Corrosion-Fouling of 316 Stainless Steel and Pure Iron by Hot Oil

Abstract

We examined the fouling and corrosion that took place when 316 stainless steel and pure iron wires were electrically heated to 540–680 °C in a liquid bath of the atmospheric bottoms fraction of a crude oil. The foulant was determined to be heterogeneous, with a thick macroscale outer layer of pitch, covering a microscale sheath of coke, which was in turn both covering and interspersed with a microscale layer of iron sulfide. This foulant was observed to delaminate from the wire surface, presumably as a result of both the generation of growth stresses and the action of gas bubbles that were evolved during the fouling process. Unexpectedly but conclusively, we observed that the underlying wire surface was heavily corroded. In the case of the stainless steel, we observed a microscale chromium oxide layer that separated the foulant from the underlying metal. This layer presumably reduced the rate of metal dissolution. The degree of corrosion was much higher in the pure iron samples, where such a layer did not exist. Our hypothesis is that there is a synergy between the measured macroscopic fouling and the underlying microscopic corrosion, where the iron from the wire reacts with the sulfur in the oil to build up the thick sulfide.