Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/38—Pretreatment of metallic surfaces to be electroplated of refractory metals or nickel
• • 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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/46—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing oxalates

Definitions

• Titanium or titanium-base alloys which are hereinafter referred to as titanium alloys for brevity, may be employed successfully in the manufacture of electrodes for electrochemical applications, provided that at least part of the surface of the titanium is coated with an adherent layer of a noble metal. However, in order to obtain satisfactory adhesion between the noble metal and titanium, it is necessary to pretreat the surface of the titanium priorto coating.
• hydrochloric acid As examples, at ambient temperatures the action of hydrochloric acid is slow and variable, treatment times of three days or more being required;'alternatively, the acid may be used at its boiling point, thus shortening the treatment time to 30 minutes, but considerable difficulty is then encountered in dealing with the copious acidic fumes evolved and in containing the extremely corrosive acid.
• titanium when titanium is etched in hydrochloric acid it becomes fouled with a black smutty deposit composed mainly of titanium hydride, which interferes with the subsequent coating process. It is, therefore, necessary to remove the deposit before attempting to electroplate the etched titanium, the only practicable method being to scour the etched surface with a mild abrasive. This is time-consuming and demands considerable skillon the part of the operator to remove the deposit without damaging the delicate structure of the etchedtitanium surface.
• a process for treating at least part of a titanium alloy" article to provide a surface suitable for the deposition of an adherent coating thereon comprises treating said part with an aqueous solution of oxalicacid at a temperature greater than ambient temperatures.
• the concentration of the solution should be greater than percent weight/volume and its temperature should be maintained at at least 70 C, in order to produce the desired surface condition in a conveniently short time, but the optimum concentration and temperature will dependon the material to be etched and the surface required. For example, treatment with a solution containing percent of oxalic acid at 80 C. (3 5 C.) will produce a satisfactory surface on commercial purity titanium sheet in 16 hours, whereas a similar surface may be produced in 8 hours by treating the metal in a l5 percent solution at 95-l00 C. Thus it will be appreciated that etching conditions are not critical provided that they are kept fairly constant during the treatment.
• the process of the invention provides a matt etched surface on titanium which acts as a suitable key for a variety of metallic or nonmetallic coatings.
• metals may be deposited by means of electrolysis or thermal or chemical decomposition of metal salts; and nonmetallic coatings may be deposited by gas phase polymerization, vacuum decomposition or vacuum evaporation.
• nonmetallic coating material is polytetrafluoroethylene which may be used to provide a low friction film on the titanium surface.
• the titanium'articles On removal from the oxalic acid etching solution, the titanium'articles normally require only a simple water rinse before being subjected to the coating operation. However, on occasions, articles emerge from the solution with a light grey bloom on their surface, but unlike the smutty deposit previously referred to in connection with the hydrochloric acid treatment, this grey bloomcan be removed by light brushing during the rinsing operation. In fact its removal is not essential for satisfactory electroplating.
• Oxalic acid etching is lesssensitive to variations in both metallurgical condition of the titanium and purity of the etching solution, compared with etching in cold hydrochloric acid.
• commercially pure titanium articles having grain sizes from 0.01 (i.e., fine) to 0.08 mm. were etched satisfactorily in oxalic acid solution; whereas the fine grained commercially pure titanium is only etched with difficulty in hydrochloric acid.
• titanium containingup to 0.2 percent by weight of iron may be successfully etched, whereas even lower amounts of iron in titanium render it quite unsuitable for etching by hydrochloric acid. This latter difference is of economic importance, since it allows the use of technical quality rather than Analytical Reagent" quality oxalic acid.
• the metal loss of the titanium sheet etched is approximately 0.023g. per square centimeter of surface.
• the limit of solubility of titanium in 10 percent oxalic acid is 7-8g. per liter and, when this is exceeded, a very tenacious insoluble layer of crystals forms on the surface of the titaniumbeing treated, it is advisable to limit the concentration of titanium salts in the solution.
• a practical limit of six g. per liter is recommended, equivalent to a throughput 'of about 260 square centimeters of titanium surface per liter solution.
• titanous oxalate which is, up to the limit mentioned above, freely soluble forming a brown solution.
• the titanous titanium is readily oxidized to the colorless titanic form, and this inhibits further attack on metallic titanium. No difficulty arises from this effect in the 16 hour treatment, but because of this oxidation partly used solutions rapidly lose-their potency on subsequent contact with air. This may be largely prevented by covering the surface of the etching solution with plastic chips or balls to exclude air.
• the titanium oxalate may be kept in the titanous form by continuous electrolytic reduction using a porous diaphragm, or by chemical reduction by means of pure aluminum sheet.
• etching pretreatment can, with experience, be 'made by visual and microscopic examination, but is made more often by determining how successful the coating proves to be in its subsequent application.
• Quantitive laboratory methods of assessment include measurement of surface roughness, adhesion of coatings subsequently deposited by electrodeposition, thermal decompositon and the like. Yet another method is to assess the durability of coatings in electrochemical applications.
• Adhesion Test coating so as to expel all air bubbles between the tape and the coating and then removing the tape in a single sharp movement.
• the comparative results of the test were assessed by visual examination of the test were assessed by visual examination of the tested coating.
• Example 1 A sheet of commercially pure titanium was etched in a 10 percent aqueous solution of technical grade oxalic acid maintained at 80 C. C. for 16 hours. The sheet was then removed from the solution, rinsed in water and dried.
• the surface roughness (CLA) of the sheet was then measured and found to be 150 micro inches.
• the CLA value before etching was 55 micro inches.
• Example 2 Sheets of the same titanium stock material were heattreated so that half had a grain size of 0.013 mm. and the remainder had a grain size of 0.080 mm. Sheets from each group were then etched in percent oxalic acid solution using varying temperatures and times of immersion, and equivalent sheets etched in concentrated hydrochloric acid at room temperature for 72 hours.
• the etched sheets were electroplated with platinum to a thickness of 100 micro inches and the plated sheets subjected to the aforementioned adhesion test.
• the results are tabulated ,below:
• the etched sheets were then electroplated with platinum to a thickness of 100 micro inches and each sheet was inserted as anode in an electrolytic cell containing five liters of saturated sodium chloride solution as electrolyte and having a titanium cathode.
• the cells were operated continuously at an anode current density of 1,000 amperes per square foot for one week, at the end of which time the electrolyte was drained and the adhesion of the platinum coating on the anode assessed by means of the aforementioned Stripping Test.”
• the electrolyte in the cell was then renewed and the electrolysis continued for another week, this procedure was repeated to the end of the respective test.
• the results of the tests are tabulated below:
• the adhesion value is the mean of three tests.
• Example 3 Two stocks of commercial purity titanium were selected, one having higher impurities than the other. Sheets were then cut from each stock and heat-treated so that half had a grain size of 0.020 mm. and the remainder 0.080 mm. The sheets were then etched in 10 percent oxalic acid at 80 C. for 16 hours or in concentrated hydrochloric acid at room temperature for 72 hours.
• the etched sheets were coated with platinum to a thickness of micro inches by thermal decomposition of a platinum organic base paint composition which was applied to the surface of the sheets.
• the coated sheets were subjected to the adhesion test, the results being tabulated below:
• a process according to claim 1 wherein the temperature for contacting said part with the solution is at least 70 C. and up to 100 C.
• a process according to claim 1 comprising degreasing said part before contacting the same with the aqueous solution of oxalic acid.
• a process according to claim 1 comprising rinsing said part with water after contact with the aqueous solution of oxalic acid.
• a process for providing a matt etched surface on at least a part of a titanium alloy" article by contacting said part with an acid the improvement which comprises using, as the acid, an aqueous solution of oxalic acid wherein the concentration of oxalic acid is greater than five percent weight/volume, and contacting the article with said oxalic acid solution at a temperature of at least 70 C. and for a period of at least four hours, said temperature and time being sufficient to give said matt etched surface.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
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• Organic Chemistry (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• ing And Chemical Polishing (AREA)
• Electrolytic Production Of Metals (AREA)
• Electroplating Methods And Accessories (AREA)

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Cited By (15)

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Citations (2)

• 1965
  • 1965-07-01 GB GB27931/65A patent/GB1105388A/en not_active Expired
• 1966
  • 1966-06-29 NL NL6609029A patent/NL6609029A/xx unknown
  • 1966-06-30 DE DE1521799A patent/DE1521799C3/de not_active Expired
• 1970
  • 1970-04-27 US US29769A patent/US3650861A/en not_active Expired - Lifetime

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