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 NaN03 and KN03 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

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tests were conducted for approximately 7000 hours with Types 304 and 3 16 stainless steels at 570°C and A36 carbon steel at 3 16°C in seven mixtures of NaN03 and KN03 containing variations in impurity concentrations. Corrosion tests were also conducted in a ternary mixture of NaN03, KN03, and Ca(N03)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. Results of these tests indicated that the short-term corrosion rates of the stainless steel specimens in many of the mixtures could be described in terms of parabolic kinetics. However, no single rate law could be assigned to the corrosion kinetics resulting from exposure in all of the mixtures. For engineering applications, corrosion rates over the entire exposure period are best described as linear with respect to time. In the binary nitrate mixtures, the annualized rates of metal loss were found to be between 6 and 15 prn/yr for the stainless steel specimens at 570°C depending on the particular mixture. Metal loss for the carbon steel specimens immersed in these same mixtures at 316OC extrapolated to approximately 1 to 4 pm/yr. Scanning electron microscopy and X-ray diffraction revealed that the complex, multiphase surface oxides formed on the stainless steel coupons were comprised primarily of iron-chromium spinel, iron oxides, and sodium ferrite. Magnetite was the principal corrosion product formed on the carbon steel specimens. Overall, for the typical range of impurities in commercially available nitrate salts, corrosion rates for solar thermal energy applications remained acceptable for all of the materials examined.

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