Corrosion Performance of Ceramic Coatings Based on SiC and MoSi2 on Ni-Based Superalloy for Iodine Sulfur Cycle for Hydrogen Production /

Abstract

In this era, renewable energy technologies are suitable to meet the challenges of fossil fuel depletion and global warming. Thus, hydrogen fuel is gaining attention as an alternative clean fuel source that can be produced from various methods, one of them is the iodine-sulfur (IS) cycle that is a thermochemical process. One of the important components in the IS cycle is a process heat exchanger (PHE) which requires excellent mechanical properties at an elevated temperature as well as a high corrosion resistance in SO2/SO3 environment. In this work, Hastelloy X (HX) superalloy was chosen as a structural material for the sulfuric acid decomposition stage due to its excellent mechanical strength and oxidation resistance at high temperatures. However, HX cannot withstand high-temperature SO2/SO3 environments. Therefore, applying a protective coating is a viable option. For that purpose, silicon carbide (SiC) is a potential material that can tolerate this harsh environment but there is a difference in the coefficient of thermal expansion (CTE) of SiC and HX. The CTE of molybdenum disilicide (MoSi2) lies between SiC and HX and it also has high oxidation resistance. Hence, MoSi2 can be utilized as an interlayer between SiC and HX. Herein, MoSi2 and SiC films were deposited via physical vapor deposition methods. In other configurations, SiC was also fabricated through low-cost methods such as the dip-coating technique. Corrosion tests of bare HX and coated samples were performed at different temperatures (60℃, 120℃, and 300℃) in 98% sulfuric acid via the weight-loss method. Furthermore, the corrosion behavior of bare HX, MoSi2 coated HX, SiC coated HX (dip coated), and SiC/MoSi2 coated HX were analyzed through SEM, AFM, FTIR, and XRD. Bare HX exhibited minute weight change and hence, low corrosion rate at low temperature (60℃ and 120℃). However, at high temperatures (300℃) the corrosion rate increased significantly. MoSi2 coating displayed minute weight change as the corrosion resistance of MoSi2 coated HX improved by 48% as compared to bare HX at 120℃. The corrosion resistance of SiC coated HX improved by 97% in comparison to bare HX at 120℃, which considers it sustainable for corrosion protection against sulfuric acid. SiC/MoSi2 coated HX exhibited the best corrosion rate at 300℃. Optical micrographs revealed that both MoSi2 and SiC/MoSi2 coatings remained intact after corrosion at high temperatures. SEM results revealed that there were coalescent particles on the surface of the MoSi2 coated substrate after being immersed in 98% sulfuric acid at 60℃. Moreover, no voids or microcracks were observed for 60℃ and 120℃. This work demonstrates that SiC coated HX with MoSi2 interlayer can be an aspiring candidate to be utilized as a corrosion-resistant layer for IS cycle.