由于来自溶液的腐蚀性物种的离子或原子与金属上的金属离子或原子之间的强亲和力,OCTG(石油国家管状产品)在油气田的服务环境变得越来越严峻。传统技术难以准确分析OCTG在CO2-H2S-Cl-体系中的腐蚀特性,有必要研究TC4(Ti-6Al-4V)合金的原子或分子尺度的耐腐蚀性能。在本文中,用第一性原理模拟分析了CO2-H2S-Cl-体系中TC4合金TiO2(100)表面的热力学特性,并采用腐蚀电化学技术对模拟结果进行了验证。结果表明,腐蚀性离子(Cl-,HS-,S2-,HCO3-,和CO32-)在TiO2(100)表面上是桥位。Cl之间存在有力的电荷相互作用,S,Cl-中的O原子,HS-,S2-,HCO3-,CO32-,和Ti原子在TiO2(100)表面吸附后处于稳定状态。电荷从TiO2中的Ti原子附近转移到Cl附近,S,Cl-中的O原子,HS-,S2-,HCO3-,和CO32-。电子轨道杂交发生在Cl的3p5之间,S的3p4,2p4的O,和3d2的Ti,这是化学吸附。五种腐蚀离子对TiO2钝化膜稳定性的影响强度为S2->CO32->Cl->HS->HCO3-。此外,TC4合金在不同饱和CO2溶液中的腐蚀电流密度为:NaCl+Na2S+Na2CO3>NaCl+Na2S>NaCl+Na2CO3>NaCl。同时,Rs(溶液转移阻力)的趋势,Rct(电荷转移电阻),和Rc(离子吸附双层电阻)与腐蚀电流密度相反。由于腐蚀物种的协同作用,TiO2钝化膜对腐蚀物种的耐腐蚀性减弱。导致严重腐蚀,尤其是点蚀,进一步证明了上述仿真结果。因此,该研究结果为揭示OCTG的抗腐蚀机理和开发新型CO2-H2S-Cl-环境缓蚀剂提供了理论支持。
The service environment of OCTG (Oil Country Tubular Goods) in oil and gas fields is becoming more and more severe due to the strong affinity between ions or atoms of corrosive species coming from solutions and metal ions or atoms on metals. While it is difficult for traditional technologies to accurately analyze the corrosion characteristics of OCTG in CO2-H2S-Cl- systems, it is necessary to study the corrosion-resistant behavior of TC4 (Ti-6Al-4V) alloys based on an atomic or molecular scale. In this paper, the thermodynamic characteristics of the TiO2(100) surface of TC4 alloys in the CO2-H2S-Cl- system were simulated and analyzed by first principles, and the corrosion electrochemical technologies were used to verify the simulation results. The results indicated that all of the best adsorption positions of corrosive ions (Cl-, HS-, S2-, HCO3-, and CO32-) on TiO2(100) surfaces were bridge sites. A forceful charge interaction existed between Cl, S, and O atoms in Cl-, HS-, S2-, HCO3-, CO32-, and Ti atoms in TiO2(100) surfaces after adsorption in a stable state. The charge was transferred from near Ti atoms in TiO2 to near Cl, S, and O atoms in Cl-, HS-, S2-, HCO3-, and CO32-. Electronic orbital hybridization occurred between 3p5 of Cl, 3p4 of S, 2p4 of O, and 3d2 of Ti, which was chemical adsorption. The effect strength of five corrosive ions on the stability of TiO2 passivation film was S2- > CO32- > Cl- > HS- > HCO3-. In addition, the corrosion current density of TC4 alloy in different solutions containing saturated CO2 was as follows: NaCl + Na2S + Na2CO3 > NaCl + Na2S > NaCl + Na2CO3 > NaCl. At the same time, the trends of Rs (solution transfer resistance), Rct (charge transfer resistance), and Rc (ion adsorption double layer resistance) were opposite to the corrosion current density. The corrosion resistance of TiO2 passivation film to corrosive species was weakened owing to the synergistic effect of corrosive species. Severe corrosion resulted, especially pitting corrosion, which further proved the simulation results mentioned above. Thus, this outcome provides the theoretical support to reveal the corrosion resistance mechanism of OCTG and to develop novel corrosion inhibitors in CO2-H2S-Cl- environments.