Corrosion of Nickel Chromium Alloys in Molten Sodium Sulfate Salt at 900 C

Transcription

1 Corrosion of Nickel Chromium Alloys in Molten Sodium Sulfate Salt at 900 C Zack Gentry, Andrew Sakamoto, Matthew Corey, Norton Thongchua and Kishan Patel Faculty Advisor: Dr. Vilupanur Ravi Abstract Chromium is an excellent choice as an alloying element in metallic alloys for combating corrosion because it enables the formation of a protective and adherent oxide on the surface of the alloy. The role of chromium in the corrosion resistance of binary nickel-chromium alloys was studied by varying the chromium content in the alloys and exposing the specimens to molten sodium sulfate (Type I hot corrosion). Nickel-chromium samples were studied via two methods: immersion and salt drip testing. In these experiments, nickel-chromium alloys containing 2.5, 5, 7.5 and 10 wt% chromium were isothermally tested at 900 C for 25, 36, 49, 100, and 500 hours. For salt drip testing, an aqueous salt solution was used to coat the sample with approximately 3-8 mg/cm 2. For the immersion tests, the samples were exposed to molten sodium sulfate at 900 C. Corroded samples were subjected to X-ray diffraction and subsequently cross-sectioned. The sectioned samples were metallographically prepared and then characterized using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Optical microscopy was used to evaluate the extent of the attack by measuring the deepest penetrating corrosion front. The salt drip tests more closely simulated the Type I hot corrosion phenomenon depicted in literature, possibly due to the higher oxygen partial pressure in the melt as compared to immersion tests. This resulted in an identifiable corrosion front depicted as a hot corrosion attack. In this paper, the results of both tests along with the relevant characterization data are discussed. Background The corrosion of metallic alloys in contact with molten salts, known as Type I hot corrosion 1 1

2 affects gas turbine engines operating at high temperatures in marine atmospheres. Impurities, such as sulfur in fuels and sodium chloride from the atmosphere can combine to create a molten salt film on the surface of turbine blades and vanes 2. The molten salt compositions can vary considerably, and there is an ongoing need to understand the nature of their interactions with base alloys in order to commence a focused and effective mitigation effort. One approach to effectively defend against hot corrosion attack is by the addition of chromium as an alloying element. Chromium is an excellent choice because it forms a protective and adherent surface oxide layer 3. In this project, the effect of chromium additions was studied by varying the chromium content in model Ni-Cr alloys and exposing the specimens to molten sodium sulfate using both traditional 3, and new innovative techniques. Objective The overall goal of this project is to define the optimum amount of chromium needed to prevent hot corrosion of nickel chromium alloys, exposed to stagnant air and underneath a molten sodium sulfate (Na 2 SO 4 ) film at 900 C. In addition, different test methods will be developed to effectively predict hot corrosion attack. Increasing the amount of chromium in binary nickel alloys is expected to increase the ability of the alloy to resist hot corrosion attack. The nature of the test method is expected to have a significant effect on the outcome. Materials and Testing Pure nickel and nickel-chromium samples containing 2.5, 5, 7.5, and 10 wt % chromium were utilized in this study. Each individual sample was metallographically prepared to obtain a 600 grit finish using silicon carbide grit paper. The samples were exposed to molten sodium sulfate at 900 C in stagnant air for times of 25, 36, 100, and 500 hours. Three different testing methods 2

3 were employed. Immersion tests were conducted by submerging the sample in a crucible packed with an excess of reagent-grade sodium sulfate. Salt coat testing was carried out by first preparing a saturated sodium sulfate solution. The sample was placed on a hotplate, where the solution was deposited on the major face of the sample by the means of a dropper. The water vaporized leaving behind a thin adherent salt film on the surface of the sample. After testing, samples were ultrasonically cleaned, cross-sectioned, and polished to a 0.05 μm finish. Samples were characterized by using X-ray diffraction, optical microscopy and scanning electron microscopy (SEM)/electron dispersive spectroscopy (EDS). Discussion A matrix of optical micrographs comparing test time and alloy composition is shown in Figures 1 and 2. Nickel samples immersed for 25 h in Na 2 SO 4 with chromium contents in excess of 2.5 wt % produced an effective chromium-rich oxide scale which mitigated hot corrosion attack, while the 2.5 wt% Cr samples showed signs of hot corrosion attack. Salt coat tests conducted for the same amount of time showed similar results. When the immersion time was increased to 500 h Ni-2.5 wt % Cr samples showed significant hot corrosion attack, while nickel samples with a chromium content of 5 wt % and above were unaffected by hot corrosion attack. This is illustrated by the presence of thin oxide scales with no observable internal sulfidation. Conversely, samples of all chromium contents exposed to salt coat tests for 500 h were unable to deter the hot corrosion attack. Post-test analysis, using scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) was used to characterize the surface composition of the samples. The Ni-2.5 wt % Cr sample, immersed in Na 2 SO 4 for 25 h, indicated that the surface layer contained both chromium and nickel oxides (Figure 3). EDS also showed internal sulfidation beneath the 3

4 corrosion front within the substrate. This suggests hot corrosion attack due to the characteristic external oxide scale formation and internal sulfidation 4-8. X-ray diffraction of a sample containing 7.5 wt % Cr immersed for 100 h, indicated the presence of three oxides: Cr 2 O 3, NiCr 2 O 4, and NiO (Figure 4). For this same time interval, a salt coated Ni 7.5 wt % Cr sample indicated a single oxide: NiO (Figure 5). The corrosion resistance of the immersed samples may be attributed to the presence of the protective chromium oxide layer. Summary and Conclusions The corrosion behavior of a series of binary nickel chromium alloys ranging from 2.5 to 10 wt % chromium was determined using immersion and salt coat testing. The tests were conducted at 900 C in stagnant air. The samples were examined using optical microscopy, scanning electron microscopy (SEM) coupled with electron dispersive spectroscopy (EDS), and X-ray diffraction (XRD). In the immersion tests, only the Ni-2.5 wt % Cr samples showed evidence of significant hot corrosion attack after 500 hours of exposure, whereas the rest of the Ni-Cr alloys appeared to be only marginally affected. Therefore the oxide layer was effective in defending against hot corrosion attack under these conditions. All samples subjected to the salt coat tests showed evidence of more aggressive hot corrosion attack compared to the similar samples in the immersion tests. After 500 hours of salt coat testing, it is clear that even Ni-10 wt % Cr was not sufficient to defend against hot corrosion attack. The difference in results may be attributed to the partial pressures of oxidizing molecules in the molten salt. In both test protocols, increasing the Cr content was beneficial in regards to defending against hot corrosion; however, the optimum Cr content to prevent hot corrosion was quite different. Great care must therefore be exercised in the type of test that is chosen to predict hot corrosion behavior. 4

5 Future Work Future testing of binary nickel-chromium alloys will continue using the control volume technique. This testing method is expected to provide more accurate and reproducible results as compared to the immersion and salt coat tests. Initial results of this testing method has shown results similar to the salt coat tests, which may be more representative of the actual environment that the alloys would be exposed. Electrochemical testing will also be performed on the same set of binary nickel chromium alloys. This will be conducted in a controlled SO 2 -O 2 atmosphere. The electrochemical tests will allow the corrosion rate to be measured in situ. Acknowledgements We would like to extend our great appreciation to our faculty advisor Dr. Vilupanur Ravi. We also gratefully acknowledge financial support from Ms. Sylvia Hall, the LA section of NACE International, Western States Corrosion Seminar (WSCS), Western Area of NACE International, the NACE foundation and California Steel Industries, Inc. We also thank Dr. Juan Carlos Nava, Ulus Ekerman, Dr. John Klasik, David LeFay, and Armen Kvryan and past team members Charles Gepford and Stephen Schoniger, as well as Dr. Erin McDevitt (ATI), Tim Huddy (Elite Technologies), and Thurston LeVay (QB Instruments, Inc.) 5

6 Appendix 2.5 wt% Cr 5.0 wt% Cr 7.5wt% Cr 10.0 wt% Cr 25 h 500 h Figure 1: Optical micrographs of immersion tested samples 2.5 wt% Cr 5.0 wt% Cr 7.5wt% Cr 10.0 wt% Cr 25 h 500 h Figure 2: Optical micrographs of the salt coat samples 6

7 Spectrum Label %O %S %Cr %Ni Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Spectrum Figure 3: SEM/EDS analysis of a Ni wt% Cr sample, immersion tested for 100h Figure 4: XRD Scan of a Ni sample containing 7.5 wt% Cr and immersion tested for 100h Figure 5: XRD Scan of a Ni sample containing 7.5 wt% Cr and salt coat tested for 100h 7

8 References 1. R. A. Rapp, N. Otsuka, "The Role of Chromium in the Hot Corrosion of Metals," ECS Transactions, 16 (2009) G. Y. Lai, High Temperature Corrosion and Materials Applications, ASM International, Materials Park, Ohio (2007) R. A. Rapp, N. Otsuka, "Hot Corrosion of Preoxidized Ni by a Thin Fused Na 2 SO 4 Film at 900 C," ECS Transactions, 137 (1990) F. Petit, "Hot Corrosion of Metals and Alloys," Oxidation of Metals, 76 (2011) J.A. Goebel and F.S. Petit, "Na 2 SO 4 Induced Accelerated Oxidation (Hot Corrosion) of Nickel," Metallurgical Transactions,), 1 (1970) N. Otsuka, "Electrochemical Study of Hot Corrosion of Ni at 900 C," MS Thesis, The Ohio State University, (1988) Y. S. Zhang, "Solubilities of Cr 2 O 3 Fused Na 2 SO 4 at 1200K," J. Electrochem. Soc.: Solid- State Science and Technology, 133 (1986), R. A. Rapp, Hot Corrosion of Materials: A Fluxing Mechanism? Corrosion Science, 44 (2002)