Experimental Study on Thermal Alternating Corrosion Behavior of Q345R (Carbon-Manganese Steel) and 347H (UNS S34709) in KNO3-NaNO2-Na2CO3 Ternary Mixed Molten Salts

Thermal shock cycling experiments are conducted in this paper to explore the effects of extreme conditions on the corrosion rate, surface morphology, and corrosion products of both Q345R (carbon-manganese steel) and 347H (UNS S34709) in KNO3-NaNO2-Na2CO3 mixed molten salt. Scaning electron microscopy, energy dispersive spectroscopy, x-ray diffraction (XRD), and other methods are used to characterize morphologies and microstructures of the samples obtained under the static and cyclic corrosion conditions. The experimental results showed that the corrosion products of the 347H sample are mainly FeCr2O4, Fe3O4, Fe2O3, and NaFeO2, while those of the Q345R sample only include Fe3O4, Fe2O3, and NaFeO2. Besides the thermal cycling operation’s effects on the electrochemical corrosion process of metals in the molten salts, Na2CO3 addition in the molten salt is also proven to be a key factor affecting the corrosion process and further altering both phase constituent and proportion in the corrosion layer. Especially, the experiment discloses that Na2CO3 also should be responsible for the surface corrosion morphology transition of the corroded sample due to its ability to enhance the inhomogeneous distribution of the corrosion rate on the sample surface. In addition, in comparison with the Q345R steel, the 347H steel has been proven to be more sensitive to the cycled heat-treatment. Further analyses on the XRD data suggest that several reasons, including reduction of Cr2O3& FeCr2O4, increment of brittle NaFeO2 and Fe2O3 phase in the scale resulting from the cycled operation, and discrepancy in thermal property resulting from the complex phases in the scale, etc., can be used for explaining the weakened corrosion behaviors of the 347H steel in the molten salt used in experiments.