Corrosion of Two Iron-Based Aluminaforming Alloys in NaCl-MgCl2 Molten Salts at 600 °C.

Molten salts have been used as heat transfer fluids since the middle of the 20th century. More recently, molten chloride salts have been studied for use in concentrated solar power plants or molten salt reactors. However, none of the materials studied to date has been able to withstand this highly corrosive environment without controlling the salt's redox potential. The alumina-forming alloy was a promising option, as it has not yet been widely studied. To investigate this possibility, two iron-based alumina-forming alloys were corroded in NaCl-MgCl2 eutectic at 600 °C for 500 h after being pre-oxidised to grow a protective layer of α-alumina on each alloy. A salt purification protocol based on salt electrolysis was implemented to ensure comparable and reproducible results. During immersion, alumina was transformed into MgAl2O4, as shown by FIB-SEM observation. Inter and intragranular corrosion were observed, with the formation of MgAl2O4 in the corroded zones. The nature of the oxides was explained by the predominance diagram. Intragranular corrosion was 2 µm deep, and intergranular corrosion 10 µm deep. Alumina formed at the bottom of the intergranular corrosion zones. The depth of intergranular corrosion is consistent with O diffusion control at the grain boundary.

A DFT study of defects in paramagnetic Cr2O3.

Cr2O3 is not only a promising functional material, but also an essential barrier to protect chromia-forming alloys against high temperature corrosion. The Cr2O3 protecting layer grows slowly via defect-mediated diffusion. Several types of point defects could be responsible for the diffusion process depending on the oxidation environment, resulting in different semiconductor characters of chromia. According to the literature, the defect chemistry of Cr2O3 in the antiferromagnetic (AFM) state has been well studied using density functional theory (DFT) calculations but not in the paramagnetic (PM) state, which is the fundamental state of Cr2O3 above 318 K. PM Cr2O3 is simulated in this study using special quasi-random structures (SQS). The formation energies of intrinsic point defects in AFM and PM Cr2O3 are calculated to study the defect chemistry and the semiconductor properties in different oxidation environments (temperature and oxygen partial pressure PO2) using a thermodynamic model. It is found that O vacancies and insulating-type Cr2O3, in which commensurate electrons and holes are dominant before atomic defects are more favorable at high temperatures and at low PO2, while Cr vacancies and p-type Cr2O3 are more favorable at low temperatures and at high PO2, according to the calculations both in AFM and PM Cr2O3. However, the limits of dominant zones for defects and for semiconductor characters shift to higher temperatures or lower PO2 in PM state calculations.