Formation And Thermal Stability Mechanism Of Ni₂Al₃/Ni Hybrid Protective Coatings On P92Stainless Steel

Ni₂Al₃/Ni complex coatings are protective coatings used on the surface of ferritic steelscomponents of ultra-supercritical steam turbines, which leads to excellent resistance againststeam oxidation for steam turbines at or beyond650oC. However, The coatings may becomeinvalid instantly at high temperature because of inter-diffusion of elements among each coatingor between coatings and substrate. Using the activity difference of between Ni-Al and Fe-Alalloys and extreme solubility of Fe in NiAl phase to restrain from the inter-diffusion of elementsin each coating of the complex coatings to prepare reasonable structure Ni₂Al₃/Ni duplexcoatings will relieve the above problem.Nickel electro-plating and low temperature packing aluminizing technology were used toprepare high quality Ni₂Al₃/Ni duplex coatings on the surface of P92steel in the present study.The effect of packing Al content in the aluminizing agent on the growth kinetics of Ni₂Al₃coating at650oC and aluminizing4hours. Two kinds of different structure Ni₂Al₃/Ni duplexcoatings （specimen#1and#2） were prepared based on the thermodynamics analysis of Ni-Al,Fe-Al and Fe-Ni-Al alloys, and then they were annealed to study thermal stability of the coatingsat650oC under argon atmosphere. The phases of the specimens annealed for different time wereanalyzed by X-ray diffraction （XRD）. The cross-section of the cut-offs were analyzed using thetechniques of scanning electron microscopy （SEM） and energy dispersive X-ray spectroscopy（EDS） was used to analyze the concentration distribution of each element in the coatings. It isexpected to learn about the mechanization of making complex coatings keep its long-term andeffective protective function, and then put forward some advices for obtaining improvedstructure complex coatings. Based on the results of this study, the following conclusions can beobtained.（1） When packing Al content in the aluminizing agents （W） is higher than the requiredminimum level about2.5wt.%, the thickness of the outer Ni₂Al₃layer （h） depended linearly onW^1/2at constant aluminizing temperature and time, the relationship can be expressed as followingequation: h=31.7W^1/2-49.6.（2）#1and#2specimens are prepared on the surface of initial Ni coatings which thickness is26μm and11μm respectively, and the aluminizing conditions is that the composition of packingpowder mixtures is4Al-2AlCl₃-94Al₂O₃（wt.%）, aluminizing at650oC for2hours and3hoursrespectively. The structure of#1specimen contains outer Ni₂Al₃layer with26μm in thicknessand inner Ni layer with4μm in thickness, while#2specimen contains outer Ni₂Al₃layer with22μm in thickness and inner Fe-Ni layer with2μm in thickness. （3） Ni₂Al₃layer in#1specimen is transformed to NiAl completely after aluminizing549hours and the newly formed outer NiAl phase layer remains stable when annealing time isfurther increased. The thickness of NiAl phase layer is nearly not reduced in the later annealingintervals. No evident diffusion of Al in the newly formed outer NiAl phase layer into thesubstrate is observed. Fe content in NiAl phase is low in each annealing intervals because oflimitation of solubility in the layer, but there are many hole defects in the new NiAl layer.（4） The transformation from Ni₂Al₃to NiAl of the complex coating on#2specimen issimilar to#1specimen. Because there is nearly no Ni layer between Ni₂Al₃layer and substrate,Fe in the substrate diffuses into the coating evidently at initial annealing time. When thetransformation is completed, Fe content in the newly NiAl phase layer is reduced gradually, butinward diffusion of Al into the substrate is increased gradually and the thickness of NiAl layer isreduced. Hole defects appear in the substrate instead of in NiAl phase layer.