The development of high temperature solar thermal systems (also known as concentrated/ting solar power, CSP) containing molten salts as the heat transfer fluids is direly needed to replace fossil-fuel energy with solar thermal power. Even though efficiency of the CSP systems increases on elevating the working temperature, molten salt overwhelmingly corrodes containments and pipes of the systems at high temperature. Therefore, corrosion is a significant obstacle for developing high temperature CSP. Ni-alloys show superior corrosion resistance to Fe-alloys in high temperature molten salt but their high cost makes them less preferable. One of the possible ways to enhance corrosion resistance in high temperature molten salt is to apply protective coatings on economic substrates like steels or stainless steel. In this work, hot-dip aluminized coating was formed on 310S-type stainless steel and the coating degradation behavior was evaluated in eutectic ternary alkali carbonate salt mixture at 650 °C.

Hot-dip aluminized coatings were formed on 310S stainless steel coupons by immersing in Al-melt at 714 °C for 1.5 minutes and subsequently annealing at 700 °C for 40 minutes. Al-rich top layer was removed by a brief shot blasting. For enhancing the inter-diffusion resistance, these aluminized samples were further given an additional heat treatment and the details will be discussed during the meeting. All the samples were polished with SiC paper (#600 grit) to remove the outer porous layer. For diffusion and corrosion test at 650 °C, a eutectic salt mixture of 0.43 Li₂CO₃-0.32 Na₂CO₃-0.25 K₂CO₃ (mole fraction) was employed. To get mass loss due to corrosion, corrosion product was selectively etched in hot 2% CrO₃+ 5% H₃PO₄ solution. For electrochemical polarization, polished flag-shaped aluminized 310S samples were employed as the working electrode (WE); flag-shaped Pt as counter and oxygen electrode as reference electrodes. Potential was scanned from stable OCP with 10 mV s^-1. For EIS, two similar flag-shaped electrodes were used; and 10 mV AC voltage was applied to freely corroding electrodes. SEM, TEM, EPMA and XRD were used to study morphology and composition of the coating as well as that of corroded aluminized coating.

The as-aluminized coating was degraded remarkably by the inter-diffusion when it was exposed in the molten salt at 650 °C. The additional heat pretreatment prevented the coating to be damaged by the inter-diffusion. Further, the heat pretreated coating was polished to remove upper porous layer and to quantify precisely the electrochemical properties. At the same time, removal of the top layer of the coating led to expose the inner highly alloyed coating material to the surface of the sample. Electrochemical polarization, EIS and mass loss observations concluded that the coating enhanced the corrosion resistance of the stainless steel at least by two orders of magnitude. Analytical TEM study revealed that ~50 nm thin α-LiAlO₂ layer was formed due to corrosion. Role of composition of coating, formation of the passive film and resultant corrosion performance will be discussed in the meeting.

Acknowledgements

This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: Japan Science and Technology Agency, JST). Authors duly thank ‘Soken Tecnix Co., Ltd.’ for carrying out aluminizing of the specimens.