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

• the present invention concerns an improvement to processes for continuous coating of metal surfaces with chromium metal and chromium oxide. More precisely it concerns anodic pretreatment which can extend the range of metal substrates on which these coatings can be applied. Hence it is possible to apply chromium metal/chromium oxide coatings anso to bases consisting of aluminium and alloys thereof.
• Zn alloys With the introduction of Zn alloys, an attempt has been made to diversity away from conventional galvanized or paint-protected sheet. For instance Zn-Ni alloys have been proposed, but they do not appear to be capable of ensuring adequate sacrifical protection for the steel when the coating is damaged.
• multilayer coatings such as Zn/Zn-Ni or Zn-Cr/Zn-Ni or Zn-Fe/Fe-P.
• thin coatings, perforating corrosion resistance, cosmetic corrosion resistance or adhesiveness but they do not ensure all the desired properties at the same time.
• An object of the present invention is to permit the formation of a protective film of chromium metal and chromium oxide also on aluminium and alloys thereof and on coatings of aluminium and alloys thereof by means of an innovatory electrochemical process.
• Another object of this invention are to ensure an increase in resistance to corrosion, especially the cosmetic type, and to attain surface characteristics such as to provide a good appearance after painting.
• a further object of the invention is to eliminate the phosphating phase in the painting cycle, since the Cr-CrOx film replaces the functions of the phosphate layer, thus assuring cost and space savings in production, particularly in processes of the coil-coating type.
• a final object of the invention is to favour spot-weldability while increasing electrode life. More precisely, by avoiding direct contact between the zinc-based coatings and the copper of the electrodes, the life of the latter has been increased by at least six times.
• the improvement provided by this invention in which a continuous metallic body is immersed in an electrolytic solution of known type, in movement and strongly acidic due to chromic acid, is characterized by the fact that said metallic body is subjected in said electrolytic solution to an anodic pretreatment consisting in the imposition of at least one pulse of current having a density between 5 and 50 A/dm2 for a time between 0.5 and 3 seconds, and subsequently in the same solution to a treatment, known per se, consisting in electrolytic deposition of chromium metal and chromium oxide, said solution having a pH between 0 and 2, at a relative velocity with respect to said metallic body between 0.5 and 4 m/s and a treatment temperature between 40 and 80 °C.
• composition of said electrolytic bath for implementation of the present invention is preferably selected in the range CrO3 from 20 to 80 g/l, 40% HBF4 from 0 to 5 ml/l, NaF from 0 to 2 g/l, 98% H2SO4 from 0 to 1 g/l.
• test technique involved exposure in the salt-spray cabinet (5%NaCl) as per ASTM B117, evaluating the time necessary for the appearance of red rust on the surface.
• the specimens with the Cr-CrOx coating obtained adopting anodic pretreatment all gave the same type of indications, resistance being at leas equal to that -already good - provided by hot-dip coatings with a thickness in excess of that of a normal electrogalvanized coating.
• Table 2 sets forth the results obtained for paint peeling, expressed in mm2, for samples of:
• the Table reveals that when an attempt is made to obtain Cr-CrOx coatings on aluminium and alloys thereof using known techniques, in the worst cases the deposits are non-adherent and easily removed during cleaning, while even in the best cases the corrosion resistance of the specimens is decidedly worse than that of similar specimens which have been subjected to a preliminary anodic pulse, so they are certainly not capable of meeting market demands.

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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)
• Other Surface Treatments For Metallic Materials (AREA)

Applications Claiming Priority (2)

Publications (2)

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ID=11264843

Family Applications (1)

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Cited By (2)

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• 1990
  • 1990-07-17 IT IT48149A patent/IT1241489B/it active IP Right Grant
• 1991
  • 1991-06-27 EP EP19910830288 patent/EP0467858A3/en not_active Withdrawn

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