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Deposition of Cerium-Based Conversion Coatings on Aluminum Alloy 380

DOI: 10.1155/2012/760284

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Cerium-based conversion coatings were deposited on as-cast aluminum alloy 380 substrates by a spontaneous immersion process. In this study, the effects of rinsing temperature prior to immersion in the coating deposition solution were studied with respect to the surface morphology, electrochemical response, and corrosion resistance of the coatings. Panels rinsed at 25°C prior to coating had large cracks and holes in the coating. In contrast, panels rinsed at 100°C prior to coating had a uniform coating morphology with fewer, smaller cracks. Electrochemical testing revealed that coatings deposited on substrates rinsed at 100°C had higher impedance (~80?kΩ·cm2) and lower corrosion current (~0.34???A/cm2) compared to coatings deposited on substrates rinsed at 25°C, which had 10?kΩ·cm2 impedance and 2.7???A/cm2 corrosion current. Finally, ASTM B117 salt spray testing showed that rinsing at 100°C prior to coating resulted in cerium-based conversion coatings that could resist the formation of salt tails for at least 8 days. 1. Introduction Aluminum cast alloys are widely used in the automotive and aerospace industries where innovative, lightweight materials and product forms are needed to improve performance [1]. Common aluminum casting alloys contain alloying additives that affect the mechanical properties, fluidity, and corrosion resistance [2]. For example, the 3xx.x series contain noble alloying elements such as Si and Cu that can promote pitting corrosion and lead to the failure of aluminum alloy components [3]. Chromate conversion coatings are used for corrosion protection for a wide variety of aluminum alloy components [4]. However, the toxicity and carcinogenic properties of hexavalent chromium (Cr6+) have caused severe restrictions to be imposed on the use of chromates [5–7]. As a result, environmentally benign alternatives to chromates have been extensively investigated [8]. Potential replacements for chromate conversion coatings include anodized coatings [9], rare-earth-based inhibitors in conversion coatings [10], and sol-gel coatings [11]. Among potential chromate replacements, rare-earth inhibitors have attracted significant attention. Hinton et al. were the first to investigate cerium-based conversion coatings (CeCCs) as an environmentally benign alternative to chromate conversion coatings [12, 13]. The corrosion resistance of CeCCs is thought to arise from a combination of barrier properties and active response to the environment [14, 15]. The barrier protection properties of CeCCs have been studied by changing processing parameters,

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