Caracterização da camada de óxido formada na superfície externa das colunas de fornos de reforma a vapor

Abstract

Steam reforming furnaces are constituted by harps of centrifugally cast HP steel tubes, added with Nb and in some cases Ti microalloyed. These tubes are exposed to severe operating conditions by working at high temperatures for a long period, resulting in the characteristic aging of the alloy and in damages, which occur mainly by creep. Therefore, studies have been carried out for the development of non-destructive inspection techniques by magnetic methods to be applied in the field. However, once the outer surface is in contact with an oxidizing environment and high temperature caused by the burners, the formation of an oxide layer occurs on it, which can influence the measurements made on the inspection. The objective of the present work is to characterize the external surface of the HP tubes removed from operation of different refining units of Petrobras and to relate their characteristics with the aging stage of the tube, as well as with its the magnetic response. The characterization of different tubes removed after long operating times was performed by optical and scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction, considering different heights of the reforming column, associated with its characteristic aging conditions. The magnetic response was analyzed by magnetic force microscopy and scanning magnetic susceptometry. The results showed that the oxidation process at high temperatures results in the formation of a complex oxide layer formed by a standard sequence of different oxides. In addition, a microstructural modification was observed in its subsurface, due to the impoverishment of chromium in this region. A relation between both the thickness of the oxide and the transformed layer with the aging state and the magnetic response of the samples was obtained. The origin of the magnetic response of the external surface was identified by magnetic force microscopy.