Currentless Electrochemical Synthesis of Functional Metallic Materials in Ionic And

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:

Abstract

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 increasinglyused 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-valenceBe+ 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.

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. Investigationsinto 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 andЭП109nickel-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 forsilicicated 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. Samplesof2Х13, 4Х13, 1Х18Н9Т, 03Х5Н45Т(ЭП218) andХ18Н36М4ТЮ (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+K2TiF6melt, 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+K2ZrF6salt 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 andШХ15 steels were prepared by immersion in KCl-NaCl-NaF+CrCl2melts 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-ZnCl2melt, 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".