Rare Earth Conversion Coatings Grown on AA6061 Aluminum Alloys: Corrosion Studies

Rare Earth Conversion Coatings Grown on AA6061 Aluminum Alloys. Corrosion Studies Article J. Mex. Chem. Soc. 2014, 58(4), 393-410 393 © 2014, Sociedad Química de México ISSN 1870-249X Rare Earth Conversion Coatings Grown on AA6061 Aluminum Alloys. Corrosion Studies Silvia Beatriz Brachetti-Sibaja,1 Miguel Antonio Domínguez-Crespo,2* Aidé Minerva Torres-Huerta,2 Edgar Onofre-Bustamante,2 and Wencel De La Cruz-Hernández3 1 Instituto Tecnológico de Ciudad Madero (ITCM), Av. 1º de Mayo s/n, Col. 1º de Mayo. Cd. Madero 89650 Tamps. México. Instituto Politécnico Nacional, CICATA-Altamira km 14.5, Carretera Tampico-Puerto Industrial Altamira. Altamira 89600 Tamps. México. . 3 Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, A. P. 2681, Ensenada 22800 B.C., México. 2 Received September 27th, 2013; Accepted June 10th, 2014 Abstract. The present work is aimed to investigate the corrosion resistance of rare earth (RE) protective coatings deposited by spontaneous deposition on AA6061 aluminum alloy substrates. Coatings were deposited from water-based Ce(NO3)3 and La(NO3)3 solutions by varying parameters such as rare earth solution concentration, bath temperature and immersion time. The values of the Tafel slopes indicate that the cathodic process is favored by concentration polarization rather than activation polarization. Keywords: Aluminum alloy, Chemical conversion coatings, Rare earth elements, Corrosion evaluation, Cerium oxide, Lanthanum oxide. Resumen. Este trabajo tiene como objetivo la investigación de la resistencia a la corrosión de recubrimientos de tierras raras mediante el proceso de inmersión, depositadas en aleaciones comerciales de aluminio AA6061. Para ello, se utilizaron diferentes soluciones de Ce(NO3)3 y La(NO3)3 evaluando parámetros tales como: la concentración de la solución, temperatura y tiempo de inmersión. Los valores de las pendientes de Tafel y su comportamiento indican que el proceso catódico está favorecido por una polarización por concentración más que la polarización por activación. Palabras clave: Aluminio, Tratamientos de conversión química, Tierras raras, Corrosión, Óxido de cerio, Óxido de lantano. Introduction tive for substituting the commonly used chromate conversion coatings, not only because of their effectiveness but also due to their low toxicity compared to chromates [12-18]. Lanthanide elements are characterized by large atomic radii, stable electronic configurations, multiple oxidation states, typically +3 and +4 and occasionally +2, reactivity with water to form a neutral oxide; formation of stable, insoluble oxides of mixed valence states; complex coordination chemistry; instability of lower valence salts in alkaline conditions with a tendency to hydrolyze and precipitate as the hydrated oxide; and an extremely low reduction potential [19-21]. The chemical properties of rare earth conversion coatings (RECCs) function mainly as barrier coatings suppressing the cathodic half-reaction. In comparison with chromate treatments, RE elements form durable aluminum alloy surface conversions but do not passivate anodic corrosion reactions. Therefore, improvements in transport control, electrochemistry, and solubility of the species involved in corrosion inhibition are required to attain the performance of chromate conversion coatings [1, 2, 22, 23]. Among the lanthanide compounds, lanthanum, cerium, praseodymium, and ytterbium have often been examined as the most prevalent and inexpensive of the rare earth metals, and of these, cerium and lanthanum salts have been used successfully as corrosion inhibitors on different metals and aluminum alloys. RE conversion coatings can be applied by different methods, which include immersion, spraying, brushing, swabbing, and electrolytic [24-29]. The solutions used in the deposition of rare earth conversion coatings generally contain a cerium Aluminum and its alloys are widely used in engineering applications such as aeronautics and construction due to their low density, favorable mechanical properties, and relatively good corrosion resistance. Aluminum alloys are very reactive materials capable of forming in moist air a robust, protective thin film of aluminum oxide to minimize extensive corrosion. However, this native oxide layer remains vulnerable under conditions different from its isoelectric point, where the metal ion or its oxo-anions are soluble, leaving bare aluminum exposed to acidic and extreme basic pH, which regularly provokes localized corrosion [1-10]. These kinds of alloys are used in a wide variety of fields such as the automobile, aeronautical, aerospace, and electronic industries, among others. In these industries, the use of pretreatments prior to painting or adhesive bonding is an essential technology to prevent local corrosion [1-10]. Each pretreatment type produces a surface coating that not only provides a first defense against corrosion, but also provides adhesion that is needed for primer and top coating performance; i.e., pretreated coatings can be used alone or in conjunction with over-coatings (organic primers and top coatings), which add physical durability and generally improved corrosion protection. Then, conversion coatings serve to structurally and chemically stabilize and control the interfacial properties of the aluminum substrate to allow a predictable, stable performance of the coated system [11]. In this way, several studies have been focused on the use of rare earth (RE), or lanthanide compounds as a green alterna- 394    J. Mex. Chem. Soc. 2014, 58(4) Silvia Beatriz Brachetti-Sibaja et al. or lanthanum salt such as cerium or lanthanum nitrate, where the deposition mechanism for these coatings involves both the oxidation of aluminum and formation of hydroxyl ions: X-ray photoelectron spectra (XPS) have been used to determine the nature and content of corrosion compounds. Al → Al3+ + 3e− (1) Results and Discussions 2H2O + 2e− → 2OH− + H2↑ (2) Electrochemical Measurements O2 + 2H2O + 4e− → 4OH− (3) NO−3 + H2O + 2e− → NO−2 + 2OH− (4) Tafel plots As it was mentioned previously, the influence of the electrolyte composition, immersion time, and temperature on the electrochemical behavior of coated aluminum was investigated. Tafel measurements were performed from the hydrogen evolution region to the anodic side at a sweep rate of 1 mV s−1. By taking into account that a reasonable criterion for steady state would be a change of less than 5 mV in Ecorr, all the corresponding measured values were obtained after 20 min, when steady state had been reached. From the overall trials, only the most outstanding electrochemical results are shown and compared for each rare earth concentration. Figures 1 and 2 show selected Tafel curves of the cerium and lanthanum conversion coatings formed on AA-6061 aluminum alloy substrates under different experimental conditions, respectively. From these plots, the corresponding Tafel slopes (...truncated)