| 其他摘要 | Ni-based superalloys are widely used as turbine blade materials for advanced engines which may suffer from corrosion problems induced by chlorides when they are in service or out of service in marine environments. In addition to the oxidation performance, the electrochemical behavior of alloys can also affect the integrity and service life of engines. Extensive research has been performed on the oxidation behavior of sputter-deposited Ni-based superalloy coatings, but few results have been reported on their electrochemical corrosion behavior. In a previous study in this laboratory, nanocrystalline M38 coatings and K52 coatings were sputter-deposited on M38 and K52 cast alloys respectively and the oxidation performance of the coatings was then studied. Cr2O3 scales formed on the former, while only a single-Al2O3 layer formed on the latter. In the present study, the effect of nanocrystallization on the electrochemical corrosion behavior of these Ni-based superalloys (M38 and K52) has been investigated using potentiodynamic polarization, capacitance and electrochemical impedance spectroscopy (EIS) measurements, in addition to XPS, SEM-EDX techniques. The protectiveness of the oxide scales as well as the rehealing ability of the oxide scales after pitting corrosion in 3.5% NaCl solution were also investigated.
The results reveal that both the M38 and K52 coatings exhibited better corrosion resistance in comparison to the respective cast alloys. This was essentially because the passive films formed on the coatings were more compact, with lower carrier density and higher stability in comparison with those of the cast alloys. What’s more, the oxide scales formed at higher temperature possessed higher protectiveness. The higher protectiveness of oxide scales may be attributed to their high chemical stability due to low carrier density.
To assess the rehealing ability of the oxide scales formed on the M38 coating and cast alloy, the samples were initially oxidized at 900℃ for 100 h and then their polarization behavior was determined. This was followed by subsequent immersion for 360 s in 3.5 % NaCl by their pitting potentials and then reoxidation at 900℃ for 100 h. This cycle was repeated three times. The M38 cast alloy had no rehealing ability even after just one cycle, but the nanocrystalline coating could re-heal itself, though this rehealing ability declined as the number of cycles increased. Subsequent experiments were undertaken to determine the effect of the initial oxidation time (1 h, 5 h, 20 h and 100 h) on the rehealing ability of the coating. The longer the initial oxidation time, the better protectiveness the coating possessed. The oxide scale formed on the sputtered coatings after 1 h initial oxidation, however, lost its rehealing ability after the third cycle. Similar experiments were performed on the nanocrystalline K52 coating, but in this case the initial oxidation time was fixed at 20 h, with varying immersion time in the 3.5 % NaCl solution (600s and 1800 s) and the reoxidation time (100h and 150h). The oxide scales formed on the coating also exhibited rehealing ability after pitting corrosion, and the coating still had excellent corrosion resistance. The rehealing ability was enhanced with prolonged re-oxidation time, but declined with longer immersion time in the chloride solution. EDX analyses revealed that the oxide scales within the pits were composed of mixed-oxides (Cr2O3, Al2O3 and TiO2) though with different compositions.
Tests were also undertaken to assess the barrier properties of the high-temperature oxide scale on the nanocrystalline M38 coating in the chloride solution over a time interval of 0 – 20 months using electrochemical impedance spectroscopy (EIS) techniques. The penetration of chloride ions through the oxide scale may be divided as three stages. In the initial stage (0 – 1 month), the oxide scale acted as a barrier layer. The electrical resistance, however, decreased with increasing immersion time. In the second stage (1 – 7 months), the oxide scale acted as a diffusion barrier and the penetration of chloride ion through the oxide scale was the controlling step. In the third stage (7 – 20 months), the oxide scale lost its protection, but the M38 coating seemed to possess certain self-passivation characteristics. |
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