Influence of coke thickness on the on-line SiO2/S coating preparation and its thermal shock property

SiO2/S coating was prepared on the surface of HP40 specimen with different thicknesses of coke. The influence of coke on the coating deposition and thermal shock property was studied. Scanning electron microscopy, X-ray diffractometry and X-ray energy dispersive spectroscopy were used for microstructure and microchemical analysis. The morphologies of cokes and SiO2/S coatings were investigated. The results show that the oxide layer of HP40 specimen formed at 850 degrees C for 2 hour is porous and mainly composed of (Cr, Fe)-spinel and a small amount of Fe2O3. The coking rate initially is high, then decreases with increasing cracking time, and finally reaches a stable value after 20 minutes. The diameter of coke changes from 1 mu m to 0.2 mu m, and then is about 0.5 mu m. The thickness of the coke layer changes from 0.56 mu m to 2.58 mu m. The thickness of coke layer has an important influence on the morphology and thermal shock property of the on-line SiO2/S coating. With increasing the thickness of the coke layer, the coating becomes rough and incompact. When the thickness of the coke layer is 0.56 mu m, the coating can bear 13 thermal shock tests. And the coating is out of service when the coke layer is 2.58 mu m. It is suggested that a thorough decoking should be carried out to improve the adhesion between the on-line coating and the inner surface of the cracking tube.