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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38 NOVEMBER 2018 W W W.MATERIALSPERFORMANCE.COM CHEMICAL TREATMENT The surface morphologies of the cor- roded SS in various salt mixtures at 750 °C are shown in Figures 3(a) and (b). EDS con- firmed that the composition of the corro- sion scales formed on the SS surface in the presence of LiKCl at both temperatures consisted of a granular iron oxide layer with nickel segregation at the grain boundaries. The oxide layer was brittle and it spalled at various places to expose the steel surface, with leaching of chromium to the surface. For the CaLiKCl salt, scattered films of cal- cium and oxygen were observed. Leaching of Cr left pores in the iron oxide matrix. The steel surface exposed to NaLiKCl consisted of oxide fibers of chromium(III) oxide (Cr 2 O 3 ), with nodules of iron oxide and Ni segregation on th e sur face. Such pro- nounced features were observed more at the higher temperature, 750 °C. The three salt mixtures were mixed with a proprietary inhibitor package devel- oped by the group. Representative SEM in Figures 4(a) and (b) demonstrated a crystal coating on the steel surface. Almost full coverage of these crystals was observed on the steel surface for the LiKCl and CaLiKCl salts at 750 °C. Based on the presence of the underlying Cr 2 O 3 , these crystals at 750 °C showed ~98.7% coverage for LiK, and 97% coverage for CaLiK. For similar inhibitor concentrations, the coverage at 650 °C dropped to 93% for LiK and CaLiK. For NaLiK, the coverage was scattered and var- ied from 68 to 97% at both temperatures. The XRD results from the heat-treated SS s a m p l e s c o n f i r m e d t h e f o r m a t i o n of perovskite crystals on the surfaces of all the samples examined. Corrosion testing on the MgCl 2 - and ZnCl 2 -based salt blends showed severe cor- rosion predominantly through the SS grain boundaries. Summary and Conclusions Suitable lithium-containing binary and ternary chloride salt mixtures with favor- able thermo-physical properties (low melt- ing point <400 °C and thermally stable >750 °C) were identified. The ternary composi- tions provided more economically viable solutions as compared to the binary salts because of lower lithium content. It was observed that the minor addition of certain compounds to the salt blends can increase FIGURE 2 A representative x-ray diffraction plot of CaLiKCl salt before and after heat treatment. FIGURE 3 SS surface morphologies with corrosion products (uninhibited salt exposure). (a) (b) Results and Discussion The primary objective was to identify salt compositions for C SP applications that have low melting points (<400 °C), are economical (≤$6/lb), and have low degra- dation. Figure 1 shows the comparison of salt price, melting point, and degradation (chloride loss). Li- and Zn-based salts showed a melting point <400 °C. Although the zinc chloride salt blend is cheaper than the lithium blend, higher chloride loss for zinc and MgCl 2 indicated that at tempera- tures >700 °C, there can be major loss of the salt due to its high vapor pressure. The Li-based salts showed ver y low chloride vapor loss over time. XRD of various salts before and after degradation was performed. A representa- tive XRD is shown in Figure 2. None of the chloride salt blends utilized in this study exhibited any significant signs of degrada- tion. KCl was a predominant phase in all of them . No new high-intensity peak was observed in any of the degraded samples, indicating that th e phases did not go through any major phase transition or decomposition. A point to be noted here is that LiCl is most likely present in small amounts in the degraded salt (below the detection limit). LiCl is highly hygroscopic, so it is possible that most of it is dissolved in its own water of crystallization.

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