MICROSTRUCTURAL STUDY OF Fe-Cr-Al/Al COMPOSITE COATINGS DURING OXIDATION AND SULFIDATION AT 900 ℃

The microstructure of a composite coating system, which was composed of an inner layer of Fe-Cr-Al and an outer layer of aluminum, was studied after it was respectively oxidized and sulfurdized at elevated temperatures. Apart from the Al2O3 scale formed on the surface, the microstructure of the composite coatings exposed at 900℃ in air for 4h was a three-layer structure. The first layer consisted of a solid solution of Cr and Fe in α aluminum and an intermetallic compound FeAl3, while the second layer was a single phase of the aluminide and the third layer still remained the same appearance as the original Fe-Cr-Al coating.The microstructural observation of the specimen tested at 850-900 ℃ at low oxygen pressure and high sulfur pressure for 576h revealed that the surface coatings of the specimen had transformed into a duplex structure containing an outer layer and a thicker aluminide layer beneath. X-ray diffraction results showed that the out layer was composed of Al2S3 and Al2O3 and that AlCrFe2 was the main phase composition of the aluminide layer, with a few of Al2S3 and Al2O3 accompanied.