| 其他摘要 | The corrosion behaviors of a Ni-based superalloy with polycrystalline, single-crystalline (SC) and nanocrystalline (NC) have been studied in aqueous solution at normal temperature and at high temperature. In order to study the corrosion behavior of three materials under the synergistic effect of NaCl and water vapor at middle temperature, some effective electrochemical techniques had been explored with pure metals as specimens. The influence of microstructure on the corrosion behavior of Ni-based superalloy had been studied in details under different corrosive environments. These results are very useful for people to understand the corrosion mechanism of Ni-based superalloy and afford the theoretical base to improve the corrosion resistant of this alloy in future.
The electrochemical corrosion behavior of a Ni-based superalloy with polycrystalline, single-crystalline (SC) and nanocrystalline (NC) have been studied in 0.25M Na2SO4 + 0.05 M H2SO4 and 0.5 M NaCl + 0.05 M H2SO4 solution respectively. The corrosion behaviors of three materials were semblable in Na2SO4 acidic solution, while the NC coating had superior resistance to pitting corrosion among three materials in NaCl acidic solution. The corrosion behaviors of the cast alloy and the SC alloy were similar in NaCl acidic solution. The semiconductive type of passive film of the NC coating was p-type in both acidic solutions, while, that of cast alloy and SC alloy changed from p-type in Na2SO4 acidic solution to n-type in NaCl acidic solution. XPS results indicated that no chloride ion was found in the passive film of NC coating while it was in the passive films of cast alloy and the SC alloy. The chloride ions adsorbing on the surface of cast alloy and SC alloy incorporated into the passive film, which induced the formation of n-type oxide film. The nanocrystallization of Ni-based superalloy obviously weakened the adsorption of chloride ions on surface, which decreased the susceptibility of pitting corrosion in acidic solution.
The electrochemical corrosion behavior of three Ni-based superalloy nanocrystalline(NC) coatings with the same composition and different grain size, fabricated by a magnetron sputtering technique, has been investigated in 0.5 M NaCl + 0.05 M H2SO4 solution. The NC coating with 100 nm grain size formed a porous passive film in NaCl acidic solution which permitted the chloride ions to penetrate the film, and thus increased the susceptibility to pitting corrosion. The NC coating with 10 nm and 50nm grain size formed a compact non-porous passive film in NaCl acidic solution which increased the resistance to pitting corrosion. The small gain size produced by the nanocrystallization had three beneficial effects: it gave a uniform elemental distribution, which decreased the amount of chloride ions adsorbed on the surface, it promoted the formation of compact passive film and it increased the repassivation ability of the superalloy. All of the effect of small grain size greatly increased material’s resistance to pitting corrosion in acidic solution.
The electrochemical corrosion behavior of three materials has been studied in 3.5% NaCl solution. The results indicated that among three materials, the corrosion resistance of the NC coating was the highest, that of the SC (200) alloy was in the next place and that of the cast alloy was the lowest. XPS results showed the composition of passive film on three materials was different. Except Cr2O3 and TiO2, there were a little of NiO in the passive film on the cast alloy, little in that of the SC (200) alloy and none in that of the NC coating. The double-log plots showed the compact property of the passive film formed on the cast alloy was the worst, that on the SC (200) alloy was the second one and that on the NC coating finally was the best one among three materials. The micro-structure influences both of the composition of passive film and the initial growth of passive film. Those determined the compact property of the film and lead to the difference of the corrosion behaviors of three materials.
The corrosion behavior of pure Fe and pure Cr at 600℃ under a deposit of solid NaCl was accelerated by the involvement of water vapor into the system. The corrosion current was tested, which testified the existence of electrochemical reactions during the whole reaction. In EIS measurement, only one capacitive loop obtained on the pure Fe and Cr coated with solid NaCl and gave the information of oxide layer on them. For the oxide in air, there is a good relationship between the Rox and the reaction rate for both pure Fe and Cr with different oxide time. The lower the Rox is, the higher the reaction rate is. Although no good relationship can be set up between the Rox and the reaction rate when water vapor presented, its trend with oxide time for both metals is generally in accordance with that of the corrosion rate measured by the mass gain curves. The electrochemical technique is an effective method for studying corrosion performance at high temperature.
The oxidation behavior of three materials was studied at 1000℃. The isothermal oxidation results indicated that the oxidation rate of nanorystalline coating was the lowest among three materials. At the initial stage, the oxidation rate of single-crystal alloy was faster than that of cast alloy, while at later stage, it was slower than that of cast alloy. A non-uniform external oxide of which some locations were nodule-like scale was formed on surface of cast alloy after oxidation 50 hours at 1000℃. The nodule-like parts consisted of TiO2, Cr2O3 and serious internal oxidation of Al, and rest flat surface was an Al2O3 layer. The non-uniform oxide layer and internal oxidation which produced by defects and large scale grain boundaries on the cast alloy, which decreased the oxidation resistance. For single-crystal alloy (200), uniform oxides scale consisted of external Cr2O3 with little TiO2 and internal continuous Al2O3, which lead to good oxidation resistance. For sputtered nanocrystalline coating, a continuous Al2O3 layer formed, which increased the material’s oxidation resistance.
The cyclic oxidation results indicated that oxidation layer on the SC alloy scaled off, but a continuous Al2O3 layer formed at later stage, therefore the mass gain had a stable value. A non-uniform external oxide and internal oxidation decreased the resistance to cyclic oxidation of the cast alloy. The continuous Al2O3 scale has good adhesion, which increased the resistance to cyclic oxidation of the NC coating.
The experiments in H2O indicated that the difference of microstructure influence the effect of H2O to materials. H2O reacted with Cr2O3 on the SC alloy to form volatile products because there were a lot of Cr2O3 and little TiO2 formed on it. A large amount of TiO2 formed on the surface of the cast alloy, which inhibited the reaction of Cr2O3 with H2O. H2O prolonged the time to form a continue Al2O3 layer on the NC coating. The crystal structure influenced the oxidation and corrosion mechanism and resulted in different oxide scale for the three materials. |
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