THE INFLUENCE OF CERIA COATINGS ON THE MANGANESE SULFIDE TEXTURE

Ceria coatings applied by a sol-gel method, strongly modify the micros tructure and texture of alpha-MnS scales grown on pure manganese in H2 S/H-2 environments at 700 degrees C. Under the imposed sulfidizing con ditions, the manganese sulfide scale always exhibit the (111)-texture when blank specimens were sulfidized, whereas, a (110)-texture was obs erved for coated specimens. The (110) crystallographic orientation cor responds to a fine grained sulfide scale, while, the (111) orientation is associated with large grains expanding in the direction parallel t o the interfaces (''lateral growth''), A small grain size is maintaini ng an important grain boundary network in the scale. Kinetic results s how that the scale growth mechanism, controlled by the bulk diffusion in the sulfide scale for blank specimens, becomes limited by the exter nal interfacial reaction for coated specimens. The benefic effect of c eria coatings on adherence, is attributed : 1) to the posibility of ce rium introduction into the manganese sulfide scale as the cerium sulfi de compound beta-Ce2S3; 2) to the smaller sulfide grain size inducing better mechanical properties; 3) to the vacancies coalescence in the c erium sulfide rich area (which acts as vacancy sinks) instead of the v acancy coalescence at the metal-scale interface.