CURRENT-LESS ELECTROCHEMICAL SYNTHESIS
OF FUNCTIONAL METALLIC MATERIALS IN IONIC
AND IONIC-ELECTRONIC SALT MELTS Yu.P. Zaikov, V.V. Chebykin, A.I. Anfinogenov Institute of High-Temperature Electrochemistry, Ural Branch RAS, 22 S.Kovalevskaya St., 620219 Ekaterinburg GSP-146, Russia
Tel.: (343) 3745089; E-mail: dir@IHTE.uran.ru
Ample information concerning the interaction of metals in ionic and ionic-electronic melts is presented. The mechanism and the character of the oriented spontaneous transport of metals by their ions in salt melts under no-electrolysis conditions are determined. It is exemplified how this phenomenon can be used for deposition of diffusion coatings (aluminum, beryllium, boron, zinc, titanium, chromium, silicon, etc.) and two-component coatings (aluminum-titanium, aluminum-chromium, or boron-silicon) on metals and alloys and diffusion alloys ((samarium-cobalt, cobalt-platinum, iron-palladium).
Introduction In addition to traditional methods for making of aluminum, magnesium, titanium, alkali, alkali-earth and rare-earth metals by electrolysis or the use as nonoxidation quenching baths, molten salts are increasingly used for thermochemical treatment, galvanoplastics in salt melts and high-temperature inorganic and organic syntheses or as electrolytes in high-temperature chemical current sources, etc.
This study focuses on the current-less electrochemical formation of diffusion coatings and alloys, which present interest for practical applications, in ionic and ionic-electronic melts.
In the majority of cases, the in-service failure of components of machines and mechanisms begins on the surface or in near-surface layers. Therefore, reliability of machinery components considerably depends on their thermochemical treatment, which imparts desired properties to the components by surface alloying.
In some cases, deposition of protective coatings is most efficient and, sometimes, the only means of solving complicated engineering problems related to improvement of the strength and the wear, heat and corrosion resistances of metals and alloys. The use of protective coatings often allows replacing expensive and scarce metals by more available materials without a considerable sacrifice of the serviceability of components, units and structures.
More and more interest is attached currently to diffusion protective coatings, because their binding to the base metal by diffusion of the deposited element into the crystal lattice of the protected material is much stronger than the binding of non-diffusion coatings.
SPONTANEOUS TRANSPORT REACTIONS IN MELTS For 45 years specialists at the Institute of High-Temperature Electrochemistry (IHTE), Ural Branch RAS, have been working on deposition of diffusion coatings in ionic and ionic-electronic melts by a no-electrolysis liquid method. Results of the research conducted in the first decades are generalized in monographs (1, 2), which deal mainly with the current-less transport of metals in ionic melts.
From results of their own research and the review of the literature, our specialists deduced the fundamental character of the oriented transport of metals by their ions in salt melts in the absence of electrolysis. The motive force of the transport, its thermodynamics and kinetics, phase compositions and properties of diffusion coatings were studied (1, 2). It was found that more electronegative metals are spontaneously transported to more electropositive ones in a given salt solvent when the metals form intermetallics or solutions. Otherwise, the transport does not take place or is realized by another mechanism, e.g., the temperature differential.
The character and the rate of interaction between metals and salt melts (corrosion) and between metals (alloying) need be known for the scientifically substantiated selection of salt melts, temperature and time intervals of the saturation processes. The interaction between metals and salt melts is important in high-temperature physical chemistry, electrochemistry, electrometallurgy, and thermochemical treatment. The most significant aspects of this interaction are the metal solubility, the form of the dissolved metal in the salt phase, the reactivity of the metal with other metals in the melt, and the coexistence of ions of different valences in contact with metals.
Our studies demonstrated that if two different metals, which can form an alloy by diffusion, are placed in a molten electrolyte containing ions of the more electronegative metal, the last metal dissolves and is transported through the electrolyte to the more electropositive metal. These metals form a surface diffusion alloy-coating. It is also possible that the more electronegative metal is transported from an alloy, where the metal reactivity is higher, to an alloy, where the metal reactivity is lower. Since the coating is made by diffusion, its phase composition is determined by the equilibrium diagram of the system of these metals.
In the absence of external oxidizing and reducing agents, the redox potential of the salt medium equals the equilibrium electrode potential of the metal relative to its ions of all possible oxidation levels. In this case, the salt melt is capable of the interaction peculiar to the metal itself.
Two metals are alloyed through a salt melt because the melt cannot be in thermodynamic equilibrium simultaneously with two metals having different reactivities. The redox potential of the medium near the metals is different. Some part of the melt, which comes to equilibrium with the more electronegative element, enters into interaction with the more electropositive metal upon making contact with the latter. Therefore, the redox potential of the salt medium near the more electropositive metal rises because the concentration of donor electrons diminishes. A characteristic feature of this transport is its orientation from the more electronegative to the more electropositive metal. The reverse transport is virtually absent because ions of electropositive metals have an extremely small reactivity in melts with a low redox potential. The transport rate depends on the surface of contact between the salt melt and the metals, the metal-to-metal distance, the diffusion rate of metal ions in salt melts, and the diffusion rate of atoms in the metal phase.
An element can be transported to form the coating either by its subions or subions of alkali and alkali-earth metals if its ions of the highest oxidation level are only in equilibrium with the electronegative element. Thus, the spontaneous transport process can be divided into three stages: corrosion of the electronegative element in its own dilute salt and formation of ions having different oxidation levels; transport of ions through the molten salt from the electronegative toward the electropositive metal; redox reactions of disproportionation or exchange on the surface of the electropositive metal and formation of the alloy-coating.
Reactions involving formation of lowest-valence ions and disproportionation usually do not encounter considerable kinetic difficulties at high temperatures. Interdiffusion coefficients in solid metals are 5 to 10 orders of magnitude smaller than ion diffusion coefficients in the electrolyte. Therefore, diffusion formation of the alloy-coating most frequently determines (being the slowest stage) the total rate of the process.
The observed phenomenon can be exemplified by the transport of beryllium to nickel and other metals in ionic melts when beryllium and the metal do not come into electronic contact. Lowest-valence Be+ ions are formed in the melt by the reaction
Bе + Bе2+(melt) 2B+(melt). 1
Then they disproportionate on nickel with an energy gain resulting from the formation of intermetallics:
2yBe+(melt) + xNi yBe2+(melt) + NixВey. 2
The current-less transport of metals in ionic melts is driven by quite certain forces: the alloying energy and thermodynamically determined gradients of concentrations (more precisely, reactivities) of ions having different oxidation levels in the electrolyte. Of course, all the processes involved in thermochemical treatment of metals cannot be reduced just to the disproportionation reaction. In each case, one has to consider thermodynamic data and assess the probability of particular reactions, necessarily including the free Gibbs energy of the alloy formation. Most frequently, a specific process is realized not by one reaction, but by several parallel or consecutive reactions, which depend on temperature and mass transfer conditions in liquid and solid phases. However, making of diffusion coatings is based on chemical transport reactions, which can be realized under isothermal conditions.
Thus, the possibility of the current-less transport of an element to a metal can be apprehended if one knows the mutual position of elements in the electric series and their standard potentials in a given medium, the equilibrium diagram of binary alloys from the viewpoint of the diffusion formation of phases having constant or variable compositions, the presence and the ratio of ions with different oxidation levels in the melt, and the fraction of reduced forms of the solvent in a given melt.
In some cases, the fraction of lowest oxidation levels of the transported element is very small, limiting the rate of the diffusant transport to the substrate surface.
Therefore, the saturating element is often taken as a powder for practical purposes and its surface in contact with the electrolyte is increased multiply, ensuring the maximum saturation of the melt with ions of the lowest oxidation level. In this case, it is not excluded that the element directly contacts the metals and a multitude of short-circuited galvanic cells participate concurrently in the transport process.
Mention should be made of studies conducted by Belorussian investigators (3, 4) who also generalized results of their work and proposed a saturation mechanism, which, in their opinion, satisfies all cases of the liquid saturation. It allows formulating practical recommendations on selection of saturation systems in ionic melts and calculating the intensity of saturation processes. According to this mechanism, the aforementioned galvanic pairs are formed. In these pairs, the cathodic and the anodic formation of active atoms takes place on the substrate and reducer surfaces respectively.
DIFFUSION COATINGS AND ALLOYS Studies concerned with making of diffusion coatings in molten salts, which contain powders of saturating metals, are classified by Dubinin as liquid methods for thermochemical treatment and synthesis of diffusion coatings on metals (5). This classification of the methods was advanced and expanded in a monograph by Shatinsky and Nesterenko (6). In 1949-1960 Russian and other researchers published papers dealing with production of boride, silicide, titanium and chromium coatings on metals in ionic melts without electrolysis.
In 1960 Smirnov and Krasnov (IHTE, UB RAS) were the first to propose the hypothesis of the current-less transport of metals by the example of the spontaneous transport of titanium through a molten salt to titanium carbide. The central idea of the hypothesis is the reaction of disproportionation of Ti2+ ions to titanium carbide (7). Their work was continued under the supervision of Prof. N.G. Ilyuschenko (1, 2, 8-13). An analogous explanation can be found in studies by Gopienko and Anufrieva (14, 15).
The technology for deposition of coatings on metals is chosen considering the consumed material, the saturation rate, the depth and the structure of layers, and requirements imposed on protective coatings with respect to high-temperature oxidation, corrosion, heat and wear resistance. The method efficiency depends on the range of treated parts, their operating conditions, dimensions and tolerances, availability of equipment, and profitability.
The table gives experimental results obtained at the laboratory of alloys (IHTE, UB RAS) with respect to the spontaneous transport of chemical elements to different metal substrates in ionic and ionic-electronic salt melts.
Table. Spontaneous electrochemical transport processes in ionic and ionic-electronic melts
Given below are experimental results obtained for specific processes involved in deposition of diffusion coatings on metals and alloys in molten salts, which were developed at the Institute of High-Temperature Electrochemistry, Ural Branch RAS.
Beryllium coatings. The most valuable property of beryllium coatings is their resistance to high-temperature oxidation, resulting from formation of a BeO oxide film on the surface. Investigations into the interaction between beryllium and metals in salt melts, which contained beryllium salts, led to development of a method for diffusion deposition of beryllium on metals and alloys (161718). Coated samples of the ЖС6К alloy did not fail at 1000 C for 360 h; some samples remained intact after 560 h and 70 heat cycles at 1100 C. Beryllated nickel-alloy blades of gas-turbine engines had good resistance in fuel combustion products containing sulfur and vanadium and under sea atmosphere. At 900 C the oxidation rate of beryllated titanium was 25 times slower than the oxidation rate of uncoated titanium. The surface of beryllated niobium remained unchanged after 30 h during heat resistance tests, whereas samples of uncoated niobium oxidized in several minutes.
Aluminum coatings. The surface saturation of metals with aluminum is used in practice for improvement of heat and corrosion resistances. In particular, aluminum coatings improve resistance of steel parts to high-temperature oxidation by a factor of 10 at 950-1000 C and 20 at lower temperatures. Protection is provided by -Al2O3 oxide films formed on the coating surface during heating in oxidizing atmosphere.
Armco iron and steels of the 55, У8 and 3X13 types were subject to low-temperature saturation with aluminum in a LiCl, KCl melt with addition of AlF3 and powdered aluminum at 540-630 C (19, 20). The mass increment of the coated samples made of the 2X13 and the У8 steel was 70 and 35 times less, respectively, than the mass increment of uncoated samples.
BT-1 titanium was aluminized in a BaCl2, KCl, NaCl melt with addition of AlF2 and powders of ferroaluminum FeAl3 and aluminum. Heat resistance of the coated titanium at 800 C increased 25-30 times.
Improvement of scale and wear resistances of aluminized copper presents great practical interest. Low-temperature aluminizing of copper and its alloys by immersion in a salt melt was performed using powdered aluminum and ferroaluminum (21). The mass increment of aluminized copper was 20 times less at 500 C and 70 times less at 700 C than the mass increment of uncoated copper during heat resistance tests. Wear resistance tests of aluminized copper under dry sliding friction conditions at a load of 80 kg demonstrated that the wear decreased by a factor of almost 2.5 as compared with the wear of uncoated copper.
Resistance of aluminized copper nozzles and tips of A-537 semiautomatic welding machines was 2 to 4 times as high (Uralmash plant). Metal spray did not adhere to the nozzles.
Heat resistance of aluminized blades of gas turbine engines (types ЖС6К, ЭИ867 and ЭП109 nickel-based alloys) was 2-6 times higher after long-time holding under oxidizing atmosphere at temperatures of 900-1000 C.
Silicon coatings. Silicide coatings on molybdenum, tungsten and niobium were made in melts of alkali metal chlorides with addition of sodium fluoride, sodium fluorosilicate and powdered metallic silicon at temperatures of 800-950 C. Heat resistance tests demonstrated the following durability of the coated materials: 180 h at 1100 C and 40 h at 1200 C for silicicated molybdenum; 130 h at 1100 C for silicicated tungsten; 80 h at 1100 C and 35 h at 1200 C for silicicated niobium. If uncoated, these metals began oxidizing at the same temperatures immediately after the test start.
Titanium and zirconium coatings. Samples of 2Х13, 4Х13, 1Х18Н9Т, 03Х5Н45Т (ЭП218) and Х18Н36М4ТЮ (ЭИ702) steels, molybdenum, tungsten, tantalum, niobium and niobium alloys were saturated with titanium at temperatures of 800, 900 and 1000 C in a KCl-NaCl+K2TiF6 melt, which was in equilibrium with the titanium powder. The coating was 20 to 80 m thick.
Zirconium coatings were deposited on metals (Ni, Co, Cu, Nb) in a Kl-NaCl+K2ZrF6 salt melt, which was in equilibrium with the zirconium powder at 900 C. The coating was 20 to 105 m thick.
Vanadium, chromium, and manganese coatings. Vanadizing is used to increase hardness of steels and improve wear resistance of tools. We studied formation of vanadium-containing layers on iron, nickel and niobium in molten chlorides and bromides of alkali metals at temperatures of 700-950 C. Continuous diffusion layers, which strongly adhered to the substrate, had the maximum thickness of up to 100 m and comprised solid solutions of the metals with vanadium, were obtained.
Diffusion chromium coatings on molybdenum, cobalt, nickel, ЖС6К alloy, and 30ХГСА and ШХ15 steels were prepared by immersion in KCl-NaCl-NaF+CrCl2 melts with addition of powdered metallic chromium at temperatures of 900-1000 C. A melt based on sodium octoborate with addition of chromium oxide, sodium chloride and fluoride and powdered metallic chromium was studied for liquid chroming of steel articles at temperatures of 850-1100 C.
Saturation with manganese improves hardness, wear resistance and corrosion resistance of metals and alloys. Coatings can be made on rubbing parts of machines operating in conditions of intensive wear. A melt containing KCl-NaCl-MnF2 with addition of powdered manganese was proposed for formation of manganese coatings. Coatings 25 to 180 m thick were made on carbon steels.
Zinc coatings. Zincing is one of the most efficient methods for protection of steels against corrosion in industrial atmosphere, tropical and maritime climates, salt mist and sea water or in conditions of hydrogen-sulfide corrosion. High protective properties of zinc coatings in combination with simplicity and diversity of zincing methods favor their wide use in practical applications.
Samples of 3, 35, 45, 35Л, ЭИ10, А12 and 30ХГСА steels were used for liquid zincing in a KCl-ZnCl2 melt, which was in equilibrium with a zinc powder, at 300-350 C. A three-layer coating 20-40 m thick made up of FeZn13, FeZn and Fe3Zn10 compounds was formed on the surface. The surface of zinc-coated and passivated samples remained intact after 3-month corrosion tests in humid atmosphere at 40-90 C.
The comparison of results of accelerated corrosion tests of diffusion zinc coatings on cast steels, which were performed for 3 months at temperatures of 20-40 C and the relative humidity of 98-100% in a salt mist containing sodium, magnesium and calcium chlorides, demonstrated that the time to corrosion of the diffusion coating was twice as long as that for galvanic coatings from water solutions. The corrosion resistance of welds in the 45Л steel, which were tested for 56 days in an atmosphere simulating the tropical climate, was referred to the "very strong" group (State Standard GOST 13819-68) and was given the corrosion resistance number "2".
Rare-earth-metal coatings. Lanthanum and cerium were deposited on iridium in salt melts. With respect to their thermoemission properties, cathodes of lanthanated iridium are not inferior to electrodes made of a smelted alloy. The surface deposition of coatings allows producing articles of any shape, including foils and wire, which cannot be made from a smelted alloy because of its brittleness. These coatings emit currents of up to 100 A/cm2 in the pulsed regime.
Boron coatings. The current-less transport of boron in ionic melts first to iron and later to other metals and alloys has long been used under the name of liquid borating. Diffusion borating of parts of machines and mechanisms provides a multifold increase in their hardness and wear resistance.
Numerous borating ionic melts can be divided into three groups: halogenide, oxide and mixed ones.
We began our investigations into borating of metals in ionic melts in 1965 (2). The transport of boron to iron in molten sodium tetraborate (borax) was studied in a galvanic cell of the amalgam type. The emf of the system
(-) BNa2B4O7FeBx Fe (+) 3
was determined. The cell emf strongly changed at the beginning of the experiment. The decrease in the emf points to the formation of the surface B-Fe alloy. The depth of the surface diffusion layer depends on the interelectrode spacing. The shorter the distance between boron and iron, the quicker the alloying process. The boron-to-iron distance can be decreased if compact boron is replaced by a boron powder. In this case, the fine boron powder is suspended in molten borax because they have nearly equal densities and the viscosity of molten borax is relatively high.
We synthesized boride layers on iron, steels, cobalt, nickel, their alloys, titanium, vanadium, zirconium, niobium, tantalum, molybdenum and tungsten in borate and mixed borate-halogenide melts. The optimal composition of the melt – 79 Na2B4O7, 15 NaCl and 6 B (mass %) – was selected and parameters of the boron diffusion to iron were calculated for liquid borating (2).
The boride layer on steels usually consists of two phases of needle borides (external FeB and internal Fe2B phases with the microhardness of 18500-20000 and 16500-18000 MPa respectively) or one phase (Fe2B). The number of boride phases in the coating depends on the method and parameters of saturation, as well as many kinetic and chemical factors.
To run the washing water in a closed circuit and eliminate the emission of toxic wastes, specialists at IHTE developed a technology for strengthening of machine parts and tools by their borating in a calcium chloride melt with addition of an amorphous boron powder in electrode salt baths, which are used in industry for nonoxidation quenching heating of parts (22, 23).
Borating of iron and steels in a calcium chloride melt with a boron powder can be pictured as follows. An oxide film of B2O3 is always present on the surface of a fine powder of amorphous boron. When the amorphous boron powder is charged into the calcium chloride melt, the oxide film dissolves in the melt. At high temperatures ions of dissolved trivalent boron are reduced by boron to the lowest-valence ions:
2 В3+(melt) + В (solid) 3 В2+(melt). 4
Bivalent boron ions disproportionate to form iron alloys by the scheme
The process is realized in commercial medium-temperature electrode baths of the CBC type at temperatures of 850-980 C during 0.5-5 h depending on the steel grade and the required thickness of the layer.
The technology is ecologically harmless because the borating bath and the quenching bath contain one and the same salt, namely calcium chloride. Calcium chloride and the amorphous boron powder, which stick to treated parts, are transferred with hot parts to the quenching bath where they are readily washed off the parts during quenching. Excess calcium chloride (above the preset concentration), which accumulates in the quenching bath, and the amorphous boron powder are removed from the bath, are evaporated, calcined, and returned to the borating bath. This scheme makes the technology efficient and ecologically friendly. Borated and quenched parts can be easily washed in water.
This borating technology was tested on constructional (20, 45, 40Х, ШХ15), tool (У8А, 9ХС, ХВГ, Х12Ф), hot-deformation tool (5ХНВ, 4ХМФС, 3Х2В8) and other steels. These steels were chosen because they are materials of drawing, bending, molding, cupping and knurling tools, forming rolls and other parts, which can be borated.
Wear resistance of borated parts depends not only on the depth of the boride layer, but also on its quality and structure. Wear resistance of boride layers under sliding friction conditions usually is determined by their phase composition, structure and microhardness. The increase in wear resistance of borated steels is due to a high hardness of boride coatings. Two-phase boride layers are 3 to 4 times more wear resistant than one-phase layers. Layers made up mostly of Fe2B borides and 20-40% inclusions of the FeB phase have the best wear resistance. However, in some situations (e.g., on exposure to thermal shocks) one-phase boride layers may prove to be much more efficient. Abrasive wear of borated parts is 2-10 times smaller than that of quenched parts.
The increase in the lifetime of some borated machine parts and tools is as follows: 2 to 10 times for cold- and hot-deformation dies (cupping, bending, molding and stamping tools); 2 to 3 times for loose material molds; 2 to 10 times for drawing and knurling tools; 2 to 4 times for parts of oil equipment (impellers and flywheels of pumps, swivel washpipes, line valves); 2 to 4 times for parts of atomizers in production of mineral fertilizers (diffusers, confusers, nozzles); 10 times for parts of machining attachments (gripping and feed collets, jigs); 3 times for thread guides in textile industry; 2 to 4 times for lining plates of brick-molding presses; 2 to 6 times for parts of machines and mechanisms operating in abrasive conditions (parts of crawler tractors, agricultural machinery, transporters, chains); 5 times for parts of casting machines and molds for casting of nonferrous metals and alloys.
Making of calcium-lead alloys. To reduce the hydrogen discharge and extend the lifetime of maintenance-free lead-acid cells by addition of calcium to the lead alloy, a technology was developed for production of lead-calcium alloying compositions (up to 2% Ca in the alloy). The technology consists in the spontaneous transport of calcium to lead through the molten salt by the scheme:
The technology was introduced at NIISTA (Podolsk) and Chepetsk Mechanical Plant (Glazov). The lifetime of the lead-acid cells increased 1.5-2.0 times.
Making of powdered hard magnetic alloys. Pilot batches of hard magnetic powders (samarium-cobalt, cobalt-platinum, iron-palladium, etc.) were produced by the method of current-less transport of more electronegative to more electropositive metals in molten salts. They are characterized by a more rectangular hysteresis loop and a higher magnetic energy as compared to those of the smelted alloys.
TWO-COMPONENT COATINGS AND ALLOYS Properties of one-component coatings not always and fully satisfy stringent modern requirements imposed on protective coatings. Therefore, complex coatings, which are synthesized by simultaneous or consecutive saturation of a substrate with two or several elements, are finding increasing use now. Thanks to saturation with two or more elements, the surface layer combines properties imparted by individual elements and, hence, the protective layer can satisfy high requirements. As distinct from one-component diffusion saturation, the interaction of metals with a multicomponent saturating medium is among the most complicated and, hence, least known processes.
Borosiliconizing. To improve surface hardness and erosion resistance of the main phase in the silicicated coating on tungsten (WSi2), the material was saturated in sequence with boron (Na2B4O7-NaCl-B, 1050 C, 1 h) and silicon (KCl-NaCl-NaF-Na2SiF6-Si, 950 C, 10 h), and then vice versa. Tungsten disilicide disappeared almost completely during borating and tungsten boride WB was formed on the surface. This coating withstood heat for not over 3-5 h at 1100 C. Borated and silicicated tungsten withstood heat for 10-17 h at 1100 C and 2-7 h at 1200 C during heat resistance tests.
Alumochroming. The main advantage of alumochromium coatings is their high corrosion resistance at elevated temperatures in air and liquid-fuel combustion products. Niobium was alumochromized in molten salts containing powdered aluminum-chromium alloys (NaCl-AlF3-CrCl2 + (Al-Cr) alloy at 950, 1000 and 1050 C). Coatings, which are synthesized by the complex saturation of niobium with aluminum and chromium, are thicker than coatings obtained by chroming and are slightly thinner than aluminized coatings. Alumochromium coatings protect niobium from oxidation at 1100 C better than aluminum coatings do.
Alumotitanizing. Pearlitic steels of the 25Х1МФ (studs) and 35XM (nuts) types were alumotitanized at 620 C in the BaCl2-KCl-NaCl-AlF3-K2TiF6 melt with addition of Al and Ti powders. Samples with alumotitanium coatings deposited in molten salts, samples with aluminum coatings synthesized by the powder method and uncoated samples were subject to tensile and impact tests at 20 and 350 C in order to determine their mechanical properties (ultimate strength, yield stress, hardness, relative elongation, contraction ratio, and impact elasticity). Corrosion tests of studs made of the 25Х1МФ steel and nuts made of the 35ХМ steel in a heat and humidity chamber demonstrated that their corrosion resistance improved without impairment of the mechanical characteristics of the base metal. Corrosion resistance of the coatings synthesized in a salt melt and by the powder methods was the same.
It was studied if niobium and the BH-3 alloy can be alumotitanized simultaneously in salt melts containing powders of an alumotitanium alloy and 35, 45 or 84% Ti at 950, 1000 and 1050 C. Complex coatings on niobium were thinner than those made by pure titanizing or aluminizing in molten salts. Uncoated niobium failed almost immediately, while titanium- and aluminum-alloyed niobium failed after 30-40 hours of the heat resistance test at 1000 C.
To improve wear resistance of copper, it was proposed to alumotitanize it by immersion in the BaCl2-KСl-NaCl-AlF3 salt melt with a powdered alumotitanium alloy at temperatures of 500, 600, 700 and 900 C. Wear resistance of coated copper was 10 times better than wear resistance of uncoated copper samples. Experimental data on heat resistance showed that alumotitanium coatings with 63-83 mass % aluminum provide a better protection of the copper surface from oxidation as compared with aluminide coatings.
COATINGS AND COMPOUNDS IN IONIC-ELECTRONIC MELTS Solutions of alkali, alkali-earth and rare-earth metals in their halogenides are referred to ionic-electronic melts, because dissolved metals dissociate to cations and delocalized electrons:
Ме Меn+ + n e. 7
As a result, these melts possess some electronic conduction, which increases with the concentration of the dissolved metal. The degree of the electron localization on metal particles is determined by the redox potential of the system, i.e. ultimately by the concentration of these particles. Such melts can enter into chemical reactions with other metals and materials in contact with the melts. Therefore, ionic-electronic melts are intermediate between ionic and metallic (electronic) melts.
We studied the isothermal transport of d-metals (Mn, Ni, Co, Cr, Mo, etc.) to iron, manganese and boron to titanium, and boron, carbon and silicon to refractory metals in LiCl-Li, CaCl2-Ca and BaCl2-Ba ionic-electronic melts (24). Saturated vapors of these melts are of low pressure at temperatures of up to 1000 C and, therefore, the melts can be handled at atmospheric pressure. The substrate metal was mainly Armco iron, because it interacts little with Li, Ca and Ba. The experiments were performed in argon. The transported metals, diffusant metalloids Me1 and iron samples were spatially separated by an ionic-electronic melt:
Me1 / MeClx(m) + Me(m) + Me1(m) / Fe. 8
In the experiments the saturation of the salt melt with lithium, calcium or barium was a maximum and the melt was brought in contact with the excess liquid alkali or alkali-earth metal. The presence of the transport was determined by the change of the mass of the substrate samples before and after experiments, and also from the X-ray phase and the X-ray microspectrum analysis of the surface layer and metallographic sections of the samples. The layer of d-metals on iron was 28 to 56 m thick in LiCl-Li at 900-1000 C, nearly 45 m thick in CaCl2-Ca at 1000 C, and 12 to 44 m thick in BaCl2-Bа at 1000 C. With respect to the mass transfer to iron in the LiCl-Li melt, the transported diffusant metals can be arranged in the series
Mn Ni Co Mo Cr Fe. 9
Manganese coatings on titanium. Titanium anodes with diffusion manganese coatings were tested in an experimental prototype installation for production of manganese dioxide in a MnSO4 + H2SO4 solution at the anodic current density of 100 to 500 A/m2. The operating time was 48 to 336 hours. The current yield of manganese dioxide changed between 100% (IA = 100 A/m2) and 88% (IA = 200 A/m2). The bath voltage increased not more than 30% by the end of the test. The total operating time of the anodes was 1475 hours. The current yield of the test anodes differed little from the current efficiency of commercial anodes, but at 160 and 175 A/m2 the current yield of the former was a little higher.
Production of refractory carbide powders. A new method is proposed for low-temperature synthesis of carbides of refractory tantalum, niobium, titanium and tungsten from powders of the corresponding metals or their oxides. The use of ionic-electronic melts allows reducing the synthesis temperature by 200-500 C. Fineness of the synthesized carbide powders is tens of micrometers to hundredth fractions of a micrometer, while their specific surface is up to 30-50 square meters per gram.
The obtained experimental data on the mass transfer demonstrated that ionic-electronic melts can serve as media for production of new materials, diffusion coatings, powdered alloys and compounds.
Review of the thermochemical formation of coatings. To observe main trends in synthesis of diffusion coatings and thermochemical treatment, we analyzed publications, including certificates of authorship and patents, over the period from 1949 till 2003 (25).
The analysis was made considering such parameters as the publication dynamics and the ratio of papers dedicated to thermochemical treatment processes (aluminizing, borating, chroming, siliconizing, zincing, titanizing).
Papers dealing with synthesis of coatings by the liquid method in molten salts account for 9.1% of all the publications dedicated to application of coatings by different methods. Out of these papers, 39.0% is dedicated to borating, 5.5% to aluminizing, 8.2% to chroming, 4.8% to siliconizing, 5.0% to titanizing, 28.8% to coatings of other metals, and 8.7% to multicomponent coatings. Since studies into synthesis of coatings by the liquid method are performed mainly at the Institute of High-Temperature Electrochemistry, the Belorussian Polytechnical Institute and in Japan, we compared these studies with respect to diffusant elements. Papers published by specialists from the Institute of High-Temperature Electrochemistry, the Belorussian Polytechnical Institute and Japan account for about 45%, 14% and 41% respectively. Studies into liquid borating in halogenide melts, which were conducted at the All-Union Research Institute of Tools (Moscow), are known well.
The largest number of papers dealing with diffusion coatings were published in 1980-1984 (20.4%), 1975-1979 (16.7%), and 1970-1974 (16.1%). Later the number of publications decreased and accounted for 3.6% in 1995-1999.
Specialists at the Institute of High-Temperature Electrochemistry were granted 28 certificates of authorship and 4 foreign patents (France, FRG, Great Britain, and Japan) for diffusion coatings synthesized by the liquid method.
CONCLUSION The obtained data on the mass transfer of metals and nonmetals show that ionic and ionic-electronic melts can serve as media for production of new materials, diffusion coatings, powdered alloys and compounds. At present, promising methods include synthesis of diffusion coatings in powdered media and salt melts (with and without electrolysis) for improvement of heat resistance of metals and alloys (Be, Al, Cr, Si, Al+Cr), their corrosion resistance (Zn, Al, Ti, Al+Cr, Al+Ti) and wear resistance (B, Cr, B+Si). Protective coatings will play an ever increasing role in reliability and lifetime of parts of machines and mechanisms as industry and engineering achieve new technical standards.
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