| 氧化对两种镍基高温合金及其纳米涂层电化学腐蚀行为的影响 | |
| Alternative Title | Effect of elevated temperature pre-oxidation on electrochemical behavior of two Ni-based alloys and their sputtered coatings |
| 白会平 | |
| Subtype | 博士 |
| Thesis Advisor | 王福会 |
| 2007-06-28 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料学 |
| Keyword | 镍基高温合金 晶粒尺寸 电化学腐蚀行为 修复能力 保护性能 |
| Abstract | Ni基高温合金是应用于飞机或燃气机涡轮叶片上的重要材料。当飞机在海面上服役于或停泊在近海时，涡轮叶片亦会面临海洋大气中氯离子引发的腐蚀。合金的抗高温氧化或抗热腐蚀性能是影响燃气机工作寿命的关键因素，对此已有许多现象。而停机状态下合金的常温腐蚀性能同样是影响飞机日历寿命的关键因素。近年来，通过磁控溅射表面纳米化(微晶化)的Ni基高温合金所表现出来的优异的抗高温氧化性能逐渐引起了人们的重视，但是研究的热点主要集中在高温腐蚀上。对于其常温下以及常温-高温交替作用下的电化学腐蚀性能却鲜有报道。 本文选用了磁控溅射的M38合金和K52合金，前者应用于飞机涡轮叶片上，高温氧化生成了Cr2O3膜，后者应用于舰涡轮叶片上，高温氧化生成了单一的Al2O3膜。以经过高温氧化的镍基高温合金在3.5%NaCl溶液中的电化学腐蚀行为为研究背景，通过电化学方法和表面分析技术，研究了纳米化对合金在水溶液中腐蚀行为的影响，高温氧化膜的保护性能，常温-高温交替作用下氧化膜的修复性。 纳米化对高温合金在3.5%NaCl溶液中的耐蚀能力影响的研究表明，纳米化并没有改变表面钝化膜的半导体类型（p型），但是增强了钝化膜的致密度，降低了钝化膜中的载流子密度，提高了钝化膜的稳定性，增强了钝化膜耐氯离子侵蚀能力。所以，涂层耐蚀能力高于合金。 M38涂层和K52涂层的氧化膜在3.5 % NaCl溶液中的保护性能均优于各自合金氧化膜的保护性能。晶粒细化的涂层在高温生成的氧化膜更为致密均匀，在水溶液中有较强的物理阻挡作用，有较大的极化电阻，致使氯离子的扩散传输非常困难。电容测试结果表明，利用Mott-Schottky测试方法得到的载流子密度和平带电位均随着频率的变化而变化，说明不同频率下电容的来源不同；而且涂层和合金两者的氧化膜在溶液中均呈现p型半导体特征，但涂层氧化膜有较低的载流子密度，保护性能更强。对M38高温合金在700-900℃生成的氧化膜研究结果表明，越高温度下形成的氧化膜在溶液中具有更好的保护性能。 对M38纳米涂层和合金的氧化膜发生点蚀坑后的修复性研究表明，M38合金氧化膜不具备修复能力，纳米涂层氧化膜具有良好的修复性，而且三个循环后，仍然具有一定的抗腐蚀能力，而修复能力随着循环次数的增加降低。对涂层氧化膜的进一步研究表明，最初的氧化表面形态是影响涂层保护修复能力的关键。不同初始氧化时间的氧化膜的修复能力呈以下顺序：1h<5h<20h<100h。 纳米涂层氧化100小时生成的氧化膜具有最好的保护修复性能。经过三次循环，900℃ 氧化 1小时的氧化膜已经失去保护作用，发生了全面腐蚀；氧化5小时以上的氧化膜仍是点蚀为主的局部腐蚀，而且新的点蚀坑出现在已经修复的点蚀坑附近。K52纳米涂层生成的氧化膜具有良好的修复性，即一次氧化膜遭点蚀破坏后，点蚀坑内高温二次氧化生成了混合氧化物（Al2O3 ，Cr2O3与TiO2），此修复膜依然具有优良的抗电化学腐蚀性能。而且击破时间越短，二次氧化时间越长，耐蚀性能越好。K52涂层遭点蚀破坏后，点蚀坑内的氧化修复膜也是Al2O3 ，Cr2O3与TiO2混合氧化物，此混合膜仍具有一定的抗电化学腐蚀性能。 利用交流阻抗方法对纳米M38涂层高温氧化膜在3.5%NaCl水溶液的保护机制研究发现，氯离子在氧化膜中的传质过程可以分为三个阶段：浸泡前期（0-1月）- 氧化膜作为一个物理阻挡层，随着浸泡时间的延长，极化电阻越来越小；浸泡中期（1-7月）- 氯离子已经到达基体，在氧化膜中的扩散成为控制步骤； 浸泡后期（7-20月）- 氯离子自由传输于氧化膜中，但是涂层具有自钝化能力，在基体涂层与溶液界面形成了钝化膜。 |
| Other Abstract | 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. |
| Pages | 112 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/16990 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 白会平. 氧化对两种镍基高温合金及其纳米涂层电化学腐蚀行为的影响[D]. 金属研究所. 中国科学院金属研究所,2007. |
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