ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2014, Vol. 50, No. 3, pp. 360–362. © Pleiades Publishing, Ltd., 2014. Original Russian Text © V.S. Rudnev, D.L. Boguta, T.P. Yarovaya, P.M. Nedozorov, 2014, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2014, Vol. 50, No. 3, pp. 313–315.
NANOSCALE AND NANOSTRUCTURED MATERIALS AND COATINGS
Coatings Based on Niobium Oxides and Phosphates Formed on Niobium Alloy V. S. Rudneva, b, D. L. Bogutaa, T. P. Yarovayaa, and P. M. Nedozorova a
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, pr. 100Letiya Vladivostoka 159, Vladivostok, 690022 Russia b Far Eastern Federal University, ul. Sukhanova 8, Vladivostok, 690950 Russia email:
[email protected],
[email protected] Received April 16, 2013
Abstract—Coatings were formed by plasma electrolytic oxidation in electrolytes containing Ni(II) hexamet 6− ]/[Ni2+] molar ratio in the electrolyte, layers with spe aphosphate complexes. Depending on the n = [P6O18 cific niobium, phosphorus, and nickel contents and of a particular thickness were produced. Upon annealing in air at a temperature of 500 or 800°C, either Nb2O5, or Nb2O5 + NbO(PO4) + P2O5 ⋅ 9Nb2O5, or Nb2O5 + NbO(PO4), or Nb2O5 + Ni2P2O7 crystal phases are formed in the coatings depending on the electrolyte com position and temperature of annealing. These surface systems formed on niobium for 10 min at room tem perature as a result of a onestage process in ecologically safe electrolytes can be catalytically active. DOI: 10.1134/S2070205114030149
INTRODUCTION Catalysis is a field of application of niobium com pounds [1]. Diverse oxide and/or phosphate niobium compounds, including mixed ones (with different MI and MII metals, particularly NiII) are used as catalysts and catalyst carriers in condensation, hydration, and esterification reactions, as well as in photolysis of water. In scientific literature, one can find numerous publications devoted to the investigation of the cata lytic activity of niobium compounds in different model processes and the development of new methods of their production [1–4]. Plasma electrolytic oxidation (PEO) of metals and alloys is a relatively simple method of the formation of inorganic coatings on substrates of diverse, including complex geometrical shapes [5, 6]. PEO of the anodi cally or alternately anodically cathodically polarized surfaces of metals and alloys is carried out at voltages that cause spark and arc electric discharges near anode zones. Discharges initiate the formation of hightem perature phases within the growing oxide layer, inclu sion of electrolyte components in the layer, synthesis of compounds based on the electrolyte components and substrate elements, and inclusion of thermolysis products of the electrolytic deposit and solid particles from the electrolyte into the coating [6, 7]. One developed method of the production of oxide–phosphate coatings on the surfaces of valve metals (Al, Ti, Mg, Zr, Nb, etc.) is the plasmaelectro lytic oxidation in electrolytes containing polyphos phate complexes of М2+, М3+, or M4+ metals [8–10]. In this case, the thickness, composition, and mor
phology of coatings can be controlled by varying the n = [polyphosphate]/[bi, tri, or tetravalent metal cation] molar ratio in the electrolyte. Regularities of plasmaelectrolytic oxidation in electrolytes containing polyphosphate complexes of variously charged metal cations have been studied pre viously chiefly on aluminum and titanium specimens. The object of this work is to investigate composition and certain characteristics of coatings formed by plasma electrolytic oxidation on niobium in electro lytes containing polyphosphate Ni(II) complexes. EXPERIMENTAL Aqueous electrolyte contained 30 g/L Na6P6O18 sodium hexametaphosphate and certain amounts of Ni(CH3COO)2 acetate to provide the desired n = 6− [P6O18 ]/[Ni2+] ratios. The calculated n = 6− [P6O18 ]/[Ni2+] molar ratio of the components in the electrolyte varied in a range from 10 to 0.5. Electro chemical cell for anodizing involved a 1L glass vessel, a hollow coil pipe cathode made of nickel alloy, and a magnetic agitator. Cold tap water was run through the coil pipe for cooling the solution. The temperature of the electrolyte during the PEO procedure did not exceed 30°C. Coatings were formed under anodic galvanostatic conditions at current density j = 0.05 A/cm2 for 10 min with the use of a computercontrolled multifunctional current source based on a commercial TER4/460N thyristor (Russia). Specimens were plates with a size of 10 × 50 × 0.15 mm made of niobium alloy (96% Nb),
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which were not subjected to preliminary treatment (chemical cleaning) of the surface. The thickness of the layers was measured with a VT201 vortexcurrent gauge (Russia). Elemental composition was determined with a JXA 8100 Xray spectral microanalyzer (Japan) with an INCA energy dispersive system (United Kingdom). To prevent sur face charging, specimens were preliminarily spray coated with carbon. Xray patterns were obtained with a D8 Advance diffractometer (Germany) in СuКα radiation. For Xray phase analysis, EVA search pro grams with the PDF2 database were used.
n = 10 n = 7.5 n = 5
361
n=3
n=2
n = 1.5
n=1
n = 0.5
Fig. 1. Images of coatings formed on niobium alloy in elec trolytes with different n molar ratios.
40
2.0 1.5
C, wt %
With an increase in the nickel acetate content in the electrolyte, the color of coatings formed changes from gray to dark brown (Fig. 1). Concurrently, the fraction of electrolyte elements in the coatings increases and the coatings become thicker at the same charge spent on the process (Figs. 2, 3). The most sub stantial changes in the thickness and elemental com position of the coatings are observed when electrolyte becomes colloidal (at n < 1.5); i.e., when solid colloi dal particles of hydrated Ni(II) polyphosphates are precipitated [8–10]. On the whole, the general shapes of the dependences of the elemental composition and thickness of coatings formed on niobium on n = 6− [P6O18 ]/[Ni2+] ratio are similar to the dependences previously obtained for coatings formed on aluminum and titanium alloys in the electrolytes containing Na6P6O18 and acetates of different M(II), M(III), or M(IV) metals [8–10]. Nevertheless, by contrast to the coatings formed in the same electrolyte on aluminum alloys [11], there are certain peculiarities of those pro duced on niobium alloy. (i) The content of substrate metal in coatings obtained from electrolytes with small n values (n < 3) is increased. For example, in an electrolyte with n = 0.5, coatings on aluminum contain less than 1 wt % aluminum, whereas, in the case of niobium, its con tent is never smaller than 10 wt %. (ii) The content of phosphorus in coatings formed in electrolytes with n > 3 is heightened. In the case of aluminum alloy, with a decrease in n from 10 to 3, the phosphorus concentration monotonically increases from 3–4 to 12–13 wt %, while, in polyphosphate electrolytes, which contained acetates of foreign M(II) metals (Mg, Mn, Pb, or Zn), it remains around 4–5 wt %. In the case of niobium alloy under the same conditions, the concentration of the metal remains about 10–11 wt % and begins to increase only with a further decrease in the n ratio in the electrolyte. (iii) The thickness of coatings in the whole n range considered is larger. Thickness of coatings formed on aluminum alloys in electrolytes containing Na6P6O18 and nickel acetate (or acetates of foreign metals) under the same conditions (the same current density
20
1.0
CNa, wt %
RESULTS AND DISCUSSION
0.5
2
4 Nb
6 Ni
n
8 P
Na
Fig. 2. Elemental composition of coatings formed on nio bium alloy according to the data of Xray spectrum analy sis (the residual is oxygen).
h, μm
40
20
2
4
6
8
n
Fig. 3. Thickness of coatings formed on niobium alloy depending on the n ratio in the electrolyte.
and the same duration of the process) is 8–10 µm within an n range of 10–3 and monotonically increases to 20–30 µm depending on the nature of M(II) metal with a decrease in n to 0.5. The thickness of the coatings on niobium alloy is twice as large at n of 10–5. In the h = f(n) dependence (Fig. 3), there is a maximum at n ≈ 3. When the electrolyte becomes col loidal, the thickness of coatings sharply increases to exceed 40 µm.
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Phase composition of coatings formed on niobium alloy n
Phase compositions of original and annealed coatings original
300°C
500°C
X/a X/a Nb2O5 X/a X/a Nb2O5 X/a X/a Nb2O5 X/a X/a Nb2O5 X/a X/a Nb2O5 X/a X/a Nb2O5 X/a X/a X/a X/a X/a X/a X/a denotes Xray amorphous specimens; specimens were annealed in air for 2 h.
10 7.5 5 3 2 1.5 1 0.5
800°C Nb2O5 Nb2O5 + NbO(PO4) + P2O5 ⋅ 9Nb2O5 Nb2O5 + NbO(PO4) + P2O5 ⋅ 9Nb2O5 Nb2O5 + NbO(PO4) Nb2O5 + NbO(PO4) Nb2O5 + NbO(PO4) Nb2O5 + Ni2P2O7 Nb2O5 + Ni2P2O7
Results of Xray analysis of the coatings, including those upon annealing at different temperatures, are listed in the table. The original specimens, as well as those annealed at 300°С, are Xray amorphous. Annealing at 500°С results in crystallization of Nb2O5 phase within coatings formed in electrolytes with n ratios of 10–1.5. The phase composition of the coat ings annealed at 800°С substantially depends on the n value of the electrolyte (table). Additionally to Nb2O5, the coatings contain NbO(PO4) oxyphosphate and P2O5 ⋅ 9Nb2O5 double oxide at n ratios of 7.5–1.5. In the coatings formed at n = 1 and 0.5, Nb2O5 oxide and Ni2P2O7 nickel pyrophosphate are crystallized.
ture of niobium oxide and M(II) phosphate are crystal 6− lized within coatings depending on n = [P6O18 ]/[Ni2+] molar ratio in the electrolyte. The compounds crystal lized under these conditions are seemingly present in the original coatings in an amorphous state. The sur face systems, which contain either niobium oxide solely or niobium oxide and M phosphates, produced on niobium during 10 min at room temperature as a result of onestage plasmaelectrolytic oxidation can be catalytically active.
Taking into account the previously obtained phase compositions of coatings formed on aluminum and titanium [8–11] and the results of this work, we can state that it is Xray amorphous PEO layers that are formed on aluminum, titanium, and niobium in elec trolytes containing polyphosphates and acetates of different M(II) metals. Crystalline oxides and phos phates are found in the coatings only upon hightem perature annealing.
This work was financially supported by the Presid ium of the Far East Branch of the Russian Academy of Sciences.
Thus, plasmaelectrolytic oxidation of niobium in electrolytes containing Ni(II) hexametaphosphate complexes enables one to produce coatings with spe cific niobium, phosphorus, and nickel contents and particular thickness depending on the n = 6− [P6O18 ]/[Ni2+] molar ratio in the electrolyte. The reg ularities of the changes in the elemental composition (EC = f(n)) and thickness (h = f(n)) of the coatings are generally similar to those observed on aluminum and titanium with certain differences. Based on data obtained previously for aluminum and titanium spec imens [8–11], one can expect the formation of coat ings on niobium that involve metals of different nature and their mixtures as a result of plasmaelectrolytic oxidation in electrolytes containing polyphosphate complexes of variously charged metals. As follows from the results obtained, upon annealing in air at temperatures of 500 or 800°C, either niobium oxide, or a mixture of niobium oxide and phosphate, or a mix
ACKNOWLEDGMENTS
REFERENCES 1. Chernyshkova, F.A., Russ. Chem. Rev. 1993, vol. 62, p. 743. 2. Nowak, I. and Ziolek, M., Chem. Rev., 1999, vol. 99, no. 12, p. 3603. 3. Tanabe, K., Catal. Today, 2003, vol. 78, nos. 1–4, p. 65. 4. Carniti, P., Gervasini, A., Biella, S., and Auroux, A., Catal. Today, 2006, vol. 118, nos. 3–4, p. 373. 5. Yerokhin, A.L., Nie, X., Leyland, A., et al., Surf. Coat. Technol., 1999, vol. 122, p. 73. 6. Walsh, F.C., Low, C.T.J., et al., Trans. Inst. Metal Fin ish., 2009, vol. 87, no. 3, p. 122. 7. Rudnev, V.S., Prot. Met., 2008, vol. 44, no. 3, p. 263. 8. Rudnev, V.S., Yarovaya, T.P., Boguta, D.L., et. al., Russ. J. Electrochem., 2000, vol. 36, no. 12, p. 1291. 9. Rudnev, V.S., Yarovaya, T.P., Boguta, D.L., et. al., J. Electroanal. Chem., 2001, vol. 497, nos. 1–2, p. 150. 10. Rudnev, V.S., Boguta, D.L., Kilin, K.N., et al., Russ. J. Phys. Chem. A, 2006, vol. 80, no. 8, p. 1350. 11. Boguta, D.L., Rudnev, V.S., Yarovaya, T.P., et al., Russ. J. Appl. Chem., 2000, vol. 73, no. 8, p. 1368.
Translated by Y.V. Novakovskaya
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 3 2014
