US3726772A - Method for removing iron impurities contained in a salt bath for nitrogenation - Google Patents

Method for removing iron impurities contained in a salt bath for nitrogenation Download PDF

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Publication number
US3726772A
US3726772A US00111906A US3726772DA US3726772A US 3726772 A US3726772 A US 3726772A US 00111906 A US00111906 A US 00111906A US 3726772D A US3726772D A US 3726772DA US 3726772 A US3726772 A US 3726772A
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Prior art keywords
salt bath
iron impurities
iron
nitrization
cathode
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Expired - Lifetime
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US00111906A
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English (en)
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Y Shimizu
S Takahashi
T Uehara
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Mitsubishi Motors Corp
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Mitsubishi Motors Corp
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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
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/48—Nitriding
    • C23C8/50—Nitriding of ferrous surfaces

Definitions

  • the present invention relates to a process for surely and economically removing iron impurities contained in a salt bath for nitrization comprising cyanide and cyanate by electrochemically depositing the iron impurities on a cathode using an anodic and a cathodic electrode provided in the salt bath.
  • FIGS. l-3 are sectional views of an apparatus suitable to practising a process of the present invention for removing iron impurities and preventing an increasement of iron impurities, FIG. 1 being that suitable to practising a process of the present invention for intermittently effecting before or after nitrization and FIG. 2 being that suitable to practising a process of this invention for effecting in parallel with nitrization; FIG. 3 being that suitable to continuously practising thereof; FIGS. 4-6 are diagrams showing test results for removing the iron impurities according to the process of the present invention taking a treating time as abscissa and a degree of purification (calculated on a weight basis of sodium ferrocyanide) as ordinate.
  • the present invention relates to the process for removing the iron impurities contained in the salt bath for nitrization mainly used for impregnating steel products to be treated.
  • nitrization for metals is effected in a molten bath for nitrization namely Taftride Bath, trade name, consisting of 30-65% by weight of cyanide (calculated on the weight basis of NACN) and 15-45% by weight of cyanate (calculated on the weight basis of KACNO) and residue of alkali carbonate at a temperature of about 500600 C., preferably around 570 C., for 1.5-2 hrs. during blowing air or oxygen in the salt bath for nitrization.
  • Taftride Bath trade name, consisting of 30-65% by weight of cyanide (calculated on the weight basis of NACN) and 15-45% by weight of cyanate (calculated on the weight basis of KACNO) and residue of alkali carbonate at a temperature of about 500600 C., preferably around 570 C., for 1.5-2 hrs. during blowing air or oxygen in the salt bath for nitrization.
  • the iron impurities have a bad effect on an efiiciency for nitrization and make worse properties of the nitriding, layer formed on the products to be treated and make weaker the treating capacity of the salt bath for nitrization as the iron impurities content increases.
  • Such phenomena is accelerated remarkably as the iron impurities contained in the molten salt bath increase, especially in case that the products to be treated is iron products or iron alloy products having a forged skin.
  • the step for removing the iron impurities contained in the salt bath may often account for about 30% of the entire nitriding process.
  • the maximum permissible concentration of the iron impurities in the salt bath is usually less than about 0.15% by weight calculated on the weight basis of sodium ferrocyanide Na Fe(ON) of course, the smaller the content of the iron impurities the better.
  • the conventional process for removing the iron impurities contained in the salt bath usually comprises the following two steps which are both carried out by mechanical operations:
  • German Pat. Nos. 608,257 and 1,255,438 well known electrochemical processes for nitrization and carbonitrization.
  • nitrization must be electro-chemically efiected for long time by using iron or iron alloy products as the anode and the salt tank as the cathode at the temperature of less than 721 C.
  • the treating temperature of more than 721 C. will be necessary to accelerate a diffusion of nitrogen into a difiusion area.
  • nitrization must be electrochemically effected by using the bath tank as the cathode and the auxiliary electrode as the anode.
  • nitrization is electrochemically eifected using the salt bath as the cathode and titanium or aluminium electrode as the anode whereby depositing the iron impurities on the wall of the salt bath. Since the efiiciency of nitrization becomes bad as the iron impurities in the salt bath as well as those deposited on the wall of the salt bath increase, there is no way but exchanging salt bath. It is a disadvantage of the invention to redissol've the iron impurities deposited on the Wall of the salt bath by means of an interruption of electric current and a stop of circularization.
  • nitrization is eletrochemically eifected using the salt bath as the cathode and iron or iron alloy electrode as the anode thereby dissolving the wall of the salt bath when a voltage of more than a decomposition voltage is charged.
  • the cathode is easy to exchange, while the anode is difiicult to exchange.
  • the efficiency of the nitrization will decrease as that of the item 1).
  • the present invention resides in a process for removing the iron impurities with good efficiency by using easily removable electrodes and circulating a direct electric current between both electrodes provided in the salt bath for nitrization, whereby depositing the iron impurities contained in the salt bath on the surface of the cathode.
  • FIGS. 1 and 2 illustrate an embodiment of a process for removing the iron impurities or preventing an increasement of the iron impurities contained in the salt bath tank according to the present invention, the process of FIG. 1 being effected before and after nitrization interminently and also the process of FIG. 2 being effected together with nitrization.
  • the anode 3 and the cathode 4 are disposed in a molten salt 2 of the salt bath 1 for nitrization, and a direct electric current from an electric source E is applied to said electrode thereby to cause a deposition of the iron impurities on the surface of the cathode 4, the latter 4 being taken out of the bath from time to time to wash away the iron impurities deposited on the cathode in the course of treatment for a repeated use thereof.
  • the article 5 that is being subjected to nitriding may be continuously treated during circulating the electric current between the electrodes provided in the salt bath for removing the iron impurities or may be taken out of the salt Without any hindrance of nitrization.
  • FIG. 3 shows another embodiment of a continuous process for removing the iron impurities contained in the salt bath according to the process of the present invention wherein the salt bath 1 for nitrization and the tank 6 for removing iron impurities are provided separately, said both tanks being communicated with each other through conduits 7, 8, whereby the molten salts 2, 2' in said tanks are smoothly transferred and circulated by means of a pump 9 thereby performing the continuous nitrization treatment in which nitrization for the products 5 as well as the removal of the iron impurities or the prevention for increasing the iron impurities are conducted separately but simultaneously.
  • the actual capacity of the nitriding tank 1 increases in the comparison with that of FIG. 2 thereby the capacity for nitrization increases.
  • the anode 3 used in the process of the present invention is made of a material such as platinum of carbon rod, which is highly resistant to corrosion by the molten salt bath and does not contain any harmful impurities such as iron, but usually, a carbon rod for electrode is suitable for use.
  • the cathode 4 may be made of any suitable material having electro-conductivity property, but preferably, an iron-made wire gauze or the like which is easy to deposit the iron impurities on the surface thereof.
  • Both electrodes may be suitable configuration and arrangement if they are disposed opposite to each other so as to produce best workability.
  • the salt bath tank 1 and the iron removal treatment tank 6 in FIG. 3 are usually iron pots, so that it is indispensable matter to provide a lining of titanium material in the prior process to present undesirable radissolution of tank wall.
  • the present invention has an advantage of omitting such titanium lining.
  • DC-source may be used, but a pulsating current or a half-wave rectified current obtained by rectifying an alternate current through a selenium rectifier may be used.
  • the DC voltage applied is usually less than 40 volts.
  • the most suittable voltage range is about 1 to 10 volts, while the current density used is less than about 20 amp./dm. preferably less than about 10 amp./dm.
  • Example 1 The electrodeposition was effected in the molten salt bath (7504 x 1600 mm.) comprising 48% by weight of potassium cyanide and 43% by weight of potassium cyanate as shown in FIG. 1 using the carbon rod (50 mm. dia x 750 mm. length) as the anode and a cylindrical wire gauze (430 mm. dia x 930 mm. length) as the cathode and handing the product to be treated between the anode and the cathode provided in the molten salt bath in order to remove the iron impurities contained in the salt bath.
  • the carbon rod 50 mm. dia x 750 mm. length
  • a cylindrical wire gauze 430 mm. dia x 930 mm. length
  • An initial concentration of the iron impurities was about 0.2% by weight calculated on the weight basis of sodium ferrocyanide.
  • the test result as shown in FIG. 4 was obtained using the current density of l amp./dm. and an electric current of 2.5 volts and amperes.
  • the iron impurities decreased to 0.07% after treating for 60 minutes.
  • Example 2 The electrodeposition was effected using the same molten salt bath and the treating conditions as shown in the Example 1.
  • the initial concentration of the iron impurities was about 0.16% by weight calculated on the weight basis of sodium ferrocyanide.
  • the test result as shownin the FIG. 5 was obtained using the current density of 2.5 amp./dm. and the electric current of 6.0 volts and 250 amperes.
  • the iron impurities decreased to 0.05% after treating for 30 minutes.
  • the treating efficiency for nitrization increases by about 30% in comparison with that in case of a prior mechanical removing process using same bath in capacity.
  • Example 3 The electrodeposition was effected using the same molten salt bath and the treating conditions as shown in the Example 1 using the apparatus (800 mm. dia x 1600 mm. height) as indicated in the FIG. 2 and six carbon rods (26 mm. dia x 700 mm. length) as the anode and the cylindrical iron wire gauze (700 mm. dia x 930 mm. length) as the cathode charging goods to be treated every 2 hrs. continuously.
  • the goods to be treated are made of cast iron.
  • the total weight amounts to kg.
  • the initial concentration of the iron impurities was about 0.08% by weight calculated on the weight of sodium ferrocyanide.
  • the test result as shown in FIG. 6 was obtained using the current density of 0.4 amp/dmfi.
  • the continuous operation for 24 hours was effected by charging eleven times of the products to be treated without any increasement of the iron impurities.
  • the removal of the iron impurities may be effected by removing the iron impurities deposited and accumulated on the cathode at an interval of the treating time.
  • the iron impurities dissolved in the salt bath may be removed electrochemically without elevating the bath temperature, so that there is no fear of inviting excess reduction of a service life of the pot lining or early aging of the bath composition as observed in the conventional process.
  • the electrode is easy to exchange since the salt bath is not used as the electrode, while the efiiciency for removing the iron impurities is high as the current applied is not limited.
  • a process for removing iron impurities contained in a molten salt nitriding bath that is maintained at a temperature of from about 500 C. to about 600 C. including the steps of disposing a removable cathode in contact with the molten salt bath and causing a unidirectional electric current to flow between an anode and said cathode thereby to deposit on the cathode iron impurities contained in the salt bath.
  • a process as claimed in claim 1 wherein the removal of iron impurities is eifected employing a current density of less than 20 amp/dm. and a voltage of less than 40 volts.
  • a process as claimed in claim 1 wherein the removal of iron impurities from the molten salt bath is effected by the use of a current density of less than 10 amp/dm. and a voltage of from 1 to 10 volts.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Water Treatment By Electricity Or Magnetism (AREA)
US00111906A 1970-02-09 1971-02-02 Method for removing iron impurities contained in a salt bath for nitrogenation Expired - Lifetime US3726772A (en)

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JP45011287A JPS4945981B1 (de) 1970-02-09 1970-02-09

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3879274A (en) * 1971-12-09 1975-04-22 Nippon Sheet Glass Co Ltd Elimination of harmful substances from molten salt used in the ion exchange treatment of glass articles
US3953308A (en) * 1974-03-27 1976-04-27 Institut De Recherches De La Siderurgie Francaise (Irsid) Process and apparatus for desulfurizing of liquid metals
US4042427A (en) * 1974-03-21 1977-08-16 Daimler-Benz Aktiengesellschaft Process for controlling fused salt nitridation of metals with a solid electrolyte electrode
US4332653A (en) * 1980-06-13 1982-06-01 Shinzoh Satoh Method of nitriding by high temperature electrolysis
US4744875A (en) * 1985-05-21 1988-05-17 The United States Of America As Represented By The United States Department Of Energy Steel refining with an electrochemical cell

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3879274A (en) * 1971-12-09 1975-04-22 Nippon Sheet Glass Co Ltd Elimination of harmful substances from molten salt used in the ion exchange treatment of glass articles
US4042427A (en) * 1974-03-21 1977-08-16 Daimler-Benz Aktiengesellschaft Process for controlling fused salt nitridation of metals with a solid electrolyte electrode
US3953308A (en) * 1974-03-27 1976-04-27 Institut De Recherches De La Siderurgie Francaise (Irsid) Process and apparatus for desulfurizing of liquid metals
US4332653A (en) * 1980-06-13 1982-06-01 Shinzoh Satoh Method of nitriding by high temperature electrolysis
US4744875A (en) * 1985-05-21 1988-05-17 The United States Of America As Represented By The United States Department Of Energy Steel refining with an electrochemical cell

Also Published As

Publication number Publication date
JPS4945981B1 (de) 1974-12-07
DE2105816C3 (de) 1975-12-04
DE2105816A1 (de) 1972-01-05
DE2105816B2 (de) 1975-03-06

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