Materials Performance

NOV 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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37 MATERIALS PERFORMANCE: VOL. 57, NO. 11 NOVEMBER 2018 An additive package was developed by the group that would enable the use of cor- rosive molten chloride salts for CSP. The inhibitor package would prevent corrosion of SS in the molten chloride salt and allow the continued use of more economical alloys instead of costly superalloys in the CSP plants. The low-cost molten salt ther- mal storage, cheaper materials of construc- tion, and higher efficiency would decrease the cost of construction and maintenance of the CSP plant and increase the lifespan of the components. Studies have shown that in extreme con- ditions, chloride salts attack SS alloys (450 to 900 °C), leading to intergranular corro- sion and intergranular stress corrosion cracking. 7 No reports were found on the development of a corrosion inhibitor that can protect the steel surfaces in these con- ditions. Though thermal spray, sol-gel coat- ings, and chemical vapor deposition 8 have been studied in the literature, these meth- ods would be difficult to apply in miles and miles of piping in CSP systems. The addition of rare earth and alkaline earth metal oxides 9-10 has been shown to prevent chro- mium ion leaching when added to a chlo- ride salt melt but no coating formation was obser ved. Use of the designed inhibitor package would lead to an in-situ formation of a protective coating on the steel surface. For this current study, various binary and ternary chloride salt mixtures showing favorable thermo-physical properties, low cost, and low corrosivity were selected for inhibitor package addition. High-tempera- ture corrosion testing was performed on the SS alloys using the salt mixtures with and without the inhibitor package (consist- ing of rare earth metal and alkaline metal oxides). Experimental Procedure Technical grade chloride salts (sodium chloride [NaCl], potassium chloride [KCl], magnesium chloride [MgCl 2 ], lithium chlo- ride [LiCl], calcium chloride [CaCl 2 ], zinc chloride [ZnCl 2 ], etc.) were heated under f lowing nitrogen at 120 °C to remove the last traces of moisture and other volatile contaminants. The salts were weighed and mixed proportionally to form a binar y, LiKCl (LiCl and KCL), ZnNaKCl (ZnCl 2 , NaCl, and KCl), MgNaKCl (MgCl 2 , NaCl, and KCl), CaLiKCl (LiCL, KCl, and CaCl 2 ), and NaLiKCl (LICl, KCl, and NaCl). Type 316L SS (UNS S31603) alloy rods were procured with approximate composition (by weight): 0.03% C, 2.0% Mn, 0.75% Si, 0.045% P, 0.03% S, 16-18% Cr, 2-3% Mo, 10-14% Ni, 0.10% N, and the rest Fe. The samples were polished to 1,200 grit and cleaned by ultrasonication in acetone, rinsed in ultra-pure distilled water, and then stored in a desiccator. Melting points of the salt blends were measured using an electrothermal melting point apparatus. The stabilities of the can- didate salt mixtures were studied by heat- ing the salts in an inert atmosphere for 100 h at 800 °C, higher than the anticipated operating temperature (700 to 750 °C), to account for hot spots and long service life. The degradations of the salt mixtures were analyzed by measuring the chloride con- tent in the effluent gas and comparing the phase composition before and after various heat treatments. Chloride vapors forming due to degradation or breakdown of the salt mixture were captured using a 0.2 wt% sodium hydroxide (NaOH) solution. The chloride content in the fluid was then mea- sured at intervals using chloride test strips, titration, and ion chromatography. X-ray dif fraction (XRD) of the salts was per - formed to determine if they decomposed during heat treatments. A dynamic corrosion setup was fabri- cated to mimic the conditions when the salt f lows inside the SS piping in a CSP plant. A rotating Type 316L SS rod was inserted in the molten salt mixture inside an alumina tube within an upright tube fur- nace. The steel rod was rotated at 10 rpm within the salt. The testing was done for 100 h at 650 and 750 °C. Following the heat treatment, the metal rods were extracted from the salt mixture and cooled down under an inert gas blanket to prevent any corrosion in air. After cleaning the rods, small pieces of the rod were cut off and charact eri zed using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). FIGURE 1 Salt melting point vs. chloride loss and price.

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