Preparation Of Ni₂Al₃/Ni Hybrid Coating And Effect Of Inner Ni Layer Thickness On Its Thermal Stability For Protecting Creep Resistant Ferritic Steels

The creep resistant ferritic steels of type9-12Cr (wt.%) are used widely for thecomponent of supercritical steam turbine. At650°C, they have performed high creepresistance but low oxidation resitace in the steam environment. Ni2Al3/Ni hybridcoatings are desighed to overcome this disadvantage. This paper focused on the researchof the coating making and the coating thermal stability at650°C.Electroplating nickel and pack aluminizing at low tempeture are combined to makeNi2Al3/Ni hybrid coatings. The aluminisation process was carried out in the temperaturerange of525～650°C. The pack powder mixtures were prepared using Al as depositingsource, Al2O3power as inert filler and NH4Cl/AlCl3(anhydrous) as activator. Theresults reveal that the coatings made as such consisted of uniform single Ni2Al3phasewithout defects. The coating thickness (h) and the sample weight gain (Δm) satisfied thelinear relationship: h=5.01m0.50. When using2wt.%AlCl3as activator, thekinetics of low temperature pack aluminizing of electroplating nickel on creep resistantsteel could be quantified as follow: h=9968W1/2t1/2exp(-6625/T). At this condition, theactivated energy of Ni2Al3growth was55.08kJ/mol. The results also present AlF3andNaCl were not promising activators for low temperature pack aluminizing.Hybrid coatings of three representive types were made for the long term annealingand diffusion experiment in which their thermal stability would be studied respectivelyso as to establish the kinetic model for the phase evolution in outer Ni2Al3layer andcertain the optimal thickness ratio of outer Ni2Al3layer versus inner Ni layer. It wasfound that the hybrid coatings, with thin inner Ni layer in which existed the pure Nilayer, exhibited excellent thermal stability, thus the optimal thickness ratio ofNi2Al3/Ni certained in this paper was in the scope of1.75～1.82. In this case, AlNprecipitated at the interface of NiAl(Fe)/Fe-Ni-Al which hindered heavily the elementdiffusion of Fe and Al and hence aroused good thermal stability of coating. Allcoatings in this paper had completed the transition from Ni2Al3phase to NiAl phase.And all the transition processes were available with the kinetic model: d=kt1/2.