Abstract
The sulfur distributions in the iron sulfide scale formed at 700°C under various sulfur pressures were determined by X-ray diffraction.
The gradient of the sulfur distribution in the scale is not linear, different from the results in the literature. A very sharp decrease in sulfur content and some flat range of composition of about 51.5 at%S are observed near the Fe/FeS interface and at the inner part of the scale, respectively.
The inter- and self-diffusivities of iron were calculated according to Fick’s second law and Darken’s relation as the function of distance and composition, based on the sulfur distributions and were an order of 10−6 and 10−8 cm2/sec, respectively. The self-diffusivity of iron was also calculated by use of several other methods and their results were compared with each other.
The extraordinary high diffusivities were obtained at the composition of about 51.5 at%S which corresponded well to the composition of the maximum value of a0 parameter of FeS. This behavior may be attributed to the generation of the stress by deformation of the scale, the variation of the a0 parameter with sulfur content in the scale, and also the annealing effect on the stress during corrosion.
The effect of the inner scale on the sulfur distributions in the outer scale was also examined and showed to result in the higher sulfur content in the outer scale.
The SEM observations of the inner scale fractured mechanically revealed the formation of voids in it, suggesting contours of the perforating dissociation of the outer scale crystal.
