Corrosion performance of alloy 800H and alloy 625 for potential use as molten salts solar receiver materials in concentrating solar power tower plants

Abstract

Two high corrosion resistant super-alloys, alloy 800H and alloy 625, were evaluated for corrosion compatibility with molten nitrate salts at 565 °C under air atmosphere since these super-alloys are possible candidates for the manufacturing of molten salts solar receivers in the CSP tower technology. Both alloys are tested in two different molten nitrate salts grades to identify how the impurities of the final mixture affect to corrosion damage. Accordingly, a technical grade molten nitrate salt (Solar_Salt_T) and refined molten nitrate salt (Solar_Salt_R) are selected as test media. In addition to corrosion rates calculation, techniques such as XRD, EDS, optical and scanning electron microscopy are used to identify the corrosion morphology and oxides layers chemistry. Alloy 800H and alloy 625 show uniform corrosion after testing without detecting localized phenomena such as pitting, stress corrosion cracking, crevice, or intergranular corrosion. While alloy 800H develops a duplex oxide layer consisting of iron oxides in its external part, and chromium oxide in its innermost layer, alloy 625 generates a compact and highly adherent oxide layer consisting mainly of nickel oxide. Corrosion rates decrease with time, being higher for alloys exposed to Solar_Salt_T mixtures. Moreover, alloy 625 shows lower corrosion rates than alloy 800H in all conditions tested in this study.