Hot Corrosion of a Nickel Chromium Alloy Underneath a Molten Sodium Sulfate Salt at 900 C
Authors:Nicholas R Agee-Acosta, Karim M Ali, Armando R Coronado, Charles Gepford, Stephen F Schoniger, Norton Thongchua, Obed Villalpando
Mentor:Vilupanur Ravi, Professor and Chair, Chemical and Material Engineering, California State Polytechnic University Pomona
The corrosion of metallic alloys in contact with molten salts is known as “hot corrosion”. The composition of the salts, and the alloys in contact with them, vary considerably; however, there is an ongoing need to understand the nature of these interactions so that mitigation efforts can be launched in a focused and meaningful way. Nickel and nickel-chromium alloys exposed to molten sodium sulfate in air are being studied to understand the role of chromium and to determine the minimum amount of chromium in nickel-chromium alloys necessary to achieve protection against hot corrosive environments. Chromium is an excellent choice as an alloying element for combating hot corrosion because it enables the formation of a protective and adherent oxide on the surface of the alloy. The samples were either immersed in sodium sulfate (Na2SO4), or coated with a thin film of the salt. The samples were heated in air to 900℃ with exposure times varying between 1 – 25h. Mass change data were plotted as a function of exposure times to obtain empirical corrosion equations which showed parabolic behavior. Parabolic rate constants were established for pure nickel in sodium sulfate. In addition, exposed samples were characterized using X-ray diffraction before and after removal of the surface salt layer. Samples were then sectioned, mounted, ground and polished according to ASTM G 31. The polished cross-sections were examined using optical microscopy. Test data from experiments with Ni – 20 wt% Cr and Ni – 30 wt% Cr and relevant literature in this area will be included as a part of the discussion.