Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
• • 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
• • 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
  • C23C8/52—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 more than one element being applied in one step
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D21/00—Processes for servicing or operating cells for electrolytic coating
  • C25D21/12—Process control or regulation
  • C25D21/14—Controlled addition of electrolyte components

Definitions

• the invention relates to a process for nitriding ferrous metal parts, giving them increased resistance to corrosion, in which the parts are treated by immersion for a suitable period in a bath of molten salts essentially consisting of cyanates and carbonates of alkali metals.
• Salt baths have been known for a long time capable of causing metalloids, essentially nitrogen and possibly carbon and sulfur, to diffuse into the surface layers of parts made of ferrous metals to improve their resistance to wear and seizure.
• metalloids essentially nitrogen and possibly carbon and sulfur
• Patent documents FR-A-2 171 993 and FR-A-2 271 307 describe such baths, in which the presence of lithium among the alkali metals, and of sulfur species in small quantities, led to nitrided layers of improved quality.
• FR-A-2 271 307 describes a process for regenerating baths by introducing regenerating salts comprising, next to nitrogen-supplying compounds, at least one substance comprising in its formula a carboxyl group, whereby the content in cyanide was maintained in trace amounts, the sulfur catalyzing the action of the regenerant.
• nitriding brings an improvement in resistance to corrosion.
• the invention provides a process for nitriding ferrous metal parts, giving them increased resistance to corrosion, in which the parts are treated by immersion for an appropriate period in a bath of molten salts consisting essentially of alkali metal cyanates and carbonates, and containing an amount of at least one sulfur species, characterized in that, during their immersion in the bath, the parts are brought, with respect to a counterelectrode immersed in the bath, to a potential positive electric current such that a substantial current crosses the bath of the parts at the counterelectrode, and the content of cyanides formed in secondary reaction is maintained at a value less than 6%.
• the bath is contained in a metal crucible forming a counterelectrode.
• the extent and shape of the crucible promotes electric field configurations in the molten salt bath which regulate the current density on the workpieces, and decrease the current density on the counterelectrode and therefore reduce the importance of secondary redox phenomena at the salt bath / crucible wall interface.
• the average current passing through the bath is kept substantially constant during the treatment of the parts. It has in fact been found that the properties of the layers formed on the parts by the treatment vary as a function of the current density which creates them. The results can therefore, be reproducible only if the current must be kept constant during treatment.
• Appropriate current density values are in the range of 300 to 800 A / m2, the preferred range being 450 to 550 A / m2. If we use the unit (non-standard), conventional in industrial electrochemistry, with current density, ie A / dm2, the ranges are 3 to 8 A / dm2, better 4.8-5.5 A / dm2.
• the bath temperatures being, conventionally in the range 450 ° -650 ° C, and better still 550-600 ° C.
• the treatment duration can range from 10 to 150 min., The most effective durations ranging from 30 to 100 min.
• the preferred baths have a composition corresponding substantially to the compositions according to FR-A-2 171 993, with, more precisely the following anionic and cationic contents: CNO ⁇ CO32 ⁇ K+ Na+ Li+ 30 to 45% 15 to 25% 20 to 30% 15 to 25% 0.5 to 5%
• Their CN ⁇ cyanide content is less than 2% and they additionally contain at least one sulfur species, in an amount such that the S2 ⁇ content of the bath is between 1 and 6 ppm.
• FR-A-2 271 307 it is preferred, in accordance with FR-A-2 271 307, to maintain the bath substantially to its original composition by additions of regenerating agents, and to homogenize it, the homogeneity being preferably obtained by blowing in air.
• thermochemical nitriding process For the study and development of the process according to the invention, the tests were carried out while trying to vary only one parameter at a time. Given that, compared to known nitriding processes, the contribution of the invention consisted in making a process cooperate electrochemical to a thermochemical nitriding process, without knowing a priori the interactions that could occur between these two processes, we chose to keep fixed the thermochemical parameters, bath composition and temperature, and to vary the electrochemical parameters, current density and amount of electrical charge passing through the bath.
• this charge quantity parameter is equivalent, at constant current density, to the time of current flow in the bath, which is also a thermochemical parameter.
• the operation was carried out in a metal crucible containing 400 kg of molten salts in accordance with FR-A-2 171 993, heated in operation to 570 ° C.
• the chemical composition was kept constant according to the teachings of FR-A-2 271 307 by periodic metered addition of regenerating salts and potassium sulfide. Air was blown into the crucible for the purpose of homogenization, at a flow rate of 250 l / min.
• Periodic filtration ensured that the rate of suspended solids was kept at a suitable value.
• test pieces were plates of 100 x 100 mm in area (ie 2 dm2 for both sides) and 1 mm thick, made of XC38 steel, which were fixed to a metal bar mounted isolated across the upper opening of the crucible.
• a direct current source consisting of a potentiostat capable of delivering up to 10 amps, adjustable and stabilized, either in voltage or in current, was connected by one pole to the crucible and by the other to the supply bar of current where the part is fixed.
• the plate-shaped parts were degreased with trichlorethylene vapor; leaving the bath after the treatments, the pieces were cooled for 2 min. with calm ambient air (to avoid thermal shocks), then rinse with hot water (> 60 ° C) for 10 minutes, the water being stirred by blowing air, then drying with hot air.
• the current passing through the bath was therefore increased in stages between series of tests.
• the current will be expressed in current density, which is a substantially invariant parameter for the transposition of dimension of parts.
• the active surface of the rooms was 2 dm2. So we set the current at 2, 4, 6, 8 and 10 amps, or 1, 2, 3, 4 and 5 A / dm2.
• test pieces were subjected to corrosion tests. These were carried out according to two methods; measurement of corrosion potential in deaerated 3% NaCl solution; and determination of the duration of exposure to normalized salt spray before the appearance of traces of corrosion. It will be noted that, for these tests, the edges of the plates were protected by a varnish, to avoid that anomalies in the surface state in the immediate vicinity of sharp angles do not disturb the tests. The results are reported in Table 1 below:
• the dense gray layer has already started to form after 30 minutes of treatment. At 60 minutes it is comparable to that obtained at 90 minutes under 4 A / dm2. It then continues to grow, but begins to become porous at 120 minutes while the deep white layer shows signs of degradation.
• the corrosion resistance tests corroborated those of the first series of tests, namely that the parts whose layers formed comprise a dense gray layer have a corrosion resistance much higher than that of the nitrided layers without intervention. current, and which is in the range of resistance to corrosion obtained by a bath treatment of oxidizing salts after the conventional nitriding treatment without current.
• the bath of oxidizing salts is, for example, a bath in accordance with FR-A-2,525,637.
• the nitrided layer degrades and becomes porous over its entire thickness, which leads to a reduction in the resistance to corrosion and to wear of the treated parts.
• the nitriding / oxidation treatment was carried out under the conditions of Example 1, at a current density of 5 A / dm2, respectively for 30 minutes (mark A) and for 60 minutes (mark B). Sliders treated for 90 minutes without current intervention (reference C) according to the teachings of FR-A-2 171 993 were used in comparison.
• the current flowing through the bath does not appear necessary for the current flowing through the bath to be a strict direct current, and could be a non-filtered unidirectional current, or a pulsed current.
• the surface condition of the parts, as well as the composition of the surface layers, would be favorable for the attachment of a varnish or a wax, beneficial for certain applications.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Mechanical Engineering (AREA)
• Electrochemistry (AREA)
• Automation & Control Theory (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
• Preventing Corrosion Or Incrustation Of Metals (AREA)
• Superconductors And Manufacturing Methods Therefor (AREA)

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• 1993
  • 1993-08-06 FR FR9309706A patent/FR2708623B1/fr not_active Expired - Fee Related
• 1994
  • 1994-07-15 TW TW083106466A patent/TW315389B/zh active
  • 1994-07-22 US US08/273,151 patent/US5518605A/en not_active Expired - Fee Related
  • 1994-07-22 EP EP94401694A patent/EP0637637B1/de not_active Expired - Lifetime
  • 1994-07-22 DE DE69402272T patent/DE69402272T2/de not_active Expired - Fee Related
  • 1994-07-22 ES ES94401694T patent/ES2099552T3/es not_active Expired - Lifetime
  • 1994-07-22 AT AT94401694T patent/ATE150802T1/de not_active IP Right Cessation
  • 1994-07-28 CA CA002129061A patent/CA2129061C/fr not_active Expired - Fee Related
  • 1994-07-29 BR BR9403000A patent/BR9403000A/pt not_active IP Right Cessation
  • 1994-08-04 PL PL94304555A patent/PL177659B1/pl not_active IP Right Cessation
  • 1994-08-05 JP JP6184526A patent/JP3056951B2/ja not_active Expired - Fee Related
  • 1994-08-05 CN CN94114998A patent/CN1054890C/zh not_active Expired - Fee Related
  • 1994-08-06 KR KR1019940019423A patent/KR100294861B1/ko not_active Expired - Fee Related
  • 1994-08-06 MY MYPI94002052A patent/MY117886A/en unknown

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