Abstract
In order to elucidate the sulfidation behavior of pure iron at high temperature, kinetic studies were carried out as functions of sulfur pressure and temperature, and the sulfide phases of the scale formed were examined.
The sulfidation obeys a parabolic rate law, but the slopes of the log-log plots of sulfur pressures and corrosion rate constants change in the range 1/3.5 to 1/5.7 depending on the sulfur pressure.
The phases and preferred growth orientations of the scale were identified by the X-ray diffraction method. The results showed that the scale was composed of the pyrrhotite, troilite, and Fe7S8 phases, and that at higher pressures the (00l) planes were normal to the scale growth direction, while at lower pressures the (110) and/or (10l) planes were normal to that direction.
The SEM observation of the surface of the scale formed at 800°C revealed that the hexagonal step-like feature of the surface changed into a more flattened one with decreasing sulfur pressure.
From the growth mechanism of the scale it was evident that these sulfidation behavior was closely related to the vacancy configulation and diffusivity of iron ions in the scale and also to the adsorption of sulfur atoms on the particular sites of the scale surface.
