海洋工程用E690高强钢薄液环境应力腐蚀行为及机理

发布时间：2018-05-30 03:24

本文选题：海洋 + 薄液　；参考：《北京科技大学》2015年博士论文

【摘要】：对海洋资源的开发利用离不开高强钢的发展及应用,随着对海洋资源,尤其是南海等高温、高湿及高盐雾的严酷环境海域开发的不断深入,海洋工程用高强钢需求量不断增大,强度要求不断升高,其应力腐蚀(SCC)问题及风险逐渐增加,制约了海洋工程的发展。但目前对海洋环境尤其是海洋薄液环境下高强钢的SCC行为研究较少,缺少深入的薄液环境下SCC敏感性和机理研究,对不同环境因素与scc敏感性的关系认识也不充分。通过建立严酷海洋干湿交替环境下电化学及SCC的室内模拟研究方法,对模拟海洋干湿交替环境中高强钢的电化学腐蚀行为及scc关键影响因素、行为及机理进行研究。结果表明：模拟海洋干湿交替环境中,随实验周期延长,E690钢阴极电流密度逐渐增加,阳极电流密度逐渐减小,耐腐蚀性能先减小后略有增大。pH值增大及干湿交替频率的升高,有利于腐蚀产物的富集和致密化,导致锈层下均匀腐蚀减弱,局部阳极溶解作用加强,SCC敏感性升高；Cl-浓度增大,SCC敏感性先增加,后减少,再增加,NaCl浓度为3.5%时SCC敏感性相对最高。E690高强钢SCC机理为阳极溶解和氢脆的混合控制机制,裂纹扩展模式为典型的穿晶扩展。对E690高强钢在模拟海洋全浸和薄液环境中的电化学和SCC机理的对比研究表明：薄液环境中E690钢会发生明显的SCC,薄液环境SCC敏感性高于海水环境的本质原因为,薄液环境更有利于氧的扩散传质、促进了局部氧的去极化过程和腐蚀产物层的致密化,该过程与Cl-在腐蚀产物内层的富集过程存在协同作用,加之应力集中作用,促进了锈层下的局部阳极溶解和点蚀底部及裂纹尖端的析氢作用,促进了SCC萌生和扩展。建立了模拟薄液环境中力学-电化学交互作用的原位测量装置,对模拟海洋薄液环境下O2、环境充氢对E690钢SCC行为的影响规律及作用机制进行分析。结果表明：O2的扩散传质对海洋薄液环境下的SCC起到关键作用,O2浓度升高,E690钢的阳极和阴极电流密度逐渐升高,耐腐蚀性能逐渐降低；SCC敏感性先增大,后减小。导致模拟薄液环境中E690钢SCC敏感性增大的临界氧浓度值为21%,当低于临界值时,氧浓度升高,氧扩散速度加快,促进阴极和阳极反应过程,加速了局部阳极溶解,当高于临界值时,裂纹被腐蚀消耗,同时氧还原降低了氢的析出,减小了氢的破坏作用。随预充氢电流密度升高,E690钢的阳极和阴极电流密度逐渐升高,耐腐蚀性能逐渐降低；氢加速了E690钢的阳极溶解作用,并与阳极溶解协同作用促进了E690钢SCC。随预充氢电流密度的增加,E690钢的SCC机理由以阳极溶解为主的混合机制向以氢脆为主的机制转变。结合有限元分析方法,通过不同应变量下模拟海洋薄液环境中的电化学及SCC试验,研究了应变对模拟海洋薄液环境中E690钢电化学和SCC行为的影响,结果表明：应变量增大,E690钢阳极和阴极电流密度逐渐升高,耐腐蚀性能逐渐减小,塑性应变区此现象尤为明显。应变量增大促进了E690钢阳极溶解,提升了SCC敏感性。模拟海洋干湿交替及薄液环境下,E690钢腐蚀产物都以Fe304为主,伴有α-FeOOH、β-FeOOH, γ-FeOOH、γ-Fe2O3、α-Fe2O3等,试验周期延长,有FeOCl、Fe2(OH)2Cl生成。锈层的结构和种类,对SCC敏感性影响明显,Cr促进了锈层的致密化,降低了均匀腐蚀,C1-富集在腐蚀产物内层,促进了锈层下的局部腐蚀,有利于SCC发生和扩展。验证了模拟薄液环境中外加电位对抑制SCC的可能性,确定了模拟薄液环境中E690钢SCC敏感性减小的最佳外加电位为-850mV (vs Ag/AgCl)。OCP电位升高或降低均能增大SCC敏感性,OCP-850mV为阳极溶解和氢脆混合机制,低于-850mV为氢脆机制。牺牲阳极保护方法减缓了E690钢的腐蚀,降低了SCC敏感性。
[Abstract]:The development and application of marine resources can not be separated from the development and application of high strength steel. With the development of marine resources, especially the high temperature in the South China Sea, high humidity and high salt fog, the demand for high strength steel is increasing, the demand for strength is increasing, and the problem of stress corrosion (SCC) and the risk are increasing gradually. However, there are few studies on the SCC behavior of high strength steel in the marine environment, especially in the marine environment, and lack of the study on the sensitivity and mechanism of SCC in the deep liquid environment, and the understanding of the relationship between the different environmental factors and the sensitivity of the SCC is not fully understood.
The electrochemical corrosion behavior of high strength steel in the simulated ocean dry wet alternate environment and the key influencing factors, behavior and mechanism of SCC are studied by establishing the electrochemical and SCC simulation methods under the dry and wet alternate environment. The results show that in the simulated ocean dry and wet alternate environment, the cathodic electricity of E690 steel is prolonged with the experimental period. The current density gradually increases, the anode current density decreases gradually, the corrosion resistance decreases first and the.PH value increases and the dry and wet alternate frequency increases slightly, which is beneficial to the enrichment and densification of corrosion products, which leads to the weakening of the uniform corrosion under the rust layer, the enhancement of the local anodic dissolution and the increase of the sensitivity of the SCC, the increase of the Cl- concentration and the increase of the sensitivity of the SCC first. Adding, then decreasing and increasing, when the concentration of NaCl is 3.5%, the SCC mechanism of SCC sensitivity relative to the highest.E690 high strength steel is a mixed control mechanism of anodic dissolution and hydrogen embrittlement, and the crack propagation mode is a typical transgranular expansion.
The comparison of the electrochemical and SCC mechanism of E690 high strength steel in simulated marine full immersion and thin liquid environment shows that the E690 steel will have obvious SCC in the thin liquid environment, and the SCC sensitivity of the thin liquid environment is higher than that in the seawater environment. The thin liquid environment is more beneficial to the diffusion of oxygen and promotes the depolarization process and corrosion production of the local oxygen. The densification of the material layer has synergistic effect with the enrichment process of Cl- on the inner layer of corrosion products, and the stress concentration effect promotes the local anodic dissolution under the rust layer and the hydrogen evolution at the bottom of the pitting and crack tip, and promotes the initiation and expansion of the SCC.
An in-situ measurement device was established to simulate the interaction of mechanical and electrochemistry in the simulated thin liquid environment. The influence law and mechanism of the E690 steel SCC behavior under the environment of O2 in the simulated marine environment were analyzed. The results showed that the diffusion mass transfer of O2 played a key role in the SCC in the marine thin liquid environment, the concentration of O2 increased, and the anode of E690 steel was the anode. With the increasing of the current density and the cathode current density, the corrosion resistance gradually decreases, and the sensitivity of SCC increases first and then decreases. The critical oxygen concentration of the SCC sensitivity of E690 steel in the simulated thin liquid environment is 21%. The oxygen concentration increases, the oxygen diffusion speed is accelerated, the reaction process of the cathode and the anode is accelerated, and the local anodic dissolution is accelerated. When the critical value is higher than the critical value, the crack is consumed, and the oxygen reduction reduces the hydrogen precipitation and reduces the hydrogen damage. With the increase of the hydrogen current density, the anode and cathode current density of E690 steel increases and the corrosion resistance gradually decreases, and hydrogen accelerates the anode dissolution of E690 steel, and promotes the synergistic effect with the anodic dissolution. With the increase of pre charge hydrogen current density of E690 steel SCC., the SCC mechanism of E690 steel changed from the mixed mechanism of anodic dissolution to the mechanism of hydrogen embrittlement.
The effect of strain on the electrochemical and SCC behavior of E690 steel in simulated marine environment was studied by the finite element method and the electrochemical and SCC tests were used to simulate the marine thin liquid environment. The results showed that the anode and cathode current density of the E690 steel increased gradually, the corrosion resistance decreased and the plasticity decreased. This strain is particularly evident in the strain zone. The increase of strain rate promotes the anodic dissolution of E690 steel and enhances the sensitivity of SCC.
The corrosion products of E690 steel are mainly Fe304, including alpha -FeOOH, beta -FeOOH, gamma -FeOOH, gamma -FeOOH, gamma -Fe2O3, alpha -Fe2O3 and so on. The test period is prolonged, FeOCl and Fe2 (OH) 2Cl are generated. The structure and types of rust layer have an obvious influence on the sensitivity of the SCC sensitivity, which promotes the densification of the rust layer and reduces the uniform corrosion. The inner layer of corrosion product promotes local corrosion under rust layer, which is favorable for the occurrence and expansion of SCC.
The possibility of inhibiting the SCC by the applied potential of the simulated thin liquid environment is verified. The optimum applied potential of the SCC sensitivity of E690 steel in the simulated thin liquid environment is determined by the increase or decrease of the -850mV (vs Ag/AgCl).OCP potential, which can increase the sensitivity of SCC, OCP-850mV is the mechanism of anodic dissolution and hydrogen embrittlement, and the mechanism of hydrogen embrittlement is lower than -850mV. The polar protection method has slowed down the corrosion of E690 steel and reduced the sensitivity of SCC.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG172

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