This property only applies for the following field types: ArcGIS.Core.Data.FieldType.Single and ArcGIS.Core.Data.FieldType.Double. It will be ignored for all other field types. This property is ignored for File, Mobile, and Memory geodatabases, and it will be stored on the resulting field as 0. The Scale must always be less than or equal to the Precision.

Reference

FieldDescription Class
FieldDescription Members