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
Abstract
Ample information concerning the interaction of metals in ionic and ionicelectronic 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, aluminumchromium, or boron-silicon) on metals and alloys and diffusion alloys ((samariumcobalt, 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 nondiffusion 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
210
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 hightemperature 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
211
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.
212
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, 813). 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 ionicelectronic melts
Coating element
Li
Be
B
N
Mg
Al
Si
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Base material (substrate)
Ionic melts ** Diffusion alloys **
Cu, Al, Ag
Zr, Ti, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Cu, Ag
Be, Ti, Nb, Mo, W, Fe, Co, Ni, Steels
Ti, (Ti) Stainless steels, (Ti) Steels
Cu, Ni
Ti, Mn, Nb, Mo, W, Fe, Ni, Cu, Steels
Ti, V, Nb, Mo, W, Fe, Re
Cu, Pb
Al
Zr, V, Nb, Ta, Mo, W, Cr, Fe, Co, Ni, Cu, C
Nb, Fe, Co, Steels, (C) Steels
Nb, Mo, Fe, Co, Ni, Steels
Nb, Mo, W, Fe, Co, Ni
Co, Ni, Pd, Pt
Pd, Pt
Mo, W
Ni
213
Zn
Y
Zr
Mo
Cd
In
La
La-Ce
Nd
Sm
Eu
W
B-Al
B-Si
Al-Si
Al-Cr
Al-Ti
Zr-Si
W-Si
B-C-Si-Cr-Fe
B
C
N
Al
Si
V
Cr
Mn
Co
Ni
Cu
Mo
Ni-Cr
Fe, Steels, (Cu) Steels
Re
V, Nb, Ta, Mo, W, Cr, Fe, Co, Ni, Cu, Stainless steels
Ni
Ag
Pd, Ni, Cu, Ag
Ir, Ni, Stainless steels
Ir
Ir
Co
Ir
Ni
Fe
Mo, W, Ni, Alloys of Ni, Steels
Mo
Mo, Nd, Ni, Alloys of Ni
Nb, Fe, Cu, Alloys of Ni
Ni, Alloys of Ni, Fe, Steels
Ni, Alloys of Ni, Fe, Steels
Ni
Ionic-electronic melts ** Diffusion alloys **
Fe, Ti, Zr
Nb, Ta, Ti, Zr
Ti, Ta
Fe, Stainless steels
Fe, Ni, Ti, Zr, Nb, Mo, W
Fe, Steels
Fe, Steels
Ti
Fe, Steels
Fe, Steels
Ni
Fe, Steels
Fe, Steels
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
214
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 lowtemperature 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 KlNaCl+K2ZrF6 salt melt, which was in equilibrium with the zirconium powder at 900
C. The coating was 20 to 105 m thick.
215
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.
216
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
(-) BNa2B4O7FeBx  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).
Bivalent boron ions disproportionate to form iron alloys by the scheme
3 В2+ (melt) + 2 Fe (solid)  2 В3+(melt) + Fe2B (solid),
4
5
3 B2+(melt) + Fe2B (solid)  2 В3+(melt) + 2 FeB (solid).
6
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.
217
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:
Ca(Cualloy) + CaCl2 → 2CaCl; 2CaCl + Pballoy → CaCl2 + Ca(Pballoy).
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
218
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-NaFNa2SiF6-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:
219
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 lowtemperature synthesis of carbides of refractory tantalum, niobium, titanium and
tungsten from powders of the corresponding metals or their oxides. The use of ionicelectronic 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 ionicelectronic 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).
220
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.
REFERENCES
1. Ilyuschenko N.G., Anfinogenov A.I., Shurov N.I. Interaction of Metals in Ion
Melts. Moscow, Nauka, 1991. 176 p.
2. Chernov Ya.B., Anfinogenov A.I., Shurov N.I. Borating of Steels in Ionic Melts.
Ekaterinburg, Ural Branch RAS, 2001. 224 p.
3. Lyakhovich L.S., Kosachevsky L.N., Dolmanov F.V., Krukovich M.G.
Thermochemical Treatment of Metals and Alloys. Minsk, BPI. 1971. 164 p.
4. Lyakhovich L.S., Kosachevsky L.N., Dolmanov F.V., Krukovich M.G. Liquid noelectrolysis processes of thermochemical treatment // Metallovedenie i
Termicheskaya Obrabotka Metallov, 1972, No. 2. pp. 60-61.
5. Dubinin G.N. Classification of Methods for Diffusion Saturation of Alloy Surface
with Metals. In: Diffusion Coatings on Metals. Kiev, Naukova Dumka. 1965. pp.
3-12.
6. Shatinsky V.F., Nesterenko A.I. Protective Diffusion Coatings. Kiev. Naukova
Dumka. 1988. 272 p.
221
7. Smirnov M.V., Krasnov Yu.N. Electrochemical behavior of titanium carbide in
chloride melt. Zh. Neorgan. Khimii. 1960. v. 5, No. 6. pp. 1241-1247.
8. Ilyuschenko N.G., Anfinogenov A.I., Kornilov N.I. Transport of metals in molten
salts and determination of metal reactivity by emf method. In: Physical Chemistry
and Electrochemistry of Salt Melts and Slags. Leningrad, Khimiya, 1966. pp. 118122.
9. Ilyuschenko N.G., Anfinogenov A.I., Belyaeva G.I., Plotnikova A.F., Kornilov
N.I. Diffusion coatings of metals in molten salts. In: High-Temperature and Heat
Resistant Coatings. Leningrad, Nauka, 1969. pp. 105-120.
10. Ilyuschenko N.G., Anfinogenov A.I., Belyaeva G.I., Plotnikova A.F., Kornilov
N.I. Study of current-less deposition of coatings in molten salts. In: HighTemperature Protective Coatings. Leningrad, Nauka, 1972. pp. 248-253.
11. Belyaeva G.I., Anfinogenov A.I., Ilyuschenko N.G. Synthesis of multicomponent
diffusion aluminum-, titanium- or chromium-based coatings on niobium in ionic
melts. In: Protective Coatings on Metals. Kiev, Naukova Dumka, 1976. No. 10.
pp. 59-62.
12. Ilyuschenko N.G., Belyaeva G.I., Anfinogenov A.I. Current-less transport of
metals in molten salts and its practical applications. In: Protective Coatings on
Metals. Kiev, Naukova Dumka, 1977. No. 11, pp. 94-96.
13. Ilyuschenko N.G., Anfinogenov A.I., Shurov N.I., Chebykin V.V., Zyryanov V.G.
Transport of metals in ionic-electronic Li-LiCl melt. Zh. Prikl. Khimii. 1995. v.
68. No. 6. pp. 1027-1029.
14. Gopienko V.G., Podushkin D.I. Formation of titanium coatings on refractories,
ceramics and some metals. In: Investigations in Chemistry of Silicates and Oxides.
Moscow-Leningrad, Nauka, 1965. pp. 234-241.
15. Anufrieva N.I. Effect of lowest titanium chlorides on electrode potentials of some
metals in molten salts. Elektrokhimiya, 1966. v. 2, No. 6. pp. 729-731.
16. Kornilov N.I., Ilyuschenko N.G. Interaction of nickel with monovalent beryllium
ions in molten salts. Proc. Institute of Electrochemistry, Ural Branch of USSR
Academy of Sciences. No. 8. 1966. pp. 73-78.
17. Ilyuschenko N.G., Kornilov N.I., Belyaeva G.I. Interaction of beryllium with
metals in molten salts. Proc. Institute of Electrochemistry, Ural Branch of USSR
Academy of Sciences. No. 12. 1969. pp. 78-84.
18. Ilyuschenko N.G., Anfinogenov A.I., Belyaeva G.I. Improvement of heat
resistance of ЖС6K alloy by liquid beryllation. In: Inorganic and Organic-Silicate
Coatings. Leningrad, Nauka, 1975. pp. 229-233.
19. Ilyuschenko N.G., Anfinogenov A.I., Belyaeva G.I., Plotnikova A.F., Kornilov
N.I. Diffusion coatings of metals in molten salts. In: High-Temperature and Heat
Resistant Coatings. Leningrad, Nauka, 1969. pp. 105-120.
20. Ilyuschenko N.G., Belyaeva G.I. Low-temperature aluminizing of steels in molten
salts. Metallovedenie i Termicheskaya Obrabotka Metallov. 1968. No. 4. pp. 1417.
21. Belyaeva G.I., Anfinogenov A.I., Ilyuschenko N.G. Low-temperature aluminizing
of copper in molten salts. Proc. Institute of Electrochemistry, Ural Scientific
Center, USSR Academy of Sciences. No. 26. 1978. pp. 30-34.
22. Chernov Ya.B., Anfinogenov A.I., Ilyuschenko N.G. Method for thermochemical
treatment of steel parts. Pat. RU 2004617, C 23 c 8/42 Filed 15.06.92. Publ.
15.12.93. Bull. No. 45-46.
222
23. Chernov Ya.B., Anfinogenov A.I., Schemelev A.V., Prudnikov A.N., Kharchenko
N.G., Kereshun R.T. Melt for liquid borating. Pat. RU 2007498 C 23 c 8/42 Filed
16.07.90. Publ. 15.02 94. Bull. No. 3.
24. Chebykin V.V., Chernov Ya.B., Anfinogenov A.I. Methods for thermodiffusion
treatment of metals and alloys. Pat. RU No. 2221898 dated 19.11.2001.
25. Anfinogenov A.I., Chebykin V.V., Chernov Ya.B. Review of thermochemical
treatment of metals and alloys. Rasplavy. 2005. No. 3. pp. 40-52.
About Authors
Zaikov Yury Pavlovich, professor, doctor of chemical sciences, director
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
Chebykin Vitalij Vasiljevich, candidate of chemical sciences, Leader lab. Alloys
Institute of High-Temperature Electrochemistry, Ural Branch RAS, 22
S.Kovalevskaya St., 620219 Ekaterinburg GSP-146, Russia, Tel.: (343)3623543;
e-mail: V.Chebykin@IHTE.uran.ru
Anfinogenov Alexander Iavanovich, candidate of chemical sciences
Institute of High-Temperature Electrochemistry, Ural Branch RAS, 22
S.Kovalevskaya St., 620219 Ekaterinburg GSP-146, Russia, Tel.: (343)3623543;
e-mail: V.Chebykin@IHTE.uran.ru
223