S.V. Gnedenkov, S. L. Sinebryukhov, O.A. Khrisanfova
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences,
Vladivostok, Russia
Abstract.
The protective composite coatings possessed by practically important properties (anticorrosive, antiscale, bioinert/bioactive and etc.) on titanium, aluminium, magnesium alloys as well on the steel and nitinol surfaces were obtained by means of the unipolar or bipolar modes of the plasma electrolytic oxidation (PEO) in different water electrolytes. The application of superdispersed polytetrafluoroethylene in the coatings composition allows to increase their stability in corrosion active media. The chemical composition, morphology, electrochemical properties and protection against scale formation in the sea water of composite coatings were investigated by different modern physical-chemical methods. The investigation of PEO and composite layers with using impedance spectroscopy method allowed to determine their morphology structure, homogeneity and to research the charge transfer mechanism which is realized on the oxide heterostructure/electrolyte interface. The composite coatings obtained may be used in shipbuilding, aircraft industry, machinery construction, and implant surgery as well.
1. INTRODUCTION
The development of highly efficient methods for the protection of metals and alloys against corrosion, scale deposition is a complex research and engineering problem [1, 2]. If in the construction of the apparatus working in the sea water the direct contact of few different metals (e.g. titanium and steel) take place then it is necessary to protect this device from contact (galvanic) corrosion [1, 2]. The solution of this problem depends on the completeness and comprehensiveness of collected information about the mechanisms of these processes. Despite the practical importance of this problem, until now, simple and highly effective protection methods of metals and alloys against the corrosion destruction have been absent. One promising solution lies in the use of constructive methods to prevent corrosion, salt deposition and mechanical destruction. The surface layers formed by the plasma electrolytic oxidation (PEO) on metals and alloys and composite layers formed using
PEO coatings, along with a considerable change in the surface state, providing some new physicochemical properties, essentially broaden the practical field of use of the processed material [3, 4]. At the same time, an additional treatment of the metal-oxide surface using Forum® superdispersed polytetrafluoroethylene (SPTFE) [5] is proposed to increase its hydrophobicity and to decrease the coating roughness, which sets up the possibility of further reducing the intensity of the corrosion and scale formation. The fractions obtained in the result of SPTFE thermal destruction using the technique developed at the Institute of Chemistry, Far Eastern Branch, Russian
Academy of Sciences [5] are different from the thermodynamic stability in the temperature ranges realized in the practice.
38

2. THE COMPOSITE ANTISCALE COATINGS ON THE TITANIUM
SURFACE
Antiscale coatings may be formed in the phosphate electrolyte [1, 4]. These layers consist of titanium dioxide in the anataze modification and contain 10 mass % of phosphorus. Such coatings decrease the scaling intensity on the surface of titanium
VT1-0 heat-exchanger, working in the sea water with a heat flow of 0.2 to 0.5
MW/m
2
. The additional treatment of PEO-coating by superdispersed polytetrafluoroethylene was made for creating of a hydrophobic layer on the surface, decreasing its roughness. It is known that the less is film surface roughness the less scale centers crystallization there are on it. The processing of a non-oxidized surface with the polymer powder was not effective because the powder had the poor adhesion to metal. The PEO-coating unlike non-oxidized metal, possessing some roughness and porosity, guarantees good polymer adhesion. The experiment for each type of surface layer was carried out during 96 hours. The statistical processed results for titanium alloy VT1-0 are shown in the Fig. 1.
220 q (kW/m
2
)
200
3 d = 50 m m
(Mg
6
FeCO
3
(OH)
13

4H
2
O+NaCl)
180 2 d = 76 m m
(Mg
6
Fe CO
3
(OH)
13
 4H
2
O+NaCl)
160
1 d = 100 m m
(CaCO
3
+Mg(OH)
2
) t (hour)
0 20 40 60 80 100 120
Fig. 1. Alteration of specific heat flow on titanium VT1-0 (pure 99.4 %) heatexchanger with different surface layers: the natural oxide film (1); PEO-coating (2);
PEO-coating after treatment by superdispersed polytetrafluorethylene (3) in dependence on the time; d – thickness of scale; the composition of scale is written in the bracket.
The following conclusions can be made: the composition of PEO-coatings
(TiO
2
– anataze, 10 mass % P) obtained in the phosphate electrolyte, and their electrochemical (Electrode potential in the sea water E = 0.3 V(NHE)) and semiconductive ( F = – 4.35 eV,

FB
= – 0.05 V(NHE), N
D
= 4.7

10
16 cm
-3
) properties influence on the thickness and composition of scale as comparison with the natural oxide film ( E = 0.1 V(NHE), F = –4.17 eV,

FB
= –0.23 V(NHE), N
D
= 5.9

10
19 cm
-3
).
The more N
D
is, the higher is the intensity of scaling because the concentration of charge carriers as well as energy-band structure and semiconductive properties have essential influence on the adsorption and exchanging processes at the phase boundary between the electrode and solution. But this influence is not the single contribution in the salt-scaling process. It was ascertained that roughness, density, composition of coatings, the hydrophobic properties of surface influence on the scaling process also.
The treatment of coatings by superdispersed polytetrafluoroethylene powder also increases the protective properties of the PEO -coatings (Fig. 1). The treatment by
39

superdispersed polytetrafluorethylene powder decreases the roughness and increases the hydrophobic properties of surface layer (contact angle increase up to 112

,
Table1). Due to composite coating on the heat-exchanger surface the intensity of scaling processes on the phase boundary sharply reduced (Fig. 2) [4] .
Table 1. Wettability of the composition coatings .
Kind of the surface layer Contact angle,

Natural oxide
PEO-coating
PEO-coating processed by PTFE
52
87
99-112 а) b) c)
m = 218 g
m = 197 g
m = 30 g
Fig. 2. The general view of the heat-exchanger and amount of the scale on the its surface with natural oxide film (a); with PEO-coating obtained in phosphate electrolyte (b); with PEO-coating obtained in phosphate electrolyte with subsequent processed by PTFE (c).
3. THE COMPOSITE PROTECTIVE COATINGS ON THE NITINOL
SURFACE
As a result of the performing search for the formation of the protective surface layers the electrolyte containing sodium aluminate NaAlO
2
, sodium carbonate
Na
2
CO
3
, sodium phosphate Na
3
PO
4
∙12H
2
O was chosen. On the basis of earlier developed directed plasma electrolytic synthesis principles [6] the suggestion that in the electrolyte of the given composition on the nitinol surface can be created layers containing the compounds of aluminium (oxides and phosphates) and titanium dioxide TiO
2
. Besides the dimethylglioxime was added into the electrolyte composition as it is known associates nickel ions Ni compound
– nickel dymethylglioxymate Ni(HC
4
2+ in the solution into chelate
H
6
N
2
O
2
)
2
which has red color [7].
It was suggested that nickel cations Ni
2+
forming in the electrolyte solution during the oxidation process under the nickel anodic release from nitinol will connect by dimethylglioxyme in the complex compound which is stable and insoluble in the water electrolyte. According to the proposed mechanism the deposition will stimulate the porosity decreasing of the formed oxide layer and make better its anticorrosion properties. About the dimethylglioxyme presence in the electrolyte composition the colored deposition existence as in the electrolyte cell volume as well on the surface of
40

the sample under anodizing does testify. In accordance with X-ray analysis data in the coating composition the aluminium phosphate AlPO
4
and double oxide of nickelaluminium NiAl
2
O
4
were found.
The PEO layer thickness is about 20 µm. From the SEM images analysis it follows that the surface of layers formed on the nitinol are heterogeneous: there are pores and cracks. It is possible that coatings have cluster character. The analysis of the electrochemical behavior of samples in the artificial physiological solution in accordance with view of polarization curves (Fig. 3) shows that by the most protective properties possess the composition coatings with PEO-layer processed with superdispersed polytetrafluoroethylene (curves 6 and 7).
Fig. 3.
Polarization curves obtained in the artificial physiological solution. Sample markings are in accordance with the Table1 in the article text.
Table 2. Corrosion properties of the investigated nitinol samples with different coatings [8].
№ sample
1
2
Coating forming regime
Without coating
SPTFE (heat treatment at 100°С, 1 h)
R
P
,
 ·сm 2
1.57

10 5
2.34

10 5
I
C
,
А/сm 2
1.66

10 -7
1.11

10 -7
E
C
,
V
-0.215
-0.261
3 PEO unipolar mode , electrolyte: Na
3
PO
4
·12H
2
O – 10 g/l,
NaAlO
2
– 20 g/l, Na
2
CO
3
– 10 g/l
6.39

10 5 4.09

10 -8 -0.262
4
5
6
7
PEO, bipolar mode, electrolyte: Na
3
PO
4
·12H
2
O – 10 g/l,
NaAlO
2
– 20 g/l, Na
2
CO
3
– 10 g/l
6.60

10 6
8.12

10 5 PEO unipolar mode, electrolyte: Na
3
PO
4
·12H
2
O – 10 g/l,
NaAlO
2
– 20 g/l, Na
2
CO
3
– 10 g/l, dimethylglioxyme – 1 g/l
As the sample № 3 + SPTFE
As the sample № 4 + SPTFE
6.05

10 6
3.95
2.34
4.31



10
10
10
-9
-8
-9
9.11

10 6 3.67

10 -9
-0.273
-0.080
-0.055
-0.082
41

Such treatment leads to refinement of open circuit potential (free corrosion potential) ( E
C
) of the sample and to decreasing of corrosion currents ( I
C
) . The addition into the electrolyte solution of dymethilglioxyme leads to some increasing of protective properties (the conclusion was made from the comparison of samples № 3 and 5). The compared corrosion characteristics of nitinol samples processed by various manners are presented in the Table. 2. Ibid the value of the polarization resistance of the sample ( R
P
) calculated on the linear plot of the polarization curve in the open circuit potential range is presented [8].
As it is shown from the experimental results of investigations [4], after pores filling with SPTFE powder which can be used as polymer for the obtaining of smooth hydrophobic surface layers, the thermo - treatment is necessary. In this case polymer seals pores in which it can previously introduce medicines, and thereby does decrease their diffusion from the surface layer. It increases the duration of the therapeutic medicine influence which is in pore. Besides the formed anticorrosion protective coatings to decrease significantly nickel ions release from nitinol, prevent its accumulation in human’s tissues.
4. THE COMPOSITE PROTECTIVE COATINGS ON THE STEEL,
ALUMINIUM AND MAGNESIUM SURFACE
Our data of investigations allows talking about a possibility of the obtaining and application of the composite (including polymer-containing) oxide coatings as anticorrosion protectors of such a material as steel. As a rule the formation of the protective anticorrosion films on the steel surface by means the electrochemical methods is a quite difficult task. Moreover, for the corrosion protection of the galvanic couples such as titanium/steel and titanium/aluminium, used in sea water, the most reliable way is a formation of heterooxide composite insulating coatings on a surface of both a cathode (titanium) and anode (steel, aluminium). One of the essential reasons of decreasing of the protective properties for a coatings obtained in aqueous solutions is their high porosity. So it is the important task a creation of the composite protective films by means of two-stage procedure. First of them is the formation of the developed surface layer with use of the PEO method. Second stage is a subsequent treatment of the obtained layer by superdispersed polytetrafluoroethylene.
The developed early basics of the directed PEO synthesis [6] were taken into account in this work for choose of the electrolyte composition permitting to obtain the protective surface coatings on the steel. As steel intensively corrodes in the acid medium, the oxidation was carried out in the alkaline solutions. The possibilities of the formation on the surface of the steel sample of insoluble in aqueous media compounds which consist of aluminium oxides and spinels, phosphates and carbonates of iron and aluminium as well of another chemical elements including in the electrolyte were taken into consideration. In the work [9] a different electrolyte components included in the electrolyte composition for obtaining protective coatings on the steel surface were considered and found.
As a result of investigation [9] and analysis of experimental data the best electrolyte compositions for formation of the protective coatings have been chosen. In these electrolytes the surface layers have been formed on the low-carbon steel in the direct and alternative current modes of the plasma electrolytic oxidation. In subsequent experiments these coatings were used for formation of the composite coatings with using of the SPTFE.
42

In the table 3 the phase composition and anticorrosion properties of the steel samples with different protective coatings have been shown. According to X-ray analysis the coatings which formed on low-carbon steel St3 by PEO method consist of iron oxides (Fe
2
O
3
, Fe
3
O
4
), iron (III) hexacyanoferrate (II) Fe
4
[Fe(CN)
6
]
3
, aluminum oxides (γ-, δ-Al
2
O
3
), aluminium phosphate (AlPO
4
), and iron and aluminium double oxide (FeAl
2
O
4
). Nevertheless, formed PEO-coatings have a through (i.e. openended) porosity and bad protection properties.
Table 3. Composition and anticorrosion properties of the surface coatings on the St3 low-carbon steel.
№
Mode of treatment, sample electrolyte
Phase composition

R
P
,
·сm 2
I
C
,
А/сm 2
E
C
V
,
1
2 composition, g/l
Without coating
Without PEO, of coating
–
SPTFE
0.873∙10
0.952∙10
3
3 2.988∙10
2.739∙10
–5
–5
–0.551
–0.572 treatment by SPTFE
(200°С, 1 h),
3 monopolar potentiodynamic
Fe
4
[Fe(CN)
6
]
3
0.786∙10
3
3.319∙10
–5 –0.463 mode,
Na
6
P
6
O
18
– 15;
K
4
[Fe(CN)
6
] – 5
4 monopolar potentiodynamic mode,
Ca(H
2
PO
2
)
2
– 15;
Al
2
(C
2
O
4
)
3
– 8.5;
X-ray amorphous phase
0.452∙10
3
5.771∙10
–5 –0.472
KNO
2
– 30
5 combined mode,
Ca(H
2
PO
2
)
2
– 15;
Al
2
(C
2
O
4
)
3
– 8.5;
KNO
2
– 30
Fe
3
O
4
0.871∙10
3
2.994∙10
–5 –0.473
6 monopolar potentiodynamic mode,
Na
6
P
6
O
18
– 15;
NaAlO
2
– 10, tretment SPTFE
FeAl
2
O
4
γ-, δ-Al
2
SPTFE
O
3
4.942∙10
3
5.279∙10
–6 –0.402
7 bipolar mode,
Na
3
PO
4
·12H
2
O – 10;
NaAlO
2
– 20;
K
4
[Fe(CN)
6
] – 3.8, treatment by SPTFE
AlPO
Fe
4
4
,
[Fe(CN)
SPTFE
6
]
3
1.45∙10
5
1.806∙10
–7 –0.331
8 combined mode,
Na
6
P
6
O
18
– 15;
NaAlO
2
– 10, treatment by SPTFE
Fe
2
O
3
δ-Al
2
O
3
SPTFE
4.656∙10
3
5.604∙10
–6 –0.439
In accordance with the table data the formation of the surface coatings in bipolar mode of PEO and subsequent treatment of them by SPTFE lead to creation of
43

the heterostructures (samples 6, 7, 8) with more higher anticorrosion properties than the samples (samples 3, 4, 5) which were not treated by SPTFE. Composite coating with best protective properties (sample 7) consists of aluminium phosphate and iron
(III) hexacyanoferrate (II). Probably, the pores of the coating are filled by insoluble precipitate during PEO process and a subsequent treatment of this layer by SPTFE leads to also increasing its anticorrosion properties.
Fig. 4.
Potentiodynamic curves for the St3 low-carbon steel samples after different types of surface treatment: 1 – without coating, 2 – with polymer coating, without pretreatment by PEO method, 3 – with PEO-coating (sample 3 in Table 3), 4 – with composite coating (sample 6 in Table 3), 5 – with composite coating (sample 7 in
Table 3), 6 – with composite coating (sample 8 in Table 3).
Fig. 5.
Bode plots (dependence of the impedance magnitude (a) and phase angle (b) on frequency) for the St3 low-carbon steel samples after the different surface treatments (notation in Fig. 4).
In order to examine further the effect of the surface treatment on the electrochemical behavior of the samples, potentiodynamic polarization curves were measured in 3 % NaCl (Fig. 4). The analysis of experimental data shows that the composite coatings obtained in alternation (bipolar) PEO mode and treated by SPTFE
(curves 4, 5, 6 in Fig. 4 correspond to samples 6, 7, 8 in Table 3) have a better
44

protective properties than a samples without coating (curve 1), samples treated by
SPTFE only (curve 2 in Fig. 4), and samples formed by PEO method only (curve 3 in
Fig. 4). The differences in the electrolyte compositions and in the mode of PEO, which used for obtaining of samples 3, 4, 5 (Table 3), did not change a location of polarization curves for these samples. Therefore, in Fig. 4 the curve only for the sample 3 have been shown. The complex treatment of the steel surface by PEO method with subsequent application of SPTFE shifts the open circuit potentials ( E
C
) to more noble values and decreases a corrosion current ( I
C
).
Electrochemical impedance spectroscopy allows to investigate the electrode/electrolyte interface. In the Fig. 5 the experimental impedance spectra in
Bode plots (the dependence of impedance magnitude | Z | and phase angle theta versus frequency f ) have been shown for the low-carbon steel samples after various types of the treatment. According to the obtained results, the bipolar mode of PEO for steel leads to increasing of its resistance in corrosion active media. The impedance spectrum for the samples without of coating coincides actually with the one for the coating consisting of SPTFE only (without forming PEO layer) (Fig. 5, curves 1 and
2). Analysis of the impedance spectra and potantiodynamic curves allow to make the following conclusion: the treatment of St3 low-carbon steel surface by superdispersed polytetrafluoroethylene without the creation an appropriate sublayer influences negligible on the state of electrode/electrolyte interface. This treatment decreases only insignificantly the anodic current (Fig. 4). Detail analysis in a combination with the data about the morphological structure of the investigated subjects allows the assuming of the presence of two time constants (excepting the sample without the coating № 1). Such behavior may be explained by low adhesion of the polymer to metal substrate and with imperfection of the continuity of obtained protection layer.
These time constants characterize the heterogeneity of films composition and features of the morphology structure [10]. Creation porous surface may be an additional advantage of PEO method, because developed surface allows realizing the insertion of the various composites and substances in the pores (e.g. hydroxyapatite in the case of the implant surgery) or sealing of the pores in result of polymer fillings.
The impedance magnitude for the sample with PEO-coating after the treatment by
SPTFE becomes higher on two orders in comparison with the sample before treatment. Therefore, processing by SPTFE and as result the filling of the pores by polymer lead to formation of the additional barrier layer which prevents a release of metal ions from steel in a corrosion-active medium.
During present investigation the estimation of the influence of the surface processing of the electrodes of galvanic couple steel St3/VT1-0 by powder of SPTFE on the values of the contact corrosion current was carried out. The steel samples both without coating and with PEO coating processed by SPTFE (samples № 6 in the
Table 3 and curve 4 in the Fig. 6) in the couple with titanium (VT1-0) without and with PEO coating processed by SPTFE were tested. Minimum values of the contact corrosion current were realized in galvanic couples where both surface of the anode and cathode were processed by PEO with subsequent treatment by SPTFE.
The protective properties of the composite layer (PEO+SPTFE) on the surface of steel (curve 2, Fig. 6) are smaller in comparison with the same prepared coating on titanium surface. Nevertheless this composite coating after 7 days of the galvanic couple's work provides the reduction of the corrosion current more than 2 times as compared with not processed steel anode (curve 1, Fig. 6).
45

Fig. 6.
Dependence of the galvanic corrosion current versus time for galvanic couples of low-carbon steel St3/titanium VT1-0: 1 – steel without coating/titanium without coating; 2 – steel with composite coating/titanium without coating; 3 – steel without coating/titanium with composite coating; 4 – steel with composite coating/titanium with composite coating.
Galvanic corrosion data (Fig. 7) testify also about expediency of the using of the oligomeric fractions of the polytetrafluoroethylene for obtaining composite coatings on the surface of the aluminium alloys previously processed by the PEO. The heat treatment is necessary for formation of the coatings with best protective properties. The temperature of this operation depends on softening temperature of the concrete oligomeric fraction and estimated by DTA-TG-DTG method . a) b)
Fig. 7.
Dependence of the galvanic corrosion current versus time for galvanic couples steel St3/aluminium alloy AMg5 – a) and commercial pure titanium VT1-0/aluminium alloy AMg5 – b) with various coatings on the surface of aluminium: 1 – without coating ; 2 – with PEO coating; 3 – with PEO coating processed by SPTFE and subsequent heat treatment at 200
 С for one hour;
4 –with PEO coating processed by high-temperature fraction of SPTFE and subsequent heat treatment at 260
 С for one hour .
Moreover, on the surface of the magnesium alloy MA8 in the silicatecontaining electrolyte and with applying bipolar polarization mode of PEO the protective layers having anticorrosion properties (Fig. 8 and Table 4) in comparison with substrate were formatted. High corrosion resistance is realized due to low
46

defectiveness including low porosity of the surface layers and presence in the coating composition high temperature phase of forsterite (Mg
2
SiO
4
) which was established by
X-ray diffractometry.
Table 4. Some electrochemical characteristics of the various layers on the surface of magnesium alloy MA8.
Kind of sample E с
, V I c
, А/сm 2
R p
,
 /сm 2
Without coating -1.56 5.33·10 -5 489.5
With PEO coating -1.50 7.84·10 -8 3.33·10 5
Thus, in the result of analysis of presented experimental data the following conclusion can be done: the composite polymer-containing coatings formed with using PEO on the surface of the metals and alloys are very perspective for practical applications in the variant fields of science, technique, medicine, etc. a) b)
Fig. 8.
Potentiodynamic curves – a) and Bode plot – b) for magnesium samples without (1), with (2) PEO coating, and (3) with PEO-coating treated by SPTFE (in the insertion of Fig. 8 a) the general views of the samples after corrosion tests are presented).
REFERENCES
1.
Gnedenkov SV, Gordienko PS, Sinebryukhov SL, Khrisanfova OA,
Skorobogatova TM: 'Anticorrosion, antiscale coatings obtained on the surface of titanium alloys by microarc oxidation method and used in seawater'. Corrosion
2000 1 24–31.
2.
Gnedenkov SV, Khrisanfova OA, Zavidnaya AG, Sinebryukhov SL, Kovryanov
AN, Scorobogatova TM, Gordienko PS: 'Production of hard and heat-resistant coatings on aluminium using plasma micro-discharge'. Surface and Coatings technology 2000 123 24–28.
3.
Gnedenkov SV, Khrisanfova OA, Zavidnaya AG, Sinebryukhov SL, Gordienko
PS, Iwatsubo S, Matsui A: 'Composition and adhesion of protective coatings on aluminium'. Surface and Coatings technology 2001 145 146–151.
4.
Gnedenkov SV, Sinebryukhov SL, Mashtalyar DV, Egorkin VS, Tsvetnikov AK,
Minaev AN: 'Charge transfer at the antiscale composite layer–electrolyte interface'.
Protection of Metals 2007 7 667–673.
47

5.
Tsvetnikov AK, Uminskiy AA: Method of the processing of polytetrafluoroethylene, Russian Federation, Pat. No 1775419, November 1992.
(in Russian)
6.
Khrisanfova OA, Volkova LM, Gnedenkov SV, Kaidalova TA, Gordienko PS:
'Synthesis of chemical composition films on titanium in microplasma discharge conditions'. Russian Journal of Inorganic Chemistry 1995 4 558–562. (in Russian)
7.
Pavlov BA, Terent’ev AP: Course of organic chemistry. Moscow, Chemistry.
1969. (in Russian)
8.
Gnedenkov SV, Khrisanfova OA, Sinebryukhov SL, Puz’ AV, Gnedenkov AS:
'Composite protective coatings on the titanium nikelid surface'. Corrosion: materials, protection 2007 2 20–25. (in Russian)
9.
Gnedenkov SV, Khrisanfova OA, Sinebryukhov SL, Puz’ AV: 'Composite protective coatings on the steel surface'. Corrosion: materials, protection 2007 11
27–33. (in Russian)
10.
Gnedenkov SV, Sinebryukhov SL, Sergienko VI: 'Electrochemical Impedance
Simulation of a Metal Oxide Heterostructure/Electrolyte Interface: A Review'.
Russian Journal of Electrochemistry 2006 3 197–211.
48