WO1996034127A1 - Alkaline nitrate salt bath composition for oxidising a ferrous metal and improving the corrosion resistance thereof - Google Patents

Alkaline nitrate salt bath composition for oxidising a ferrous metal and improving the corrosion resistance thereof Download PDF

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Publication number
WO1996034127A1
WO1996034127A1 PCT/FR1995/000560 FR9500560W WO9634127A1 WO 1996034127 A1 WO1996034127 A1 WO 1996034127A1 FR 9500560 W FR9500560 W FR 9500560W WO 9634127 A1 WO9634127 A1 WO 9634127A1
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Prior art keywords
bath
nitrate
cations
anions
sodium
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PCT/FR1995/000560
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French (fr)
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Jean-Paul Terrat
Philippe Maurin-Perrier
Daniel Viviani
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Centre Stephanois De Recherches Mecaniques Hydromecanique Et Frottement
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Priority to BR9507428A priority Critical patent/BR9507428A/en
Priority to PCT/FR1995/000560 priority patent/WO1996034127A1/en
Publication of WO1996034127A1 publication Critical patent/WO1996034127A1/en

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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
    • C23C8/80—After-treatment
    • 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
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/70—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using melts
    • C23C22/72—Treatment of iron or alloys based thereon

Definitions

  • the invention relates to a composition of salt baths for a surface oxidation treatment of ferrous metal parts, in particular nitrides, in order to increase their resistance to corrosion, the treatment being carried out at a temperature between 320 ° and 550 ° C, composition comprising at least nitrate anions and alkali metal cations sodium and if necessary potassium.
  • Salt baths containing alkali metal nitrates are used in the usual way and for a long time to treat ferrous metal parts, in particular previously nitrided, in order to increase their resistance to corrosion, by forming a layer of magnetite Fe 3 0 4 , to protect the underlying iron.
  • Document FR-A-2 463 821 describes a process for treating nitrided ferrous metal parts which consists in immersing the parts in a bath of molten salts consisting of sodium and potassium hydroxides with 2 to 20% by weight of nitrates of these alkali metals, for 15 to 50 minutes.
  • the forecast temperatures range from 250 ° to 450 ° C.
  • the corrosion resistance of the parts thus treated is greatly increased compared to that of simply nitrided parts.
  • Document FR-A-2 525 637 describes a process of the same kind, particularly intended for parts made of ferrous metal containing sulfur, such as parts nitrided in baths containing sulfur species.
  • the oxidizing bath comprises sodium and potassium cations, and nitrate and hydroxyl anions, preferably accompanied by carbonate anions, with in addition from 0.5 to 15% of an oxygenated salt of alkali metal whose oxidative potential reduction, compared to the hydrogen reference electrode is less than or equal to - 1 volt, such as a dichromate.
  • oxygenated gas is blown into the bath, and the weight content of the bath in insoluble particles is maintained at less than 3%. Good performance in corrosion resistance is obtained (250 hours in salt spray), while the wear and fatigue resistance is not impaired, and an improvement in the resistance to seizing is observed. in dry friction.
  • the enrichment of carbonates in the baths due to the oxidation of cyanates in nitriding baths, and to the absorption of atmospheric carbon dioxide causes precipitation of carbonates, forming sludge at the bottom of the bath.
  • the removal of this sludge gives rise to entrainment of active compounds from the bath.
  • the invention relates to oxidizing bath compositions based on nitrates of alkaline earth metals which have a reliable and repetitive oxidizing power.
  • the invention therefore provides a composition of salt baths for a surface oxidation treatment of ferrous metal parts, in particular nitrides, in order to increase their resistance to corrosion, the treatment being carried out at a temperature of between 320 ° and 550 ° C., composition comprising at least nitrate anions and sodium and if appropriate potassium alkali metal cations, characterized in that it comprises lithium cations substituted for sodium or potassium cations, at a weight content, based on the mass of the bath, of between 0.1 and 5%.
  • the Applicant has found that the substitution of lithium for sodium and possibly potassium in the proportions indicated above led, unpredictably, to baths which formed, on ferrous metal parts, layers of magnetite of uniform black, the corrosion potential of the treated parts being systematically at least 1000 mV, even on parts made of materials known to be refractory to the oxidation treatment, such as nitride cast irons.
  • the chemical properties of the alkali metals are very close, so that those skilled in the art usually consider that the alkali metals can be substituted for one another due to circumstances such as availability, cost of the compounds, purity or stability.
  • the cationic combination is often chosen so that the melting temperature of the mixture is fairly low, and the viscosity of the bath is, at working temperature, sufficiently low.
  • the Applicant has moreover not elucidated in a certain and precise manner the physicochemical mechanisms which lead, for the baths according to the invention, to the formation of perfectly tight magnetite layers with ordered crystallization, manifested by the appearance uniform black of the surface of the parts, and by the corrosion potential.
  • the lithium content by weight is between 0.5 and 1.75%; it is in this area that the results of corrosion resistance are the most reliable and reproducible.
  • the preferred bath compositions comprise, in stoichiometric equilibrium with the alkali metal cations, in addition to nitrate anions, carbonate and hydroxyl anions, the weight proportions of carbonate anions C0 3 2 ⁇ , nitrate N0 3 " and hydroxyl OH ⁇ , based on the active or liquid mass of the bath, being included within the following limits, in percentages:
  • this aforementioned composition contains significant weight proportions of potassium.
  • the Applicant has also found that the presence of lithium in the baths containing nitrate, hydroxyl and carbonate anions reduced the amount of sludge formed by the precipitation of carbonates. This effect seems particularly marked when the contents of lithium and potassium cations and of carbonate or nitrate anions substantially correspond to a carbonate or ternary nitrate eutectic of alkali, sodium, potassium and lithium.
  • the lithium content being determined to ensure the formation of magnetite layers with ordered crystallization, the contents of carbonate or nitrate anions and potassium cations are linked to the lithium content by the relationships for the carbonate eutectic:
  • the sodium content satisfies the stoichiometry.
  • Example 1 An oxidizing salt bath is produced by melting a mixture of 365 kg of sodium nitrate, 365 kg of sodium hydroxide, 90 kg of sodium carbonate, 90 kg of potassium carbonate and 90 kg of lithium carbonate , and bringing it to 450 ° C.
  • ionic contents are therefore the following, in percentages: cation anions
  • test pieces presented a particularly uniform and decorative black color.
  • a crystallographic analysis of these test pieces by X-ray diffraction shows that the predominantly present species is magnetite Fe 3 0 4 ; there is a minor proportion of mixed oxide Li 2 Fe 3 0 4 .
  • 1,300 mV corresponds in fact to the inherent oxidation potential of the NaCl solution; thus, an actual corrosion potential cannot be measured, since it is at least as high as the oxidation potential of the test solution.
  • the first bath included 330 kg of sodium nitrate, 330 kg of sodium hydroxide, 330 kg of sodium carbonate and 10 kg of sodium dichromate, which gives the ion contents in percent: cation anions
  • the second bath included 150 kg of sodium nitrate, 530 kg of sodium hydroxide and 320 kg of sodium carbonate, that is to say in ionic composition (in%): cation anions N0 3 ⁇ 11 Na + 48.3
  • the treatment conditions (temperature 450 ° C., duration 5 minutes) were repeated from the first example. The following observations were made:
  • test pieces treated in the first comparison bath are of a uniform black; their corrosion potential is between 1000 and 1300 mV, and it can be concluded that the oxide layer is very passive.
  • test specimens treated in the second comparison bath are mostly black, but some have brown reflections.
  • the corrosion potential varies between 250 and 1,300 mV. It can be concluded from this that the layer of magnetite is of irregular quality from one test piece to another, and that the second comparison bath does not have sufficient reliability.
  • Example 2 In the aspect of mechanical and tribological qualities, the bath of Example 1 and the first comparison bath give completely equivalent results.
  • Example 2 In the aspect of mechanical and tribological qualities, the bath of Example 1 and the first comparison bath give completely equivalent results.
  • Example 2 In the aspect of mechanical and tribological qualities, the bath of Example 1 and the first comparison bath give completely equivalent results.
  • Example 2 In the aspect of mechanical and tribological qualities, the bath of Example 1 and the first comparison bath give completely equivalent results.
  • the nitrates are distributed between the three alkaline cations in proportions NaN0 3 14.9%, KN0 3 66.8%, LiN0 3 18.3% corresponding substantially to the ternary eutectic.
  • the corresponding ionic contents by weight are as follows, in percentages: cation anions
  • Example 2 In this bath, with the same operating conditions as in Example 1 and the comparison examples, nitrided cast iron test pieces were treated. These specimens, after treatment, are of a uniform black, the surface layer is mainly magnetite Fe 3 0 4 , and their corrosion potential is in the range 1000-1 300 mV.
  • a bath A comprised 48.5% KN0 3 , 39.5% NaN0 3 and 12% LiN0 3 , with the ionic composition (in%): anion cations
  • a comparison bath B is made up, comprising 55% NaN0 3 and 45% KN0 3 , ie in ionic percentage: cation anion
  • test pieces treated in bath A all have a deep black surface layer, while the test pieces treated in bath B have a gray surface layer with brown reflections.
  • the corrosion potentials determined as above, are in the range of 1000-1 300 mV for the specimens treated in bath A, and in a range of 300-900 mV for those which have been treated in bath B, with the expected consequences on corrosion resistance.
  • Example 3 It will be noted, with regard to Example 3, that the parts to be treated must have been carefully freed from the residues of nitriding bath, the baths of pure nitrates being liable to react violently on contact with reducing substances.
  • the rule to best associate the two effects will be to choose the appropriate lithium content for the formation of the protective magnetite layer, and starting from this content, to determine the potassium and carbonate or nitrate anion content, from the ternary eutectic composition of this anion.
  • the carbonate anion we then have:
  • the sodium cation will be in excess of the composition of the ternary eutectic, due to the presence of anions other than the anion taken into consideration for the eutectic, and to the fact that the bath must be in stoichiometric equilibrium.

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  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
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Abstract

A salt bath composition is provided for forming on the surface of ferrous metal, particularly pre-nitrided, parts, an Fe3O4 magnetite layer so as to protect the underlying iron against corrosion, said layer being a sealing and ordered crystal layer, thus providing a deep black coloration and a corrosion potential of more than 1 000 mV. The composition includes at least nitrate anions and sodium and lithium cations in a range of 0.1-5 % by weight of the bath, and preferably of 0.5-1.75 %. The preferred compositions contain sodium, potassium and lithium cations, and nitrate, carbonate and hydroxyl anions within the following percentage ranges: 8.5 ≤ CO32- ≤ 26, 15 ≤ NO¿3?- ≤ 41,5, 4.7 ≤ OH- ≤ 21.5. In addition to the exceptional reliability of the oxidizing treatment, the presence of lithium combined with sodium and potassium and nitrate or carbonate anions reduces the formation of carbonated slurries in the bath, probably due to the formation of low melting point ternary nitrate or carbonate compounds.

Description

"Composition de bains de sels à base de nitrates alcalins pour oxyder du métal ferreux et ainsi améliorer sa résistance à la corrosion""Composition of salt baths based on alkaline nitrates to oxidize ferrous metal and thus improve its resistance to corrosion"
L'invention se rapporte à une composition de bains de sels pour un traitement d'oxydation de surface de pièces en métal ferreux, notamment nitrurees, en vue d'augmenter leur résistance à la corrosion, le traitement s'effectuant à une température comprise entre 320° et 550°C, composition comportant au moins des anions nitrate et des cations alcalins sodium et le cas échéant potassium.The invention relates to a composition of salt baths for a surface oxidation treatment of ferrous metal parts, in particular nitrides, in order to increase their resistance to corrosion, the treatment being carried out at a temperature between 320 ° and 550 ° C, composition comprising at least nitrate anions and alkali metal cations sodium and if necessary potassium.
On utilise des bains de sels comportant des nitrates de métaux alcalins de façon usuelle et depuis longtemps pour traiter des pièces en métal ferreux, notamment préalablement nitrurees, en vue d'augmenter leur résistance à la corrosion, par formation d'une couche de magnetite Fe304, pour protéger le fer sous-jacent.Salt baths containing alkali metal nitrates are used in the usual way and for a long time to treat ferrous metal parts, in particular previously nitrided, in order to increase their resistance to corrosion, by forming a layer of magnetite Fe 3 0 4 , to protect the underlying iron.
Le document FR-A-2 463 821 décrit un procédé de traitement de pièces en métal ferreux nitrurees qui consiste à immerger les pièces dans un bain de sels en fusion constitué d'hydroxydes de sodium et potassium avec 2 à 20 % en poids de nitrates de ces métaux alcalins, pendant 15 à 50 minutes. Les températures prévues s'échelonnent entre 250° et 450°C. La résistance à la corrosion des pièces ainsi traitées est fortement augmentée par rapport à celle de pièces simplement nitrurees.Document FR-A-2 463 821 describes a process for treating nitrided ferrous metal parts which consists in immersing the parts in a bath of molten salts consisting of sodium and potassium hydroxides with 2 to 20% by weight of nitrates of these alkali metals, for 15 to 50 minutes. The forecast temperatures range from 250 ° to 450 ° C. The corrosion resistance of the parts thus treated is greatly increased compared to that of simply nitrided parts.
Le document FR-A-2 525 637 décrit un procédé de même genre, particulièrement destiné à des pièces en métal ferreux contenant du soufre, telles que des pièces nitrurees dans des bains contenant des espèces soufrées. Le bain oxydant comprend des cations sodium et potassium, et des anions nitrate et hydroxyle, de préférence accompagnés d'anions carbonate, avec en plus de 0,5 à 15 % d'un sel oxygéné de métal alcalin dont le potentiel d'oxydo- réduction, par rapport à l'électrode de référence à hydrogène est inférieur ou égal à - 1 volt, tel qu'un bichromate. En outre, on insuffle un gaz oxygéné dans le bain, et on maintient la teneur pondérale du bain en particules insolubles à moins de 3 %. On obtient de bonnes performances en résistance à la corrosion (250 heures en brouillard salin), tandis que l'on n'altère pas la tenue à l'usure et à la fatigue, et l'on observe une amélioration de la résistance au grippage en frottement sec.Document FR-A-2 525 637 describes a process of the same kind, particularly intended for parts made of ferrous metal containing sulfur, such as parts nitrided in baths containing sulfur species. The oxidizing bath comprises sodium and potassium cations, and nitrate and hydroxyl anions, preferably accompanied by carbonate anions, with in addition from 0.5 to 15% of an oxygenated salt of alkali metal whose oxidative potential reduction, compared to the hydrogen reference electrode is less than or equal to - 1 volt, such as a dichromate. In addition, oxygenated gas is blown into the bath, and the weight content of the bath in insoluble particles is maintained at less than 3%. Good performance in corrosion resistance is obtained (250 hours in salt spray), while the wear and fatigue resistance is not impaired, and an improvement in the resistance to seizing is observed. in dry friction.
Toutefois, on s'est aperçu que ces performances ne pouvaient être en fait atteintes avec les degrés de fiabilité et de reproductibilité requis par les exigences industrielles. En laboratoire, les écarts de performances sont relativement peu visibles. Ils deviennent par contre beaucoup plus nets dès lors qu'il s'agit de traiter des séries industrielles. Ils sont particulièrement observés lorsque l'on doit conditionner, selon la technologie dite du "vrac", de grandes quantités de petites pièces, ou bien des pièces dont les états de surface sont imparfaits : la présence de zones perturbées telles que bavures d'emboutissage ou de poinçonnage, replis de sertissage ou de pliage, hétérogénéités de soudage est autant de sources de défauts, donc d'amorces de corrosion.However, it has been found that these performances cannot in fact be achieved with the degrees of reliability and reproducibility required by industrial requirements. In the laboratory, differences in performance are relatively invisible. However, they become much sharper when it comes to dealing with industrial series. They are particularly observed when it is necessary to condition, according to the so-called "bulk" technology, large quantities of small parts, or else parts whose surface conditions are imperfect: the presence of disturbed areas such as stamping burrs or punching, crimping or folding folds, welding heterogeneities are all sources of faults, and therefore of corrosion.
Or, sur des pièces telles que des tiges de vérins ou d'amortisseurs, ou bien encore des axes d'essuie-glaces ou de démarreurs automobiles, une tenue aléatoire à la corrosion est absolument inacceptable. La solution a longtemps été de procéder à des retouches successives des bains au cas par cas et selon les comportements plus ou moins aberrants observés. Toutefois, cette solution n'est pas satisfaisante compte tenu notamment des exigences industrielles expliquées précédemment. On a cherché, par ajustage des proportions de constituants du bain (hydroxydes, carbonates, nitrate, bichromate) , à améliorer la fiabilité et les performances de résistance à la corrosion. Les études de la Demanderesse ont fait ressortir que, pour présenter d'excellentes performances en résistance à la corrosion (soit plus de 400 heures d'épreuve en brouillard salin avant apparition de traces de corrosion) , la surface des pièces devait présenter une teinte uniforme de noir profond, typique de la formation d'une couche de magnetite Fe304 de cristallisation bien ordonnée. Parallèlement une détermination de potentiel de corrosion dans une solution de NaCl à 30 g/1, par rapport à l'électrode au calomel saturé devait accuser une valeur de 1 000 à 1 300 mV, correspondant à une passivation complète.However, on parts such as rods of cylinders or shock absorbers, or even axes of windscreen wipers or car starters, random corrosion resistance is absolutely unacceptable. The solution has long been to carry out successive retouching of the baths on a case-by-case basis and according to the more or less aberrant behaviors observed. However, this solution is not satisfactory, in particular given the industrial requirements explained above. Attempts have been made, by adjusting the proportions of constituents of the bath (hydroxides, carbonates, nitrate, dichromate), to improve the reliability and the performance of corrosion resistance. The Applicant's studies have shown that, in order to exhibit excellent performance in corrosion resistance (ie more than 400 hours of salt spray test before the appearance of traces of corrosion), the surface of the parts must present a uniform shade of deep black, typical of the formation of a layer of magnetite Fe 3 0 4 of well-ordered crystallization. At the same time, a determination of the corrosion potential in a NaCl solution at 30 g / 1, relative to the saturated calomel electrode, had to show a value of 1000 to 1300 mV, corresponding to a complete passivation.
On observera la corrélation entre le potentiel d'oxydo-réduction du sel oxygéné tel qu'un bichromate, et le potentiel de corrosion souhaitable.The correlation between the redox potential of the oxygenated salt such as a dichromate and the desirable corrosion potential will be observed.
Toutefois, l'efficacité de tels bains comprenant des hydroxydes, des nitrates, des carbonates et du bichromate ou permanganate de métal alcalin demande un contrôle fréquent de la composition du bain, et des réglages des conditions opératoires spécifiques des pièces. De plus, les modifications de composition des bains par consommation des réactifs, apports de souillures des pièces dues à des traitements antérieurs, et réaction de ces souillures avec les composants du bain, entraînement des composants du bain par les pièces retirées du bain de sel, et réaction des hydroxydes du bain avec le dioxyde de carbone atmosphérique, entraînent des variations de performances, malgré les réajustements périodiques de composition. Notamment la teneur on oxydant fort (bichromate) est relativement critique, pour des applications spécifiques.However, the effectiveness of such baths comprising hydroxides, nitrates, carbonates and dichromate or permanganate of alkali metal requires frequent control of the composition of the bath, and adjustments of the specific operating conditions of the parts. In addition, the changes in composition of the baths by consumption of the reagents, contributions of soiling of the parts due to previous treatments, and reaction of these soiling with the components of the bath, entrainment of the components of the bath by the parts removed from the salt bath, and reaction of the hydroxides of the bath with atmospheric carbon dioxide, cause variations in performance, despite periodic readjustments in composition. In particular, the content of strong oxidant (dichromate) is relatively critical, for specific applications.
Plus particulièrement, l'enrichissement en carbonates des bains, dû à l'oxydation de cyanates de bains de nitruration, et à l'absorption du dioxyde de carbone atmosphérique entraîne une précipitation de carbonates, formant des boues en fond de bain. L'enlèvement de ces boues donne lieu à entraînement de composés actifs du bain.More particularly, the enrichment of carbonates in the baths, due to the oxidation of cyanates in nitriding baths, and to the absorption of atmospheric carbon dioxide causes precipitation of carbonates, forming sludge at the bottom of the bath. The removal of this sludge gives rise to entrainment of active compounds from the bath.
L'invention vise des compositions de bains oxydants à base de nitrates de métaux alcalino-terreux qui présentent un pouvoir oxydant fiable et répétitif. L'invention propose donc une compositon de bains de sels pour un traitement d'oxydation de surface de pièces en métal ferreux, notamment nitrurees, en vue d'augmenter leur résistance à la corrosion, le traitement s'effectuant à une température comprise entre 320° et 550°C, composition comportant au moins des anions nitrate et des cations alcalins sodium et le cas échéant potassium, caractérisée en ce qu'elle comporte des cations lithium substitués à des cations sodium ou potassium, à une teneur pondérale, rapportée à la masse du bain, comprise entre 0,1 et 5 %.The invention relates to oxidizing bath compositions based on nitrates of alkaline earth metals which have a reliable and repetitive oxidizing power. The invention therefore provides a composition of salt baths for a surface oxidation treatment of ferrous metal parts, in particular nitrides, in order to increase their resistance to corrosion, the treatment being carried out at a temperature of between 320 ° and 550 ° C., composition comprising at least nitrate anions and sodium and if appropriate potassium alkali metal cations, characterized in that it comprises lithium cations substituted for sodium or potassium cations, at a weight content, based on the mass of the bath, of between 0.1 and 5%.
La Demanderesse a constaté que la substitution de lithium au sodium et éventuellement potassium dans les proportions indiquées ci-dessus conduisait, de façon imprévisible, à des bains qui formaient, sur des pièces en métal ferreux, des couches de magnetite d'un noir uniforme, le potentiel de corrosion des pièces traitées étant d'au moins 1 000 mV de façon systématique, et ce même sur des pièces en matériaux réputés réfractaires au traitement d'oxydation, tels que des fontes nitrurees.The Applicant has found that the substitution of lithium for sodium and possibly potassium in the proportions indicated above led, unpredictably, to baths which formed, on ferrous metal parts, layers of magnetite of uniform black, the corrosion potential of the treated parts being systematically at least 1000 mV, even on parts made of materials known to be refractory to the oxidation treatment, such as nitride cast irons.
On observera que les propriétés chimiques des métaux alcalins sont très proches, de sorte que l'homme du métier considère habituellement que l'on peut substituer les métaux alcalins les uns aux autres en raison de circonstances telles que disponibilité, coût des composés, pureté ou stabilité. En bains de sels, la combinaison cationique est souvent choisie pour que la température de fusion du mélange soit assez basse, et que la viscosité du bain soit, à la température de travail, suffisamment faible.It will be observed that the chemical properties of the alkali metals are very close, so that those skilled in the art usually consider that the alkali metals can be substituted for one another due to circumstances such as availability, cost of the compounds, purity or stability. In salt baths, the cationic combination is often chosen so that the melting temperature of the mixture is fairly low, and the viscosity of the bath is, at working temperature, sufficiently low.
La Demanderesse n'a d'ailleurs pas élucidé de façon certaine et précise les mécanismes physico-chimiques qui conduisent, pour les bains conformes à l'invention, à la formation de couches de magnetite parfaitement étanches à cristallisation ordonnée, manifestée par l'aspect noir uniforme de la surface des pièces, et par le potentiel de corrosion.The Applicant has moreover not elucidated in a certain and precise manner the physicochemical mechanisms which lead, for the baths according to the invention, to the formation of perfectly tight magnetite layers with ordered crystallization, manifested by the appearance uniform black of the surface of the parts, and by the corrosion potential.
Elle a soupçonné toutefois, à partir des résultats constatés, que la petitesse du rayon atomique du lithium pouvait jouer un rôle déterminant. On sait que, grâce à ce faible rayon atomique, le lithium est susceptible de pénétrer dans la maille cristalline de la magnetite pour former un composé cristallin Li2Fe304, de paramètres de maille cristalline bien définis et constants. Il est alors possible que le cation lithium joue, lors de la formation de la magnetite, un rôle stabilisateur de la maille cristalline de ce composé.However, it suspected from the results observed that the small atomic radius of lithium could play a decisive role. We know that, thanks to this small atomic radius, lithium is capable of penetrate into the crystal lattice of magnetite to form a crystalline compound Li 2 Fe 3 0 4 , of well defined and constant crystal lattice parameters. It is then possible that the lithium cation plays, during the formation of magnetite, a stabilizing role for the crystal lattice of this compound.
De préférence, la teneur pondérale en lithium est comprise entre 0,5 et 1,75 % ; c'est dans ce domaine que les résultats de résistance à la corrosion sont les plus fiables et reproductibles.Preferably, the lithium content by weight is between 0.5 and 1.75%; it is in this area that the results of corrosion resistance are the most reliable and reproducible.
Les compositions de bain préférées comportent, en équilibre stoechiométrique avec les cations de métaux alcalins, outre des anions nitrates, des anions carbonate et hydroxyles, les proportions pondérales en anions carbonate C03 2~, nitrate N03 " et hydroxyle OH~, rapportées à la masse active ou liquide du bain, étant comprises dans les limites ci-après, en pourcentages :The preferred bath compositions comprise, in stoichiometric equilibrium with the alkali metal cations, in addition to nitrate anions, carbonate and hydroxyl anions, the weight proportions of carbonate anions C0 3 2 ~ , nitrate N0 3 " and hydroxyl OH ~ , based on the active or liquid mass of the bath, being included within the following limits, in percentages:
8,5 < C03 2~ < 268.5 <C0 3 2 ~ <26
15 < N03 ~ < 41,5 4,7 < OH" < 21,515 <N0 3 ~ <41.5 4.7 <OH " <21.5
Ces limites ont été déterminées de façon expérimentale, pour assurer, avec une faible susceptibilité de réactions incontrôlées en présence d'agents réducteurs, des conditions de fluidité convenables aux températures de travail, tout en tenant compte des teneurs relatives possibles des cations.These limits have been determined experimentally, to ensure, with a low susceptibility to uncontrolled reactions in the presence of reducing agents, fluidity conditions suitable for working temperatures, while taking into account the possible relative contents of the cations.
De préférence, cette composition précitée contient des proportions pondérales significatives en potassium.Preferably, this aforementioned composition contains significant weight proportions of potassium.
La Demanderesse a également constaté que la présence de lithium dans les bains contenant des anions nitrate, hydroxyle et carbonate réduisait la quantité de boues formées par la précipitation de carbonates. Cet effet semble particulièrement marqué lorsque les teneurs en cations lithium et potassium et en anions carbonate ou nitrate correspondent sensiblement à un eutectique carbonate ou nitrate ternaire en alcalins, sodium, potassium et lithium. La teneur en lithium étant déterminée pour assurer la formation de couches de magnetite à cristallisation ordonnée, les teneurs en anions carbonate ou nitrate et en cations potassium sont liées à la teneur en lithium par les relations pour l'eutectique carbonate :The Applicant has also found that the presence of lithium in the baths containing nitrate, hydroxyl and carbonate anions reduced the amount of sludge formed by the precipitation of carbonates. This effect seems particularly marked when the contents of lithium and potassium cations and of carbonate or nitrate anions substantially correspond to a carbonate or ternary nitrate eutectic of alkali, sodium, potassium and lithium. The lithium content being determined to ensure the formation of magnetite layers with ordered crystallization, the contents of carbonate or nitrate anions and potassium cations are linked to the lithium content by the relationships for the carbonate eutectic:
9 x Li+ < C03 2" < 11 x Li+ 2,7 X Li+ < K+ < 3,2 X Li+ pour l'eutectique nitrate : 30 X Li+ < N03~ < 36 X Li+ 9 x Li + <C0 3 2 " <11 x Li + 2.7 X Li + <K + <3.2 X Li + for the nitrate eutectic: 30 X Li + <N0 3 ~ <36 X Li +
10 x Li+ < K+ < 12,5 x Li+ 10 x Li + <K + <12.5 x Li +
Dans tous les cas, la teneur en sodium satisfait à la stoechiométrie.In all cases, the sodium content satisfies the stoichiometry.
Des caractéristiques et avantages de 1• invention ressortiront d'ailleurs de la description qui va suivre, illustrée d'exemples. Exemple 1 On réalise un bain de sels oxydant en faisant fondre un mélange de 365 kg de nitrate de sodium, 365 kg d'hydroxyde de sodium, 90 kg de carbonate de sodium, 90 kg de carbonate de potassium et 90 kg de carbonate de lithium, et en le portant à 450°C.Characteristics and advantages of the invention will moreover emerge from the description which follows, illustrated with examples. Example 1 An oxidizing salt bath is produced by melting a mixture of 365 kg of sodium nitrate, 365 kg of sodium hydroxide, 90 kg of sodium carbonate, 90 kg of potassium carbonate and 90 kg of lithium carbonate , and bringing it to 450 ° C.
Les teneurs ioniques sont donc les suivantes, en pourcentages : anions cationsThe ionic contents are therefore the following, in percentages: cation anions
N03 " 26,6 Na+ 34,7N0 3 " 26.6 Na + 34.7
C03 2" 16,3 K+ 5,1C0 3 2 " 16.3 K + 5.1
0H~ 15,6 Li+ 1,70H ~ 15.6 Li + 1.7
On traite dans ce bain, pendant 5 minutes, des éprouvettes en acier non allié à 0,38 % de carbone, préalablement suifonitrurees conformément aux enseignements des documents FR-A-2 171 993 et FR-A-2 271 307 (immersion pendant 90 minutes dans un bain de sels à 570°C, contenantIn this bath, for 5 minutes, non-alloy steel specimens containing 0.38% carbon, previously sulfonitrided, are treated in accordance with the teachings of documents FR-A-2 171 993 and FR-A-2 271 307 (immersion for 90 minutes in a salt bath at 570 ° C, containing
37 % d'anions cyanate et 17 % d'anions carbonate, les cations étant K+, Na+ et Li+, le bain contenant en outre 10 à 15 ppm d'ions S2-) . Après traitement, les éprouvettes présentaient une couleur noire particulièrement uniforme et décorative. Une analyse cristallographique de ces éprouvettes par diffraction de rayons X montre que l'espèce majoritairement présente est la magnetite Fe304 ; on note une proportion mineure d'oxyde mixte Li2Fe304.37% cyanate anions and 17% carbonate anions, the cations being K + , Na + and Li + , the bath also containing 10 to 15 ppm of S 2- ions). After treatment, the test pieces presented a particularly uniform and decorative black color. A crystallographic analysis of these test pieces by X-ray diffraction shows that the predominantly present species is magnetite Fe 3 0 4 ; there is a minor proportion of mixed oxide Li 2 Fe 3 0 4 .
Un test électrochimique de corrosion par voltampérimétrie dans une solution aérée de NaCl à 30 g/1, le potentiel de corrosion mesuré par rapport à l'électrode au calomel saturé est compris dans une fourchette de 1 000 à 1 300 mV, ce qui est l'indice de passivité totale des pièces, conformément à ce que la Demanderesse a rassemblé comme informations techniques pour l'évaluation de la qualité de traitement aux bains de sels oxydants. On observera que les potentiels relevés de 1 000 àAn electrochemical corrosion test by voltamperimetry in an aerated NaCl solution at 30 g / 1, the corrosion potential measured with respect to the saturated calomel electrode is within a range of 1000 to 1300 mV, which is l 'total passivity index of the parts, in accordance with what the Applicant has gathered as technical information for the evaluation of the quality of treatment with oxidizing salt baths. It will be observed that the potentials raised from 1000 to
1 300 mV correspondent en fait au potentiel propre d'oxydation de la solution de NaCl ; ainsi, on ne peut mesurer un potentiel réel de corrosion, dès lors qu'il est au moins aussi élevé que le potentiel d'oxydation de la solution d'essai.1,300 mV corresponds in fact to the inherent oxidation potential of the NaCl solution; thus, an actual corrosion potential cannot be measured, since it is at least as high as the oxidation potential of the test solution.
Lorsque le bain de sels de l'invention est utilisé quotidiennement en production, un décrassage hebdomadaire pour enlever les boues déposées au fond du creuset conduit à éliminer 70 kg de sels contenant 60 % en poids de carbonates.When the salt bath of the invention is used daily in production, a weekly scrub to remove the sludge deposited at the bottom of the crucible leads to the elimination of 70 kg of salts containing 60% by weight of carbonates.
On observera que l'eutectique ternaire de carbonates de sodium, potassium et lithium présente la compositionIt will be observed that the ternary eutectic of sodium, potassium and lithium carbonates has the composition
Na2C03 33,2 %, K2C03 34,8 % et Li2C03 32 %. La composition des carbonates mis dans le bain (soit 33,3 pour chacun) est très voisine de celle de l'eutectique.Na 2 C0 3 33.2%, K 2 C0 3 34.8% and Li 2 C0 3 32%. The composition of carbonates put in the bath (33.3 for each) is very close to that of eutectic.
Essais comparatifs.Comparative tests.
On a réalisé deux bains expérimentaux sans lithium.Two experimental baths were carried out without lithium.
Le premier bain comportait 330 kg de nitrate de sodium, 330 kg d'hydroxyde de sodium, 330 kg de carbonate de sodium et 10 kg de bichromate de sodium, ce qui donne les teneurs ioniques en pour-cent : anions cationThe first bath included 330 kg of sodium nitrate, 330 kg of sodium hydroxide, 330 kg of sodium carbonate and 10 kg of sodium dichromate, which gives the ion contents in percent: cation anions
N03 " 24,1 Na+ 42,3N0 3 " 24.1 Na + 42.3
OH" 14 C03 2" 18,8 cr2o7 2- 0,8OH " 14 C0 3 2" 18.8 cr 2 o 7 2- 0.8
Le second bain comportait 150 kg de nitrate de sodium, 530 kg d'hydroxyde de sodium et 320 kg de carbonate de sodium, soit en composition ionique (en %) : anions cation N03 ~ 11 Na+ 48,3The second bath included 150 kg of sodium nitrate, 530 kg of sodium hydroxide and 320 kg of sodium carbonate, that is to say in ionic composition (in%): cation anions N0 3 ~ 11 Na + 48.3
OH" 22,5 C03 2" 18,2OH " 22.5 C0 3 2" 18.2
Les conditions de traitement (température 450°C, durée 5 minutes) ont été répétées du premier exemple. On a fait les constatations suivantes :The treatment conditions (temperature 450 ° C., duration 5 minutes) were repeated from the first example. The following observations were made:
Toutes les éprouvettes traitées sont recouvertes d'une couche noire constituée de magnetite Fe304.All the treated specimens are covered with a black layer consisting of magnetite Fe 3 0 4 .
Les éprouvettes traitées dans le premier bain de comparaison sont d'un noir uniforme ; leur potentiel de corrosion est compris entre 1 000 et 1 300 mV, et l'on peut conclure que la couche d'oxyde est bien passive.The test pieces treated in the first comparison bath are of a uniform black; their corrosion potential is between 1000 and 1300 mV, and it can be concluded that the oxide layer is very passive.
Les éprouvettes traitées dans le second bain de comparaison sont en majorité noires, mais certaines présentent des reflets bruns. Le potentiel de corrosion varie entre 250 et 1 300 mV. On peut en conclure que la couche de magnetite est de qualité irrégulière d'une éprouvette à l'autre, et que le second bain de comparaison ne présente pas une fiabilité suffisante.The test specimens treated in the second comparison bath are mostly black, but some have brown reflections. The corrosion potential varies between 250 and 1,300 mV. It can be concluded from this that the layer of magnetite is of irregular quality from one test piece to another, and that the second comparison bath does not have sufficient reliability.
L'utilisation quotidienne en production des deux bains expérimentaux conduit, lors du décrassage hebdomadaire, à éliminer environ 150 kg de boues à 60 % environ de carbonate.The daily use in production of the two experimental baths leads, during the weekly cleaning, to eliminating approximately 150 kg of sludge containing approximately 60% carbonate.
Sous l'aspect des qualités mécaniques et tribologiques, le bain de l'exemple 1 et le premier bain de comparaison donnent des résultats tout à fait équivalents. Exemple 2In the aspect of mechanical and tribological qualities, the bath of Example 1 and the first comparison bath give completely equivalent results. Example 2
On réalise un bain de sels oxydant à partir des composants NaOH 365 kg, Na2C03 270 kg, NaN03 62 kg, KN03 277 kg et LiN03 76 kg. Les nitrates sont répartis entre les trois cations alcalins dans des proportions NaN03 14,9 %, KN03 66,8 %, LiN03 18,3 % correspondant sensiblement à l'eutectique ternaire. Les teneurs pondérales ioniques correspondantes sont les suivantes, en pourcentages : anions cationsIs made an oxidizing bath salts from the components 365 kg NaOH, Na 2 C0 3 270 kg NaN0 3 62kg KN0 3277 kg LIN0 3 76 kg. The nitrates are distributed between the three alkaline cations in proportions NaN0 3 14.9%, KN0 3 66.8%, LiN0 3 18.3% corresponding substantially to the ternary eutectic. The corresponding ionic contents by weight are as follows, in percentages: cation anions
N03 " 28,2 Na+ 34,3N0 3 " 28.2 Na + 34.3
C03 2" 15,4 K+ 10,8C0 3 2 " 15.4 K + 10.8
OH" 15,5 Li+ 0,77OH " 15.5 Li + 0.77
Dans ce bain, avec les mêmes conditions opératoires qu'à l'exemple 1 et les exemples de comparaison, on a traité des éprouvettes en fonte nitrurees. Ces éprouvettes, après traitement, sont d'un noir uniforme, la couche superficielle est majoritairement de la magnetite Fe304, et leur potentiel de corrosion se situe dans la fourchette 1 000-1 300 mV.In this bath, with the same operating conditions as in Example 1 and the comparison examples, nitrided cast iron test pieces were treated. These specimens, after treatment, are of a uniform black, the surface layer is mainly magnetite Fe 3 0 4 , and their corrosion potential is in the range 1000-1 300 mV.
Des éprouvettes semblables en fonte nitrurees ont été traitées dans les premier et second bains de comparaison décrits ci-dessus, et présentaient des couleurs brun rouge irrégulières. Une analyse par diffraction de rayons X montre que la couche superficielle est majoritairement constituée de magnetite, mais que la cristallisation doit être irrégulière, le spectre de diffraction X présentant des anomalies par rapport aux spectres étalons de la magnetite (ASTM) . Dans le bain de l'exemple 2, à 0,77 % de lithium, en exploitation quotidienne de production, le décrassage hebdomadaire enlève environ 80 kg de boues. Exemple 3Similar nitrided cast iron specimens were treated in the first and second comparison baths described above, and exhibited irregular reddish brown colors. An analysis by X-ray diffraction shows that the surface layer is mainly made up of magnetite, but that the crystallization must be irregular, the X-ray diffraction spectrum presenting anomalies compared to the standard spectra of magnetite (ASTM). In the bath of Example 2, at 0.77% lithium, in daily production operation, the weekly scrub removal removes about 80 kg of sludge. Example 3
On a constitué deux bains expérimentaux ne comportant d'anions que nitrate. Un bain A comportait 48,5 % de KN03, 39,5 % de NaN03 et 12 % de LiN03, avec la composition ionique (en %) : anion cationsTwo experimental baths were formed with only nitrate anions. A bath A comprised 48.5% KN0 3 , 39.5% NaN0 3 and 12% LiN0 3 , with the ionic composition (in%): anion cations
N03 " 70,3 Na+ 13,1N0 3 " 70.3 Na + 13.1
K+ 15,4 Li+ 1,2 On constitue un bain B de comparaison, comportant 55 % de NaN03 et 45 % de KN03, soit en pourcentage ionique : anion cationsK + 15.4 Li + 1.2 A comparison bath B is made up, comprising 55% NaN0 3 and 45% KN0 3 , ie in ionic percentage: cation anion
N03 " 67,6 Na+ 14,9N0 3 " 67.6 Na + 14.9
K+ 17,5 On traite dans ces bains des éprouvettes en fonte préalablement nitrurees. On immerge les pièces pendant 15 minutes à 400°C.K + 17.5 In these baths, cast iron test specimens, previously nitrided, are treated. The pieces are immersed for 15 minutes at 400 ° C.
Les éprouvettes traitées dans le bain A présentent toutes une couche superficielle d'un noir profond, tandis que les éprouvettes traitées dans le bain B ont une couche superficielle grise avec des reflets bruns.The test pieces treated in bath A all have a deep black surface layer, while the test pieces treated in bath B have a gray surface layer with brown reflections.
Les potentiels de corrosion, déterminés comme précédemment, se situent dans la fourchette de 1 000-1 300 mV pour les éprouvettes traitées dans le bain A, et dans une fourchette de 300-900 mV pour celles qui ont été traitées dans le bain B, avec les conséquences attendues sur la résistance à la corrosion.The corrosion potentials, determined as above, are in the range of 1000-1 300 mV for the specimens treated in bath A, and in a range of 300-900 mV for those which have been treated in bath B, with the expected consequences on corrosion resistance.
On observera que les exemples de comparaison correspondant aux exemples 2 et 3 confirment les difficultés connues de protection contre la corrosion des fontes nitrurees, et font ressortir par contraste l'efficacité des bains de l'invention.It will be observed that the comparison examples corresponding to Examples 2 and 3 confirm the known difficulties of protection against corrosion of nitrided cast irons, and bring out by contrast the effectiveness of the baths of the invention.
On notera, à propos de l'exemple 3, que les pièces à traiter doivent avoir été soigneusement débarrassées des résidus de bain de nitruration, les bains de nitrates purs étant susceptibles de réagir violemment au contact de substances réductrices.It will be noted, with regard to Example 3, that the parts to be treated must have been carefully freed from the residues of nitriding bath, the baths of pure nitrates being liable to react violently on contact with reducing substances.
Pour en revenir à la réduction de formation de boues carbonatées des bains contenant des hydroxydes, des nitrates et des carbonates, on a constaté que la réduction de formation de boues paraissait passer par un optimum lorsque les teneurs pondérales d'un anion nitrate ou carbonate, en conjonction avec les teneurs en cations potassium et lithium, correspondaient à la présence dans le bain d'un eutectique ternaire de l'anion avec les cations Na+, K+ et Li+. Comme l'efficacité de formation d'une couche de magnetite à cristallisation ordonnée dépend de la teneur pondérale en lithium, la règle pour associer au mieux les deux effets sera de choisir la teneur en lithium appropriée à la formation de la couche de magnetite protectrice, et partant de cette teneur, de déterminer la teneur en potassium et en anion carbonate ou nitrate, à partir de la composition eutectique ternaire de cet anion. Pour l'anion carbonate, on a alors :To return to the reduction in the formation of carbonated sludge from baths containing hydroxides, nitrates and carbonates, it has been found that the reduction in the formation of sludge appears to pass by an optimum when the contents by weight of a nitrate or anion carbonate, in conjunction with the contents of potassium and lithium cations, corresponded to the presence in the bath of a ternary eutectic of the anion with the cations Na + , K + and Li + . As the efficiency of formation of a magnetite layer with ordered crystallization depends on the weight content of lithium, the rule to best associate the two effects will be to choose the appropriate lithium content for the formation of the protective magnetite layer, and starting from this content, to determine the potassium and carbonate or nitrate anion content, from the ternary eutectic composition of this anion. For the carbonate anion, we then have:
9 x Li+ < C03 2~ < 11 x Li+ 2,7 X Li+ < K+ < 3,2 X Li+ 9 x Li + <C0 3 2 ~ <11 x Li + 2.7 X Li + <K + <3.2 X Li +
Et pour l'anion nitrate :And for the nitrate anion:
30 X Li+ < N03 " < 36 x Li+ 30 X Li + <N0 3 " <36 x Li +
10 x Li+ < K+ < 12,5 x Li+ 10 x Li + <K + <12.5 x Li +
Bien entendu, dans tous les cas, le cation sodium sera en excès de la composition de l'eutectique ternaire, en raison de la présence d'autres anions que l'anion pris en considération pour l'eutectique, et du fait que le bain doit être en équilibre stoechiométrique.Of course, in all cases, the sodium cation will be in excess of the composition of the ternary eutectic, due to the presence of anions other than the anion taken into consideration for the eutectic, and to the fact that the bath must be in stoichiometric equilibrium.
Il va de soi que l'invention n'est pas limitée aux exemples décrits, mais en embrasse toutes les variantes d'exécution, dans le cadre des revendications. It goes without saying that the invention is not limited to the examples described, but embraces all the variant embodiments thereof, within the scope of the claims.

Claims

REVENDICATIONS
1. Composition de bains de sels pour un traitement d'oxydation de surface de pièces en métal ferreux, notamment nitrurees, en vue d'augmenter leur résistance à la corrosion, le traitement s'effectuant à une température comprise entre 320° et 550°C, composition comportant au moins des anions nitrate et des cations alcalins sodium et le cas échéant potassium, caractérisée en ce qu'elle comporte des cations lithium substitués à des cations sodium ou potassium, à une teneur pondérale, rapportée à la masse du bain, comprise entre 0,1 et 5 %.1. Composition of salt baths for a surface oxidation treatment of ferrous metal parts, in particular nitrides, in order to increase their resistance to corrosion, the treatment being carried out at a temperature between 320 ° and 550 ° C, composition comprising at least nitrate anions and alkaline cations sodium and optionally potassium, characterized in that it comprises lithium cations substituted for sodium or potassium cations, at a weight content, based on the mass of the bath, between 0.1 and 5%.
2. Composition selon la revendication 1, caractérisée en ce que la teneur pondérale en cations lithium est comprise entre 0,5 et 1,75 %.2. Composition according to claim 1, characterized in that the content by weight of lithium cations is between 0.5 and 1.75%.
3. Composition selon l'une des revendications 1 et 2, caractérisée en ce qu'elle comporte, en équilibre stoechiométrique avec les cations de métaux alcalins, outre des anions nitrate, des anions hydroxyle et carbonate, les teneurs pondérales en anions carbonate C03 2~, nitrate N03~ et hydroxyle OH", rapportées à la masse active ou liquide du bain, étant comprises, en pourcentages, dans les limites ci-après :3. Composition according to one of claims 1 and 2, characterized in that it comprises, in stoichiometric equilibrium with the cations of alkali metals, in addition to nitrate anions, hydroxyl and carbonate anions, the contents by weight of carbonate anions C0 3 2 ~, nitrate N0 3 ~ and hydroxyl OH " , relative to the active or liquid mass of the bath, being included, in percentages, within the following limits:
8,5 < C03 2" < 268.5 <C0 3 2 " <26
15 < N03 ~ < 41,515 <N0 3 ~ <41.5
4,7 < OH" < 21,5 4. Composition selon la revendication 3, caractérisée en ce qu'elle contient une teneur pondérale significative en cations potassium.4.7 <OH " <21.5 4. Composition according to claim 3, characterized in that it contains a significant weight content of potassium cations.
5. Composition selon la revendication 4, caractérisée en ce qu'elle comporte des teneurs pondérales en anions carbonate C03 2~ et cations potassium K+, liées à la teneur pondérale en cations lithium Li+ par les relations :5. Composition according to Claim 4, characterized in that it contains weight contents of carbonate anions C0 3 2 ~ and potassium K + cations, linked to the weight content of lithium cation Li + by the relationships:
9 X Li+ < C03 2" < 11 x Li+ 2,7 x Li+ < K+ < 3,2 x Li+, la teneur en sodium satisfaisant la stoechiométrie. 9 X Li + <C0 3 2 " <11 x Li + 2.7 x Li + <K + <3.2 x Li + , the sodium content satisfying the stoichiometry.
6. Composition selon la revendication 4, caractérisée en ce qu'elle comporte des teneurs pondérales en anions nitrate N03 " et cations potassium K+, liées à la teneur pondérale en cations lithium par les relations :6. Composition according to claim 4, characterized in that it comprises weight contents of anions nitrate N0 3 " and potassium K + cations, linked to the weight content of lithium cations by the relationships:
30 x Li+ < N03 " < 36 x Li+ 10 x Li+ < K+ < 12,5 x Li+, la teneur en sodium satisfaisant la stoechiometrie. 30 x Li + <N0 3 " <36 x Li + 10 x Li + <K + <12.5 x Li + , the sodium content satisfying the stoichiometry.
PCT/FR1995/000560 1995-04-28 1995-04-28 Alkaline nitrate salt bath composition for oxidising a ferrous metal and improving the corrosion resistance thereof WO1996034127A1 (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3847685A (en) * 1970-02-11 1974-11-12 Texas Instruments Inc Oxide coated metal discs and method of making the same
FR2463821A1 (en) * 1979-08-23 1981-02-27 Degussa METHOD FOR RAISING THE CORROSION RESISTANCE OF NITRIDE PARTS IN FERROUS MATERIAL
FR2525637A1 (en) * 1982-04-23 1983-10-28 Stephanois Rech Mec PROCESS FOR TREATING FERROUS METAL PARTS IN OXIDIZING SALT BATTERS TO IMPROVE CORROSION RESISTANCE, PARTS CONTAINING SULFUR
EP0196093A1 (en) * 1985-03-27 1986-10-01 Liu Yu Zhong A process for treating an article made of metal selectec from tantalum, niobium and tantalum-niobium alloy

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3847685A (en) * 1970-02-11 1974-11-12 Texas Instruments Inc Oxide coated metal discs and method of making the same
FR2463821A1 (en) * 1979-08-23 1981-02-27 Degussa METHOD FOR RAISING THE CORROSION RESISTANCE OF NITRIDE PARTS IN FERROUS MATERIAL
FR2525637A1 (en) * 1982-04-23 1983-10-28 Stephanois Rech Mec PROCESS FOR TREATING FERROUS METAL PARTS IN OXIDIZING SALT BATTERS TO IMPROVE CORROSION RESISTANCE, PARTS CONTAINING SULFUR
EP0196093A1 (en) * 1985-03-27 1986-10-01 Liu Yu Zhong A process for treating an article made of metal selectec from tantalum, niobium and tantalum-niobium alloy

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
PENJAGINA: "Passivierung von Eisen und Nickel in geschmolzenen Carbonaten", WERKSTOFFE UND KORROSION, vol. 23, no. 11, 1 November 1972 (1972-11-01), WEINHEIM DE, pages 1046 *
ZASHITA METALLOV, vol. 8, no. 3, MOSKOU, pages 312 - 1314 *

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