Mechanisms of Sulfidation Attack in a Low Oxygen Partial Pressure-High Sulfur Partial Pressure Environment

A series of nickel-base and iron-base alloys has been subjected to corrosion in CO₂/CO/SO₂ gaseous environments at ⁽¹⁾ oxygen partial pressure of 10^−11.3 atm and sulfur partial pressure of 10^−5.2 atm and ⁽²⁾ oxygen partial pressure of 10⁻¹³ atm and sulfur partial pressure of 10^−5.2 atm corresponding to CaO/CaSO₄ and CaS/CaO phase boundaries at 1650 F, respectively. At the two conditions chosen, the thermodynamically stable phases on the alloys are nickel sulfide, iron oxide, chromium oxide and aluminum oxide. Sulfidation attack could be initiated within 15 minutes on all the alloys containing nickel, while the FeCrAlY alloys did not suffer any sulfidation up to 250 hours. Although the conditon corresponding to the CaS/CaO phase boundary is thermodynamically more favorable to sulfidation than the condition corresponding to the CaO/CaSO₄ phase boundary, the experimental results indicated that the alloys suffered sulfidation attack at a slower rate in the former condition. Although the partial pressures of sulfur in the two environments were the same, the partial pressure of sulfur dioxide was approximately 2 orders of magnitude smaller in the former case. Kinetic considerations, in particular the flux of the sulfur-carrying gaseous phase, rather than the thermodynamic considerations, in particular the partial pressure of oxygen in the environment, appear to be dominant factors in these corrosion reactions.