Classifications

• • 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

• ABSTRACT A prefused and solidified salt for use in forming a metal-conditioning bath and for additives to existing baths.
• the slat is a uniform solid solution of an alkali metal hydroxide, a nitrate salt, and water.
• the removal of the scale requires some type of pickling operation, conventionally in an acid medium.
• this scale is of such a character that it is either difficult or impossible to remove merely by pickling in an acid or various combinations of acids; or, the time required to remove such scale solely by pickling in an acid maybe so long as to be economically undesirable.
• a large tank is initially supplied with a physical mixture of the alkali metal hydroxide and the nitrate salt and water in the desired proportions and then the bath is heated to the desired operating temperature fusing and dissolving the hydroxide and nitrate in the water, and the bath then is maintained at this elevated temperature for scale conditioning.
• the metal to be treated is immersed and then removed from the bath, after which the conditioned scale is removed by acid pickling. This immersing and removing of the metal from the bath results in certain losses to the bath.
• One of the principal factors contributing to this loss is the so called dragout phenomenon.
• This technique of bath additions has several undesirable aspects and also causes certain undesirable bath characteristics.
• One undesirable characteristic of adding the separate ingredients is that the constituents are rather slow to dissolve in the bath, thus delaying the time which it takes them to become active inasmuch as they are not active until they are in a fused form.
• Another undesirable aspect is that when additions to the bath are made as separate constituents the composition of the bath tends to change, even though the additions are of the same proportions as the constituents of the bath. This normally is characterized by a decreasing percentage or proportion of the nitrate and obviously has the disadvantage of causing the bath to be nonuniform. This in turn varies its scale-conditioning characteristics which causes variations in time necessary for the treatment. This changing of constituent proportions of the bath also causes certain other undesirable aspects such as a tendency toward pitting and nonuniform scale treatment.
• Another object of this invention is to provide an improved scale-conditioning bath material which is suitable for additions to said baths which comprises a uniform solid solution of the desired material composition.
• Still a further more particular object of this invention is to provide an improved method of preventing or reducing the tendency toward change of constituent proportions in scaletreating baths.
• the present invention contemplates the provision of a material which is adapted to form or be added to existing scale-conditioning baths of the alkali metal hydroxide-nitrate salt water type maintained at elevated temperatures.
• the invention is especially applicable although not limited to baths of aqueous solution of potassium hydroxide and potassium nitrate which are utilized for scale conditioning of titanium, stainless steel, super alloys and other metals and alloys having scale which is difi'lcult to remove by acid pickling alone.
• the invention contemplates the provision of a prefused solidified material having a uniform composition which is a solid solution of the alkali metal hydroxide and the nitrate salt with water.
• the invention will be described in relationship to the potassium hydroxide, potassium nitrate baths of the type disclosed in the U.S. Pat. Nos. 3,254,0ll and 3,l2l,026.
• the invention is not limited to the precise compositions disclosed in these patents but is adapted for use for other scale-conditioning baths which include alkali metal hydroxide and nitrate salt combinations in aqueous solution.
• the potassium hydroxide is the basic or principal constituent for the scale conversion function.
• the potassium nitrate is present in order to avoid certain harmful effects to certain alloys which may result from the use of potassium hydroxide alone. These surface effects include such phenomenon as hydrogen pickup by titanium alloys and a propensity toward pitting of stainless steel and other alloys.
• the potassium nitrate also aids in the conversion of the scale of stainless steel and certain super alloys. There should be at least 5 percent potassium nitrate to perform these functions effectively, but there should be no more than 30 percent potassium nitrate, inasmuch as concentration of more than 30 percent of potassium nitrate do not provide any additional beneficial results and indeed are detrimental in that there is a reduced amount of available potassium hydroxide to perform the basic scale converting function. Therefore, the potassium hydroxide percentage should be between 5 and 30 percent. In
• the finished salt or, the salt as it is to be-used in the descaling temperatures in the bath there should be at least 5 percent water in order to allow the bath to be liquid at temperatures below 500 F. Without at least 5 percent water the salt in effect becomes a nonaqueous solution and must be heated well above 600 F. for fusion. The 5 percent water provides for the aqueous-type solution and fusion at temperatures below 500 F. There should not be any more than 20 percent water in the final salt as it is to be used, inasmuch as this tends to make the salt damp and sticky which is undesirable which will be explained in detail hereinafter.
• One method of manufacturing a flaked product with an approximate composition of about 75 percent potassium hydroxide, about I 15 percent potassium nitrate and about percent water and normal impurities is as follows.
• a solution of 55 percent water and 45 percent potassium hydroxide is placed in a reaction vessel and the heating started. As the water evaporates more of the original material is added, and the heating continues until the temperature of the vessel reaches about 645 F.
• the material and vessel are then cooled to about 500 F. at which time the required amount of potassium nitrate is added, this being in proportion of parts potassium nitrate to 75 parts of potassium hydroxide by weight.
• the heat is then adjusted to maintain a temperature of about 465 F. until all of the potassium nitrate has gone into solution. At this point, the temperature is reduced to 435 F. at which temperature the material is flaked in conventional flaking equipment.
• a uniform solid solution of a salt according to this invention containing approximately 75 percent potassium hydroxide, 15 percent potassium nitrate and 10 percent water has a melting point of approximately 284 F. with a boiling point well over 500 F.
• potassium nitrate has a melting point of 630 F
• potassium hydroxide has a melting point of 680 F.
• water has a boiling point of 212 F.
• the potassium hydroxide, potassium nitrate and water form a solution which is somewhat eutectic in nature.
• each particle merely melts and changes from a solid solution to a liquid solution of uniform composition with no chemical action or reaction taking place.
• each particle encounters a bath having a temperature which is below its melting point. These particles therefore will remain in the solid state until the nitrate and hydroxide particles encounter each other in sufficient proportions to go into the liquid solution at this reduced temperature.
• nitrate is the constituent that is normally lower than would be expected from the added proportions, it is believed that there is more of a propensity for the nitrate particles to collect on the contaminates and go into the sludge than there is for the hydroxide particles.
• a prefused solid solution product according to this invention provides a particle of uniform composition which goes into solution merely by a phase change from solid to liquid and thus does not provide separate particles of the separate constituents which can be prevented from going into solution during this dissolving process.
• the bath maintains such a constant composition balance that the frequent analyses of the bath, which in the past had been required, can be eliminated, or drastically reduced in number.
• a prefused and solidified solid solution of an alkali metal hydroxide and nitrate salt with water provides a superior material both for forming a new bath for treating metal oxide scale and also it provides a superior constituent for additions to existing salt baths, which in both cases the improved results are obtained from the use of such a prefused solid solution.
• said material having been solidified from a liquid sufficient] rapid] to form a uniform solid solution comprising an alka 1 metal ydroxide, a nitrate salt, and water.
• alkali metal hydroxide is selected from the group consisting of sodium hydroxide and potassium hydroxide.
• nitrate salt is an alkali metal nitrate.
• alkali metal nitrate is selected from the group consisting of sodium nitrate and potassium nitrate.
• a method of maintaining a preselected chemical content in a scale-conditioning bath which is an aqueous solution of an alkali metal hydroxide and a nitrate salt maintained at elevated temperatures, and wherein periodic additions of the constituents is required, which method comprises, providing a fused and solidified material which is a uniform solid solution of said alkali metal hydroxide and said nitrate salt in the desired relative proportions and water and adding quantities of said material to said bath as required.
• the bath is a potassium hydroxide, potassium nitrate, water bath and wherein the material to be added has the potassium hydroxide and potassium nitrate in substantially the same proportions of the bath, and has from 5 to 20 percent water.
• the method of claim 10 wherein the material to be added has from 6 to 90 percent potassium hydroxide, from 5 to 30 percent potassium nitrate, and no more than 10 percent water.
• a method of forming a material adapted for conditioning metal scale which material is solid at ambient temperature comprising the steps of heating an alkali metal hydroxide, a nitrate salt and water to a temperature sufficiently high and for a sufficient time to provide a uniform solution of said hydroxide, said nitrate and water, which is liquid at said temperature and solid at ambient temperature, and cooling said solution to ambient temperature sufficiently rapidly to form a uniform solid solution of said constituents.

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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)

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Cited By (6)

Citations (6)

• 1968
  • 1968-01-22 US US699342A patent/US3625900A/en not_active Expired - Lifetime
  • 1968-12-30 SE SE17969/68A patent/SE358424B/xx unknown
• 1969
  • 1969-01-07 DE DE19691900466 patent/DE1900466A1/de active Pending
  • 1969-01-20 GB GB3055/69A patent/GB1195815A/en not_active Expired
  • 1969-01-21 AT AT62669A patent/AT294522B/de not_active IP Right Cessation
  • 1969-01-21 BE BE727215D patent/BE727215A/xx unknown
  • 1969-01-22 FR FR6901128A patent/FR2000554A1/fr not_active Withdrawn

Patent Citations (6)

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