Development and Evaluation of Nanostructured Coatings for Long-Term Corrosion and Oxidation Performance

Nanocrystalline coatings were developed on austenitic alloys to meet the high temperature oxidation and corrosion requirements of the waterwalls and superheater/reheater sections of ultra-supercritical (USC) boilers. Nanocoatings were deposited on 304 stainless steels alloys using Plasma Enhanced Magnetron Sputtering (PEMS) process. Coatings of Fe-18Cr-8Ni-xAl (where x = 0, 4, 10%) were deposited on all 304L steel substrates. The as-deposited Fe-based nano-crystalline coatings exhibited ultra-fine grain structure. The long-term oxidation behavior of nanocrystalline coatings was evaluated through cyclic oxidation tests. The results showed that the fine grain structure improved oxidation resistance, promoted selective oxidation of Cr or Al. High temperature corrosion tests were performed in simulated combustion and gasification environment with and without the presence of deposits such as coal ash, alkali sulfates, and sodium chloride. The weight loss or gain for the exposed samples was measured followed by microstructural characterization. Exposed samples were sectioned and characterized by energy dispersive x-ray analysis to identify the corrosion-product phases in the specimen scales. The results highlighted the importance of bonding between the coating and base alloys to resist the coal ash corrosion. Increasing amount of Al content is required to resist sulfur corrosion. Evaluation of samples tested in flue gas environment with SO₂ showed an increase in corrosion resistance with increase in aluminum content in the coating.