DK169354B1 - Melting bath and method for electrolytic surface coating with refractory metals from fluoride-containing salt melts - Google Patents

Abstract

The invention relates to a molten bath for plating with high-melting metals, in particular niobium and tantalum. The bath consists of an alkali metal fluoride melt, which contains oxide ions and ions of the metal to be precipitated. The molar ratio between the metal to be precipitated and the oxide ions, or the other cations in the melt, must be held within given ratios. The redox level must be held at a value which corresponds to that which is reached when the molten bath is in contact with the particular high-melting metal in the metallic form. The invention also relates to a process in which the bath in question is used.

Description

in DK 169354 B1

The invention relates to a melt bath of the kind set forth in the preamble of claim 1.

Refractory metals (niobium, tantalum, zircon, molybdenum, wolfram, etc.) are generally very resistant to corrosion in acidic and oxidizing media, for example Nb and Ta are attacked only to a small extent by 200 ° C hot concentrated sulfuric acid and of elemental chlorine . In addition, they can withstand high temperatures (melting points> 2000 ° C) under non-oxidizing atmosphere.

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The applications of niobium and tantalum metal coatings for the corrosion protection of specially exposed parts of valves, flowmeters, pumps and the like will be great in the chemical and other industries where 15 high corrosion resistance is required.

Refractory metal coatings can be electrolytically precipitated from chloride and fluoride-containing salt melts. Methods are described in the literature (G.W. Mellors and S.

20 Senderoff, U.S. Pat. 3,444,058 (1969), J.E. Perry, US Pat.

3,371,020 (1968), G.P. Capsimalis, E.S. Chen, R.E. Peterson and I. Ahmad, J. Appl. Electrochem. 17, 253 (1987), P.

Los and J. Joslak, B. Electrochem. 5, 829 (1989), P. Taxil and J. Mahenc, J. Appl. Electrochem. Γ7, 261 (1987)), but in practice it has proved difficult to obtain technically and economically satisfactory results, especially with regard to niobium coatings. The problems include avoiding precipitates containing alkali metals, oxides and complex oxo salts of the refractory metals.

30 Common to the previously described bath types for electrolytic coating are that as anions they contain only fluoride and complex fluorides and in some cases chlorides (VI Konstantinov, EG Polyakov and PT Stan-35 grit, Electrochemica Acta 26. 445 (1981 ), AN Baimakov, SA Kuznetsov, EG Polyakov and PT Stangrit, Elektrok- DK 169354 B1 2 him 2J-, 597 (1985)). However, baths containing chlorides have most often produced dendritic precipitates or have resulted in the formation of lower positive oxidation steps of the refractory metals. It has been hitherto assumed that even small amounts of oxide are detrimental to the precipitation quality when pure fluoride baths were used. However, processes have described in the literature that niobium precipitates on the cathode from mixed fluoride chloride melts added with K2NbF7 and Nb205 (VI Konstantinov, EG Polyakov and PT Stangrit, Elec-10 trochemica Acta 26, 445 (1981), AN Baimakov, SA Kuznetsov, EG Polyakov and PT Stangrit, Elektrokhim 21, 597 (1985)).

U.S. Patent No. 1,815,054 also discloses a process wherein refractory metals, especially tantalum, can be prepared by electrolysis of alkali metal halide melting agents (especially fluoride), with double alkali metal refractory metal halide and an ionizable oxygen-containing salt of the refractory metal. . In this process, the refractory metal can only be obtained in powder form and not as a coherent surface layer on the cathode. No reduction of the melt bath is used and the oxygen-containing compound is decomposed at the anode.

The present invention, which is of the kind set forth in the preamble of claim 1, is characterized by the characterizing part of claim 1. The use of salt melt baths according to the invention avoids the known drawbacks such as impure and incoherent surface layers, and the plating 30 with said metals can be carried out continuously with an economically and technically satisfactory result.

The invention also relates to a method as claimed in the preamble of claim 4, which is characterized by the characterizing part of claim 35 4.

in 3 DK 169354 B1

The salt melt baths of the invention contain, in addition to fluoride anions, a substantial amount of oxide anions. Baths of this composition can be used for electrolytic plating of fine crystalline, coherent and adhesive surface layers of refractory metals.

Plating with refractory metals from fluoride baths with oxide anions has not been known in the past, and the use of these baths, as mentioned, brings great advantages both technically and economically. This has been demonstrated by the current yield and by electron microscopy and EDX analyzes of the precipitated metal.

In plating, the molten bath's content of metal ions 15 of the refractory metal to be precipitated must be between 1 and 8 atomic% and the molar ratio of oxide to metal must be in the range 0.1 to 1.5 to obtain coherent surface layers of pure metal at working temperatures between the melting point and approx. 900 ° C.

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The redox level of the melt bath must be maintained at an appropriate value by the addition of a redox agent. This may be the refractory metal in metal form or a compound having a similar effect.

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The electrolytic precipitation must be carried out in an inert, non-oxidizing atmosphere of, for example, argon, neon, dry nitrogen or under vacuum.

The bath composition according to the invention is no more corrosive than can be used for containers and the like of any material which does not significantly react with the melt, e.g. glassy carbon, graphite, stabilizing zirconium oxides, nickel and nickel-containing materials, sialones 35 and aluminum nitride. .

DK 169354 B1 4

The cathode on which metals are deposited must consist of an electrically conductive solid which does not react excessively with the melting electrolyte. This may be, for example, steel, alloy steel, graphite, nickel, nickel-containing alloys 5 or copper.

The anode may consist of the metal to be precipitated, for example in the form of rods, metal foil or plate in various geometric designs. Thus, the anode acts as a source of the metal to be precipitated, and further maintains the oxidation step of the refractory metal in the melt bath at the desired size.

The electrolyte bath can also be used as a metal source. In such cases, an inert anode, for example, of graphite, vitreous carbon or platinum, may be used. When the electrolyte bath is used as a metal source, metal ions must be added to the melt so that the concentration of the precipitated metal is kept within the desired range. In addition, a reducing agent, such as the refractory metal in question, must be added in order for the oxidation step to be correct.

The bath composition according to the invention is based on the addition of alkali fluoride melt mixtures as an electrolyte with added niobium / tantalum fluorides, niobium / tantalum oxides, niobium / tantalum oxofluorides or mixtures thereof and sufficient oxide to keep the metal / oxide ratio in the desired ratio. interval.

The preferred base melt (solvent) is the eectectic mixture of LiF-NaF-KF. This mixture is added to niobium / tantalum in the form of fluorides, oxofluorides, complex fluorides / oxofluorides or oxides. In order to obtain the correct oxide content in the melt, it is optionally adjusted with the addition of oxides from the first or second main group and / or oxides DK 169354 B1 5 or oxofluorides of the metal to be precipitated. These components make up the electrolyte bath.

Example 1.

Niobium was plated on a low carbon steel rod from a melt containing 2.7 mole% niobium and 2.7 mole% oxide with eutectic LiF-NaF-KF as the primary melt. Niob was added as K2NbF7 and the oxide as Na2O. The anode consisted of 1 mm thick niobium plate. Process temperatures 700 ° C, current density (cathodic) 77 10 mA / cm2. Prior to electrolysis, the niobium anode was immersed for 3 hours in the electrolyte bath. The cathodic current yield was 95%.

The precipitated surface layer was crystalline, coherent and adhered well to the low carbon steel substrate. EDX analysis revealed that it consisted of 100% niobium.

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Example 2.

Niobium was plated on carbon steel under the same process conditions as in Example 1. The melt electrolyte was a eutectic mixture of LiF-NaF-KF with a niobium content of 3.2 mole% and an oxide content of 3.2 mole%. The precipitated surface layer consisted of pure niobium (EDX analysis), was fine crystalline, coherent and adhered well to the substrate. The cathodic current yield was 77%.

Example 3.

Niobium was precipitated on carbon steel under the same process conditions as in Example 1. The melt electrolyte was a eutectic mixture of LiF-NaF-KF added to oxide-containing NbF5. Niobium and oxide contents were 2.7 and ca.

3.2 mole%. The precipitated layer was coherent, fine crystalline and adhered well to the substrate of steel. EDX analyzes showed that the surface layer was 100% niobium. The cathodic current yield was 56%.

Example 4.

DK 169354 B1 6

Tantalum was precipitated on carbon steel from a basic melt of eutectic LiF-NaF-KF added with K2TaF7 and Na2O. Mol% of tantalum and oxide were 2.7 and ca. 2.0. As an anode, a cylinder of 1 mm thick tantalum film is used. The anode was immersed for 3 hours before electrolysis was initiated. The process temperature was 700 ° C. The precipitated surface layer consisted of pure tantalum metal, was coherent, fine crystalline and adhered well to the low carbon steel substrate. The cathodic current yield was 78%.

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Example 5

Zirconium was precipitated from a eutectic LiF-NaF-KF melt added with K2ZrF6. Mol% Zr and oxide were both 2.7%. As the anode, 1 mm thick zircon foil was used. The cathodic current density was 40 mA / cm 2 and the process temperature 700 ° C. The precipitated zircon metal layer adhered well to the low carbon steel substrate. The current yield was 58%.

Claims (4)

1. Electrolytic surface coating melt bath with a 5 refractory metal selected from Nb, Ta; Zr, W or Mo, especially niobium or tantalum, on the basis of a salt melt of alkali metal fluorides and a fluoride of the refractory metal, characterized in that the bath content of metal ions of the refractory metal is between 1 and 8 atom%, that the bath contains oxide anions and is in contact with the refractory metal in metal form or contains a corresponding redox agent and that the molar ratio of oxide to the refractory metal is in the range 0.1 to 1.5. 15 Bath according to claim 1, characterized in that it contains oxide anions in the form of an alkali metal oxide or an oxide, oxofluoride or complex oxofluoride of the refractory metal. 20 Bath according to claim 1 or 2, characterized in that it contains the refractory metal in the form of a fluoride, a complex fluoride, an oxofluoride or a complex oxofluoride of the refractory metal. 25 Method of electrolytic surface coating with a refractory metal selected from Nb, Ta ', Zr, W or Mo, especially niobium and tantalum, in an inert, non-oxidizing atmosphere or under vacuum, characterized in that a bath is used for the electrolysis with a composition according to any one of claims 1-3.