Abstract: The black deposit inside the steel tube of the heat exchanger in the oil refining system was analyzed to ascertain the composition and the origin by gas chromatography-mass spectrometry (GC/MS) and environmental scanning electron microscope (ESEM). The carbon, ferric and ferrous oxide, halogen, inorganic salt of the potassium etc., and satisfied fatty hydrocarbon presented in the black deposit. The results indicated that the mixed gas in the steel tube changed into the condensate and sediment. The sediment reacts with the steel wall. The character of the medium in heat exchanger was changed for the deposit corrosion. The serious consequence was of the steel tube to burst violently under the internal normal pressure.