Effect of Micro Arc Oxidation Coatings on Corrosion Resistance of 6061-Al Alloy Nitin P. Wasekar, A. Jyothirmayi, L. Rama Krishna, and G. Sundararajan (Submitted June 14, 2007; in revised form January 8, 2008) In the present study, the corrosion behavior of micro arc oxidation (MAO) coatings deposited at two current densities on 6061-Al alloy has been investigated. Corrosion in particular, simple immersion, and potentiodynamic polarization tests have been carried out in 3.5% NaCl in order to evaluate the corrosion resistance of MAO coatings. The long duration (up to 600 h) immersion tests of coated samples illustrated negligible change in weight as compared to uncoated alloy. The anodic polarization curves were found to exhibit substantially lower corrosion current and more positive corrosion potential for MAO-coated specimens as compared to the uncoated alloy. The electrochemical response was also compared with SS-316 and the hard anodized coatings. The results indicate that the overall corrosion resistance of the MAO coatings is significantly superior as compared to SS316 and comparable to hard anodized coating deposited on 6061 Al alloy. Keywords Al 2 O 3 coating, corrosion, hard anodizing, micro arc oxidation, potentiodynamic polarization 1. Introduction Micro arc oxidation (MAO) is an emerging ecofriendly coating technique capable of forming ceramic coatings on metals such as Al, Mg, Ti, and their alloys (Ref 1-5). The ceramic coatings deposited using MAO technique exhibit superior tribological properties (Ref 6-9). Previous studies on MAO coatings have been mostly devoted to the synthesis, analysis, and tribological behavior of these coatings (Ref 7-11). Much of the corrosion-related studies have been confined to MAO coatings formed on Mg and Ti alloys substrate (Ref 12-15). However, there exist a few studies investigating the corrosion behavior of MAO coatings, formed on Al alloys (Ref 16-18). Kuhn (Ref 16) demonstrated the improved corrosion resistance of MAO coatings compared to hard anodized coatings under salt spray test conditions. Nie et al. (Ref 17) studied the corrosion properties of 250-lm-thick MAO coating deposited at 0.1 A/cm 2 on BS 6082 Al alloy for different immersion periods in 0.5 M NaCl solution up to 48 h. Though the presence of occluded porosity in the MAO coating was confirmed by TEM, it did not affect the corrosion resistance. Recent study by Barik et al. (Ref 18) illustrates the importance of sealing the pores in MAO coatings by sol–gel technique in enhancing the short-term corrosion resistance in 0.6 M NaCl solution. The above investigation also revealed that the MAO coatings (unsealed) did not improve the corrosion resistance of Al alloy because of the presence of through thickness coating defects. Thus, there exist conflicting data on the effectiveness of MAO coatings formed on Al alloys against the aqueous corrosion. It has already been demonstrated that the current density plays a significant role on the MAO coating deposition kinetics and development (Ref 7, 10). It has been found that a-alumina content increases with an increase in current density in MAO coatings (Ref 19). So far the effect of current density on long- term corrosion behavior of MAO coatings deposited on Al alloys has not been studied. Moreover, a recent study by Curran et al. (Ref 20) found that the MAO coatings characterized by up to 20% of surface connected porosity which is expected to substantially influence the corrosion properties of the coatings. The MAO coatings deposited at the authorÕs laboratory were found to be dense and have exhibited excellent tribological properties (Ref 7, 21). However, the corrosion properties of these coatings need to be studied in order to establish the suitability of these coatings for applications requiring corrosion resistance in addition to wear resistance. In the view of above, the major objective of the present study is to evaluate the overall effectiveness of MAO coatings in terms of resistance to aqueous corrosion. Toward the above objective, MAO coatings have been deposited on a 6061 Al alloy at current densities of 0.1 and 0.3 A/cm 2 and their corrosion behavior evaluated in 3.5% NaCl solution. The corrosion resistance has been evaluated using weight loss (immersion) and the potentiodynamic polarization technique. In addition, the corrosion performance of MAO coatings has been compared with hard anodized coatings deposited on 6061 Al alloy as well as SS316L. 2. Experimental Details 2.1 MAO Coating Procedure A 6061-T6 Al alloy having the nominal composition 1.1% Mg, 0.7% Si, 0.1% Cu, 0.6% Fe, 0.15% Mn, 0.2% Zn, 0.016% Ti, Nitin P. Wasekar, A. Jyothirmayi, L. Rama Krishna, and G. Sundararajan, International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Balapur (P.O.), Hyderabad 500005, India. Contact e-mail: nitin.arci@gmail.com. JMEPEG (2008) 17:708–713 ÓASM International DOI: 10.1007/s11665-008-9222-8 1059-9495/$19.00 708—Volume 17(5) October 2008 Journal of Materials Engineering and Performance