Effect of Sealing Treatments on the Corrosion Behavior of Micro-arc Oxidation Coating on Aluminum Alloy in Acid NaCl Solution

doi:10.15541/jim20140582

Abstract

Abstract:

Ceramic coatings were fabricated on 7A85 aluminum alloy by unipolar positive pulsed micro-arc oxidation (MAO) treatment for enhancing corrosion resistance. A bi-layer coating was formed with large pores between the layers. The outer layer contained several micro pores and cracks. Therefore, cerium nitrate, potash bichromate and SiO₂ sol were adopted to seal the MAO coatings. Scanning electron microscope, X-ray diffraction and electrochemistry instrument were used to investigate the effect of sealing treatments on the surface morphology, coating structure and corrosion resistance of MAO coatings. Results show that the MAO coating can not protect aluminum alloy from acid NaCl solution for the existing of defects. The defects can be sealed by precipitations produced by cerium nitrate and potash bichromate sealing treatment, and corrosion resistance of the MAO coating is improved. However, the sealed coatings also fail in the acid NaCl solution for dissolution of the precipitation. After sealing with SiO₂ sol, the MAO coating is covered with a SiO₂ gel layer has become a compact barrier layer to protect aluminum substrate from corrosion in acid NaCl solution.

Key words: aluminum alloy, micro-arc oxidation, seal treatment, electrochemistry, corrosion resistance, SiO2 sol

CLC Number:

TQ174

YE Zuo-Yan, LIU Dao-Xin, LI Chong-Yang, ZHANG Xiao-Hua, ZANG Xiao-Ming, LEI Ming-Xia. Effect of Sealing Treatments on the Corrosion Behavior of Micro-arc Oxidation Coating on Aluminum Alloy in Acid NaCl Solution[J]. Journal of Inorganic Materials, 2015, 30(6): 627-632.

Figures/Tables 9

Fig. 1 XRD pattern of the MAO coating on the aluminum alloy

Fig. 2 Section morphology (a) and elements distribution along cross section (b) of MAO coating A-substrate, B-inner layer, C-outer layer

Fig. 3 Surface morphologies of MAO coating before and after sealing (a) MAO; (b) MAO+Ce; (c) MAO+Cr; (d) MAO+Si

Fig. 4 EDS patterns of the precipitations

Fig. 5 Electrochemical impedance spectroscopy (EIS) spectra of MAO coatings (a) MAO with and without sealing treatments; (b) EIS fitting curves of MAO coating; (c) EIS fitting curves of MAO+Ce and MAO+Cr; (d) EIS fitting curves of MAO+Si

Fig. 6 Scheme of MAO coating structure and equivalent circuit of EIS (a) MAO, MAO+Ce and MAO+Cr; (b) MAO+Si

Table 1 Calculated impedance parameters of MAO coatings

	MAO	MAO+Ce	MAO+Cr	MAO+Si
R_out/ (Ω·cm²)	810.9	5.723×10⁴	1.344×10⁵	6.851×10⁵
R_in/ (Ω·cm²)	4236	1.416×10⁴	2.230×10⁵	6.851×10⁵

Fig. 7 Surface morphologies of samples immersed for 24 h (a) MAO; (b) MAO+Ce; (c) MAO+Cr; (d) MAO+Si

Fig. 8 Optical microscope images of MAO and MAO+Si cross- sectional topography (a) MAO; (b) MAO+Si

References 15

[1]	WANG D, NI D R, MA Z Y.Effect of pre-strain and two-step aging on microstructure and stress corrosion cracking of 7050 alloy.Materials Science and Engineering: A, 2008, 494(1): 360-366.
[2]	GUO Q, JIANG B, LI J, et al.Corrosion resistance of micro-arc oxidized ceramic coating on cast hypereutectic alloy.Transactions of Nonferrous Metals Society of China, 2010, 20(11): 2204-2207.
[3]	KUNG K, LEE T, CHEN J, et al.Characteristics and biological responses of novel coatings containing strontium by micro-arc oxidation.Surface and Coatings Technology, 2010, 205(6): 1714-1722.
[4]	RAJ V, MUBARAK ALI M.Formation of ceramic alumina nanocomposite coatings on aluminium for enhanced corrosion resistance.Journal of Materials Processing Technology, 2009, 209(12): 5341-5352.
[5]	GUO H F, AN M Z, HUO H B, et al.Microstructure characteristic of ceramic coatings fabricated on magnesium alloys by micro-arc oxidation in alkaline silicate solutions.Applied Surface Science, 2006, 252(22): 7911-7916.
[6]	WEI T, YAN F, TIAN J.Characterization and wear- and corrosion- resistance of microarc oxidation ceramic coatings on aluminum alloy.Journal of Alloys and Compounds, 2005, 389(1): 169-176.
[7]	TIAN J, LUO Z, QI S, et al.Structure and antiwear behavior of micro-arc oxidized coatings on aluminum alloy.Surface and Coatings Technology, 2002, 154(1): 1-7.
[8]	YU JIE, WEI DONG-BO, WANG YAN, et al.Structure and property of micro-arc oxidation coating modified by laser melting and solidifying on aluminum alloy.Journal of Inorganic Materials, 2013, 28(8): 859-863.
[9]	CHU C L, HAN X, XUE F, et al.Effects of sealing treatment on corrosion resistance and degradation behavior of micro-arc oxidized magnesium alloy wires.Applied Surface Science, 2013, 271: 271-275.
[10]	ZHANG Y, BAI K, FU Z, et al.Composite coating prepared by micro-arc oxidation followed by Sol-Gel process and in vitro degradation properties.Applied Surface Science, 2012, 258(7): 2939-2943.
[11]	ZHU M H, CAI Z B, LIN X Z, et al.Fretting wear behaviors of micro-arc oxidation coating sealed by grease.Wear, 2009, 267(1): 299-307.
[12]	CHENG Y, XUE Z, WANG Q, et al.New findings on properties of plasma electrolytic oxidation coatings from study of an Al-Cu-Li alloy.Electrochimica Acta, 2013, 107: 358-378.
[13]	DECROLY A, PETITJEAN J.Study of the deposition of cerium oxide by conversion on to aluminium alloys.Surface and Coatings Technology, 2005, 194(1): 1-9.
[14]	VEYS-RENAUX D, ROCCA E, MARTIN J, et al.Initial stages of AZ91 Mg alloy micro-arc anodizing: Growth mechanisms and effect on the corrosion resistance.Electrochimica Acta, 2014, 124: 36-45.
[15]	YE Z, LIU D, LI C, et al.Effect of shot peening and plasma electrolytic oxidation on the intergranular corrosion behavior of 7A85 aluminum alloy.Acta Metallurgica Sinica (English Letters), 2014, 27(4): 705-713.