Effect of Gas Switching on Corrosion Behavior of 304 Stainless Steel in Carbonate Melt at 650 °C
Authors: Sah Santosh Prasad , Tada Eiji , Nishikata Atsushi 
Published: February 2020

Concentrated solar power (CSP) technology is getting sheer attention for substituting thermal power plants. Cleanliness, voluminous capacity, energy-on-demand and easy-to-hybridize (with thermal plants) are the significant features of CSPs. Unfortunately, the cost of the electricity produced by CSP is much high. Developing high temperature CSPs is likely to make the electricity cheaper. Solar heat is generally preferred to contain in molten salts, and the high temperature molten salts often cause severe corrosion to the containment (metallic pipes and vessels) materials. Therefore, understanding and mitigating corrosion is very important for developing high temperature CSPs. Stainless steels are popular corrosion resistant materials, but they show higher corrosion rates in high temperature melts [1, 2]. One of the parameters that enhances corrosion of stainless steels in the melt of oxy-salts can be the chemical dissolution of corrosion products [3]. The extent of corrosion accelerated by such a chemical dissolution is not yet estimated. In this study, an interesting phenomenon of control of corrosion was observed when the gas environment of carbonate melt was changed during corrosion of 304 stainless steel in the melt at 650 ºC.
Corrosion products of different composition and microstructure were formed on 304 stainless steel by changing the gas environment of a carbonate melt at 650 ºC. N2-O2 and CO2-O2 mixed gases were used to control the chemistry and corrosivity of the melt. Electrochemical impedance spectrum (EIS) was observed when corrosion occurred in the melt with the same gas environment and with the changed one. SEM, EDS, XRD and gravimetry were observed to find the effect of gas environment that controlled the progress of corrosion.
When CO2-O2 was used as the gas environment of the melt from the very beginning of corrosion, a single layer of the corrosion products was formed that contained Cr-rich mixture of LiCrO2 and LiFeO2. In this case, corrosion was controlled by diffusion processes, i.e., the stainless steel was corroded severely. A bi-layered corrosion product was formed when N2-O2 was the gas environment from the beginning. In this case, the top layer of the corrosion products contained only LiFeO2, and the inner layer contained a mixture of LiCrO2 and LiFeO2. In this case too, the corrosion was a diffusion–controlled process. Interestingly, when the environment of N2-O2 was changed to CO2-O2, the diffusion impedance was completely disappeared, and a very large semicircle was observed from the Nyquist plot. Therefore, the bi-layered corrosion product formed with N2-O2 was turned very protective when the gas environment of the melt was switched to CO2-O2. The observed corrosion behavior will be discussed in terms of morphology and composition of the corrosion products
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