IMPROVEMENT OF THE OXIDATION RESISTANCE OF ALUMINA-FORMING ALLOYS BY NITRIDING

THE CORROSIVE AND EROSIVE ASPECTS OF COAL CONVERSION AND UTILIZATION LIMIT OPERABILITY AND RELIABILITY IN ENERGY SYSTEMS USING COAL. TO IMPROVE COMPONENT LIFETIME, MATERIALS HAVING INCREASED RESISTANCE TO CORROSION DEGRADATION UNDER OPERATIONAL CONDITIONS ARE REQUIRED. ALLOYS THAT PRODUCE PROTECTIVE OXIDES ON THEIR SURFACE ARE WIDELY UTILIZED FOR HIGH TEMPERATURE APPLICATIONS. ALPHA-ALUMINA IS ONE OF SEVERAL METAL OXIDES THAT CAN BE EFFECTIVE IN PROVIDING PROTECTION FOR ALLOYS AGAINST SULFIDATION ATTACK UNDER CERTAIN ENVIRONMENTAL CONDITIONS FOUND IN FOSSIL ENERGY SYSTEMS. HOWEVER, THE ALPHA-ALUMINA SCALE USUALLY HAS POOR ADHESION AND THUS HAS LIMITED PROTECTION CAPABILITIES, PARTICULARLY IN APPLICATIONS SUBJECT TO THERMAL CYCLING. A NOVEL TECHNIQUE FOR IMPROVING THE ADHERENCE OF METAL OXIDE SCALES TO THEIR SUBSTRATE HAS BEEN DEMONSTRATED PREVIOUSLY IN FERRITIC ALLOYS. THE TECHNIQUE INVOLVES THE GENERATION OF A NITRIDEDLAYER ON THE SURFACE OF THE BASE MATERIAL PRIOR TO EXPOSURE TO THE OXIDIZING ENVIRONMENT. INCOHERENT NITRIDES IN THE SURFACE LAYERS SUBSEQUENTLY IMPROVE THE ADHERENCE OF THE OXIDE SCALE, RESULTING IN IMPROVED SPALLING RESISTANCE AND LEADING TO EXTENDED SERVICE LIFETIMES. THIS PROJECT WILL ATTEMPT TO USE THESE PRINCIPLES TO IMPROVE THE SCALE ADHERENCE, AND THUS THE SERVICE LIFETIME, OF FE-CR-AL ALLOYS. THE OBJECTIVE WILL BE TO EMPLOY A MODIFICATION OF AN ACCEPTED SURFACE TREATMENT (NITRIDING) TO PRODUCE INCREASED CORROSION RESISTANCE AND EXTENDED LIFETIMES OF COMPONENTS WITHOUT RECOURSE TO MORE EXPENSIVE EXOTIC ALLOYS. THE DEVELOPED PROCESS WILL BE ECONOMICALLY ATTRACTIVE AS IT WILL RELY ON RELATIVELY INEXPENSIVE PROCESSING OF ALLOYS IN CURRENT USE. WIDESPREAD APPLICATIONOF THE PROCESS IS ANTICIPATED TO COMPONENTS IN FOSSIL ENERGY TECHNOLOGY AND OTHER HIGH TEMPERATURE PROCESSES.