The Relation Between the Microstructure of Aluminum Alloy 7075-T6 and the Type of Cerium Salt in the Formation of the Cerium Conversion Layer

Abstract

The deposition of the cerium conversion layer on aluminum alloy 7075-T6 proceeds by a simple procedure of immersion, but it is affected by several factors which govern its corrosion protectiveness. The study aimed to investigate two crucial relationships: (i) how the type of cerium salt used for conversion affects the composition, thickness, and electrochemical characteristics of the deposited layers, and (ii) how the deposition depends on the type of intermetallic particles. Four sets of samples were prepared by immersion in 0.1 M NaCl with and without adding 3 mM Ce(III) salts: chloride, nitrate, and acetate. Samples were analyzed by scanning electron microscopy with chemical analysis at the same site before and after immersion. Therefore, this study is site-specific and targets the mode of corrosion and deposition of the Ce-conversion layer as a function of the intermetallic particle’s type. The type of anion of Ce salt affects the path of conversion layer formation. The corrosion protectiveness is the highest for acetate salt. The model was postulated, describing the precipitation of a mixed layer by the dynamic “two-way road” process of selectively dissolving elements from the substrate and precipitating Ce-hydroxide. Dealloying of intermetallic particles is the governing mechanism for the precipitation of large Ce-containing deposits.