THE INFLUENCE OF CERIA COATINGS ON THE MANGANESE SULFIDE TEXTURE

Ceria coatings applied by a sol-gel method, strongly modify the microstructure and texture of alpha-MnS scales grown on pure manganese in H2S/H-2 environments at 700 degrees C. Under the imposed sulfidizing conditions, the manganese sulfide scale always exhibit the (111)-texture when blank specimens were sulfidized, whereas, a (110)-texture was observed for coated specimens. The (110) crystallographic orientation corresponds to a fine grained sulfide scale, while, the (111) orientation is associated with large grains expanding in the direction parallel to the interfaces (''lateral growth''), A small grain size is maintaining an important grain boundary network in the scale. Kinetic results show that the scale growth mechanism, controlled by the bulk diffusion in the sulfide scale for blank specimens, becomes limited by the external interfacial reaction for coated specimens. The benefic effect of ceria coatings on adherence, is attributed : 1) to the posibility of cerium introduction into the manganese sulfide scale as the cerium sulfide compound beta-Ce2S3; 2) to the smaller sulfide grain size inducing better mechanical properties; 3) to the vacancies coalescence in the cerium sulfide rich area (which acts as vacancy sinks) instead of the vacancy coalescence at the metal-scale interface.