Corrosion of alloys in molten nitrates

ABSTRACT:
As a portion of a broad based effort involving studies of heat transfer fluids for solar collectors, we initiated work .in support of efforts by Sandia National Laboratory and others to utilize molten nitrates as such fluids. Initially, the intent had been to study the behavior of a

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variety of metal ions at high temperatures in the (K,Na)N03 binary molten salt. In all this work, a gold electrode of relatively large area was employed as a reference electrode and was checked for stability against a Ag/Ag(I) electrode. We first investigated the behavior of glassy carbon as a potential indicator electrode; it behaved rather similarly to prior investigations of Pt and Au indicator electrodes in these melts in that evidence indicated that N03- reduction, passivation and dissolution processes at the glassy carbon electrodes were about the same as those found for Pt or Au electrodes. On the other hand, the anodic limit of the melt, at the electrode, at relatively low temperatures «300°C), was shifted to more cathodic values than at either Pt or Au, and, as the temperature was raised, the behavior of the electrode became sufficiently complex that further studies were abandoned. The behavior of platinum and gold as indicator electrodes was studied up to 550°C. Although both electrodes behave in a similar manner, with increased complexity of the electrode reactions taking place in the pure melt as the temperature is raised, it was determined that Pt appeared to behave in a simpler, more reproducible manner than did gold. Both electrodes showed evidence of attack if prolonged reduction of N03- were performed at the electrode surface.

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