Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc
  • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
• • 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
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32

Definitions

• a method for the surface treatment of an alloy consisting predominantly of zinc and aluminium, and containing 60-85% by weight of zinc by immersing a surface of the alloy in dilute aqueous sulphuric acid, comprising subjecting the alloy to anodic oxidation at a current density of from 50 to 200 milliamperes per square centimetre of surface.
• the alloy may include up to 0.5% by weight of copper or magnesium as a third metal.
• This invention relates to the surface treatment of zinc/ aluminium alloys to produce thereon a protective and/ or decorative surface finish.
• aluminium alloys containing a few percent of zinc by Weight can be anodised in a 15% solution by weight of sulphuric acid to produce aluminium oxide films containing zinc values in approximately the alloying proportions. It has been found impossible to anodise zinc in any sulphuric acid solution or a 55/45 zinc/aluminium alloy in 4 N sulphuric acid solution at a current density of 10 ma./ sq.
• the invention consists in a method for the surface treatment of an alloy consisting predominantly of zinc and aluminium, and contaiining 60-85% by weight of zinc, comprising immersing a surface of the alloy in dilute aqueous sulphuric acid and subjecting it to anodic oxidation at a current density of from 50 to 200 milliamperes per square centimetre of the surface.
• zinc/aluminium alloys containing 60-85% by weight of zinc may be anodically oxidised in aqueous solutions of sulphuric acid to produce films which are basically aluminium oxide with only a small content of zinc, probably as oxide, of about 510% by weight of the anodised film.
• sulphuric acid aqueous solutions of sulphuric acid
• this film confers corrosion protection on the alloy and that sealing treatment of the type 3,729,391 Patented Apr. 24, 1973 "Ice applied to anodised films on aluminium also has a beneficial effect on these films.
• the alloy is usually either an alloy of high-purity zinc, or commercial-purity zinc (i.e. containing only incidental impurities, as well known to the man skilled in the art) with correspondingly high-purity or commercial-purity aluminium. While the alloy can have a composition containing up to by weight of zinc, i.e. as little as 15% by weight of aluminium, in practice it is most preferred to use compositions containing 60-80%, or even 60- 70% of zinc by weight. Although the alloys to be treated in the present method consist predominantly of zinc and aluminium it is possible to include minor amounts of a ternary metal as deliberate additions, e.g. up to 0.5% of copper or magnesium may be added.
• the microstructure of the alloy is usually that produced by casting and mechanical working, and will generally comprise alternate regions, e.g. of approximately 1,u. diameter (i.e. 10- metres) of zinc-rich and aluminium-rich phases.
• Zinc may be removed from the surface of the alloy prior to anodizing e.g. by evaporation in vacuo.
• anodizing e.g. by evaporation in vacuo.
• it is a valuable feature of the present method that by suitable control of the anodizing conditions the preferential leaching of a zinc phase and the anodizing of an aluminium-rich phase may be carried out simultaneously in the same bath of sulphuric acid.
• the sulphuric acid concentration is preferably between 1% and 40% (W./v.) and the most preferred range is between 10% and 20% (w./v.) H 80 in water.
• the anodizing is preferably carried out at a temperature within the range 030 C. Above 30 C. the anodized layer becomes less adherent.
• sealing can be effected in boiling distilled water.
• the voltage began to rise slowly after a region of very low voltage (12 V.) lasting 5 minutes, and then more steeply to give a peak of 50 v. after 25 minutes. The voltage then dropped slowly to give 15 v. after 34 minutes, and then rose slowly to give 35 v. after 60 minutes.
• ALLOY F At 150 ma./cm. alloy F exhibited a region of very low voltage (1-2 V.) lasting 2 minutes followed by a voltage rise to a plateau of 8 v. after 3 minutes. This value was maintained until a sharp rise started after 7.5 minutes to give 30 v. after 12.5 minutes. The voltage then rose more gradually to 45 v. after 22.5 minutes. At this stage a voltage surge to over 100 v. occurred at about 25 minutes. The voltage then dropped sharply to 30 v. and thereafter fluctuated gently about this value.
• the voltage rose to a platen of 4 v. after 6 minutes after an initial low voltage region of about 12 v. lasting 5 minutes. After about 16 minutes a sharp rise occurred reaching 30 v. after 22.5 minutes. The voltage then rose more gradually to about 45 v. after 35 minutes. It then dropped gradually to about 40 v. after 47.5 minutes and then more sharply to 25 v. after 50 minutes. Thereafter the voltage remained approximately at this value.
• Tests were also carried out on commercial purity zinc/ aluminium alloys containing small amounts of magnesium and copper. Two alloy samples were treated as summarised below.
• the voltage rose gradually to 3 v. after 20 minutes. This rise steepened to 30 v. after 35 minutes. The voltage then dipped to 28 v. after 42.5 minutes and rose again to 37 v. after 60 minutes.
• the voltage rose to 4 v. after 6 minutes after an initial low voltage region of 1-2 v. lasting 5 minutes. The voltage then rose increasingly sharply to give 26 v. after 25 minutes. It then rose gradually to 35 v. after 52.5 minutes and then dropped to 25 v. after 60 minutes.
• SPECIMEN FILM THICKNESS-TIME DATA The figures in the table below refer to the thickness of the anodic film (in microns) formed during anodising of Zn-22% Al at 100 ma./cm. in w./v. sulphuric acid at C. for various times.
• the alloy includes up to 0.5% by Weight of a th rd metal selected from the group consisting of copper and magnesium.
• a method as claimed in claim 1 further comprising sealing the anodized film produced in boiling distilled water.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Mechanical Engineering (AREA)
• Prevention Of Electric Corrosion (AREA)
• Electrolytic Production Of Metals (AREA)
• Electroplating Methods And Accessories (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=9773212

Family Applications (1)

Country Status (6)

Cited By (6)

• 1971
  • 1971-02-09 GB GB423171A patent/GB1322077A/en not_active Expired
• 1972
  • 1972-02-03 DE DE2204958A patent/DE2204958C3/de not_active Expired
  • 1972-02-04 NL NL727201517A patent/NL147794B/xx unknown
  • 1972-02-08 BE BE779103A patent/BE779103A/xx unknown
  • 1972-02-08 SE SE7201476A patent/SE372565B/xx unknown
  • 1972-02-09 US US00224979A patent/US3729391A/en not_active Expired - Lifetime

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