Release and transformation of incombustible inorganic substances during petrochemical industrial wastewater incineration form a large number of ash-forming species, which deposit and accumulate on the surfaces of heat transfer tubes, and can pose some severe operational problems, such as fouling, slagging, and even low heat transfer efficiency. Ash deposits characterization has been performed to investigate ash transformation and deposition behavior in a large-scale petrochemical industrial wastewater incineration plant using analytical techniques, including XRF, SEM-EDS, and XRD for elemental composition, morphology, and mineralogy. The results show that this volatile element has a dominated contribution to form varied ash deposits in a wide temperature range from 1000 to about 260 degrees C. Nickel and iron mainly in the oxide forms are transported by ash particles, have strong deposition propensity at high temperatures, and can be captured by the sticky initial deposition layer. Meanwhile, sulfur is enriched at low temperatures due to condensation, nucleation of alkali metal sulfates, and sorption of SO2/SO3. In the temperature range of 550-900 degrees C, eutectic mixtures which are temperature-dependent are formed in the molten phase and an "evolving" branched structure detected from SEM indicates the formation of a sintered deposition layer. Acidic salt sodium sesquisulfate is generated at low temperatures and should be responsible for the initiation of low temperature corrosion.