EFFECT OF THICKNESS ON THE MORPHOLOGY AND CORROSION BEHAVIOR OF CERIUMBASED CONVERSION COATINGS ON AZ31B MAGNESIUM ALLOY

摘要

Cerium-based conversion coatings (CeCCs) were deposited ontoAZ31B magnesium alloy substrates using a spontaneous reactionof CeCl_3, H_2O_2 and gelatin in a water-based solution. Thecoating thickness was adjusted by controlling the immersion timein the deposition solution. Prior to deposition, the AZ31Bsubstrates were treated using an acid pickling in nitric acid andthen an alkaline cleaning in sodium metasilicate pentahydrate.After deposition, the coated samples were immersed in aphosphate bath that converted cerium oxide/hydroxide into ceriumphosphate. Electrochemical impedance spectroscopy,potentiodynamic polarization and neutral salt spray testingstudies indicated that ～100 nm thick CeCC had better corrosionperformance than ～400 nm coatings. Characterization of theCeCCs by transmission electron microscopy (TEM) revealed athree layer structure with different compositions.