t g"\ lktented June 3, 1952 such as sodiumor potassium UNITED STATES PATENT OFFICE PICKLING CHROMIUM ALLOYS William E. (Daugherty, Tarentum, Pa., assignor to Allegheny Ludlum Steel Corporation, a corporation of Pennsylvania No Drawing. Application July 18, 1947, Serial No. 762,014
11 Claims. (Cl. 134-3) This invention pertains to a method of pickling and removing scale from chromium-iron alloys, generally termed, straight chromium alloys.
Considerable difficulty has been encountered in removing the scale from alloys of the above type,
and particularly,v in removing the scale after annealing operations. Schulte, in Patent No. 1,657,228, discloses a two-step method of pickling which involves the preliminary step of first introducing the alloy into a hydrofluoric acid bath and then into a nitric acid bath However, such a two-step operation is time-consuming, diiflcult to control and rather expensive.
It has thus been an object of my invention to -rovide a simple and effective one-step procedure i or pickling straight chromium alloys without producing detrimental etching effects.
After considerable experimental work in which various pickling solutions were tested, I finally discovered that a solution containing proper proportions of nitric acid and a solution-soluble fluoride (salt) would produce exceptionally improved results. I determined that n it rig ,aeidgyil1 n ot, itself, attack the sgalgand that a fluoride cgm- 13Wfilt isneededitoschange the character of'th escale so that the nitric acid can be made effective. I found that a solution of hydrofluoric and nitric acids was too strong and will severely etch the metal; also, that a bifiuoridz qau sed etchin Etching produces a dullsurface as con tfasted to the high or extremely bright finish obtained by my procedure.
A fluoride of somewhat intermediate activity, (univalent, alkali earth metal) fluoride, has, been successfully employed- Such a fle ridesminimzsssseh i e iron., A pickling solution of 5% to 10% nitric acid and about 1% to 2% sodium or potassium fluoride was used and a high satisfactory result was obtained without any etching or pitting effect upon the metal.
My procedure also saves considerable time in the pickling operation. For example, I have been able to effect a saving of time of about 50% over the Schulte procedure. Of course, in any pickling bath, as a the solution gets older, the. time progressively increases and it is advisable to replenish it from time to time. I have successfully pickled 40,000 pounds of chromium alloys at one time, employing a temperature of about 130 to 160i F and an average time of 8 to30 'Iiiiniit'sTThE metal has a dark scum which inattack upon the metal its li, and at the 'sfine I have found that the pickling solution should preferably contain about 5% to 10% nitric acid and about 1% to 2% of sodium or potassium fluoride. Although less tli'ad'about 5% nitric acid arid 1% fluoride may be employed, the above ranges provide optimum results.
The above aqueous pickling solution has been found suitable for chromium-iron alloys of the stainless or rustless type containing up to 20% chromium as a maximum. For an alloy containing over 20% chromium, the strength or percentage of the fluoride can be increased up to about 4% to 8%, but some slight etching may ocdicates scale removal without etching. It is advantageous to employ a hot solution for best re sults.
Although sodium or potassium fluorides are preferable, any other suitable fluoride of the same general character may be employed provided it is water or acid soluble, in the sense of being somewhat readily soluble in the pickling solution employed, that is, solution-soluble.
After the pickling operation, the alloy is taken out of the solution and scrubbed with hot water.
It will be apparent to those skilled in the art that some variations in the described procedure may be employed without departing from the spirit and scope of my invention as indicated by the appended claims.
What I claim is: a
1. A method of the character described for removing scale from straight chromium alloys to provide a bright surface finish which comprises, immersing such alloys in an aqueous solution consisting of nitric acid and a univalent solutionsoluble alkali-metal fluoride salt, the fluoride salt being in a suflicient minimum amount of about 1% to act upon the scale and make it subject to effective action by the nitric acid and being within a maximum amount such that etching of the alloys will be substantially eliminated and not above about 8%, the nitric acid being in a sufficient minimum amount of about 5% to remove the scale and within a maximum amount of about 10%, and the proportloning of the fluoride salt and nitric acid being such that a substantially complete removal of the scale is effected.
2. A method as defined in claim 1, wherein the fluoride salt is of the class of sodium and potassium fluorides.
3. A method as defined in claim 1 which comprises, hot-water scrubbing the alloys after scale has been removed therefrom by the nitric acid and fluoride salt aqueous solution treatment.
4. A method as defined in claim 1, wherein the nitric acid and fluoride salt aqueous solution is maintained at a temperature of about 130 to 160 F.
5. A method as defined in claim 1, wherein the nitric acid and fluoride salt aqueous solution has a temperature of about 130 to 160 F., and the alloys are subjected to the solution for an average time of about 8 to 30 minutes.
6. A method as defined in claim 5, wherein the alloys are removed from the nitric acid and fluoride salt aqueous solution'after scale has been dissolved, and are then water-scrubbed.
7. A method of the character described for removing scale from straight chromium alloys to provide a bright surface finish containing up to about 20% chromium as a maximum which comprises, immersing such alloys in an aqueous solution consisting of nitric acid and a univalent solution-soluble alkali-metal fluoride salt, the fluoride salt being in a sufficient minimum amount of about 1% to act upon the scale and make it subject to effective action by the nitric acid and being within a maximum amount of about 2%, the nitric acid being in a sumcient minimum amount of about 5% to remove the scale and within a maximum amount of about and the proportioning of the fluoride salt and nitric acid being such that a substantially complete removal of the scale is effected.
8. A method of the character described for removing scale from straight chromium alloys which comprises, immersing such alloys in an aqueous solution consisting of nitric acid and a univalent solution-soluble alkali-metal fluoride salt, the fluoride salt being within a range of about 1 to 2% and the nitric acid being within a range of about 5 to 10%.
9. A method of the character described for removing scale from straight chromium alloys which comprises, immersing such alloys in an aqueous pickling solution consisting of nitric acid and a solution-soluble fluoride salt of the class of sodium and potassium fluorides, the fluoride salt being within a range of about 1 to 2% and the nitric acid within a range of about 5 to 10%.
10. A method as defined in claim 9, wherein the pickling solution is maintained at a temperature of about to F.
11. A method as defined in claim 9, wherein the alloys are removed from the pickling solution after the scale has been dissolved therefrom, and then washed.
- WILLIAM E. CAUGHERTY.
REFERENCES CITED The following references are of record in the Germany Apr. 25, 1926