中国科学院海洋研究所机构知识库

近年来，国内外对于绿色发展的需求不断增加。随着绿色能源天然气越来越广泛利用，石油天然气工业设施建设也在高速发展。LNG（液化天然气）储罐是液化天然气储运过程中的重要组成部分，而9Ni钢是LNG储罐内罐的主体材料。水压试验是储罐建造中不可或缺的工序，出于节约资源和环境保护考虑，工程应用中开始使用海水代替淡水试压，而海水的强腐蚀性必将对罐体材料9Ni钢产生危害。因此，LNG储罐用9Ni钢在海水中的腐蚀行为及防护方法研究就显得尤为重要。

本论文采用浸泡失重法以及金相显微测试、X射线衍射、拉曼光谱、红外光谱、扫描电子显微镜、EDS能谱等表征分析手段，同时结合开路电位、电化学阻抗谱和动电位极化曲线等电化学测试方法，探讨了LNG储罐用9Ni钢在我国四大海域天然海水环境中的腐蚀行为，研究了304L不锈钢作为储罐内支架材料对9Ni钢造成的电偶腐蚀影响，阐明了模拟海水（3.5%NaCl）中温度、Cl^-浓度对9Ni钢腐蚀的影响规律以及9Ni钢的腐蚀机理。评价了缓蚀剂保护、牺牲阳极法、外加电流法三种腐蚀防护技术的保护效果。

研究结果证明，在我国四大海域海水中，9Ni钢的腐蚀特征相似，主要发生局部腐蚀，其产物疏松易脱落，腐蚀差异性主要与各个海域的水质电导率和盐度相关，海水盐度、电导率由大到小分别为南海、黄海、渤海、东海，对应的腐蚀速度分别为0.0761、0.0742、0.0712、0.0627g/m²·h，且随着浸泡时间的延长，腐蚀速度加快，30天内并未生成保护性腐蚀产物层。研究也发现304L不锈钢和镍基焊缝都要比9Ni钢稳定，其中304L不锈钢与9Ni钢之间存在约±0.56V的电偶电位差，二者连接必然引发电偶腐蚀，对主体材料9Ni钢起到加速腐蚀的作用。

在模拟海水（3.5%NaCl）中，9Ni钢的腐蚀速度随着温度（5-35°C）的上升而增大，随着Cl^-浓度（0.06-0.6M）的增大而增大。经形貌和成分分析发现，9Ni钢的马氏体和逆转变奥氏体微观组织结构使得其主要发生选择性腐蚀，其外层腐蚀产物为γ-FeOOH、α-FeOOH和Fe₃O₄，而中间层出现富Ni层且并未对9Ni钢基体起到保护作用，反而加速了基体的腐蚀。

此外，通过浸泡失重实验和电化学测试方法对三种腐蚀防护方法的保护效果进行对比研究，牺牲阳极保护法是9Ni钢储罐海水试压最佳临时保护技术，保护度可达到85%以上。

In recent years, the demand for green development has continuously increased. With the rapid increase of the application of green energy and natural gas, the construction of oil and gas industry facilities is also developing. LNG (Liquefied natural gas) storage tanks are important for the LNG storage and transportation process, and 9Ni steel is the main material of LNG storage tanks. Hydraulic pressure test is an indispensable process in tank construction, and due to the demand for environmental protection and resource conservation, seawater is used instead of freshwater. However, seawater will cause severe corrosion of 9Ni steel tank. Therefore, it is very important to study the corrosion behavior and protection methods of 9Ni steel in seawater.

In this experiment, the corrosion behavior of 9Ni steel in the natural seawater from the China's four sea areas, the South China Sea, the Yellow Sea, the Bohai Sea and the East China Sea, was studied via immersion weightlessness test, metallographic microscope, X ray diffraction, Raman spectroscopy, Infrared spectroscopy, scanning electron microscope, EDS spectrum analysis, and open circuit potential measurement, electrochemical impedance spectrum, potentiodynamic polarization and so on. The effect of galvanic corrosion between 9Ni steel and 304L stainless steel as holders within tanks had also been studied in Yellow seawater. Meawhile, the influence of temperature and Cl^- concentration on the corrosion of 9Ni steel in simulated seawater and the corrosion mechanism were clarified. The three corrosion protection methods including corrosion inhibitor, sacrificial anodes and impressed current had also been compared.

The experimental results showed that the corrosion characteristics of 9Ni steel were similar in the natural seawater from the China's four sea areas. Selective corrosion of 9Ni steel mainly occured, and the product was loose and easy to fall off. The difference was mainly related to the water conductivity and salinity of each sea area. It was found that the salinity and conductivity decreases in the order of the South China Sea, the Yellow Sea, the Bohai Sea and the East China Sea, and the corrosion rates were 0.0761, 0.0742, 0.0712, and 0.0627 g/m²·h, respectively. The 9Ni steel surface did not produce a protective product layer within 30 days, and the corrosion rate accelerated as the immersing time increased. The 304L stainless steel holder and weld in storage tank were relatively stable compared to 9Ni steel, and there was a galvanic potential of approximately ±0.56 V between 304L stainless steel and the 9Ni steel, which may cause galvanic corrosion and accelerate the corrosion of the 9Ni steel.

In 3.5% NaCl simulated seawater, the corrosion of 9Ni steel became more severe as the temperature increases from 5-35°C. Corrosion rate of 9Ni steel slightly increased with the increase of Cl^- concentration in the range of 0.06-0.6 M. The morphology and composition analysis revealed that the martensite and reversed austenite microstructures of 9Ni steel mainly caused selective corrosion, and the outer layers of corrosion products were γ-FeOOH, α-FeOOH and Fe₃O₄. The presence of a Ni-rich layer in the middle layer did not protect 9Ni steel. Instead, it accelerated the corrosion of the substrate.

According to the comparative study of three corrosion protection methods, the best protection technology was sacrificial anode cathodic protection and the degree of protection can reach more than 85%.