US3544368A - Process and the descaling of metals - Google Patents

Process and the descaling of metals Download PDF

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Publication number
US3544368A
US3544368A US866808A US3544368DA US3544368A US 3544368 A US3544368 A US 3544368A US 866808 A US866808 A US 866808A US 3544368D A US3544368D A US 3544368DA US 3544368 A US3544368 A US 3544368A
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Prior art keywords
metal
bath
molten
sodium
hydroxide
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Expired - Lifetime
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US866808A
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English (en)
Inventor
Rudolf Kuhn
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Evonik Operations GmbH
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Degussa GmbH
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    • 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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/28—Cleaning or pickling metallic material with solutions or molten salts with molten salts

Definitions

  • the invention relates to the removal of metal oxide scales from metals.
  • salt bath processes have been developed which are significantly short and have more limited process times than is required with the acid treatment processes.
  • Such salt bath processes include those involving the use ot molten alkaline salts, in which the scales are more or less given a preremoval treatment with the aid of direct electrical current in order to assure the quick removal of the scales in a subsequent heat treatment in the molten salt, as well as processes in which a reducing agent, for example, sodium hydride, is used in order to extensively remove the scales.
  • a reducing agent for example, sodium hydride
  • a caustic soda melt which contains metallic sodium in suspended form is used as the reducing agent.
  • a reducing agent such as sodium hydride be sprayed or scattered on the surface of the steel band which is to be treated for the removal of scales therefrom.
  • An object of the present invention is to provide a process whereby metal oxide scales can be readily removed from metal surfaces.
  • Another object of the present invention is to provide a process for the removal of metal oxide scales from metal surfaces which does not require the use of an extensive acid pickling treatment of the metal surface before or after the process of this invention.
  • a further object of the present invention is to provide an installation in which to conduct the novel process of this invention.
  • a further object of the present invention is to provide an installation in which to conduct the novel process of this invention.
  • the essence of the process of the present invention involves treating the metal to be descaled with molten sodium metal, which treatment is preferably both preceded and followed by treating the metal with molten alkali metal hydroxide.
  • FIG. l is a side view of an installation in which the process of the present invention may be conducted.
  • FIG. 2 is an end view of the installation of FIG. 1.
  • the molten sodium metal treatment is preferably conducted at 230 to 700 in about 60 to 5 seconds.
  • the molten sodium treating process can be further improved if the metal to be treated is pretreated or dipped in molten alkali metal hydroxide for a short time before the molten metal treatment.
  • the alkali metal hydroxide used is preferably sodium hydroxide. Molten mixtures of alkali metal hydroxides and alkali chlorides can also be used.
  • the pretreatment with alkali metal hydroxide p is preferably conducted at 230 to 700 C. for 5 to 60 seconds. The wetting and resulting descaling of the surface of the metal being treated is so free of objectionable features that the descaling process is completed within a very short time without any trouble.
  • the alkali metal hydroxide bath which is used in conjunction with the actual pickling process, which latter process is conducted in the molten sodium metal, can be used as means for sealing the sodium metal processing chamber from the atmosphere.
  • An alkali metal hydroxide bath can also be used as a post-descaling treatment in order to act as a scavenger and remove the metal object being processed not only pieces of sodium metal that may adhere to the metal object as it leaves the sodium metal bath, but also sodium oxide reaction products that may form during the descaling step.
  • the pieces of sodium metal and sodium oxide reaction products that are taken up by the alkali metal hydroxide bath can be readily converted into sodium hydroxide upon contact with moisture in the atmosphere.
  • the molten sodium metal bath is preferably blanketed with an inert gas which will not react with the sodium metal,
  • This inert gas is primarily to seal ⁇ ofl the molten sodium metal bath from gases or other components of the atmosphere which would either react with the sodium metal or with the metal being processed under the prevailing process conditions.
  • gases or other components of the atmosphere which would either react with the sodium metal or with the metal being processed under the prevailing process conditions.
  • the undesired gases orV other compounds are oxygen, hydrogen, water.
  • Nitrogen is the preferred inert gas to be used asthe blanketing medium.
  • a metal processing installation in which the process of the present invention can be carried out can be constructed in such a way that the molten sodium metal is allowed to oat on top of the molten alkali metal hydroxide bath.
  • the sodium metal bath can be isolated from the atmosphere by means of a tower type structure erected over the sodium metal bath.
  • Elongated metal objects, such as, wire and bands, to be treated in the sodium metal bath can be fed continuously through the bath and and the tower type structure surrounding the bath by means of feeder devices, such as guide rollers.
  • a particularly advantageous installation for carrying Aout the process of the present invention is one in which the alkali metal hydroxide bath is used in a trough like container and the molten sodium bath is changed into a funnel shaped container which latter container is inserted into the alkali metal hydroxide bath in such a fashion that the alkali metal hydroxide bath completely surrounds the sides of the funnel shaped container.
  • the funnel shaped container has a wide open top and a narrow open bottom whereby an interface between molten sodium metal and molten alkali metal hydroxide sets in inside ⁇ the narrow open end of the funnel shaped container.
  • FIG. 1 shows a side view of the installation
  • FIG. 2 shows an end view ofthe same installation.
  • the installation consists essentially of a furnace which has a wall 1 and a heating chamber 1a, an open topped trough or vat 2 for the molten alkali metal hydroxide 2a, an open topped container 3 for the molten sodium metal bath 3a, and air tight housing 4 and feeder or guide rollers 5.
  • Walls 4a of housing 4 extend on all sides down into bath 2a and housing 4 provides in combination with the molten alkali metal hydroxide 2a, an air tight area 7. Area 7 is thus bounded on its top and sides by housing 4 and at its base by the top of bath 2a. Area 7 completely isolates bath 3a from contact with the atmosphere, and during the operation of the installation area 7 is charged with an inert gas. 'I 'he portions of the top of bath Za which are outside walls 4a of housing 4 are in contactwith the atmosphere.
  • An elongated metal object O which is to be treated using the process of the present invention is fed into and through the installation, in the direction of the arrows, by means of guide rollers 5.
  • the metal o'bject passes first through the NaOH bath, then through the Na bath and again through the NaOH bath before it leaves the installation.
  • the furnace maintains both the NaOH bath and the Na bath in a molten condition.
  • the NaOH bath is heated directly through its container wall 2 by means of heat generated in heating chamber 1a.
  • the sodium is in turn heated by the molten NaOH.
  • the sodium bath 3a is housed in funned shaped container 3.
  • Container 3 may be suspended from above in the NaOH bath or it may be supported by a base which is itself resting on the bottom of trough 2.
  • the walls of the lower half of container 3 slope inwardly to provide a narrow, neck 6 which is open at the bottom thereof.
  • Neck 6 extends into the NaOH bath 2a in such a way as to leave a distance, on all sides of it, between it and the walls and base of trough 2.
  • neck 6 has a definite length
  • care must be taken during the operation of the installation to provide that uctuations in the amounts of sodium and sodium hydroxide in their respective containers do not cause the interface between the sodium and the sodium hydroxide to be located outside of neck 6.
  • the moisture in the air can enter the sodium hydroxide Where the molten sodium hydroxide is exposed to the atmosphere, i.e., that portion of the molten sodium hydroxide bath which is outside walls 4a. 'Ihe usual impurities which are present or formed during the descaling step, notably reduced metal, and which eventually fall to the bottom of trough 2, can be removed mechanically from the trough.
  • the narrowing of the interface ⁇ surface between the molten sodium and the molten sodium hydroxide by means of narrow neck 6 has the purpose of preventing losses of sodium by reason of the sodium dissolving in the sodium hydroxide.
  • the relatively narrow interface provided by neck 6 has the effect of providing a relatively small reaction surface between the two materials and thereby prevents a large scale dissolution of the sodium in the alkali metal hydroxide.
  • the molten sodium hydroxide can be used asan air tight sealing means, in combination with air tight housing 4, in order to keep moisture and other materials in the air from reaching and reacting with the molten sodium.
  • Housing 4 is so constructed that continuous metal wire or bands can be processed therein under an inert atmosphere from the time the wire or band enters the molten sodium hydroxide for the first time until it leaves the molten sodium hydroxide at the end of the process. In this way slits are not needed in walls 4a in order to provide ingress and egress means for the Wire and bands. Particles of sodium metal that may adhere to the band after the band leaves the molten sodium bath are readily taken up in the molten sodium hydroxide the second time the band passes through bath 2a, and thus before the band passes out into the outside atmosphere.
  • housing 4 is constructed Wide enough so that there is sufcient open space on the surface of bath 2a between walls 4a of housing 4 and Walls 3b of container 3 as to allow for the removal of impurities in the form of sludge from under container 3 in bath 2a through such open surface space.
  • two separate alkali metal hydroxide baths placed in separate troughs can also be used.
  • the sodium melt could also be housed in a separate trough shaped container of its own and the three troughs could still all be housed under a single air tight housing.
  • Those elements of construction which are to be used in the building of the installation and which are to come in contact with molten sodium during the operation of the installation, such as the inside of container 3 or the guide rollers which are partly immersed in bath 3a, must be made of a material which will not be attacked by the molten sodium.
  • Such elements of construction should, therefore, be constructed of a sodium resistant material such as nickel, or be plated with such sodium resistant materials.
  • the elements of construction which are to come in contact with the molten alkali metal hydroxide should be such as are capable of withstanding attack from such hydroxides.
  • the sealing of the guide roller shafts 8 which are to pass through housing 4 can be done with stuffing boxes in order to maintain the air tight integrity of housing 4.
  • the heat generated in the heating area 1a of the furnace can be generated electrically, as by using resistance elements, or by burning gas or oil. Heat generating electrodes can also be inserted directly into the baths.
  • EXAMPLE 1 A test piece of ST-high grade steel was heat rolled and annealed in the atmosphere and oxide scales appeared on the surface of the test piece. The surface of the test piece was then treated for 5 to 10 seconds at 400 C. in molten sodium hydroxide (100% pure) and then for 5 to seconds at 450 C. in molten sodium metal (100% pure). The surface of the thus treated metal piece was clear of oxide scales.
  • test pieces of a highly allow chromesteel identied as workpiece numbers 4006 to 4021 were heat rolled and annealed in the atmosphere whereupon oxide scales appeared on the surface of the test pieces.
  • the surface of some of the test pieces were then treated for 5 to 10 seconds at 500 C. in molten sodium hydroxide (100% pure) and then for 5 to l0 seconds at 400 to 500 C. in molten sodium metal pure).
  • the surface of the thus treated metal pieces were clear of oxide scales.
  • EXAMPLE 3 A test piece of highly alloyed chrome-nickel steel identified as workpiece number 4300 was hot rolled an annealed in the atmosphere whereupon oxide scales appeared on the surface of the test piece. The surface of the test piece was then treated for 5 to 10 seconds at 400 C. in molten sodium hydroxide (100% pure) and then for 5 to 10 seconds at 500 C. in molten sodium metal (100% pure). The layer of oxide scales was thus removed from the surface of the test piece.
  • the molten sodium bath should contain sodium which is at least 100% pure.
  • the alkali metal hydroxide bath should contain alkali metal hydroxide which is at least 100% pure. While sodium hydroxide is the preferred alkali metal hydroxide, there can be used potassium hydroxide.
  • the metals which may be descaled in accordance with the present invention include alloyed an non-alloyed iron and steel as well as copper, alloys of zirconium and titanium, iron-free alloys basing on Ni, Co and Cr.
  • the metals being processed in the process of the present invention may have any shape or conguration.
  • the process is particularly suitable to the treatment of objects which may Ibe fed into the molten sodium metal bath in a continuous fashion, such as wire, cable, strips, bands, sheets, and similar elongated objects.
  • a particularly preferred form of the invention is to utilize the sodium hydroxide pretreatment in a manner to produce the patentization effect and eliminate the customary lead bath thereby.
  • the steel wires or bands are lead through the sodium hydroxide bath at 450 to 550 C., preferably 500 C., which is in the Austenitization range.
  • the structure of the steel is changed to a perlite structure with its specific mechanical properties, e.g. improved wetting properties.
  • This preferred procedure involving perlitization is of concern only with steels.
  • vExample 2 illustrates this preferred procedure.
  • the steel should be held in the sodium hydroxide bath long enough to accomplish the desired structrual change.
  • a process for removing metal oxide scales from the surface of a metal object which comprises treating the surface of a metal object having metal oxide scales thereon with a molten bath consisting essentially of molten sodium metal.
  • a process according to claim 1 wherein the met is iron or steel and the surface is pretreated with molten alkali metal hydroxide at a temperature of 450 to 550 pretreating is C. to convert the material structure to a perlite structure and the molten sodium metal is maintained between 230 and 700 C. during said treating.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)
US866808A 1968-11-30 1969-10-16 Process and the descaling of metals Expired - Lifetime US3544368A (en)

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Application Number Priority Date Filing Date Title
DE19681812035 DE1812035A1 (de) 1968-11-30 1968-11-30 Verfahren zum Entzundern von Metallen,insbesondere von Staehlen

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DE (1) DE1812035A1 (pt)
FR (1) FR2027562A6 (pt)
GB (1) GB1296849A (pt)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4581073A (en) * 1984-04-10 1986-04-08 Allegheny Ludlum Steel Corporation Method for descaling metal strip
US20230002912A1 (en) * 2020-06-18 2023-01-05 Jiangsu Xcmg Construction Machinery Research Institute Ltd. Workpiece cleaning apparatus and cleaning method

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2188930A (en) * 1938-09-08 1940-02-06 Du Pont Metal cleaning process
US2311099A (en) * 1938-10-21 1943-02-16 Tainton Urlyn Clifton Metal treatment
GB565567A (en) * 1942-10-16 1944-11-16 Du Pont Improvements in and relating to the removal of surface impurities from metals by fused alkali baths
US2717845A (en) * 1947-01-09 1955-09-13 Roy E Carter Metal descaling methods
US3424614A (en) * 1963-02-28 1969-01-28 Schloemann Ag Cleaning,particularly de-scaling,of metal articles

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2188930A (en) * 1938-09-08 1940-02-06 Du Pont Metal cleaning process
US2311099A (en) * 1938-10-21 1943-02-16 Tainton Urlyn Clifton Metal treatment
GB565567A (en) * 1942-10-16 1944-11-16 Du Pont Improvements in and relating to the removal of surface impurities from metals by fused alkali baths
US2717845A (en) * 1947-01-09 1955-09-13 Roy E Carter Metal descaling methods
US3424614A (en) * 1963-02-28 1969-01-28 Schloemann Ag Cleaning,particularly de-scaling,of metal articles

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4581073A (en) * 1984-04-10 1986-04-08 Allegheny Ludlum Steel Corporation Method for descaling metal strip
US20230002912A1 (en) * 2020-06-18 2023-01-05 Jiangsu Xcmg Construction Machinery Research Institute Ltd. Workpiece cleaning apparatus and cleaning method

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Publication number Publication date
FR2027562A6 (pt) 1970-10-02
DE1812035A1 (de) 1970-09-17
GB1296849A (pt) 1972-11-22

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