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
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32

Definitions

• the present invention relates to an anodizing bath and a process for producing adherent, corrosion and abrasion resistant coatings on surfaces composed of zinc and zincbase alloys.
• a process for applying a corrosion and abrasion resistant coating to zinc and zinc-base alloy surfaces comprising the step of subjecting to electrolysis an aqueous solution which contains chromate, phosphate, and fluoride anions provided by compounds selected from chromic acid, phosphoric acid, hydrofluoric acid, and the ammonium salts thereof.
• the solution has a pH value within the range of from about pH 6 to about pH 8 and is substantially free from metal ions.
• the surface of at least one of the electrodes comprises zinc or a zinc-base alloy to be coated. Either alternating current or direct current can be used. With alternating current, the preferred current density is about 50-200 amperes per square foot of electrode surface per electrode.
• the voltage rises as the electrolysis proceeds, due to the increasing resistance of the coating as it is formed. Electrolysis is continued until the voltage rises to a levelling, or finishing, value of at least about 200 volts, usually from about 200 to about 250 volts.
• the chromate anion according to the copending application may be replaced at least in part by tungstate, molybdate or vanadate anion.
• the use of such a coating or anodizing solution allowed the electrolysis to be conducted at a voltage substantially below that necessary for the electrolytic process of United States Patent No. 3,011,958 and thus instead of a final voltage of 200 to 250 volts, a finishing voltage of only 35 to 110 is necessary.
• anodizing bath consisting of an aqueous solution containing silicate anion calculated as SiO' in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter and alkali metal cation selected from sodium and potassium, said solution having a pH from about 11 to 13.
• the pH range of the aqueous solution, i.e. electrolyte of the anodizing bath, for satisfactory film formation is from about pH 11 to about pH 13, preferably from pH 11 to pH 12.5.
• the alkalinity of the solution may also be defined by specifying the Na O/SiO mole ratio, R.
• the Na O figure is obtained by deducting from the total Na O content of .the solution the amount of Na O associated with the other anions present, such as sodium chromate for chromate, sodium vanadate for vanadate and sodium molybdate for molybdate.
• the value of R should not be less than 0.3 and not be greater than 1.0, and more preferably be in the range of 0.4 to 0.7.
• the temperature of the bath should be maintained in the range of from 25 to 95 C, but it is preferred to maintain the temperature of the bath within the range of about C to C.
• the amount of chromate anion calculated as CrO should be in the range from about 0.05 to about 1.5 moles per liter and more preferably the amount of chromate anion should be in the range of from 0.1 to 0.3 moles per liter.
• the silicate anion concentration should be in the range of 0.2 to 2.0 moles per liter calculated as SiO and more preferably in the range of 0.2 to 0.4 moles per liter. It is immaterial which of the alkali metal cations is present, both sodium and potassium and mixtures thereof serving equally Well.
• both electrodes can and should be articles formed of or surfaced with zinc or a zinc base alloy.
• the initial current density should be in the range of from about 350 to about 1000 amps per square foot until the formation of a film is initiated and thereafter the current density should be reduced to a range of to about 300 amps per square foot of electrode surface per elect-rode and the electrolysis continued until the voltage rises to a terminating value According to the present invention therefore there is of from 55 to 250' volts.
• the initial current density is from 465 to 700 amps per square foot and the final current density is in the range 115 to 230 amps per square foot and the terminating voltage is from 100 to 200 volts.
• the chromate anion can be replaced in part or in whole by at least one anion selected from vanadate, molybd-ate, permanganate and tungstate anion.
• the surfaces of the articles to be anodized are first degreased and cleaned.
• the aqueous solution may be readily prepared by dissolving sodium or potassium salts in water and adjusting the pH to the desired value with an alkali metal hydroxide.
• Chromate anion can be conveniently added as chromic acid, and vanadate, permanganate, molybdate and tungstate anion as their alkali metal salts.
• the cleaned articles are then immersed as electrodes in the squeous solution as electrolyte and connected to a suitable power source supplying 60 cycle AC within the range of from to 250 volts.
• the electrolyte is agitated and maintained at a temperature of from 25 to 95 C. during the electrolysis.
• the terminating voltage will usually be within the range of 4 from about to about 250 volts. It is found that generally sparking commences in the range of 50 to 200 volts; under preferred conditions of electrolyte composition and temperature, sparking commences in the range of to 110 volts.
• the time required to complete film formation varies with conditions but is usually of the order of 5 to 15 minutes.
• the initial current density is for example within the range of 115 to 230 amps per square foot of electrode surface to be coated.
• the process of the present invention it is possible to apply to articles formed from zinc or zinc-base alloys, for example die-cast or rolled zinc panels, or articles electroplated or hot dipped in zinc or zinc-base alloys, a coating or film which improves the corrosion resistance and appearance of the article.
• the coating gives a hard, smooth matte finish in shades of green, grey or brown depending upon the electrolyte composition, for example, a sodium silicate-chromate electrolyte will give a green shade; a sodium silicate chromate-vanadate electrolyte will give a charcoal grey shade; and permanganate-containing electrolytes will give shades of brown.
• the coating can, if desired, be painted or lacquered.
• the coating is resistant to abrasion and corrosion by various media such as industrial atmospheres, salt water and detergent solutions.
• the process is efiiective and can be applied to surfaces of commercial grades of zinc and zinc alloys such as special high-grade zinc, and alloys used in rolling, extruding and die-casting operations.
• the present invention will be further illustrated by way of the examples in Table 1, which illustrate the conditions under which satisfactory coatings are produced by the process of the present invention, and those in Table 2 which illustrate the conditions under which satisfactory coatings were produced by the process of the invention for three electrolytes using die-cast Zinc plates which had been given a preliminary treatment in an ammonium phosphate-chromate-fiuoride bath according to United States Patent No. 3,011,958 until a potential of 110 volts AC was reached; the articles were then rinsed and allowed to dry before being anodized in the electrolyte of the present invention.
• An anodizing bath for coating zinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount from 0.2 to 0.4 mole per liter, chromate anion calculated as CrO in an amount of from 0.1 to 0.3 mole per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH in the range from 11 to 12.5.
• An anod-ic process for coating zinc and zinc-base alloy surfaces which comprises electrolysing using alternating current an aqueous solution containing silicate anion calculated as SiO in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH from about 11 to 13, using electrodes having exposed surfaces formed by a metal selected from zinc and zinc-base alloys until a coating is formed on said electrodes.
• Detergent resistance was determined by subjecting anodized panels to constant immersion in a 2.5 gm./l. solution of a standard household detergent, sold under the trade name Tide, maintained at a temperature of 185 F.
• the coatings were examined periodically and rated on the basis of leaching and discoloration, softening, adhesion and base metal corrosion. After days in test, the SSV coating had shown no deterioration and was rated as best. By comparison the SSCV, the SSC and SSCW were almost as satisfactory and were rated as good.
• the SSW, SSMo, SSCMo, SSMn and the SSCMn showed varying degrees of deterioration and relatively were rated as fair.
• An anodizing bath for coating zinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount of from about 0.2 to about 2 moles per liter, chromate anion calculated as CrO in an amount from about 0.05 to 1.5 moles per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH from about 11 to 13.
• the current density is from 35 0 to 1000 amps per square foot of electrode surface per electrode initially until the formation of a film is initiated and thereafter the current density is reduced to to 300 amps per square foot of electrode surface and the electrolysis continued with increasing voltage to a terminating value in the range of from 55 to 250 volts.
• An anodic process for coating zinc and zinc-base alloy surfaces which comprises electrolysing using alternating current an aqueous solution containing silicate anion calculated as SiO in an amount from 0.2 to 0.4 mole per liter, chromate anion calculated as CrO in an amount from 0.1 to 0.3 mole per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH in the range from 11 to 12.5, the temperature of the bath being maintained within the range from 25 C. to 95 C.
• An anodizing bath for coating zinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount of from 0.2 to about 2 moles per liter, and at least one member selected from the group consisting of chromate anion calculated as CrO vanadate, permanganate, molybdate and tungstate anion and mixtures thereof in an amount from about 0.05 to 1.5 moles per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH of from about 11 to 13.
• An anodizing bath for coatingzinc and zinc-base alloy surfaces consisting of an aqueous solution consisting essentially of silicate anion calculated as SiO in an amount of from 0.2 to 0.4 mole per liter, and at least one member selected from the group consisting of chromate anion calculated as CrO vanadate, molybdate and tun gstate anion'mixtures thereof in an amount of from 0.1 to 0.3 mole per liter, and cations of at least one alkali metal selected from the group consisting of sodium and potassium, said solution having a pH of from about 11 to 12.5.

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• Chemical Treatment Of Metals (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Other Surface Treatments For Metallic Materials (AREA)

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Citations (4)

• 0
  • NL NL128525D patent/NL128525C/xx active
• 1964
  • 1964-07-13 US US382346A patent/US3335074A/en not_active Expired - Lifetime
  • 1964-08-04 GB GB30752/64A patent/GB1020187A/en not_active Expired
  • 1964-09-11 NL NL6410636A patent/NL6410636A/xx unknown
  • 1964-10-08 BE BE654143A patent/BE654143A/xx unknown
  • 1964-10-20 DE DE19641496752 patent/DE1496752A1/de active Pending

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