A promising way of upgrading bitumen is de-asphalting with the subsequent partial oxidation of asphaltene. Sulfur, metals (V, Ni), and other minerals concentrated in the asphaltene are subjected to reactions at elevated temperatures in both oxidizing and reducing atmospheres. Upon the cooling of the raw syngas, some of the particulate and condensable material deposit surfaces and ages. This paper discusses the subsequent deposition of layers on syngas cooler tubes. The-deposit consists of a corrosion layer of FeS, a condensation layer of Ni3S2, and subsequent fouling layers composed of V2O3 and Ni3S2. Firstly, the base metal is corroded by the H2S electrochemical attack with the simultaneous Ni3S2 condensation on the cold surface. With the increasing fouling thickness of the Ni3S2 layer, the surface temperature at the process side increases, causing the condensation process to come to a gradual stop. Then infusible nanometer size V2O3 particles with Ni3S2 are deposited upon this sticky and rough surface, forming an extremely thick composite fouling layer. A high resolution inspection of this layer shows three sub-layers with distinct morphologies: (i) a fully molten liquid dense layer of Ni3S2 with V2O3 particles soaked below solidified into the smooth surface, (ii) very fine-grained (average particle size of 140 nm) spherical V2O3 particles embedded within a melting Ni3S2 sheet along with scale, where the spaces between the V2O3 particles are filled with Ni3S2, and (iii) a fire side sub-layer consisting primarily of spherical V2O3 particles wrapped by the condensed Ni3S2 glue and keeping the original morphology of the particles in the syngas. The paper discusses the mechanism in detail. (C) 2015 Elsevier B.V. All-rights reserved.