Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F3/00—Brightening metals by chemical means
  • C23F3/02—Light metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
  • C22C21/08—Alloys based on aluminium with magnesium as the next major constituent with silicon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F1/00—Etching metallic material by chemical means
  • C23F1/10—Etching compositions
  • C23F1/14—Aqueous compositions
  • C23F1/32—Alkaline compositions
  • C23F1/36—Alkaline compositions for etching aluminium or alloys thereof

Definitions

• This invention relates to an improved process for etching aluminum alloys and, more particularly, relates to a process, as aforesaid, in which the etched products have a substantially uniform gloss finish from piece to piec and in which the etchant bath has a much longer useful life than it has heretofore been possible to achieve.
• Galvanizing refers to the phenomena according to which the grain located near the surface of the aluminum extrusion becomes relatively highly visible.
• galvanizing problem appears to be limited to aluminum extrusions because aluminum alloy sheets, for example, as a result of their smaller grain size and other metallurgical factors, do not appear to be subject to galvanizing.
• aluminum alloy castings although having a larger grain size are subject to having nonuniform composition and porosity and therefore have only limited use for decorative or architectural purposes.
• 3,464,870 Patented Sept. 2, 1969 galvanizing is the result of aluminum build-up in the etchant bath. It has been considered that the maximum aluminum concentration in the etchant bath that can be tolerated, and still entirely avoid galvanizing, is approximately 25 g. of dissolved aluminum per liter.
• the gloss of aluminum extrusions can be measured by a suitable gloss meter, such as one manufactured by Photovolt Corporation. Such meters are graduated to indicate the gloss of the surface measured. Readings can be based on a perfect mirror finish having a reading of 1000.
• the aluminum alloy extrusions and sheets produced by the present invention under typical conditions have a gloss measured at 45 to the surface of about when the dissolved aluminum concentration in the bath is 50 g. per liter and they have a gloss of about 200 when the dissolved aluminum concentration is about 80 g. per liter.
• the gloss reading should be maintained within a range of not more than about :20% of the median gloss reading.
• the aluminum alloy that is being etched is the major source of the dissolved zinc, although other possible sources include the make-up water supplied, and impurities in the proprietary etchant.
• a typical value for zinc which is present in 6063 aluminum alloy is'.02%. It has been found that the presence of small amounts of sulfide ions in the etchant bath will precipitate the zinc sulfide and that such substantially eliminates the problem of galvanizing in etchant baths having a high dissolved aluminum concentration.
• a method of etching an aluminum alloy product which comprises treating the product with an alkali etching solution having dissolved therein at least about 50 g. of aluminum per liter and also having present therein a positive concentration of sulfide ions.
• the minimum dissolved aluminum concentration is 50 g. per liter.
• the etched surface of an extrusion thus treated is free of galvanizing effects caused by the etch bath and is smooth and has a uniform gloss as the bath ages further. This gloss will be substantially uniform for all aluminum alloy extrusions and sheets treated by the same type of bath under similar conditions of time, temperature and alkali concentration in the bath.
• the useful life of the bath will be of indefinite duration, provided that suitable amounts of make-up chemicals are added thereto from time to time to maintain the desired concentrations of such chemicals.
• the method according to the invention can be carried out on aluminum sheets or extrusions of the compositions referred to above, it being noted here that the invention does not pertain to the treatment of other aluminum alloys.
• An aluminum alloy which provides especially good results is the commercially available alloy 6063.
• other 6063 type aluminum alloys can be etched according to the invention to obtain similar improved results.
• the etching treatment is carried out in a hot aqueous alkali solution containing, for example, sodium or potas sium hydroxide, or other alkaline materials, sodium hydroxide, being preferred.
• the alkali concentration of the etchant bath is not critical and such concentration can range from 2 oz. alkali/ gallon of Water to oz. alkali/ gallon of water.
• the alkali solution also can contain various conventional noncritical additives, such as sodium gluconate, which are added to improve the etching effect, prevent scale formation, etc.
• the alkali solution must contain at least about 50 g. of dissolved aluminum per liter in order to achieved the desired uniform gloss on the surface of the treated aluminum alloy products. Lesser amounts of dissolved aluminum in the etchant bath will cause variations in the gloss of the etched extrusion which are diflicult to control.
• the maximum dissolved aluminum concentration in the etching bath is limited only by the ability of the etchant solution to retain the aluminum in solution. This is affected by a variety of factors, including particularly the temperature of the solution. It has been found that when a suitable temperature is maintained, the concentration of the dissolved aluminum in the etchant bath can be as high as about 90-105 g. per liter Without difficulty, and even higher dissolved aluminum concentrations can be used in appropriate cases.
• the make-up etching solution is free of aluminum, the aluminum concentration in the etchant bath is thereby maintained within fairly constant limits because the rate at which the aluminum is dissolved from the etched products becomes more or less equal to the amount of dissolved aluminum that is removed from the bath as a result of the drag out.
• the temperature of the etchant bath may range from F. to 180 F. It has been found satisfactory to use etching times of from 1 to 30 minutes.
• a positive sulfide ion concentration is maintained in the etchant bath at all times in order to precipitate Zinc as zinc sulfide. Because of the low solubility of zinc sulfide in alkaline aqueous solutions, it is possible to main tain the residual zinc concentration in the bath at a sufficiently low level that galvanizing does not occur.
• the sulfide ion concentration is continuously maintained at a level of from four to eight gms. per liter (calculated as sodium sulfide) in order to assure precipitation of the zinc. A higher level of the sulfide concentration can be maintained, if desired, but such is not necessary.
• the sulfide material is added to the bath from time to time to make up for the part thereof which precipitates with the zinc and other alkali insoluble metallic sulfides and, also, the part that leaves the bath with the etched product as a result of the drag out.
• the sulfide material will be required at a rate of one to ten gms. for each 100 gms. of the make-up etchant material added to the bath.
• sulfide ions supplying material examples include hydrogen sulfide, alkali sulfides and polysulfides, sodium sulfide being preferred. These materials may be employed individually or as a mixture of two or more of these materials.
• Gloss differences can be minimized by anodizing or other after treatment.
• a method of polishing aluminum alloy extrusions which contain minute traces of zinc for decorative or architectural purposes in an etching solution to produce a uniform gloss finish thereon, to substantially prevent galvanizing of the surfaces thereof and to substantially extend the useful life of said etching solution which includes immersing said extrusions in an alkali etching solution having dissolved therein at least about grams per liter of aluminum and having present therein sulfide ions sufficient to limit the presence of zinc to less than five parts per million.
• sulfide ions are obtained by dissolving in the etching solution materials selected from the group consisting of sulfides and polysulfides of alkali metals.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• ing And Chemical Polishing (AREA)

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Publications (1)

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Cited By (4)

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Citations (2)

• 1965
  • 1965-06-25 US US467132A patent/US3464870A/en not_active Expired - Lifetime
• 1966
  • 1966-06-10 GB GB25995/66A patent/GB1136588A/en not_active Expired
  • 1966-06-20 DE DE19661521908 patent/DE1521908B2/de not_active Withdrawn

Patent Citations (2)

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