Abstract

The pH-concentration diagram for the formation of barium molybdate films on a metallic molybdenum substrate in a barium hydroxide aqueous solution has been experimentally studied. Due to the relatively large solubility of the molybdate, the estimation of film thickness from the weight gain was corrected by considering the substrate dissolution. The molybdenum substrates had a preferential [100] crystallographic orientation, but the films exhibited no preferential orientation. It is suggested that the film grows through a dissolution-precipitation mechanism and that nucleation begins only after the solution is locally saturated with barium molybdate. Irregular intergrowth is frequent due to the high crystal growth rate. XPS measurements on samples that appeared completely coated under SEM examination exhibited only the peaks corresponding to barium molybdate. On the other hand, the substrates that appeared partially coated under SEM, when examinated by XPS exhibited the superposition of spectra of the molybdate and of the uncoated molybdenum substrate. This allows the study of the degree of coating as a function of the solution concentration and pH. © 2002 Elsevier Science Ltd. All rights reserved.