Thermal Convection Loop Study of the Corrosion of Incoloy 800 in Molten NaNO₃-KNO₃

Abstract

The corrosion behavior of Incoloy 800 was studied using a thermal convection loop that operated between the temperatures of 300 and 600 C for 5000 h. The extent of corrosion of the removable test coupons was determined by chemical descaling, which indicated a maximum loss of --5µm at 600 C during the course of the experiment. The morphologies of corrosion products were examined by electron microprobe analysis (EMA) and x-ray diffraction (XRD) and found to consist primarily of Fe304 and Fe(Fe,Cr)204 over most of the temperature range. At temperatures near 600 C, however, complex multilayer scales were observed that consisted of two magnetite spinels, a Cr-rich M 203 oxide, subsurface oxide penetration, precipitation of metallic nickel, and internal nitridation. Other observations are reported concerning the occurrence of accelerated oxide penetration on unabraded surfaces of Incoloy 800 tubing and pitting at temperatures below 400 C, at which only very thin oxide layers are formed. Although chromium was selectively removed from the alloy and dissolved in the melt, its depletion was a secondary mechanism of metal loss. A mass balance demonstrated that thermal gradient mass transfer of chromium was negligible.

Corrosion Behavior of Structural Materials for Potential Use in Nitrate Salts Based Solar Thermal Power Plants

“…• C. The yellow coloration of the salt is indicative of chromium dissolution as observed previously by Bradshaw 5 and supported by ICP-OES analysis of the salt following exposure as will be discussed later.…”

“…• C. [4][5][6] Isothermal studies were also performed on stainless steels, Cr-Mo steels, low alloy steels, aluminum-coated steels, nickel, aluminum, and titanium. Incoloy 800, aluminum and the aluminized steels saw negligible corrosion, while Inconel 600 underwent internal oxidation, titanium rapidly formed an adherent surface oxide film, and nickel experienced severe intergranular corrosion.…”

Corrosion Resistance of Stainless Steels During Thermal Cycling in Alkali Nitrate Molten Salts

“…Corrosion of Incoloy 800 has also been studied at constant temperature in a thermal convection loop. [10] Coupons exposed at the same temperature as above displayed only half as much scale formation after 5000 hours. This contrast in scale formation suggests that thermal cycling may significantly affect corrosion rates relative to isothermal exposure.…”

Progression to Compatibility Evaluations in Flowing Molten Salts

“…Thus, much of the research is misguided with low quality and the status of molten salts in the 2017 CSP Generation 3 roadmap is not surprising. Most importantly, the roadmap has no discussion about moving to a flowing salt experiment, which has long been the ultimate test for molten salt compatibility [Susskind 1960, DeVan 1979, Keiser 1979, Bradshaw 1987. Like liquid metals, molten salts represent a unique form of corrosion, which is better described as compatibility or limited (i.e.…”