Hot Chlorination Corrosion of Metallic Nickel by Chlorine Catalyzed by Sodium Chloride.

Abstract

Reducing chlorine corrosion to metals at high temperatures is a big problem for many industrial processes. Some high Ni alloys such as Hastelloy C-276 (Ni > 50 wt %) have been widely used for this purpose. Chlorine and chlorides often coexisted in many industrial processes at high temperatures, such as some industrial incinerators and metallurgical furnaces. Thus, a comprehensive experimental investigation regarding the effect of NaCl on the chlorination corrosion of metallic nickel powder by chlorine at a high temperature was performed. It was more convenient to investigate the intrinsic chlorination mechanisms and kinetics of metallic Ni if Ni powder was used instead of a Ni plate. It was found that there existed a critical chlorination temperature of 450 °C for relative safe use of Ni-based alloy in the presence NaCl. The Ni chlorination in the presence of NaCl was increased with increasing temperature and reached a maximum of 97% at 700 °C, which was about 21% higher than that in the absence of NaCl. An anhydrous NiCl2 was initially formed at about 700 °C during the chlorination process and then immediately reacted with NaCl to form a novel eutectic complex with a flaky shape, a melting point of 585 °C, and a simplified molecular formula of NiNa0.33Cl2.33 based on X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electronic microscopy (SEM), and chemical analysis with inductively coupled plasma atomic emission spectroscopy (ICP-AES). As a result, only the complex of NiNa0.33Cl2.33 and NaCl was left in the chlorinated product. At 700 °C, the chlorinated product evaporated only in the form of complex NiNa0.33Cl2.33 instead of individual NiCl2 or NaCl. The chlorination mechanisms of metallic Ni at a high temperature, for example, 700 °C, in the presence of NaCl were as follows. Step 1: Formation of initial chlorinated solid product NiCl2 (mp 1001 °C) at a high temperature; step 2: The NiCl2 reacted with solid additive NaCl (mp 801 °C) to form a final liquid product NiNa0.33Cl2.33 (mp 585 °C); step 3: External Cl2(g) was dissolved in the liquid product layer; step 4: The chlorine dissolved in the liquid product of NiNa0.33Cl2.33 reacted with the unreacted Ni core. The external Cl2(g) passed through the liquid product NiNa0.33Cl2.33 layer faster than the solid product NiCl2 layer formed in the absence of NaCl. This resulted in 21% more chlorination corrosion of metallic Ni powder with NaCl addition than that without NaCl addition.