The Effect of Traces of SO2 on Iron Oxidation: A Microstructural Study

Abstract

Pure iron has been exposed to pure O 2 and O 2 with 100 ppm SO 2 at 525°C for 1 and 24 h. The samples were investigated by FIB, SEM, TEM, EDX and EBSD. The oxide scales formed on iron at 525°C in O 2 and in O 2 + 100 ppm SO 2 are dense and adherent and consist of three layers. The outermost layer consists of hematite. Beneath it there is a duplex-magnetite scale. The two magnetite layers are separated by a straight interface. It is concluded that the inner-magnetite layer grows inward while the outer magnetite layer grows outwards. In the presence of SO 2 the inner-magnetite layer is much thinner, iron sulphate forms at the oxide surface and discrete iron sulphide grains nucleate at the metal/oxide interface. The amount of sulphide at the metal/oxide interface increases with exposure time. The oxidation of iron in oxygen at 525°C is inhibited by 100 ppm SO 2 . The inhibitive effect of SO 2 is attributed to iron sulphate that blocks active sites on the hematite surface, slowing down the formation of oxygen ions. This explains the strong inhibition of the inward growth of magnetite by SO 2 . There is also a marked effect on the morphology of the outer oxide, producing hematite whisker growth and a less porous surface in the presence of SO 2 .

Oxidation After Breakdown of the Chromium-Rich Scale on Stainless Steels at High Temperature: Internal Oxidation

“…This is in agreement with earlier studies on oxidation of iron at 500°C in H 2 O(g) and in dry air [39]. It has been reported that the growth rate of the iron oxide layer in high oxygen activity at 500-600°C is accelerated by water vapor because of changes in the transport properties of the hematite layer [40][41][42]. This may explain why the scale formed after breakaway oxidation grew slower in O 2 ?…”

“…1, 5a, 8). In contrast, addition of small amounts of SO 2 has been reported to decrease the growth rate of iron oxide scales due to formation of sulphate on top of the hematite layer, blocking the surface sites necessary for O 2 reduction [40]. Accordingly, the growth rate after breakaway oxidation in the O 2 ?…”

The influence of small amounts of KCl(s) on the high temperature corrosion of a Fe‐2.25Cr‐1Mo steel at 400 and 500°C

“…Several recent investigations of iron oxidation in this temperature range shows that the magnetite layer actually consists of a lower inward-growing part and an outer outward growing part [22][23][24][25][26][27][28][29]. A recent study of the oxidation behaviour of a binary Fe-2.25Cr steel at 6008C [30] showed a similar three-layered scale structure.…”

Comparative Study on High Temperature Oxidation of T92 Steel in Dry and Wet Oxyfuel Environments

“…In contrary to that, beneficial effects of SO 2 against oxidation of pure Fe and Fe-base alloys, if present in trace amounts (1-1000 ppm) in the temperature range of 450-650°C, has been recently reported [22][23][24]. The inhibitive effect of SO 2 was attributed to a small amount of sulphate found at the gas/oxide interface of the alloys.…”