Corrosion effects of Na2CO3/NaCl molten salt eutectic on 316L stainless steel under isothermal and cycling conditions

Abstract

Using phase-change materials in thermal energy storage systems for concentrating solar thermal plants can significantly reduce the volume of storage media and, therefore, the cost of the system. However, this approach introduces new material compatibility issues between the thermal energy storage material and the containment vessel in the form of corrosion and thermal transients, which are inherent in these systems. Rods and sheet coupons of stainless steel 316L were corroded for up to 768 h in 59.4/40.6 wt% Na2CO3/NaCl eutectic salt under isothermal (650 °C) and cycled conditions (between 600 and 650 °C). The testing methodologies correspond to actual service conditions, as isothermal degradation may not represent material loss under typical solar thermal cycling conditions. Mass losses were calculated via the Dynamic Gravimetric Analysis technique while Scanning Electron Microscopy utilizing Energy Dispersive Spectroscopy was used to analyze the corrosion layers. Isothermal mass loss plateaued at 75 mg/cm2 at 96 h with no further measurable loss after this time, while cycled samples had a fast initial mass loss rate of 60 mg/cm2 in the first 96 h and lost 0.0331 mg/cm2 every hour thereafter. The results indicate that isothermal samples corrode more slowly than the predominant industry cases of thermal cycling. Spalling is responsible for the degradation in both sets; the continual temperature changes in the cycled samples lead to additional stresses, exacerbating the effect.