Effect of hydrostatic pressure on stress corrosion susceptibility of low-cost titanium alloy plates and tubes
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摘要: 采用高压釜测试系统开展低成本Ti6411板材和Ti52管材及船用TC4 ELI和Ti80合金模拟深水环境压力(7.5 MPa)作用下的慢应变拉伸应力腐蚀试验,腐蚀介质为3.5% NaCl溶液和干燥空气,应变速率分别采用10−5、10−6/s和10−7/s,获取拉伸强度、断后延伸率和应力腐蚀敏感性指数(ISSRT)等参数,同时结合OM、SEM等方式观察原始组织与断口形貌,从而阐明低成本钛合金板管材在模拟深水压力作用下的应力腐蚀行为及其失效机制。结果表明:四种钛合金材料的应力腐蚀最佳应变速率在10−5~10−6/s内,其中Ti6411板材在10−5/s和10−6/s两种条件下,ISSRT数值分别为10.0%和9.1%,具有较为明显的应力腐蚀敏感性,而Ti52管材、TC4 ELI和Ti80合金的最大ISSRT值均低于5%,应力腐蚀敏感性不明显。片层状Ti6411板材组织特征易产生应力集中效应,导致准解离断裂,断口表面存在黑色腐蚀产物,氧化膜破裂、选择性阳极溶解和氢脆机制共同主导模拟深水环境应力腐蚀现象,施加的恒定压力活化Cl−,加速穿透氧化膜,从而破坏氧化膜稳定性,增加H含量聚合形成氢气,拉应力加载和内部氢压耦合作用可降低材料耐腐蚀性。Abstract: This paper investigates the stress corrosion cracking (SCC) susceptibility of low-cost Ti6411 plate, Ti52 tube, marine TC4 ELI, and Ti80 alloys under simulated deep-water environment pressure using a high-pressure autoclave testing system. Slow-strain-rate tests (10−5, 10−6/s, and 10−7/s) were exposed to 3.5% NaCl solution and dry air with a hydrostatic pressure of 7.5 MPa. The tensile parameters and stress corrosion susceptibility index (ISSRT) were obtained. Furthermore, the microstructures and fracture features have been observed to elucidate the stress corrosion behavior and failure mechanisms of low-cost titanium alloy plates and tubes using OM and SEM under simulated deep-water pressure. The results indicated that the optimal strain rate is within 10−5/s and 10−6/s. Specifically, the ISSRT values of Ti6411 plate are 10.0% and 9.1% with strain rates of 10−5/s and 10−6/s, respectively. A notable stress corrosion susceptibility was obtained. In contrast, all ISSRT values of Ti52 tube, TC4 ELI, and Ti80 alloys were below 5%, indicating no significant stress corrosion susceptibility. Particularly, Ti6411 plate with lamellar microstructure is sensitive to stress concentration effects and exhibits quasi-cleavage fracture. Some black corrosion products can be observed on the fracture morphologies. It is believed that the corrosion mechanisms were coupled effects of selective anodic dissolution and hydrogen embrittlement. Hydrostatic pressure can promote Cl- to weaken the passive film and increase hydrogen content. As a result, both of the tensile loading and internal hydrogen pressure reduce the corrosion resistance of the titanium alloys.
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表 1 不同试验条件下钛合金拉伸性能(平均值)
Table 1. Slow-strain-rate tensile properties of titanium alloys under different test conditions (average value)
Materials Conditions×
107/(s−1)Rm(corrosion)/
Rm(Dry air)Am(corrosion)/
Am(Dry air)ISSRT Ti6411 1 98.2 86.9 3.7 10 95.3 74.2 9.1 100 97.0 61.1 10.0 Ti52 1 103.0 89.9 −1.4 10 101.0 108.1 −2.3 100 97.7 94.5 3.2 TC4 ELI 1 100.6 101.8 −1.0 10 98.0 88.2 4.4 100 100.9 136.2 −5.1 Ti80 1 100.7 90.0 1.3 10 98.9 92.0 2.7 100 109.9 123.8 −14 -
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