Hot corrosion often takes place at elevated temperatures in gas turbines and other equipment. However, hot corrosionin ethylene cracking tubes is seldom seen. In this case, an examination of pitting in ethylene cracking tubes made ofHP 40 steel (25Cr–35Ni) which failed in weldments (Ni-base alloy) has been conducted to identify the cause of failure.Results show that these failures had been caused by hot corrosion produced by Na2SO4. A molten state was noticedfrom the morphology. Analysis revealed that pitting in tube weldment was caused by high-temperature hot corrosion(Type I corrosion and Type II corrosion) produced by Na2SO4. The mechanism of different types of hot corrosion isdiscussed. Finally, the cause of hot corrosion is discussed including the origin of Na,V, S and the formation of Na2SO4.The influence of temperature, composition and microstructure of materials on hot corrosion is discussed. To avoid suchhot corrosion, precautions should be taken, including increasing Cr content in weldments, decreasing protrusion inweldments, washing internal surfaces of tubes with hot steam regularly.