Corrosion of stainless and carbon steels in molten mixtures of industrial nitrates

Abstract

The isothermal corrosion behavior of two stainless steels and carbon steel in mixtures of NaNO3 and KNO3 has been evaluated to determine if the impurities found in commodity grades of alkali nitrates aggravate corrosivity as applicable to an advanced solar thermal energy system. Corrosion tests were conducted for approximately 7000 hours with Types 304 and 316 stainless steels at 570°C and A36 carbon steel at 316°C in seven mixtures of NaNO3 and KNO3 containing variations in impurity concentrations. Corrosion tests were also conducted in a ternary mixture of NaNO3, KNO3, and Ca(NO3)2. Corrosion rates were determined by descaled weight losses while oxidation products were examined by scanning electron microscopy, electron microprobe analysis, and X-ray diffraction. The nitrate mixtures were periodically analyzed for changes in impurity concentrations and for soluble corrosion products.

Comparison of corrosion performance of grade 316 and grade 347H stainless steels in molten nitrate salt

“…2 that the metal loss rates for the two alloys do not extrapolate at shorter times through the origin (zero metal loss at zero exposure time). Similar behavior has been previously reported [1] and is most likely due to higher short-term corrosion rates during the initial immersion period. As the original surface finish is oxidized, corrosion rates reflecting the more characteristic uniform surface oxidation is achieved.…”

“…It is not possible to determine if this separation reflects the intrinsic adhesive character of the surface scales at temperature, or is simply the result of mechanical damage due to sample handling and metallographic preparation. The general appearance and structure of the oxides were quite comparable to that found in other studies that examined similarly exposed austenitic stainless steels [1] [5].…”

“…In the present case, the reagent grade salts had an initial chloride content of only 0.03 wt%. Since chloride has been found to increase corrosion rates (1,4,5), a decision was made to intentionally dope the mixture with chloride to near 0.1 wt% reflecting the maximum concentration likely to be encountered in the salts that would eventually be used in an actual CSP facility. NaCl and KCl were added in appropriate amounts to reach a final chloride-ion content of ~0.10 wt% for the test.…”

“…al. derived annual metal losses of 6-15 um for 316 stainless steel at 565°C for salts containing various levels of chloride [1] [4]. A recent study by Kruizenga examined 347 stainless steel at 600°C and projected an annual metal loss of 10.4 um [2].…”

“…Austenitic stainless steels have been identified as prime candidates for most hot salt (>500°C) handling based on several studies [1][2][3][4][5]. The work presented here was intended to confirm the generally acceptable behavior of these materials, to develop corrosion rates for potential hot salt containment materials and to optimize the material selection from a cost standpoint (that is, to demonstrate the similarities or differences in the molten salt corrosion behavior between stabilized or non-stabilized grades of stainless steel).…”