Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
  • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
  • C25F1/02—Pickling; Descaling
  • C25F1/04—Pickling; Descaling in solution
  • C25F1/08—Refractory metals

Definitions

• This invention relates to a process for descaling and pickling the surfaces of titanium and titanium base alloy articles. More particularly, the invention is directed to a two-step bipolar electrolytic process successively employing different electrolytes for descaling and pickling the article.
• My invention provides a rapid and efficient method of descaling and pickling articles of titanium and titanium base alloys.
• a titanium or a titanium base alloy article is often exposed to relatively high temperature, and oxidation of the metal surface results. This oxidation increases the oxygen content in the surface layer of the metal which is harmful to mechanical properties and may also create an undesirable layer of scale on the surface.
• the scale consists of fused oxides of titanium and is disadvantageous to further processing and fabrication of the metal article.
• phosphoric acid When phosphoric acid is used as the electrolyte in the cathodic cycle, it may have a concentration between 10% and 70% by volume; although a concentration between about 50% and 70% is preferred since scale removal and conductivity are increased at the higher concentrations. For example, at a temperature of 85 C., a 10% aqueous solution of phosphoric acid will require about 17 volts at 20 amperes; whereas a 70% solution will require only about 6 volts at 20 amperes. Alternatively, when sulphuric acid is used as the electrolyte in the cathodic cycle, it should have a concentration between 30% and 40% by volume.
• the cathodic cycle may be carried out at any temperature down to room temperature; but I have found that this cycle is most efficient if the temperature is maintained just below the boiling point of the electrolyte, as descaling action is better and the conductivity of the electrolyte is increased
• the current density in the cathodic cycle should be maintained between about 100 and 1000 amps/ft. since hydrogen evolution is not large enough to effectively remove scale at current densities below 100 amps/ft. and at current densities above 1000 amps/ft. the action of the electrolyte is so vigorous that it may boil out of the tank.
• the initial temperature of the electrolyte not exceed about 80 C. since loss of HF and water due to evaporation is excessive above 80 C. However, the efiiciency of the reaction appears to be unimpaired up to the boiling point of the electrolyte. It is also preferred that the final temperature of the electrolyte be above about 50 C. since the conductivity of the electrolyte decreases rapidly at low temperatures, and voltage requirements are correspondingly large. Pickling will, however, proceed at room temperature, although the voltage requirements are so large as to make room temperature operation impractical.
• the anodes in the cathodic cell are preferably either graphite or platinum-plated titanium, and the cathodes in the anodic cell may be stainless steel. Stainless steel anodes may also be used, but such is not preferred because the anode material goes into solution and contaminates the electrolyte requiring frequent replacement with fresh electrolyte.
• EXAMPLE 2 A specimen of Ti75A 1 in. by 6 in. was cleaned and placed in a muflie furnace at 1450" F. for 30 minutes to simulate an annealing cycle. A dense scale was formed on the surface of the specimen.
• the specimen was then made cathodic in a 35% aqueous solution of H 80 solution for 10 minutes at a current density of 168 amps/ft. and a temperature of 85 C. At the end of this cycle, the specimen was completely descaled but the surface was a dull grey.
• Example 1 The specimen was then made anodic in a solution having the composition of Example 1 for 5 minutes at a current density of 168 amps/ft? The soultion was at room temperature initially and increased as in Example 1.
• EXAMPLE 3 Four specimens of T i5Al2.5Sn having a hard yellow hot roll scale and having a surface area of 10 in. were subjected to various treatments.
• the first three specimens were made cathodic for 10 minutes at a current density of 288 amps/ft?
• Specimens Nos. 1 and 2 were processed in an electrolyte consisting of a 70% aqueous solution of H PO at 95 C.
• specimen No. 3 was processed in a 35% aqueous solution of H 50 at C.
• Specimen No. 4 was not given a cathodic cycle.
• specimen No. 4 which had not been subjected to the cathodic treatment, was not descaled. This shows that descaling of tin-containing titanium alloys can be achieved utilizing the process of the instant application; whereas an anodic treatment will not descale tin-containing titanium base alloys.
• EXAMPLE 4 Six specimens 1 in. x 6 in. were considered in this example. Two specimens were Til3Vl lCr-3Al; two specimens were Ti8Al1MolV; and two specimens were Ti5Al-2.5Sn. Each specimen was exposed to a temperature of 1450 F. for 30 minutes to form anneal scale on the surfaces thereof.
• each of the specimens was treated in the cathodic cycle for 10 minutes at a temperature of 95 C. and a current density of 316 amps/ft
• the electrolyte in the cathodic cycle was a 70% aqueous solution of H PO
• Each specimen was then treated in the anodic cycle for 5 minutes at 200 amps/ft. during which the temperature of the electrolyte increased from room temperature to about 65 C.
• the electrolyte used for the anodic cycle was the same as in Example 1.
• EXAMPLE 5 A specimen of Ti8Al1MolV 0.1 in. thick and having a surface area of 12 in? having hot roll mill scale on the surface was treated cathodically for 10 minutes at a current density of 200 amps/ft. in a 70% aqueous solution of H PO at C.
• Example 2 The specimen was then treated anodically for 10 minutes in the same electrolyte as in Example 1 at a current density of 200 amps/ft? Complete descaling was obtained and the surface was bright and clean.

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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)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=25103384

Family Applications (1)

Country Status (4)

Cited By (7)

• 1968
  • 1968-11-12 US US775120A patent/US3632490A/en not_active Expired - Lifetime
• 1969
  • 1969-09-02 DE DE19691944388 patent/DE1944388B2/de active Granted
  • 1969-10-28 GB GB1228095D patent/GB1228095A/en not_active Expired
  • 1969-11-07 FR FR6938345A patent/FR2023051A1/fr not_active Withdrawn

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