UC Berkeley

Nitrate molten salts are widely used in applications requiring high temperature heat transfer and thermal energy storage such as in the chemical and metal industries and in concentrated solar power plants. Recently, several advanced nuclear reactor designs have proposed the use of nitrate salt as an intermediate loop or thermal energy storage system. In doing so, nitrate salt is aimed to be operated at temperatures above the current max operating limits around 570°C. At these elevated temperatures, corrosion rates of materials in molten nitrate salt environments have been observed to markedly increase, owing in large part to the decomposition of the nitrate salt. While significant research has been conducted regarding elevated temperature corrosion of materials in nitrate salt, particularly rare are long-term, elevated temperature corrosion studies which enable more detailed understanding of the interplay between nitrate salt decomposition and material corrosion behavior. In this dissertation, the corrosion performance of steel and nickel alloys in elevated temperature Solar Salt (60wt% sodium nitrate, 40wt% potassium nitrate) was investigated. Chapter 3 describes the results from a multi-alloy corrosion experiment conducted at 630°C for 600hr. Twelve different materials were investigated, revealing significantly better corrosion performance for alumina-forming austenitic steel and nickel alloys when compared to typical stainless steels such as 316SS. This is followed by a relatively brief Chapter 4 in which 316SS was exposed in Solar Salt for 645hr at elevated temperatures ranging from 570°C-650°C. Through this temperature scoping study, a temperature inflection point was discerned in the rate of increased corrosion of 316SS, hinting at a change in the rate controlling process for corrosion within the salt. The heart of the dissertation revolves around a long-term corrosion study (Chapter 5) conducted on 316SS and Alloy 601 at a range of elevated temperatures up to 5000hr. Microscopy revealed a complex evolution in corrosion scale morphology exhibited by the materials as a function of both temperature and time. Solar Salt was found to become more corrosive over time to fresh samples, and over-nitriding of the metal surface was readily observed. These and other results were synthesized in a proposed corrosion mechanism for 316SS and 601 in elevated temperature Solar Salt with significant implications for the use of these and other related materials in molten nitrate salt applications.