ass-tea United States Patent 3,007,780 TITANIUM ETCHING Jack M. Beigay, New Kensington, and Donald R. Zaremski, Cheswick, Pa., assignors to Titanium Metals Corporation of America, New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 20, 1958, Ser. No. 722,628
6 Claims. (Cl. 156-20) This invention relates to conditioning the surface of a titanium or titanium base alloy article, and to a pickling solution adapted for such conditioning.
During various processes employed in manufacturing titanium and titanium base alloy articles the surface thereof often becomes pitted, scratched or otherwise marred. Also, treatment at elevated temperatures results in formation of an oxide film or layer on the surface, and grinding operations to remove heavy scale and surface imperfections result in typical grind lines, as they are termed in the art. Removal of such oxide film and grind lines by conditioning to provide a bright, clear surface has not heretofore been readily accomplished.
It is, therefore, an object of this invention to provide an improved method for conditioning the surface of titanium and titanium base alloy articles. Another object of this invention is to provide a method for removakof grind lines from the surface of titanium and titanium base alloy articles. Yet another object of this invention is to provide an improved pickling solution adapted for conditioning the surfaces of titanium and titanium base alloy articles. These and other objects of this invention will be apparent from the following description thereof.
This invention in its broadest aspects contemplates a pickling solution comprising an aqueous solution contai ing by weight, from to 30% of acetic acid, and from 0.5% to 17% of hydrofluoric acid. Preferably the solution contains about 10% acetic acid and about 2.5% hydrofluoric acid. The method for conditioning the sur face of a titanium or titanium base alloy article comprises bringing such surface into contact with such pickling solution, preferably by immersion therein at room temperature for a period of up to about 20 minutes.
The articles suitable for conditioning treatment according to this invention may be formed in any convenient manner. The pickling solution is particularly adapted for conditioning articles manufactured from titanium or titanium base alloys. Products such as sheet, strip, bars, plate and castings as well as finished or semi-finished articles may be successfully treated. During production the metal or alloy is often subjected to elevated temperatures such as for example, hot rolling and other forming processes, as well as annealing and heat treatment, which tend to oxidize the surface. Grinding the surface to remove imperfections or heavy scale results in typical scratch marks or grind lines which may be readily eliminated by conditioning the surface employing the pickling solution of this invention.
The action of the pickling solution on the surface of titanium and titanium base alloys appears to be one in which metal is actually dissolved by the solution. It has been determined that immersion in a pickling solution containing about 10% acetic and 2.5% hydrofluoric acid, at room temperature, will remove about 1 mil thickness of titanium metal or alloy per minute. Solutions con- 3,007,780 Patented Nov. 7, 1961 taining larger amounts of hydrofluoric acid will generally attack metal at a higher rate. The acetic acid content appears to modify or slow down the hydrofluoric acid activity. The action is particularly effective for removing grind lines, and immersion for a period of from about 1 to about 5 minutes result in removal of the required amount of metal, depending on the depth of the grind line scratches, to produce a uniform, level surface. Since the pickling solution of this invention acts on the metal surface itself, that is, acts as a surface conditioner, it will not generally attack and remove heavy and tightly adhering scale. Such scale must ordinarily be removed by treatment with molten caustic compositions at elevated temperatures, the molten caustic apparently acts as a solvent for the scale. A light scale, which may be more properly considered as a surface film of oxide, such as may be produced by hot rolling or low temperature annealing may, however, be readily removed according to this invention and an excellent clear, bright surface obtained.
The acetic acid content of the pickling solution should be between about 5% and 30% and the hydrofluoric acid content should be between about 0.5% and 17%. Higher concentrations than these particularly with respect to the hydrofluoric acid content, will be found to be so strong that metal dissolution will occur at an uncontrollable rate. Weaker solutions will not provide sufficient conditioning action in a reasonable time to produce effective results. Glacial acetic acid which comprises about 99% (by weight) C H 0 balance water, is an article of commerce, as is also 52% hydrofluoric acid which contains from 48% to 52% HF (by weight), balance water. The pickling solution is readily prepared by admixing the required amounts of such commercial acids and water, measured conveniently by volume in a suitable tank or treatment container. Since the finished solution will corrode many metals it is advantageous to provide a container of corrosion resistant construction, or having an acid proof liner.
A preferred composition of the pickling solution contains by weight, about 10% acetic acid and about 2.5% hydrofluoric acid, balance water. Such a solution provides adequate strength for rapid and effective conditioning of titanium and titanium base alloy surfaces and yet its action is predictable and controllable. Elevated temperatures are not necessary when conditioning according to this invention. If the temperature of the pickling solu- =tion as employed is substantially above normal room temperature, however, a normal and expected increase in reactivity will result.
The time of contact of the metal surface with the pickling solution may vary generally up to about 20 minutes. A time period of longer than 20 minutes will generally result in gross metal removal which will not be desirable for surface conditioning treatment. Since the conditioning action is one of metal removal it will be apparent that the time of treatment will be adjusted to remove not much more than suflicient to erase or remove the imperfections or contaminating film to produce the desired surface. For conditioning to remove shallow grind lines or light scale the time of treatment may be generally from 1 to 5 minutes, which when employing the 10% acetic- 2.5% hydrofluoric preferred solution will result in removal of between about 1 and 5 mils of metal thickness.
After conditioning in the pickling solution the excess solution may be rinsed olf with water and the article surface suitably dried as in a current of warm or hot air.
Table 1, below, illustrates the results obtained by conditioning the surface of an article formed from a metal of the class or group consisting of titanium and titanium base alloys according to this invention, employing the preferred 10% acetic-2.5% hydrofluoric acid pickling solution.
Table 2, below, illustrates the practice of this invention to remove grind lines resulting from grinding with 150 grit employing the same pickling solution as the tests in Table 1.
Table 2 Time of Article Immer- Results sion, minutes Titanium alloy sheet (Ti-2% Eta-2% 3 Complete grind line M-2% Or); 150 grit grind lines. removal; 2-3 mils gage loss. Titanium alloy sheet (Ti-5% All- Do.
2.5% Sn); 150 grit grind lines. Titanium alloy sheet (Ti-6% iii-4% V); 150 grit grind lines.
removal; 3 mils gage loss.
A significant advantage of the pickling solution and method of this invention is that no appreciable increase results in the hydrogen content of the article being treated. In all the examples shown in Table 1 and Table 2, oxide and grind lines were removed without significant increase (more than a few parts per million) in the hydrogen content of the sheet or strip treated. This invention therefore provides an efiective pickling solution and a method for conditioning the surface of titanium and titanium base alloy articles. Light scale, oxide films, grind lines, and other surface imperfections are readily removed Complete grind line 4 without significant hydrogen pickup in the article being treated.
We claim:
1. A pickling solution consisting essentially of by weight, from 5% to 30% of acetic acid, and from 0.5% to 17% of hydrofluoric acid balance substantially all water.
2. A pickling solution consisting essentially of by weight, about 10% of acetic acid, and about 2.5% of hydrofluoric acid balance substantially all water.
3. A method for conditioning the surface of an article formed from a meta-l from the group consisting of titanium and titanium base alloys which comprises bringing said surface into contact with a pickling solution consisting essentialy of by weight, from 5% to 30% of acetic acid, and from 0.5% to 17% of hydrofluoric acid balance substantially all water.
4. A method for conditioning the surface of an article formed from a metal from the group consisting of titanium and titanium base alloys which comprises bringing said surface into contact with a pickling solution consisting essentially of, by weight, about 10% acetic acid and about 2.5% hydrofluoric acid balance substantially all water for a period up to 20 minutes.
5. A method for conditioning the surface of an article formed from a metal from the group consisting of titanium and titanium base alloys which comprises immersing said surface in a pickling solution consisting essentially of, by weight, from 5% to 30% acetic acid, and from 0.5 to 17% hydrofluoric acid balance substantially all water for a period up to 20 minutes.
6. A method for conditioning the surface of an article formed from a metal from the group consisting of titanium and titanium base alloys which comprises immersing said surface in a pickling solution consisting essentially of, by weight, about 10% acetic acid and about 2.5% hydrofluoric acid balance substantially all water for a period between about 1 and about 5 minutes, rinsing with water, and drying the surface of the article.
References Cited in thefile of this patent UNITED STATES PATENTS 2,724,667 MacPherson Nov. 22, 1955 2,734,837 Hands Feb. 14, 1956 2,828,192 Lan'gsfeld Mar. 25, 1958 2,847,287 Landgren Aug. 12, 1958 2,871,110 Stead Ian. 27, 1959 2,913,360 Zarernski et al. Nov. 17, 1959 FOREIGN PATENTS 589,932 Great Britain July 3, 1947