Formation of Cerium Conversion Coatings on AZ31 Magnesium Alloys

Abstract

Cerium conversion coatings, which have been used as protective coatings for aluminum alloys, are now being considered as an alternative to chromium conversion coatings for improving the corrosion resistance of magnesium alloys. This study investigated the evolution of conversion coatings on an AZ31 magnesium plate immersed in 0.05 M cerium nitrate solution. In addition to the expected growth of the conversion coating with immersion time, it was found that there may be an inherent adhesive weakness within the coating layers, which then led to partial detachment of the coatings from the magnesium plate while drying the samples at room temperature. Cross-sectional transmission electron microscopy characterization of conversion coatings revealed a three-layered structure comprising of porous, compact, and fibrous layers sequentially formed on top of the magnesium plate. Furthermore, the weakest bonding was identified as the interface between the compact and the fibrous layers. Based on the identified layer morphology and the respective composition, a possible formation mechanism for cerium conversion coating on magnesium alloy was proposed, which would serve as a basis for improving the adhesive strength of the coating on magnesium substrate.In light of their low density and high specific strength and stiffness, magnesium alloys are extensively used in electrical appliance and automobile industries. 1,2 However, most magnesium alloys are chemically reactive and tend to suffer severe corrosion during service. 2,3 Surface modification treatments are therefore indispensable for improving the corrosion resistance of magnesium alloys, which includes anodizing, 4-8 conversion coating, 4,9-17 electroless nickel plating, 18 and pure magnesium coating via physical vapor deposition ͑PVD͒. 19 Among the various surface modification techniques, conversion coating treatment is known for its low cost and simplicity in operation. For example, chromate conversion coatings have been extensively applied to magnesium alloys because of their simplicity in production and excellent corrosion resistance. 9,10 However, the toxicity of hexavalent chromium ions in the solution and exhaust fumes has imposed a strict restriction on the use of chromate conversion coatings. In seeking alternatives to chromate conversion coating, nonchromate solutions have recently been developed, such as phosphate, 4 phosphate/permanganate, 11,12 stannate, 13,14 and solutions containing salts of rare earth metals 15,16 or cobalt. 17 The rare earth conversion-coating process is recognized for its simple electrolytic constituents that usually contains nitrate, sulfate, and chloride of rare earth metals such as cerium, lanthanum, and neodymium. 15,16,[20][21][22][23] This simple electrolyte makes it easy to maintain and recycle, and more importantly, the solution is considered to be friendly to the environment. 21 Therefore, many efforts have been made to investigate how the solution composition affects the formation and properties of the conversion coatings, especia...

Corrosion Resistance of Cerium-Conversion Coated AZ31 Magnesium Alloys in Cerium Nitrate Solutions

“…Our previous study showed that proton discharge accompanying oxidation and dissolution of magnesium substrate results in a pH rise ahead of the substrate/solution interface. 11) This high pH causes the precipitation of Mg 2þ and Al 3þ ions dissolved from the substrate as a magnesium/aluminum hydroxide layer directly contacting the substrate. Further away from the substrate/solution interface, the primary precipitation is Ce 3þ ions to cerium (III) hydroxide/oxide, since Mg 2þ and Al 3þ ions are markedly depleted due to their precipitation in the vicinity of the magnesium surface.…”

“…Accompanying these oxidation reactions, the primary reduction is proton discharge, which again leads to an increase in the pH ahead of the substrate/solution interface. 11,14) Ce 4þ then can precipitate as Ce(OH) 4 which, once dehydrated, transforms to cerium (IV) oxide (reaction 1).…”

“…These cracks have been shown to be induced by the stresses generated by volume shrinkage of the coating during the drying process. 11,21) When H 2 O 2 was added to the solution, the major defects associated with the coating were the local severely-damaged areas. Long-term immersion generally results in more severely-damaged areas, while the coating is thin during the early stage of immersion.…”

“…Our previous study revealed the porous layer was primarily composed of magnesium and aluminum hydroxide/oxide; while the fibrous major overlay was presumably cerium hydroxide/oxide. 11) Since the coating displayed a grey color, this fibrous layer was likely cerium (III) hydroxide/oxide. 24) The coating formed after a longer period of immersion still comprised three layers, in which the major overly (i.e.…”

“…[8][9][10][11] For the medium containing rare-earth metal salts, the solution containing Ce 3þ has received everincreasing attention, into which Ce 3þ is usually added in the form of nitrate, sulfate or chloride. In these cerium salt solutions, AA2024 and AA7050 aluminum alloys exhibit improved corrosion resistance by the formation of an insoluble film at cathodic sites.…”