Corrosion of stainless steel 316 in a ternary nitrate salt and its composite with expanded graphite for solar thermal applications

Abstract

The most concern of using nitrate salt mixtures in a concentrated solar power plant is their corrosion to the working pipes and containers. This paper studied the corrosion behavior of stainless steel 316 in a ternary nitrate salt mixture at three different conditions using static corrosion method. The mass losses of each sample were carefully measured. The instantaneous and average corrosion rates were predicted. The surface and cross-sectional morphologies with the corrosion products of the 316SS coupons after the test were investigated using SEM (Scanning Electron Microscope) combined with EDS (Energy-dispersive X-ray spectroscopy) and XRD (X-ray Diffraction) analyses. The results showed that temperature is the main factor to affect the average corrosion rate, for example, 0.0032 mm/a at 300 °C and 0.016 mm/a at 500 °C during the test period. At 300 °C, adding 10 wt% EG can reduce the average corrosion rate by 20%. The cross-sectional morphologies revealed that the corrosion layer of 316SS coupon in the composite with 10 wt% EG was 1/3 thinner than that in pure salt mixture at 300 °C. That was because the CaCO3 phase formed by the reaction of Ca(NO3)2 and EG could work as a protective layer to prevent further corrosion to some extent.