OXIDE LAYERS OF COMPLEX COMPOSITION
ON VALVE METALS
Rudnev V.S., Yarovaya T. P., Lukiyanchuk I.V., Vasylieva M.S., Morozova V.P.
Institute of Chemistry FEB RAS, Vladivostok, Russia
Abstract. In recent years the investigations of electrochemical formation of function
oxide coatings on valve metals in electrolytes at the voltage of electric breakdowns –
sparks and arcs (plasma-electrochemical processes, PEO), are very expansive. Using
electrolytes with polyanions, metastable electrolytes or suspensions we have obtained
the coatings of a complex composition. In electrolytes contained polyphosphate
complexes of metals the films consisted of high-temperature phosphates of metals
and/or spinels are obtained. For example, MnAl2O4, ZnAl2O4, Mn2P2O7, Fe2Fe(P2O7),
ZrP2O7 and CePO4 are among them. In electrolytes with iso- and heteropolyoxianions
(IPA and HPA, for example [V10O28]6-, [PW12O40]3-) the films with WO3, WO2.9,
Al2(WO4)3, V2O4, V2O5 etc. have been formed. We carry out studies of PEO
processing in electrolytes evolving the precipitates. For example the composites
Ti/TiO2/Mn2O3+Mn3O4, Ti/TiO2+ZrO2, Ti/TiO2+CeO+ZrO2 are obtained.
Thus, using electrolytes with polyanions, as well as metastable and colloidal
electrolytes is very promising to produce the PEO coatings on valve metals and alloys
that contain oxide structures of diverse composition and destination such as
decorative, protective, catalytic layers, for medical purposes and other.
INTRODUCTION
In recent years the investigations of electrochemical formation of protective oxide
coatings on anode-polarized metals in electrolytes at the voltage of electric
breakdowns – sparks and arcs, are very expansive. The method is known as plasmaelectrolytic, microplasma, microarc or anodic spark deposition (or oxidation). Further
we use the abbreviation 'PEO' (plasma- electrochemical oxidation).
Together with electrochemical oxidation, during the PEO process the reactions
may take place at the discharge regions, that are similar to hydrothermal synthesis,
interactions in melt, solid state interactions as well as the thermolysis of adsorbed
compounds and sediment from electrolyte. In general, the composition of such
coatings depends on the substrate metal, treatment parameters and compounds
delivered from electrolyte, i.e. the electrolyte composition. Therefore, on the level
with formation of protective coatings (wear-, corrosive- and heat-resistant), the PEOprocess is promising to produce the coatings that contain oxygen-containing
compounds of different metals and non-metals besides the substrate metal oxide. The
latter may be of great importance in new materials development, for example, for the
purposes of heterogeneous catalysis.
Our investigations of the PEO processes on valve metals (mostly on Al and Ti)
have revealed, that using electrolytes with polyanions, metastable electrolytes or
suspends is promising to form the coatings of a complex composition. Analyzing the
thermolysis products of compounds (particles) from electrolyte one can predict the
coating to contain a certain compound. Possible application fields of metal/multiphase
surface PEO-structure compositions are considered.
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ELECTROLYTES WITH METAL POLYPHOSPHATE COMPLEXES
In an aqueous solution of Na6P6O18 polyphosphate, on aluminum, layers formed
preferentially consist of an oxide of the treated metal (M) (1,2). The addition of watersoluble M salts to the electrolyte to form polyphosphate-M complex ions leads to a
simultaneous and proportional incorporation of the metal and phosphorus from the
electrolyte into films (M(II): Mg, Ca, Ba, Cd, Cu, Zn, Ni, Co, Mn (1,2); M(III): Al,
Eu, La, Y, Ce (1-3); M(IV): Zr (4). The incorporation of elements from
polyphosphate complexes is determined by the molar ratio n = [polyphosphate]/[M] in
solution (Fig. 1, Table 1), or, in other words, by the solubility of complexes and the
solution state.
Fig. 1. The effect of molar ratio n = [polyphosphate]/[M] in aqueous solutions with 30
g/l Na6P6O18 and (a) Mn(CH3COO)2 or (b) Y(CH3COO)3 on the elemental
composition of layers formed on aluminum alloy. Solution state: I true; II metastable;
III precipitation of hydrated manganese polyphosphates.
A considerable change in the composition of PEO layers on valve metals (Al, Ti, Nb,
Zr) is observed when the solutions approach the state of spontaneous precipitation of
solid metal polyphosphates (in ranges II and III, Fig. 1).
Changing the solution condition, one can direct the PEO-coating composition
(Table 1), structure, thickness and properties. The layers formed in region (I) consist
of the substrate metal oxide. In region (III) the surface of coating is built of
compounds on the base of electrolyte species.
Table 1. Elemental composition of coatings formed on aluminum alloy and titanium
in electrolytes with different n = [polyphosphate]/[M cation].
ELECTROLYTE
N
10
0,5
10
2,6
10
1
30 Г/Л NA6P6O18+PB(CH3COO)2
40 Г/Л NA6P6O18+EU(CH3COO)3
30 Г/Л NA6P6O18+ZR(SO4)2
COMPOSITION, MASS %
45,5 AL; 2,8P; 0,3 PB
1,1 AL; 11,5P; 42,4 PB
40,9 AL; 3,2P; 0,8 EU
5,6 AL; 8,5 P; 34,3 EU
32,6 TI; 15,6 P; 3 ZR
19,4 TI; 11,6 P; 21.4 ZR
The majority of formed films are crystallographically amorphous. Hightemperature annealing of samples or coatings separated from the substrate, or grinded
coatings led to the crystallization of M phosphates, which include the high-
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temperature thermolysis products of M polyphosphates. In films formed in
electrolytes either metastable or capable of precipitating M hydrated polyphosphates
(n, 2–8), in addition to the oxide crystallization, the crystallization of M
pyrophosphates, spinels, orthophosphates, and polyphosphates is observed. For
example, in films on aluminum Ni2P2O7, NiAl2O4, Zn3(PO4)2, ZnAl2O4, NaZnPO4,
MnAl2O4, were formed. Layers formed on niobium and zirconium contained Mg2P2O7
and NaMgPO4, respectively. Films formed on titanium were characterized by the
crystallization of double phosphates of titanium and M in addition to M
pyrophosphates. For example, crystallization of Na0.23TiO2, NaTi2(PO4)3, Mn2P2O7,
Mn(II)Mn(III)Ti(PO4)3 was observed in films formed in electrolytes with Mn(III)
hexametaphosphate complexes, or the formation of ZrP2O7, Ti0.8Zr0.2P2O7, Ti2ZrO6,
and NaZr2(PO4)3 for layers formed in aqueous electrolytes containing Na6P6O18 and
Zr(SO4)2. The combined thermogravimetric, IR spectroscopic, X-ray spectral and
diffraction data made it possible to conclude that these phosphates were also present
in the original crystallographically amorphous films.
A comparison of data for films formed in aqueous solutions containing sodium
hexametaphosphate and tripolyphosphate, namely, Na6P6O18 (cyclic) and Na5P3O10
(linear polyphosphate) showed an insignificant effect of the polyphosphate structure
on the elemental and phase composition of coatings. Coatings with the analogous
composition can be formed in solutions containing other water-soluble linear and
cyclic polyphosphates.
The development of conditions in electrolytes for the formation of polyphosphate
complexes of two or more M made it possible to form films containing compounds of
these metals. For example, utilizing electrolytes with polyphosphate complexes of
different metals allows to form the coatings that contain these metals in a ratio
required (Fig. 2).
Fig. 2. Influence of substitution of Mg(CH3COO)2 by Ba(CH3COO)2 in electrolyte
containing Na6P6O18 on magnesium and barium content in coatings formed on
aluminium alloy.
ELECTROLYTES WITH ISO - AND HETEROPOLYAOXOANIONS
As was mentioned above, one of the main mechanisms of incorporating the
electrolyte components into the composition of PEO layers is the thermolysis of
adsorbed compounds and electrolytic deposits in the vicinity of the electric discharge
channels. Insofar as the oxides of elements in the inner coordination spheres of isoand heteropolyoxocompounds prevailed in the thermolysis products and the
decomposition temperatures were rather low (150–450°C), the use of electrolytes
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containing iso- and heteropolyoxoanions (IPA and HPA, e. g., decavanadate ion
[V10O28]6– and vanadophosphate ion [PV14O42]9–, respectively) is a promising method
for the preparation of oxide structures containing tungsten, molybdenum, vanadium,
and niobium oxides. Moreover, in addition to commercial iso- and
heteropolyoxocompounds, it is possible to develop conditions for the formation of
IPA and HPA in anodizing solutions.
Study of physicochemical trends in the formation of multiphase PEO structures on
valve metals in electrolytes containing IPA and HPA is of considerable scientific and
practical interest, because the HPA class is very wide. It is sufficient to say that more
than 65 elements of the Mendeleev Periodic System can play the role central atoms in
HPA. In HPA-containing electrolytes, one can expect the formation of PEO coatings
of diverse chemical composition.
For electrolytes with tungstate IPA and HPA (Table 2), in solutions close to
neutral, in which the conditions for the existence of different lacunar HPA forms are
developed and the dissolution rates of metal substrates are insignificant, layered
surface structures were formed, (Fig. 3 and 4) (5,6).
Table 2. The effect of the HPA nature on the phase composition of PEO layers.
Electrolyte characterization
Composition
pH
0.0091 M K8[BW11O39H] 6.5-6.9
0.0083 M Na2H[PW12O40] 4.1-7.3
0.0083 M H4[SiW12O40]
5.7-6.6
Phase composition of PEO layers
Al
Ti
WO3
WO3
WO2.9 and/or
Na0.1WO3
Na0.1WO3
H0.23WO3 + ?WO2..9
H0.23WO3 +
?WO2.9 +
?traces SiO2
Fig. 3. The coating growth stages. The white parts are the phase WO3.
As for aluminum and titanium the outer part of such structures contained WO3 or
nonstoichiometric tungsten oxide WO2.9 and/or sodium-tungsten bronze Na0.1WO3
(Table 2). In the acidic and alkaline ranges (region II in Fig. 5), the dissolution of
metals and their oxides affected the composition of oxide films. In range II, in
addition to dissolution processes, a substantial role was played by the HPA
decomposition under the action of hydroxyl ions.
The identity of the central atom in a tungtate HPA in the studied HPA series
(K8[BW11O39H], Na2[HPW12O40], H4[SiW12O40], Table 2) had virtually no effect on
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the elemental composition of coatings.
Fig. 4. The surface geometry of the coating (the surface of the phase WO3).
Fig. 5. The pH influence of the [BW11O39H]8- containing electrolyte on the elemental
composition of PEO coatings on Al.
The results of our studies confirmed that this approach can be used in syntheses of
multiplayer coatings with the external layers containing thermolysis products of
heteropoly compounds, e. g., oxides and compounds of vanadium, vanadium and
tungsten, and vanadium and molybdenum.
ELECTROLYTES WITH FLUOROCOMPLEXES OF TRANSITION
METALS
The conditions for the formation of PEO films and their elemental and phase
compositions were studied for films on a series of valve metals in electrolytes
containing water-soluble fluorocomplexes of titanium, zirconium, hafnium (K2TiF6,
K2HfF6, K2NbF7, Na2ZrF6, K2ZrF6 and (NH4)3ZrF7, Table 3). Titanium, zirconium,
hafnium, and niobium are preferentially accumulated in the external layers of coatings
that amount to approximately third of the overall film thickness. The presence of
oxides of transition metals M in films can be assumed to be due to two processes,
namely, the hydrolysis of salts to form complex anions and the subsequent annealing
in breakdown zones.
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Table 3. Examples of coating with transition metal oxides on a niobium alloy and a
aluminum alloy.
Elec
trolyte
K2Zr
F6
lloy
b
l
K2TiF6
b
l
K2H
fF6
Composition, mass %
N
T
Z
b/Al
i
r
f
N
3
27,5
5,5
A
2
4
,5
2,0
N
3
2
0,8
7,1
A
1
4
,8
9,7
N
1
9,5
0,3
A
b
H
Phases
-
δ-Nb2O5, ZrO2(т),
ZrO2(м).
-
ZrO2(к) , α-Al2O3
3
TiO2(р), TiO2(а), δNb2O5
TiO2(р), TiO2(а), αAl2O3
HfO2,. δ-Nb2O5
The synthesized films were heat resistant, stable with respect to corrosive media,
and displayed decorative qualities (Fig.6). The use of electrolytes simultaneously
containing several transition metals will apparently make it possible to form
multiphase PEO layers containing a set of transition metal oxides.
Fig. 6. The surface geometry of the coating with ZrO2 on aluminum alloy. The
coatings formed by different of the current density.
ELECTROLYTES EVOLVING RECIPITATION
To introduce the required elements or compounds into PEO layers, it was proposed
to use electrolytes-suspensions.
We have begun the studies on the use of PEO electrolytes that under certain
conditions can produce deposits in the form of dispersed solid particles of a given
composition (7-9). The following ways exist for the transition of salt solutions to their
colloidal or suspension solutions: by varying the temperature, the component
concentration, and pH; by introducing compounds-precipitants; by creating the
conditions in solutions favorable for reactions with precipitation of solid deposits; etc.
For example, mixing aqueous solutions of 0.1 M Na2B4O7 and (1.81–9.05) 10–2 M
Mn(CH3COO)2 resulted in the formation of a gel-like white deposit with a pink tint,
which contained both hydroxides and borates (7). Layers formed on titanium in these
electrolytes-suspensions contained from 5 to 40 at % manganese and crystalline
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phases of Mn2O3 and Mn3O4. The chemical composition of precipitates played the
key role in the formation of coatings containing crystalline manganese oxides on
titanium. The high temperature in the vicinity of breakdown channels caused thermal
transformations of manganese compounds in the recipitate, according to the known
scheme
The films formed on titanium from aqueous solutions-suspensions of Zr(SO4)2
under galvanostatic conditions by the plasma-electrolytic method contained
considerable amounts of zirconium (about 20 at%) and, correspondingly, crystalline
zirconium oxide (Fig.7 and 8) (9). The film composition can be controlled by
changing the pH of the electrolyte. The formed films contained a small number of
pores, were dense and poorly wetted with water. They can be of interest as probable
protective films.
Fig. 7. X-ray diffraction patterns of oxide layers formed in electrolytes with pH: (a) 2,
(b) 6, (c) 11.5. Designations: 1 - monoclinic modification of ZrO2; 2 - tetragonal
modification of ZrO2; 3 - cubic modification of ZrO2; 4- Ti; 5- TiO2.
Fig. 8. Surface organization (electron-microscopic images) and the profile of a drop
of distilled water placed at the surface of films formed at the electrolyte pH: (a, b) 2,
(c, d) 7.
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The oxide layers containing -Al2O3 and nickel and copper compounds, were
prepared on aluminum using PEO in an aqueous electrolyte-suspensions (8). Elements
such as nickel, copper, sodium, and phosphorus from the electrolyte were
concentrated on the film surfaces. Nickel occurred as Ni2+, whereas copper occurred
as Cu+ and Cu2+. The Ni2+/(Cu+ + Cu2+) on the surface was 2.3–3.0; the major portion
of copper occurred in the state Cu+. The surface contained noticeable amounts of
carbon. Formation by anodic–cathodic polarization and annealing in air resulted in
increase in the mechanical resistance of the compositions.
Based on the experiments performed, aqueous electrolytes capable of forming M
hydroxide precipitates were proposed to use when preparing PEO layers containing
compounds of bi-, tri-, and polyvalent metals on valve metals.
POSSIBLE APPLICATION FIELDS ON MULTIPHASE ANODIC FILNS
Among the promising directions is the use of the metal-oxide and metal-oxidephosphate compositions as the carriers of catalytically active compounds and noble
metals, which exhibit intrinsic catalytic activity. The important advantage of these
composites is the high heat and electric conductivity of the metal substrate. The latter
properties are important for processes that require the removal of excessive heat.
Many inorganic salts of silver, zinc, copper, and other metals, including
phosphates, exhibit antibacterial and fungicide properties (are bioactive). The PEO
layers formed on aluminum and titanium in electrolytes with polyphosphate
complexes of M contained copper, strontium, zinc, cadmium, or lead phosphates and
exhibited antibacterial properties. Such coatings are promising in the protection of
parts and structures against the development of microflora, biocorrosion, and
biofouling on their surface.
PEO layers with calcium phosphates on titanium were prepared and studied aimed
at enhancing the biocompatibility of titanium implants and biological prostheses .
Such layers can be formed in electrolytes with Ca(II) polyphosphate complexes. The
extra addition of, e. g., Zn(II), to this electrolyte may allow one to obtain
biocompatible layers with a certain antibacterial activity.
Coatings on aluminum, which were formed in electrolytes with zirconium
fluorocomplexes and contained zircomium oxides in their external part, exhibited
integral reflectivity of up to 80–81% in the wavelength range of 430–670 nm. Optical
characteristics of films remained unchanged under irradiation with hard ultraviolet.
The films were recommended as light reflectors.
It may be expected that the introduction of rare-earth metals, e. g., from
electrolytes with polyphosphate complexes M(III), will lead to the formation of films
with luminescent properties. This is confirmed by observations (1) of luminescence of
europium-containing PEO layers on aluminum under ultraviolet irradiation.
Phosphates of many metals are colored and their presence in films imparts color to the
latter. For example, films containing zinc phosphate are white and those with cobalt
compounds are blue.
At certain temperatures, vanadium oxides are characterized by metal–
semiconductor phase transitions, which allows one to use them as the active
components of thermal resistors and heat-sensing devices. According to preliminary
studies, PEO films formed on aluminum in electrolytes containing vanadate HPA and
certain compounds including vanadium oxides exhibit interesting electrical properties
(1). PEO layers formed on zirconium and containing zirconium oxide in its cubic
modification exhibited ionic conductivity with respect to oxygen and may be used as
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the active elements of gas-analyzing systems. These results may serve as the basis for
further studies of the electrical properties of the films and their purposeful
modification by varying the electrolyte composition and the formation conditions.
Several of formed surface structures were of interest for the corrosion protection of
metals and alloys, for example, zirconium-containing PEO layers on aluminum and
titan.
Films with phosphates of certain metals may be of interest as the prime coatings
for paints, varnishes, and polymers; they may exhibit hydrophilic or hydrophobic
properties, serve as solid lubricants and display refractory properties.
CONCLUSION
The results of studies summarized in this publication and the literature data
devoted to linking the electrolyte composition to the properties of the coatings show
that the method of plasma-electrolytic oxidation-deposition is very effective for the
targeted preparation of the valve-metal/surface-layer compositions with the definite
phase composition and properties. Up to date, attention was focused on the formation
of PEO coatings that prevent the mechanical wear and corrosion of structures. The
insight into the growth mechanisms of oxide structures on metals and alloys under the
action of electric discharges and the correlations between the composition and state of
electrolytes on one hand and the composition and structure of surface PEO formations
on the other hand, will allow one to use this method for the formation of layers that
would contain certain compounds, have a definite composition and structure, could be
used in medicine, catalysis, hydroacoustics (with perovskites), and exhibit certain
optical, thermal, and electric properties.
In its potentials, the PEO method resembles the high-temperature solution ceramic
process. The main difference is that the high temperature in local areas of the
electrode surface is provided by electric discharges immediately in solution.
Moreover, the average temperature in the solution volume does not usually exceed
50°C.
It cannot be denied that this method is still in the development phase. It is
necessary to systematically study the growth mechanisms of surface structures, which
should involve the use of different discharge types, the elucidation of the influence of
different components, the electrolyte state and type on the growth and composition of
films, the elucidation of relations between the preparation conditions, composition,
structure, and properties of PEO layers, finding out the behavior of such structures
under different impacts in different media, and solving many other problems. On the
other hand, from our viewpoint, this method in its present form can find application as
an unconventional method of preparing practically important functional layers on
metals and in the production of new materials and compositions.
ACKNOWLEDGEMENTS
The study was financially supported by the Russian Foundation for Basic Researches
(project no. 06-03-32184) and the Presidium of the Far East Division of the Russian
Academy of Sciences (projects no. 06-IISO-04-011 and no. 06-I-OkhNM-137).
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