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 small amount of one or more of a thiosulfate, dithionite, bisulfite, sulfite, thiosulfite, pyrosulfite, sulfide, as an auxiliary agent is added to a main electrolyte solution of chromic acid and a cathode electrolytic reduction thereof is carried out in a conventional cell.
• This invention relates to a process for producing a coating having excellent adhesion properties and/or corrosion resistance using an ordinary stationary cell and electrolytic reduction with an aqueous treating solution comprising chromic acid as the primary agent and an auxiliary agent selected from thiosulfate, sulfite, dithionite, bisulfite, thiosulfite and sulfide.
• auxiliary agents which have been used include: sulfuric acid, tartaric acid, maleic acid, oxalic acid, aromatic sul- :fonic acid having a hydroxyl group, sucrose, and glycol.
• the coating film produced by conventional electrolytic treatment generally comprises a double layer consisting of metallic chromium and hydrated chromium oxide layers deposited on the surface of steel, notwithstanding the difference in some of the electrolytic conditions or the difference in auxiliary agents. It is true that such a double layer contributes to the corrosion resistance, but problems remain with regard to adhesion or color tone of the surface.
• the object of the present invention is to provide a process for forming easily a coating which is primarily chromium oxide.
• a process for depositing chromium hydroxide on a steel surface which comprises contacting a steel surface with an aqueous chromium plating solution for from 1 to 5 seconds and a current density of to 50 A/dm.
• the solution containing from about 50 to about 100 grams per liter of chromic acid and from about 0.5 to about 1 gram per liter of an auxiliary agent selected from the group consisting of an alkali metal thiosulfate, sulfite, dithionite, bisulfite, thiosulfite, pyrosulfite, and sulfide.
• the aqueous chromium solutions employed have an usually low concentration of chromic acid.
• concentration ranges from about 50 to about 100 grams per liter (g./l.). Concentrations below about 50 g./l. are insuflicient to provide a suitable film on the steel surfaces which are treated. More than about 100 g./l. results in considerable deposit of metallic chromium, rather than the desired chromium hydroxide. With excessive deposits of metallic chromium, the resulting films suffer in adhesion properties.
• the aqueous chromium solutions also contain one or more auxiliary agents.
• These agents include water-soluble thiosulfates, sulfites, dithionites, bisulfites, thiosulfites, pyrosulfites and sulfides, particularly the alkali metal compounds of these groups.
• the concentration of such an agent or agents in the solutions is also controlled. Such concentrations range from about 0.5 to about 1 g./l. of solution. Concentrations of less than about 0.5 g./l. are insufficient to provide the desired characteristics to the steel surfaces which serve as the cathode and which are treated. And concentrations in excess of about 1 g./l. cause unsatisfactory surface efiects.
• Current density employed ranges from about 10 to about 50 A/dmfi, with about 30 preferred.
• Plating or treating time with such a current density is extremely short, from about 1 to about 5 seconds, with about 3 preferred.
• Steel such as strip steel, ordinary carbon steel sheet and the like are suitable for use in the process of this invention.
• Bending test was carried out in accordance with test method 115 Z 2203 and impact test in accordance with test method 115 Z 2242.
• the samples, the coated surfaces of which are on the outside, are pushed out of the inside of an Erichsen tester by a circular rod having a certain diameter at a definite speed, and the distance by which the samples are pushed is measured when cracks or exfoliations appear.
• the coated surface of the samples is cut with a knife in such a way as to make 100 squares, each of which has an area of 1 mm. so deeply as to reach the matrix, and the state of the exfoliation of the film is observed.
• Table 3 shows the result of chemical property tests.
• Salt spray test is carried out in accordance with test method JIS Z 2371. The result of the test is evaluated as 1 when rust appears in more than 51% of the area of the surface of the sample, and as 10 when rust does not appear (i.e. of the area).
• Shade test is carried out with regard to the colors of rusts. A sample on which the color of the rust does not change at all is evaluated as 10, and one whereon the characteristic reddish brown rust appears is evaluated as 1.
• the anti-corrosive property of a film on the surface of the samples is substantially superior to that of a film wherein H was used in the treating solution.
• a process for depositing chromium hydroxide on a steel surface which comprises contacting a steel surface as a cathode with an aqueous chromium plating solution for from about 1 to about 5 seconds and a current density of from about 10 to about 50 A/dmfi, the solution containing from about 50 to about grams per liter of chromic acid and from about 0.5 to about 1 gram per liter of an auxiliary agent selected from the group consisting of an alkali metal thiosulfate, sulfite, dithionite, bisulfite, thiosulfite, pyrosulfite and sulfide.
• a process of claim 1, wherein the agent is Na S O 5.
• a process of claim 1, wherein the agent is Na S O 6.
• a process of claim 1, wherein the agent is Na SO 7.
• a process of claim 1, wherein the agent is NaHS0' 8.
• a process of claim 1, wherein the concentration of said agent is from about 0.5 to about 0.65 gram per liter.

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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)
• Electroplating Methods And Accessories (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=12212107

Family Applications (1)

Country Status (3)

Cited By (1)

• 1968
  • 1968-04-26 GB GB09879/68A patent/GB1216770A/en not_active Expired
  • 1968-04-29 FR FR1572432D patent/FR1572432A/fr not_active Expired
• 1970
  • 1970-06-01 US US00042505A patent/US3794572A/en not_active Expired - Lifetime

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