ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2013, Vol. 49, No. 7, pp. 885–890. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.V. Oleinik, V.S. Rudnev, A.Yu. Kuzenkov, T.P. Yarovaya, L.F. Trubetskaya, P.M. Nedozorov, 2012, published in Korroziya: Materialy, Zashchita, 2012, No. 11, pp. 36–41.
CONVERSION COATINGS
Modification of Plasma Electrolytic Coatings on Aluminum Alloys with Corrosion Inhibitors S. V. Oleinika, V. S. Rudnevb, A. Yu. Kuzenkova, T. P. Yarovayab, L. F. Trubetskayaa, and P. M. Nedozorovb aFrumkin
Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russia email:
[email protected] b Institute of Chemistry, Far East Branch, Russian Academy of Sciences, pr. 100Letiya Vladivostoka 159, Vladivostok, 690022 Russia Received May 22, 2012
Abstract—Protective properties of the coatings produced by plasmaelectrolytic oxidation on different alu minum alloys in a phosphate–borate electrolyte are studied. Filling the coatings with corrosion inhibitors of IFKhAN family is shown to substantially improve their corrosion resistance in chloride environments. Mod ification of the coatings with corrosion inhibitors provides an increase in the contribution of hydrophobiza tion to the improvement of the protective characteristics of coatings. Keywords: aluminum alloys, PEO coatings, pitting corrosion DOI: 10.1134/S2070205113070113
INTRODUCTION Plasmaelectrolytic oxidation (PEO), which is anodic or anodic–cathodic electrochemical treat ment of metals under the effect of spark and arc elec tric discharges within the nearelectrode zone, is the next step in conventional electrochemical oxidation methods [1–3]. Today, the method is widely used in producing wearresistant protective coatings on recti fying metals, including aluminum alloys [4, 5]. Such coatings are composed of not only oxides of substrate metals, but also products of the physico chemical transformations of electrolyte components under the effect of electric discharges. The latter pecu liarity enables one to affect the composition and prop erties of coatings in a deliberate manner by varying the composition of the bath [6, 7]. At the same time, such coatings formed under conditions of the breakdown of an oxide film are noticeably porous. On one hand, the defectiveness (porosity) of coatings produced by the PEO method can cause the decrease in their corrosion resistance in aggressive environments; however, on the other hand, the porous structure is a substrate with a substantial adsorption capacity. As was shown in [8–10], filling conversion coatings formed by chemical oxidation on aluminum alloys with corrosion inhibitors noticeably improves their protective characteristics in chloride environments.
Therefore, the effect produced by filling PEO coatings formed on different aluminum alloys with corrosion inhibitors on the protective properties of the coatings is studied in this work. EXPERIMENTAL Coatings were formed on planar specimens (50 × 50 mm) of AMg5, D16, V95, and AMtsM aluminum alloys. Before obtaining coatings, the specimens were chemically polished in an H3PO4 : H2SO4 : HNO3 = 4 : 2 : 1 acid mixture at 110–120°C. Coatings were produced in accordance with the technique outlined in [11]. Electrolyte was prepared from chemically pure reagents taken in the following amounts (g/L): 90 Na3PO4 ⋅ 12H2O, 26 Na2B4O8 ⋅ 10H2O, and 4 Na2WO4 ⋅ H2O. Coatings were formed on the speci mens under conditions of pulse anodic–cathodic polarization at effective anodic and cathodic current densities of 5 A/dm2 and duration of pulses ta = tc = 0.02 s for 10 min. A bath made of a stainless steel served as a cathode. Upon oxidation, the specimens covered with coatings were washed in tap and, then, distilled water and dried in air. The computercontrolled multifunctional current source was assembled on the basis of a serial bidirec tional TER4/460N22UKhL4 thyristor (Russia).
885
886
OLEINIK et al.
energy of 15 keV, a size of the surface spot analyzed of 50 × 50 μm2, and a depth of analysis of 1 μm.
i, µA/cm2 20 1
3
2
4
15 10 5 0 –0.6 –0.4 –0.2
0
0.2
0.4
0.6
0.8 E, V
Fig. 1. Anodic polarization curves recorded in 0.01 M NaCl (pH 7.4) on AMtsM alloy (1) individual and (2–4) covered with PEO coating: (2) original and (3, 4) filled in (3) water and (4) IFKhAN25 solution.
The thickness of the coatings formed was measured with a VT201 calibrator (Russia), and the results are given below: Aluminum alloy AMtsM D16 V95 AMg5
Thickness, µm 14.0 ± 1.0 8.3 ± 1.7 9.9 ± 1.0 12.3 ± 3.5
The coatings were filled with the selected substance by exposing the specimens to solutions containing 1 g/L corrosion inhibitors (IFKhAN25, IFKhAN39, and molybdatophosphoric acid (MPA)) and distilled water (95–100°C) for an hour. Anodic polarization curves of the specimens (with a working surface area of 0.5 cm2) were recorded in a borate buffer solution (pH 7.36) containing 0.01 M NaCl at t = 20 ± 2°C in a conventional threeelectrode cell. An auxiliary electrode was made of pyrographite. Potential of the working electrode was measured with respect to a saturated silverchloride electrode and then recalculated to the normal hydrogen scale. Elec trode polarization (1 mV/s) was started from the free corrosion potential, which was reached in 20–30 min of exposure in the studied baths. Corrosion tests of alloy specimens covered with the coatings studied were carried out in a 3% NaCl solu tion containing 0.1% H2O2 additive. During the experiment, specimens were visually examined every day and the first signs of corrosion destruction were noted. Compositions of the coatings formed on the alloy surfaces were studied with the use of an Xray spectral microanalyzer (CAMEBAX) at an electron beam
RESULTS AND DISCUSSION The corrosion potentials of AMtsM alloy either covered with a PEO coating or not in a chloride solu tion are close and fall in a range of E = –0.35…–0.40 V. As follows from the shape of polarization curves (Fig. 1), local anodic activation potential Epf of the alloy specimen covered with a coating is only 0.2 V higher than that of the unprotected specimen. The small protective effect of the coating is seemingly determined by its noticeable porosity, including through pores. The supposition is confirmed by the decrease in Epf of the specimen covered with a coating filled in dis tilled water to the level typical of the coating on AMtsM alloy without coating. One more reason for the absence of a substantial increase in the protective abil ity of the coatings upon their filling in hot water, in contrast to conversion coatings produced by chemical oxidation [8–10] or conventional anodizing [12], appears to be the structure of aluminum oxides, which are weakly hydrated and, hence, do not make the layer more compact. However, the picture dramatically changes when PEO coatings on AMtsM alloy are filled in a solution containing IFKhAN25 corrosion inhib itor. Epf of the coatings filled in the inhibitorcontain ing solution increases to 0.8 V, and the pittingresis tance range (passivity domain) is 1.4 V (Fig. 1). The increase in the resistance of coating against local anodic activation in chloride environments is due to adsorption of the inhibitor on the coatings, which is confirmed by the Xray spectral surface microanalysis of the original and filled coatings (table). As can be seen in microimages of the coatings, the structure and porosity of the coatings remain almost unchanged upon filling, but the elemental composition of the coating now involves carbon, which indicates the presence of adsorbed organic inhibitor on the surface and in pores of the coating. The structures of coatings formed in D16 and V95 alloys (table) noticeably differ from that of the coating on AMtsM alloy. There is probably no through pores in the former coatings, and the structureforming alumi num oxides can become hydrated during their filling in distilled water, which results in a substantial increase in Epf in a chloride solution (Figs. 2, 3). The resistance of coatings that were formed on V95 alloy and filled in IFKhAN25 or IFKhAN39containing solutions to local anodic activation is still higher in the environ ment studied. The increase in Epf value of the coatings formed on D16 alloy upon exposure to either distilled water or an
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 7 2013
MODIFICATION OF PLASMA ELECTROLYTIC COATINGS ON ALUMINUM ALLOYS
inhibitorcontaining solution is less substantial than those observed on V95 alloy, which is probably related to the different structures and, hence, adsorptivities of the heterogeneous layers. This idea has been con firmed by the results of Xray spectral microanalysis of the coatings formed on V95 and D16 alloys and filled with inhibitors. For example, the surface content of carbon, the presence of which indicates the adsorption of the organic inhibitor, in a coating formed on D16 alloy is smaller compared to that in coatings on V95 alloy (table). Coatings formed on AMgM5 alloy seem to be the most compact among all the coatings studied. As a result, they are characterized by the substantially higher Epf values in the original state (Fig. 4). Upon filling such system in distilled water, the outer layer of the coating solely is probably hydrated and com pacted, which does not lead to a noticeable increase in Epf, in contrast to coatings formed on D16 and V95 alloys. However, upon adding IFKhAN25 or IFKhAN39 inhibitor to the filling solution, the Epf values of the specimens increase by 0.7 and 0.5 V, respectively. The result is provided not only by the protective effect of the inhibitors, but also by the surface activity, which promotes the hydration of aluminum oxides, as follows from the changes in the structure of the coating upon filling (table). The most substantial increase in the resistance to local anodic activation (Epf = 2.5 V) is attained upon filling the coating in a molybdatophos phoric acid solution. Being an oxidizer, the acid pro motes the passivation of defective sites of the porous structure and, in this way, creates an additional barrier to the activation with chlorides. On the whole, the results of corrosion tests of the specimens covered with the coatings studied (3% NaCl + 0.1% H2O2) agree with the data of polariza tion measurements. The first signs of corrosion destruction (pits) were noticed in 2 days on the sur faces of V95 and D16 alloy specimens covered with original coatings and in 4 or 5 days on the coating sur faces of AMg5 and AMtsM alloys. On the coatings formed on all alloys and filled in IFKhAN25con taining solution, the time period before the beginning of pit formation was 7–8 days, and, after additional hydrophobization of the coatings with lowmolecular polytetrafluoroethylene (1–2 μm), the time increased to 15–17 days. Note that a similar hydrophobization of the origi nal coatings (without inhibitor filler) did not result in an improvement of the protective properties—namely, small pits formed on the surfaces in 2–4 days. Filling the coatings formed in D16 and V95 alloys in IFKhAN39containing solution increased the time period before the beginning of pit formation to 11 and
i, µA/cm2 20
3
1
887
4
2
15 10 5 0 –0.9 –0.6 –0.3
0
0.3
0.6
1.2
0.9
1.5 E, V
Fig. 2. Anodic polarization curves recorded in 0.01 M NaCl (pH 7.4) on D16 alloy covered with PEO coating: (1) original and (2–4) filled in (2) water and (3, 4) solutions containing (3) IFKhAN25 and (4) IFKhAN39.
i, µA/cm2 20
3
2
1
4
15 10 5 0 –1.0 –0.6 –0.2 0.2
0.6
1.0
1.4
1.8
2.2 E, V
Fig. 3. Anodic polarization curves recorded in 0.01 M NaCl (pH 7.4) on V95 alloy covered with PEO coating: (1) original and (2–4) filled in (2) water and (3, 4) solutions containing (3) IFKhAN25 and (4) IFKhAN39.
i, µA/cm2 15 1 2
10
4
5
2.5
3.5 E, V
3
5 0 –1.5
–0.5
0.5
1.5
Fig. 4. Anodic polarization curves recorded in 0.01 M NaCl (pH 7.4) on AMg5 alloy covered with PEO coating: (1) original and (2–5) filled in (2) water and (3–5) solu tions containing (3) IFKhAN25, (4) IFKhAN39, and (5) molybdatophosphoric acid.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49, No. 7 2013
888
OLEINIK et al.
Surface microimages (800×) and elemental composition of coatings formed on different alloys Alloy
Filling
Surface microimages of PEO coatings
Content of main elements, wt % P 2.7; Mn 0.4; W 1.8; Na 0.4; Al 49.0
No filling
40 µm AMtsM
P 2.4; Mn 0.4; W 2.3; Na 0.4; Al 45.2; C 3.3
IFKhAN25
40 µm P 5.9; Mn 0.3; W 2.9; Na 0.7; Al 36.7
No filling
40 µm
D16
P 5.5; Mn 0.4; W 3.0; Na 1.1; Al 42.9; C 13.5
IFKhAN25
40 µm P 4.2; Mn 0.3; W 2.8; Na 0.5; Al 37.9; C 11.8
IFKhAN39
40 µm PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 7 2013
MODIFICATION OF PLASMA ELECTROLYTIC COATINGS ON ALUMINUM ALLOYS
889
Table. (Contd.) Alloy
Filling
Surface microimages of PEO coatings
Content of main elements, wt % P 5.9; W 3.0; Na 0.7; Al 42.5
No filling
40 µm
V95
P 4.7; W 2.3; Na 1.4; Al 24.5; C 27.7
IFKhAN25
40 µm P 5.4; Mg 3.4; W 0.9; Na 29.8; Al 18.8
IFKhAN39
40 µm P 4.4; Mg 0.5; W 2.9; Na 0.6; Al 39.9
No filling
40 µm AMg5
P 2.6; Mg 0.4; W 1.8; Na 0.4; Al 33.4; C 21.6
IFKhAN25
40 µm * The residual is oxygen.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49, No. 7 2013
890
OLEINIK et al.
15 days, respectively, while the coating formed on AMg5 alloy and treated in a molybdatophosphoric acid solution was resistant to pitting corrosion destruction for 15 days. CONCLUSIONS 1. Coatings formed by the PEO method in a phos phate–borate electrolyte on aluminum alloyed with manganese or magnesium (AMtsM and AMg5 alloys) exhibit a slightly higher resistance against pitting cor rosion in chloride environments than do coatings formed on D16 and V95 coppercontaining aluminum alloys. 2. Filling the coatings in an aqueous solution con taining a corrosion inhibitor substantially improves their protective characteristics in chloride environ ments. An increase in the protective properties is addi tionally provided by hydrophobization of coatings with lowmolecular polytetrafluoroethylene. REFERENCES 1. Gunterschulze, A, and Betz, G., ElektrolytKondensa toren, Berlin: Herbert Cram, 1937. 2. Zakgeim, L.N., Elektroliticheskie kondensatory (Elec trolytic Capacitors), Moscow: Gosenergoizdat, 1963. 3. Tareev, V.M. and Lerner, M.M., Oksidnaya izolyatsiya (Oxide Insulation), Moscow: Energiya, 1964.
4. Markov, G.A., Terleeva, O.P., and Shulepko, E.K., in Sbornik trudov Instituta neftekhimicheskoi gazovoi promyshlennosti im. I.M.Gubkina (Collected Papers of Gubkin Institute of Petrochemical Gas Industry), Moscow: 1985, no. 185. 5. Jiang, B.L. and Wang, Y.M., in Surface Engineering of Light Alloys, Dong, H., Ed., Cambridge: Woodhead, 2010, p. 110. 6. Gordienko, P.S. and Rudnev, V.S., Elektrokhimicheskoe formirovanie pokrytii na alyuminii i ego splavakh pri potentsialakh iskreniya i proboya (Electrochemical For mation of Coatings on Aluminum and Its Alloys at Spark and Breakdown Potentials), Vladivostok: Dal’nauka, 1999. 7. Rudnev, V.S., Zashch. Met., 2008, vol. 44, no. 3, p. 283. 8. Kuzenkov, Yu.A. and Oleinik, S.V., Korroz.: Mater. Zashch., 2008, no. 11, p. 38. 9. Zimina, Yu.M., Kuzenkov, Yu.A., and Oleinik, S.V., Korroz.: Mater. Zashch., 2010, no. 7, p. 44. 10. Kuzenkov, Yu.A., Oleinik, S.V., Karimova, S.A., and Pavlovskaya, T.G., Korroz.: Mater. Zashch., 2011, no. 10, p. 42. 11. Rudnev, V.S., Yarovaya, T.P., and Nedozorov, P.M., RF Patent no. 2263163, S25V11/02, Byull. Izobret., 2005, no. 30. 12. Sheasby, P.G. and Pinner, R., The Surface Treatment and Finishing of Aluminum and Its Alloys, New York: ASM International, 2001, vol. 2.
Translated by Yu. Novakovskaya
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 7 2013
