JOURNAL OF RARE EARTHS, Vol. 29, No. 10, Oct. 2011, P. 961 Foundation item: Project supported by National Research Network of the National Council for Science and Technology of Mexico (228198) Corresponding author: KIYOTA Sayuri (E-mail: sayuri684@gmail.com; Tel.: +52 686 566 41 50) DOI: 10.1016/S1002-0721(10)60579-0 Anticorrosion behavior of conversion coatings obtained from unbuffered cerium salts solutions on AA6061-T6 KIYOTA Sayuri 1 , VALDEZ Benjamin 1 , STOYTCHEVA Margarita 1 , ZLATEV Roumen 1 , BASTIDAS Jose Maria 2 (1. Engineering Institute of UABC, B. Juárez Blvd. and Normal St., 21280 Mexicali, B.C., Mexico; 2. CENIM-National Centre for Metallurgical Research, CSIC, 8 Gregorio del Amo Ave., 28040 Madrid, Spain) Received 18 April 2011; revised 8 August 2011 Abstract: The anticorrosive properties of cerium based conversion coatings deposited on AA6061-T6 alloy by immersion in unbuffered ce- rium chloride and cerium nitrate solutions in the presence of hydrogen peroxide were investigated and characterized by potentiodynamic po- larization (PDP) and electrochemical impedance spectroscopy (EIS) in 0.5 mol/L NaCl aqueous solution. The microstructure and chemical composition of the protective films were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). It was found that the best corrosion protection was afforded by the samples treated during 600 s in cerium chloride solution at pH values ∼5.5–4, showing higher amounts of cerium and polarization resistance values greater than 10 Ω m 2 . Moreover, an ennoblement of the corro- sion potential and decreasing of the cathodic and anodic currents were obtained compared with the cerium nitrate solutions application. This behavior was attributed to the influence of the deposition parameters such as type of the salt anion, i.e., chelating effect and chaotropic char- acteristics, pH fluctuations in the conversion solution and deposition time. Keywords: cerium based conversion coatings; EIS; PDP; anticorrosion properties; rare earths Aluminum and its alloys are widely used as structural materials due to their low cost and excellent mechanical properties [1] . However, the heterogeneous microstructure of aluminum alloys makes them liable to localized corrosion such as pitting, affecting their performance [2] . The corrosion protection of aluminum alloys components in the military, aerospace and avionics industries is provided by systems that generally consist in chromate conversion coatings, primers and paintings, where each film has a specific role in the whole protection mechanism. Chromate conversion coatings (CrCC) have been used as anticorrosive treatments in the industry for over 50 years. Despite their undeniable qualities and advantages, such conversion treatments should be re- placed since Cr 6+ is highly toxic, carcinogenic and provokes effluent disposal problems [3–6] . Environmentally benign alternatives to CrCC have been investigated as potential replacements and include anodiz- ing [7] , rare earth inhibitors and coatings [8–11] , and sol-gels [12,13] , among others. A number of studies demonstrate that the ce- rium-based conversion coatings in particular could be con- sidered as one of the more promising chromate coatings sub- stitutes, as cerium compound do not pose environmental problems and the coatings provide an efficient corrosion protection close to that of chromium [14–21] . Two different techniques of cerium-based conversion coatings (CeCC) deposition have been investigated. The first one, described in the earlier studies, involves the formation of conversion coatings onto the metal surface for several days in simple cerium salt solution at near neutral pH [16,20–29] . Obviously, the method is time-consuming. This drawback has been eliminated with the application of another tech- nique: faster treatments (<600 s) with cerium salt in the presence of hydrogen peroxide as accelerating agent at pH 2 [11,19,24,30–35] . Nevertheless, pH maintaining in both cases ei- ther by solutions buffering or pH adjustment presents nu- merous problems. For instance, Pardo et al. [36] reported the formation, at pH 6, of precipitates resulting from the interac- tion of Ce 3+ with buffer components affecting the conversion process, while the acidifying agents as HCl or HNO 3 favor the formation of complexes with Ce 3+ in solution [37] . In addi- tion, nitrate ions induce the passivation of the aluminum al- loy surface via incorporation into the surface oxide [38] . Therefore, the goal of this work was to characterize the coatings deposited from H 2 O 2 assisted cerium salt solutions without any pH correction in terms of chemical composition, morphology and anticorrosion properties in particular, and analyze the effect of changes induced by H 2 O 2 and cerium salt anions. The formulations and the substrate used were respectively CeCl 3 ·7H 2 O and Ce(NO 3 ) 3 ·6H 2 O salts, and the widely applied aluminum alloy AA6061-T6. To the best of our knowledge, investigations on the deposition of CeCC on AA6060-T6 at the above mentioned conditions and on the performances of such coatings have not been reported until now.