Microstructure Evolution and Stress Corrosion Cracking Susceptibility of 12Cr Martensitic Steel Upon Long-Term Service in Power Plants Z. Zhang, Z.F. Hu, P.M. Singh, X. Li, S. Xiong, and X.X. Fang (Submitted August 7, 2018) After 230,000-h long-term service at 550 °C/13.7 MPa, the microstructure of a kind of 12Cr martensitic steel was investigated using scanning electron microscopy, backscattering electron microscope and x-ray diffraction analysis. The results show that the precipitation and coarsening of carbides at grain/lath boundaries are the main cause of microstructure degradation. The static immersion tests and the slow strain rate test coupled with electrochemical impedance spectroscopy were conducted on the served steel in 1.0% NaCl solution, and it turns out that the pitting corrosion resistance and repassivation ability of the steel are significantly reduced as a result of microstructure degradation. The stress corrosion cracking susceptibility of the steel was also studied. Fracture morphology analysis shows that the secondary crack in conjunction with slip lines is a result of the coalescence of micro-cracks nucleated from the pits. Keywords 12Cr martensitic steel, microstructure, pitting corro- sion, stress corrosion cracking 1. Introduction 9-12%Cr martensitic steels have high strength, low thermal expansion, good corrosion resistance and good mechanical properties (Ref 1-3). These characteristics are the reasons for them being extensively used as structural materials for the steam generator components and fossil fired power plants in the past 50 years (Ref 4). The X20CrMoV12.1 (X20) steel, typically with 10-12% Cr, has been used as heat exchanger, steam pipes/tubes, boilers and turbine parts in Europe since the 1960s (Ref 5). The alloy has typical strengthening possibilities such as solid solution by high content of Cr and Mo, refined grains by formation of tempered martensitic lath structure and precipitation of the uniform dispersion of M 23 C 6 carbides that were Cr enriched and fine MX-type carbides (M denotes the metal elements, and X denotes nonmetallic elements of C and N) rich in V and Nb homogeneously. For the long-term application of the steels, it is necessary to assess the microstructural changes that are likely to occur during service exposure and to evaluate these changes on the mechanical properties of the steel, such as fatigue (Ref 6-8), creep (Ref 9- 11) and creep–fatigue interaction process (Ref 12-14). In addition to mechanical properties, corrosion resistance is also critical for the performance of the materials used in aggressive environments, such as high-temperature water (Ref 15), steam (Ref 16-18) and supercritical water (SCW) (Ref 19-21) environment. In general, steels with higher Cr contents show superior oxidation resistance as compared to those with lower Cr contents. However, the presence of aggressive species of chloride in the environment will reduce the corrosion resistance of the steels due to the localized breakdown of passivity and eventually lead to a significant decrease in its service life. During the long-term service in power plants, the products made by martensitic steel, such as tube, pipe and components, are inevitably in contact with the chloride in the external environment. Furthermore, both the storage and plant sites are often located close to marine environment; without any filtering of the sea air, the steels are exposed to a mixture of chloride and sulfate salts. Therefore, it is critical to understand the process of corrosion and stress corrosion cracking (SCC) of the steel in such chloride environment. WhatÕs more, considering that the components made of these kinds of steels must survive for an extended period of time (at least 40 years), the service of martensitic materials at elevated temperature causes a number of phenomena and changes in the microstructure, like the decrease in free dislocation density and the disappearance of some low-angle boundaries (LABs), as well as the precipitation and coarsening of carbides inside grain and along grain/lath boundaries (Ref 22-26). The change of the microstructure will also influence the mechanical, electrochemical and SCC of the steel. However, there is no general agreement on how these microstructural changes affect the corrosion resistance of steels. In the previous studies of carbon steels (Ref 27, 28), some authors consider that the microstructure not only modifies the characteristic and the kinetics of corrosion products formation but also has a significant effect on the action of corrosion inhibitors. During the exposure time, ferritic phase acts as anodic sites and dissolves, whereas iron carbide acts as a cathode, which is a consequence of the galvanic coupling that occurs between these microstructures. As for the low-carbon Z. Zhang, X. Li, S. Xiong, and X.X. Fang, School of Materials Science and Engineering, Nanjing Institute of Technology, Nanjing 211167, China; and Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, China; Z.F. Hu, School of Materials Science and Engineering, Tongji University, Shanghai 201804, China; and P.M. Singh, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta 30332-0245. Contact e-mail: 012zhangzhen@tongji.edu.cn. JMEPEG ÓASM International https://doi.org/10.1007/s11665-018-3840-6 1059-9495/$19.00 Journal of Materials Engineering and Performance