention is to provide a novel electrolyte to be used in the aforesaid process.
• the aqueous solution of chromium compound includes an aqueous solution of chromium trioxide or salt of bichromic acid, and the like.
• the water-soluble organic compound containing a thio radical used in the present invention includes, for example, thioformamide, thiourea, thiosemicarbazide, thioglycol, thioacetate, and thiosalicylate. The amounts of these compounds to be added may be a small amount.
• the surface of metal to be treated is first cleaned by pretreatment such as degreasing, pickling and, waterwashing and then an electrolytic treatment is carried out using the metal to be treated as a cathode with the following composition of electrolyte and electrolysis conditions.
• Chromium trioxide or bichromate Water soluble organic compound containing a thio radical Treatment temperature Treatment time Current density at the cathode More than a./dm.
• the product After completion of the electrolysis treatment, the product is water-washed and dried.
• An object of the invention resides in the use of the Water-soluble organic compound containing a thio radical as described above. It is considered that a strong molecular adsorption effect of this water-soluble organic compound to the surface of the metal acts effectively on the formation of film upon the metal surface on electrolysis. That is, according to the process of the invention, numerous molecules of thio compound are absorbed onto the surface of metal in the treatment solution and, under such a surface condition of the cathode when hexavalent chromium ion is electrolytically reduced to trivalent chromium ion, the aforesaid gluey complex compound thereby produced grows as, a dense and hard film. Contrary to the fact that a film deposited according to conventional methods of this kind has been confirmed to be a non-crystalline substance, it has been clarified by electron diffraction that the film formed according to the process of the invention is deposited as a crystalline substance.
• Thickness of films obtained by electrolytic treatment methods are thin in general.
• weight of film is 4 mg. to about 8 mg. per one dm.
• corrosion resistance of the treated product is remarkably improved, compared with those of conventional ones.
• the product does not become rusty when subjected to a salt water spray test in accordance with Japanese Industrial Standard Z237l, extending the spraying time until reaching 500 hours.
• color tone of treated film is delicately influenced by the electrolysis conditions such as a concentration of chromium compound in the electrolyte and temperature at the electrolyte, and the current density of cathode, and therefore the control of operating conditions for producing the product having a uniform color tone becomes diificult, especially in the continuous treatment of the steel strip.
• the color tone of film obtained in accordance with the process of the invention is scarcely dominated by the variations of the above-mentioned electrolysis conditions, whereby a film of beautiful pale blue having a strong water repellent property can be always obtained.
• the film obtained according to the invention is harder than those obtained by conventional methods, and therefore it is difficult to subject to scratching damage.
• cross-cut adhesion test was conducted on sample on which melamine-alkyd paint has been coated in order to measure adhesivity of the film to paint, and as a result, there was obtained a favorable result wherein the number of squares remaining amounted to more than 95.
• adhesivity of the film to the surface of metal material bending test, an Erichson test were conducted on various kinds of products to find that no peeling phenomenon of film in each test was observed at all.
• EXAMPLE 1 Product to be treated Low carbon steel cold rolled sheet having 0.2 mm. thickness.
• the thus treated steel sheet had a beautiful pale blue color and the weight of film was found to be 60 mg. per one dm. No rust was generated thereon after salt water spray test (Japanese Industrial Standard Z237l) carried out for a period extending up to 500 hours.
• EXAMPLE 2 Product to be treated Low carbon steel cold rolled sheet having 0.2 mm. thickness.
• Chromium trioxide 100 g./l. Thioglycol 2 cc./l. Treatment temperature 15 C. Treatment time 10 seconds.
• the thus treated steel sheets had a beautiful pale blue color and the weight of film was found to be 3.8 mg. per one dm. No rust was observed thereon after salt water spray test (Japanese Industrial Standard Z-2371) carried out for a period extending up to hours.
• EXAMPLE 3 Product to be treated Low carbon steel cold rolled sheet having thickness of 0.2 mm. Chromium trioxide 70 g./l. Thiosemicarbazide 2 g./l. Treatment temperature 30 C. Treatment time 5 seconds. Current density at the cathode 20 a./dm. Drying condition of film after water-washing At 100 C. for 30 minutes.
• the thus treated steel sheet had a beautiful pale blue color and the weight of film was found to be 2.3 mg. per one dm. No rust was observed thereon after an outdoor exposure test carried out for a period of extending up to three months.
• a process for the surface treatment of iron and steel products which comprises electrolytically forming a protective film on the surface of said metal product by carrying out electrolysis in the presence of an electrolyte solution under the conditions suitable for forming a corrosion resistant film on the surface of the metal while using the metal product as a cathode, said electrolyte solution being an aqueous solution of a chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ions, said solution also containing a water-soluble organic compound containing a thio radical.
• aqueous solution of chromium compound is an aqueous solution of chromium trioxide or salt of bichromate.
• watersoluble organic compound is a member selected from the group consisting of thioformamide, thiourea, thiosemicarbazide, thioacetate and thiosalicylate.
• a process for the surface treatment of iron or steel products which comprises'forming an electrolyte treatment solution by the addition of 0.5-2.0 g./l. of a watersoluble organic compound containing a thio radical to an aqueous solution containing 20-150 g./l. of chromium trioxide, and thereafter subjecting said product to electrolysis conditions while in contact with said electrolyte treatment solution, said product utilized as a cathode, said electrolysis conducted under a treatment temperature of 2060 C., a treatment time of at least 1 second, and a current density at the cathode of no less than 10 a./dm. whereby the dense protective film is formed on the surface of said product.
• watersoluble organic compound containing a thio radical is a member selected from the group consisting of thioformamide, thiourea, thiosemicarbazide, thioglycol, thioacetate and thiosalicylate.

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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)
• Chemical Treatment Of Metals (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)

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Citations (4)

• 1967
  • 1967-02-28 GB GB9556/67A patent/GB1179021A/en not_active Expired
  • 1967-03-03 DE DE1621096A patent/DE1621096C3/de not_active Expired
  • 1967-03-06 US US620681A patent/US3486990A/en not_active Expired - Lifetime
  • 1967-03-10 FR FR98355A patent/FR1515378A/fr not_active Expired

Patent Citations (4)

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