| Aluminide coating for elevated temperature applications rely on the formation of a continuous, and compact Oxide scales for protection in oxidizing and sulfide environment. Aluminide coating were used widely in gas turbine, rocket engine, nuclear reactor, thermal power plant, petrochemical equipment.Al-Cr alloy coating is one of Aluminide coating, which have good oxidation and sulfuration resistant also have excellent thermal erosion and Tritium permeation resistant. The common formation of Al-Cr alloy coating is above 900℃, which can case degradation in mechanical properties of substrate alloys.In this paper, the Cr/Al composite coatings were obtained on 1Cr17 stainless steel by electrodepositing Cr from aqueous solution followed by electrodepositing Al from AICl3-EMIC (molar ratio 2:1) ionic liquid. The Al-Cr alloy coating can produced by heat treatment to the Cr/Al composite coating. Effects of heat treatment on composition and phase structure of Al-Cr alloy layers were studied by using optical microscope (OM), scanning electron microscope (SEM), backscattered electron (BSE), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The mechanical properties of alloy layer were test by Koop hardness equipment.The oxidation behaviors of different Al-Cr alloy coating were studied.Effects of heat treatment on structure transformation to the 6.5μm Cr/15μm Al composite coating were studied. The result show that an Al-Cr alloy layer at the Cr/Al interface was formed at heat treatment at 540℃in 5min. In time of 5 min～16h at 640℃and 740℃, the outer layer is Al4Cr, Al11Cr4, Al9Cr4and Al8Cr5 in turn. The main phase of alloy layer is Al9Cr4, Al8Cr5 and Al8Cr5 was obtained by the heat treatment of 740℃,1h,690℃,16h and 740℃,16h, respectively. The Al8Cr5 layer formed with single phase does not obviously interdiffuse with the substrate. The voids formed in coating surface within heat treatment under nitrogen and short time are less than that formed under air atmosphere and long time. The thickness of Al9Cr4 and reacted Cr coating is linear relationship with the square root of heat treatment time from 5 min to 20 min at 740℃. The Koop hardness tests show that the AlgCrs, Al9Cr4, Al11Cr4 and Al4Cr layer in turn reduce the hardness and toughness.Study on the lower Cr-containing of 1.5μm Cr/15μm Al composite coating to the form Al-Cr alloy layer by heat treatment at the temperature below Al melting. The result show that the law of the reaction of Cr/Al composition coating between the higher and the lower Cr-containing were the same at the beginning. When the Cr layer in the lower Cr-containing of Cr/Al composition coating is react off, the Al-Cr alloy can reacted with the substrate. For 1.6μmCr,15μm Al composite coatings, the main phase of alloy layer is Al4Cr was obtained by 30min heat treatment at 600℃. The first phase formation in the Cr/Al composition coating by heat treatment is the Al7Cr.The oxidation behavior of the outer layer of Al7Cr, Al4Cr and Al8Cr5 phase were studied in the temperature at 690℃in 100 h. The surface morphology of the sandblasting sample is rough, and of the polishing sample is smooth and dense. The oxidation film of sandblasting sample is thicker than the polishing sample. The Al8Cr5 were formed by oxidation to the outer layer of Al7Cr and Al4Cr phase sample at last. The xps test result show that the surface of oxide of Al-Cr alloy is Al2O3. The containing of the Al2O3 by the oxidation of Al8Cr5, Al4Cr and Al11Cr2 is increased in turn.
|
PDF Full Text Request