Corrosion Behavior of Steam Turbine Materials for Geothermal Power Plants

Transcription

1 4th International onference on the Properties of Water and Steam in Kyoto orrosion ehavior of Steam Turbine Materials for Geothermal Power Plants Hiroshi Takaku, Li-in Niu, Hiromasa Kawanishi 2, Naoki Takamura 2, Kensuke Monma 2, Yoshihiro Sakai 3, Mitsuo Yamashita 4 and Kunio Shiokawa 5 Faculty of Engineering, Shinshu University, 4-7- Wakasato, Nagano Japan takakuh@gipwc.shinshu-u.ac.jp 2 Student of Graduate School of Eng., Shinshu University, 4-7- Wakasato, Nagano Japan 3 Fuji Electric Systems o.ltd., - Tanabeshinden, Kawasaki Japan 4 Fuji Electric dvanced Technology o. Ltd., 4-8-, Tsukama, Matsumoto ity Japan 5 Fuji Electric dvanced Technology o. Ltd., - Tanabeshinden, Kawasaki Japan To evaluate the corrosion behavior of steam turbine materials for geotherma l power plants, corrosion tests are conducted in the simulated geothermal water with corrosive chemicals. newly developed high purity 9%rMoV steel for rotor and a modified heat-treated 6r-4Ni steel for blade had a good general corrosion resistance, compared to the conventional materials. ue to the galvanic corrosion effect, rotor materials were subjected to the more severe crevice corrosion than blade materials. The S initiation and S propagation did not occur up to the 4 h corrosion test in the H 2 S free waters, although the sulfide induced stress corrosion cracking occurred in water with hydrogen sulfide. The combinations of corrosive chemicals on the electrochemical corrosion behavior were investigated for specimens with and without oxide films formed in the simulated geothermal water. The corrosion behaviors were discussed mainly based on the oxide film characteristics, material properties and the water quality.. Introduction s compared with the conventional fossil fuels, the geothermal energy is one of the clean and renewable energies because of almost no carbon dioxide (O 2 ) emission, and it could produce the vast energies most effectively by the existing technologies or by a little modification of the present technologies. The geothermal fluid, water and steam, and their mixtures, of which quality varies site to site contains many kinds of the corrosive chemicals such as chlorides, sulfates, hydrogen sulfide etc. which induce the corrosion problems. The corrosion of components of the geothermal plants is one of the most important subjects to be evaluated [-3]. On the other hands, the systematic data and knowledge have not been obtained sufficiently for maintenance, design and others of the steam turbine equipments in the geothermal power plant. In this work, some kinds of corrosion tests of the steam turbine materials for the geothermal power plants are experimentally conducted in a simulate geothermal water, to evaluate the generic corrosion characteristics. 2. Experimental Procedures Steam turbine materials tested are shown in Table. That is two rotor materials of a newly developed high purity 9%rMoV steel (steel ) and the presently used %rmov steel (steel ), Table. hemical composition of steels used (mass%) (*: not analyzed) Material Si Mu Ni r Mo V u 9%rMoV (Steel ) * %rmov (Steel ) * 3%r (Steel ) * * * 6r-4Ni (Steel ) * 3. 78

2 4th International onference on the Properties of Water and Steam in Kyoto R3 36 Fig.. onfiguration and size of lunt Notch ompact Tension (NT) specimen P Table 2. pplied maximum stress for NT specimens. Specimen Number Materials /9%rMo /%rmo /3%r /6r-4Ni Unit: MPa,.2 :.2% proof stress at 9 and two blade materials of the conventional 3%r steel (steel ) and the 6%r-4%Ni steel (steel ) subjected to the modified heat treatment. The corrosion test was conducted at 9 in a simulated geothermal water which contains 2 ppm l - 2- as Nal, 5 ppm SO 4 as Na 2 SO 4 and approximately 3 ppm O 2 by a continuous gas bubbling method as the main corrosive chemicals. The effect of corrosive chemicals on the general corrosion and also the crevice corrosion were investigated. The general corrosion test using the plate type specimen with thickness 2 mm and the crevice corrosion test using the coupled specimen with the rotor and blade materials were conducted. The stress corrosion cracking sensitivity (S) was evaluated using the two kinds of specimens shown in Figures and 2. Namely, the blunt notch compact tension (NT) specimen is used for S initiation and the double cantilever beam compact tension () specimen is for S propagation [4]. The stress is applied to the zone near the circular tip by the inserted wedge in NT specimens. Thereby, the maximum stress is formed at the inside of the hole-top on NT specimens. The Fig. 2. onfiguration and size of antilever eam () specimen. ouble maximum stress can be calculated from the following equation. 2K/( R) /2 where is the stress, K is the stress intensity factor and R is the radius of the hole. K is calculated according to the following equation [5]. K E h{3h(a.6h) 2 h 3 } /2 {4(a.6h) 3 h 2 a } - where E is the Young s modulus, is the crack opening displacement, h is the half value of the specimen height and a is the crack length. s shown in Tables 2 and 3, the levels of applied stress and stress intensity are variable for NT and specimens, respectively. nodic polarization curves were measured as one of the electrochemical corrosion tests, to evaluate the existence of the passivation state and the stability of the corrosion films during the stopping period of power plants. The measurement according to JIS G579 is carried out using the automatic equipment, in the various simulated geothermal waters with the variable corrosive chemicals. The system is composed of the three electrodes which are the sample electrode, the Pt couterpart electrode and the Hg/Hgl reference one. The scanning speed of the potential is 2 mv/min in this work. 3. Results and iscussion 3. General orrosion and revice orrosion Figure 3 shows the general corrosion rate by mdd [mg/(dm 2 day)]. It was clarified that the general 79

3 4th International onference on the Properties of Water and Steam in Kyoto Table 3. pplied stress intensity factor for specimens. Materials Specimen Number /9%rMoV K a /%rmov K a /3%r K a /6r-4Ni K a K: Stress intensity factor/mpa mm /2, a: rack length/mm 3 Steel 3 Steel 2 2 orrosion rate (mdd) 3 2 Steel 3 2 Steel orrosion time (h) Fig. 3. General corrosion rate, mdd [mg/(dm 2 day)]. corrosion rate of the blade materials (steel and ) shows smaller approximately / than that of rotor materials (Steel and ), and also that the new rotor material of steel and the modified heat-treated blade of steel have the very high general corrosion resistance. The aspect of the as-scaled and de-scaled surfaces after the 4 h. corrosion test is shown in Fig. 4. For steel, the corroded surface is very porous and also no passivation film is formed. On the other hand, for other three materials with the high r content, the thin film is formed on surfaces of under the as-scaled condition, and also the corrosion pits are observed in these materials. It is found that the corrosion films of these materials having the high general corrosion resistance are mainly composed of r 2 O 3 by the X-ray photoelectron spectroscopy (XPS). lthough these tight films may suppress the general corrosion, the localized break of these films due to l - ion may be easy to induce the corrosion pits. Figure 5 shows the typical aspect of corroded surfaces in the crevice using coupled specimens of blade materials (steels and ) and rotor steel. It is clarified that the steel was subjected to the severe corrosion as compared with the both blade materials. It is considered that this phenomenon is due to the galvanic corrosion effect in addition to the electric cell effect due to the difference in the oxygen concentration between the crevice and its outside water. 3.2 S Sensitivity s shown in Table 2, the S 72

4 4th International onference on the Properties of Water and Steam in Kyoto s-scaled escaled s-scaled escaled 25 m Fig. 4. spect of as-scaled and descaled surfaces. initiation test was conducted under the three stress levels condition of the maximum applied stress.8.2,..2 and.2.2 at 9 strengths in each material, up to the test duration 4 h. However, it is clarified that no S initiated for all materials. re shown in Fig. 6 the fractographs after the S propagation test conducted for 4 h at the applied maximum stress intensity. No cracks elongated from the precrack as S for all materials in this work. The hydrogen sulfide induced S (SS), especially for 3%r steel, occurred in our other work conducted in the water containing the H 2 S [3]. It is considered that the environmental-induced crackings such as S and corrosion fatigue may be rather suppressed when the general corrosion will be dominant in the relatively severe corrosive environment. 3.3 Electrochemical orrosion ehavior y the anodic polarization method, the corrosion tendency could easily evaluate from the existence of the passivation region, its length, the current density in passivation region, and so on. Figures 7 and 8 show effects of the l - ion concentration on the corrosion for the materials polished by Emery paper. From the figures, it is clarified that the l - ion accelerates the corrosion of all materials, and that the corrosion tendency is almost the same one tested in the simulated geothermal water at 9. Figure 9 shows the result tested in the water sampled from the corrosion loop water in which many kinds of metallic ions were dissolved. In this case, were observed the activated dissolution peaks for all materials except steel, and this water may be very severe for the materials corrosion due to the 72

5 4th International onference on the Properties of Water and Steam in Kyoto 25Ǵm Fig. 5. spect of descaled surfaces after 2 h crevice corrosion test. 2mm Fig. 6. Fracture surfaces of specimens after S test for 4 h (K=22 MPa/mm/2 ). 722

6 4th International onference on the Properties of Water and Steam in Kyoto urrent ensity ( /cm 2 ) : Steel : Steel : Steel : Steel Fig. 7. nodic polarization curve in water with 5 ppm SO ppm l - +O 2. urrent ensity ( /cm 2 ) : Steel : Steel : Steel : Steel Fig. 8. nodic polarization curve in water with 5 ppm SO ppm l - +O 2. urrent ensity ( /cm 2 ) : Steel : Steel : Steel : Steel Fig. 9. nodic polarization curve in water with 5 ppm SO ppm l - +O 2 + metallic ions. dissolved metallic ions. Figure shows the result of specimens with the scale (film) formed by the 5 h test in the simulated geothermal waters mentioned above. ompared with the other curves, it is clear that the corrosion films of all materials were extremely stable and had the good corrosion resistance in the severe corrosive water. urrent ensity ( /cm 2 ).8 : Steel.7 : Steel.6.5 : Steel.4 : Steel Fig.. nodic polarization curve of materials with scale by 5 h corrosion in water with 5 2- ppm SO 4 + ppm l - + saturated air at room temperature. (3) The corrosion by the anodic polarization measurement corresponds with that in a simulated 9 water. The films formed by corrosion in a simulated 9 water were stable even in the severe corrosive waters. 4. onclusions Essential results obtained are as follows: () 9%rMoV steel for new-developed rotor and 6r-4Ni steel for modified heat-treated blade have a good general corrosion and crevice corrosion resistance. The r content in materials controls the corrosion. (2) ll materials showed no S sensitivity in a simulated geothermal water without H 2 S, although the H 2 S in a water accelerates the S. References [] Y. Sakai, The Thermal and Nuclear Power, 5, 778 (999). [2] Geothermal Education Office, Homepage (23). [3] Y. Sakai, M.Yamashita, K. Shiokawa, L..Niu, and H. Takaku, Proc. IOPE-3, 3, 297 (23). [4] H. Takaku and H. Kimura, Proc. Inter. Symp. on Plant ging and Life Prediction of orrodible Structures, 793 (995). [5] amage Tolerant Handbook Part 2, ppendix., attelle olumbus Lab. (975). 723