抑制钛合金缝隙腐蚀性能的探索-表面梯度纳米化后离子渗氮

Titanium alloy has great application prospect in offshore engineering because of its excellent self-passivated corrosion resistance, and its corrosion resistance and wear resistance can be further improved by plasma nitriding. However, the corrosion resistance and stability of the nitrided layer are not high enough due to the relatively low compactness and interfacial adhesion of the ordinary nitrided layer. Furthermore, the self-passivation performance of titanium alloy is difficult to play the role in the low oxygen/anaerobic environment in deep sea and the structure gap is easy to be in a highly active state because of the accumulated high Cl- and low pH value, making crevice corrosion to occur and leading to failure or even catastrophe accidents. This project intends to solve this key problem of crevice corrosion of titanium alloys by preparing plasma nitrided layer with a gradient nanocrystalline microstructure on its surface. Firstly, surface shot peening and plasma nitriding processes are combined to form a gradient nanocrystalline nitrided layer on the surface of titanium alloys, and the effect of nanocrystallization on the diffusion enhancement of N atoms in different structural titanium alloys is focused and well understood so as to realize the control on gradient microstructure of nitrided layer. The compactness and interfacial adhesion of gradient nanocrystalline nitrided layer, especially its inhibition mechanism on crevice corrosion, are mainly investigated. Finally, the synergistic effect and mechanism of local corrosion and interfacial adhesion on crack initiation and propagation in the gradient nanocrystalline nitrided layer are revealed. The goal of fully improving the uniform corrosion resistance, crevice corrosion resistance and wear resistance of the titanium alloys by plasma nitrided layer is expected to be achieved through the surface gradient nanocrystalline microstructure.

钛合金优异的自钝化耐蚀性使之在海洋工程中有巨大应用前景，通过离子渗氮可进一步提高其耐磨及耐蚀性。但普通渗氮层的致密度及与基体结合力的不足导致渗层的耐蚀性和稳定性不高。另外，深海中的低氧/无氧环境使得钛合金自钝化性能难以发挥作用，而结构缝隙内容易处于高Cl-、低PH值的高活性状态，使之易发生缝隙腐蚀，导致失效甚至灾难性事故。本项目拟在钛合金表面制备梯度纳米晶结构渗氮层来解决其缝隙腐蚀这一关键问题。首先将表面喷丸与离子渗氮工艺相结合在钛合金表层形成梯度纳米晶渗氮层，并掌握纳米化对N原子在不同结构钛合金中的扩散增强作用，实现对渗氮层梯度结构的控制；重点研究梯度纳米晶渗氮层的致密性和与基体结合力及其对缝隙腐蚀的抑制作用和机理，最后综合揭示局部腐蚀和界面结合力对梯度纳米晶渗氮层中裂纹萌生与扩展的协同作用规律和机理，通过表层纳米晶梯度结构实现全面提高钛合金离子渗氮层抗均匀腐蚀、缝隙腐蚀和磨损性能的目的

钛合金优异的耐氯离子腐蚀性能使其在海洋工程中有巨大的应用前景，但其低硬度和低耐磨性也限制了它的应用。离子渗氮可以在钛合金表面生成硬度高且耐蚀性能优异的渗氮层，但传统的渗氮层在海洋环境下长期服役会发生局部剥落。特别的，结构缝隙内容易处于高Cl-、低PH值的高活性状态，而深海中的低氧/无氧环境使得钛合金难以自钝化，从而使得其发生缝隙腐蚀的倾向增大。本项目将表面喷丸纳米化与低温活性屏离子渗氮相结合，利用喷丸形成的纳米晶结构促进氮扩散实现钛合金的低温离子渗氮，生成具有梯度纳米晶结构的渗氮层。利用梯度结构改善渗氮层与钛基体的结合力，利用表面致密纳米晶渗氮层提高耐蚀性能，目标是全面改善钛合金的缝隙腐蚀性能与耐磨性能。.本项目选择应用最广泛的TC4（Ti6Al4V）双相钛合金和海洋中常用的近α相TA17（Ti4Al2V）钛合金为实验材料进行表面喷丸纳米化。以表面晶粒细化至纳米级、表面粗糙度较小和剧烈塑性变形层厚度合适为指标，获得了两种钛合金优化喷丸工艺为0.3MPa-25min（TC4）和0.2MPa-10min（TA17）。通过调控渗氮温度、渗氮气压、活性屏高度以及渗氮偏压等参数，获得了两种钛合金纳米化后活性屏离子渗氮的最优工艺为500℃-20h-200Pa-高屏(100mm)-400V偏压，该最优工艺下钛合金表面能获得最厚的渗氮层。喷丸试样中的梯度结构有效缓解了渗氮层与基体间结构与力学性能的突变，提升了渗氮层与基体的结合力。喷丸样品表面的渗氮层比原始样品的更厚、更致密，表现出更高的耐蚀性。原始-渗氮和喷丸-渗氮样品的渗氮层均为内外两层结构，其中TiN外层是通过钛活性屏上溅射出来的氮化物颗粒沉积形成的，Ti2N内层是通过氮扩散到钛基体中形成的。Ti2N层中贫Al，这可以作为钛合金活性屏离子渗氮层中TiN和Ti2N不同形成机理的证据。原始-渗氮样品的TiN层全部由柱状晶TiN组成，而喷丸-渗氮样品的TiN层由等轴TiN纳米晶和部分分散的柱状晶TiN组成。喷丸表面大量的晶界为TiN提供了大量形核位点，晶界能量也降低了TiN的形核势垒，这两者都有助于TiN快速成长为等轴纳米晶，并且纳米晶TiN由于其高的热稳定性而得以在渗氮过程中保持下来，而基体表层的纳米晶则由于低的热稳定性在渗氮过程中发生长大。此外，根据初步测定，TA17钛合金在海洋环境中的临界缝隙腐蚀温度在95℃以上。

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