Aluminizing is an effective surface modification method to improve the high temperature corrosion resistance of metal materials (such as carbon and low-alloy steels). Unfortunately, the conventional pack aluminizing method was normally performed at temperatures above 850oC. Such high processing temperatures would significantly damage the mechanical properties of the metal substrates, and thus limit the widespread applications of aluminide coatings. Therefore, it is essential to lower the aluminizing temperature. To fabricate a low temperature aluminide coating on nanocrystalline (NC) metal matrix is a preferred choice because the grain refinement significantly enhances the diffusivity of aluminum during aluminization in comparison to that in coarse-grained counterparts due to the existence of numerous grain boundaries acting as “short-circuit paths” for diffusion. In this study, a novel CeO2-dispersed ultrafine-grained (UG) δ-Ni2Al3 coating ( or ) was fabricated on a prior electrodeposition NC Ni-CeO2 composite film on a CG pure Ni or A3 plate using an NH4Cl-activated pack cementation method but at a lower temperature (500~700oC). For comparison, two CeO2-free aluminized coatings, or , and were also prepared on a NC Ni film and a coarse-grained (CG) metal (Ni or A3 steel), respectively. Their microstructural characteristics, oxidation and corrosion-erosion performances were investigated by using X-ray diffraction (XRD), scanning electron microscopy equipped with energy-dispersive X-ray analysis (SEM/EDAX), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The main results are presented below:
1. The growth kinetics of aluminized coatings on various Ni substrates was investigated at 500~700oC. It was observed that the growth kinetics of the coating thickness (h) increased significantly with the increasing aluminizing temperature, and h vs aluminizing time (t) almost obeyed the parabolic relationship under a certain temperature. The parabolic constant from highest to lowest, depending on the Ni substrates, followed the order: > > . The result indicated that NC Ni grains significantly enhanced the inward Al diffusion during aluminization. Moreover, the Al diffusion was further facilitated in due to the retarded coarsening kinetics of NC Ni grains for Ni-CeO2 composite film in comparison to pure Ni film. Compared to coarse-grained δ-Ni2Al3 coating in , the nanocrystalline Ni favored to form a UG δ-Ni2Al3 coating, the grain refinement was more significantly in the presence of CeO2 nanoparticles.
2. Oxidation behavior of various aluminide coatings was investigated in air for 20 h at 800~1100oC. Compared to CG , two UG aluminide coatings with NC Ni film pretreatment ( and ) exhibited a significantly slower oxidation rate because coating microcrystallization enhanced the nucleation process of α-Al2O3. Moreover, the alumina scale on exhibited a slower oxidation rate and spallation resistance at 1000~1100oC in comparison to . The reason may be correlated with the dispersed CeO2 playing the “reactive element effect” in reducing the growth rate of alumina scale and in increasing the scale adhesion to the coating substrate.
3. Cyclic oxidation was investigated in air at 1000 oC for 100 h in a vertical furnace. The result indicated that completely degraded as a result of easy scale spallation due to the formation of cavities at the interface of scale/coating. Compared to , degraded slowly due to the procrastination effect of grain refinement on the onset of oxidation. In contrast, exhibited the best scale spallation resistance to cyclic oxidation. The CeO2 dispersions not only significantly refined the grain structure of the aluminide but also prevented the formation of interface cavities and consequently allowed the coating to intrinsically grow adherent alumina scale.
4. The erosion-corrosion (E-C) performance of three aluminide coatings ( 、 和 )was evaluated during 100 h exposure at the running temperature of 550~850oC in a coal-fired atmosphere and concurrently in the condition of exacerbating the impact of solid particles (mainly SiO2 bed materials) through rotating the rig at a given rate. The result were summarized as follows: (1) degradation on the side of all the coatings facing high-velocity particle impact was dramatically increased; (2) offered profoundly improved E-C resistance compared to the corresponding CeO2-free ones, because of increased coating surface hardness and the development of the oxide scales with decreased growth rate and enhanced adhesion.
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