The utilization of a typical Chinese low-rank coal (Zhundong coal) usually gives rise to severe fouling and slagging in equipment due to its excessively high content of sodium-bearing corrosive substances. This study systematically clarifies the occurrences and transformation mechanism of corrosive materials during pyrolysis of Zhongdong coal by combining solid-state nuclear magnetic resonance (SSNMR), in situ high-temperature X-ray diffraction (HT-XRD), and Factsage simulation. For the first time, the experimental evidence in this study shows that the corrosive elements demonstrated distinct forms in coal and could be significantly varied by thermal treatment. Specifically, homogeneously distributed Na ions could mutually transform between inorganic and organic-bounded form under the influence of ionic force, resulting in its elutable feature. During pyrolysis, Na was successively transformed to be inorganic form and completely volatilized above 800 degrees C, thus diversifying Na-related fouling propensity in various pyrolysis stages. The Cl was unlikely to entirely exist as inorganic form; nevertheless it was strongly restrained by functional groups of coal matrix. The organic Cl-containing functional groups was gradually decomposed to volatile Cl at pyrolysis temperature higher than 500 degrees C, whereas the inorganic Cl was more stable and possibly exposed on the surface of char particle. In situ analysis further revealed that the formation of aerosol by the diffused corrosive elements was the key step leading to the deposition. More importantly, Factsage thermodynamic calculation demonstrates that the sequential release of Cl as well as S, and their interactions with Na are the prerequisite and essential factor governing the generation of low-temperature eutectic and subsequently initial corrosion.