Solar thermal irradiation cycles and their influence on the corrosion behaviour of stainless steels with molten salt used in concentrated solar power plants

In this study, the corrosion behaviour of stainless steels AISI 321 and 347 in molten nitrate salt were investigated for up to 28 days. Three different thermal conditions; isothermal, thermal cycling and thermal shock, were explored to investigate the effect of the likely temperature gradients on the corrosion characteristics of AISI 321 and 347 during the operation of Concentrated solar power plants. Gravimetric corrosion analysis was combined with micro-morphology and cross-sectional analysis of the corrosion interface; using a combination of scanning/transmission electron microscopy, energy dispersive X-ray spectroscopy and X-Ray diffraction techniques, for this study. Results from corrosion rate measurements and surface characterisation show that thermal shock induces higher material degradation rate than the two other temperature profiles. Severe spallation of interfacial corrosion oxide layers was observed under thermal shock conditions other than in isothermal and thermal cycling conditions and correlated to the overall performance after 28 days. The level of spallation of interfacial oxides across the three temperature profiles investigated in this study was linked to the induced stresses from mismatch in the thermal expansion properties between the corrosion oxide layer and the substrates.