Corrosion Behavior of Austenitic stainless steels utilized for coal-fired boilers in simulated coal-ash/high sulfur environments

The corrosion behavior of three austenitic stainless steels (Super304H, TP347H FG and HR3C) in the simulated coal-ash/high sulfur flue-gas environments were studied at 650°C. The weight changes of the tested samples were measured by a discontinuous weighting method, whereas the morphologies, compositions and phases of the corrosion products were analyzed by SEM, EDS and XRD, respectively. The results indicated that low temperature hot corrosion occurred in the three tested alloys at 650°, whereas the corrosion resistance was strongly related to the levels of Cr. The HR3C exhibited a better corrosion resistance than the other alloys due to a stable and dense Cr
₂
O
₃
scale formation. Then on-protective outer oxide scale (Cr
₂
O
₃
) formed on the lower Cr content of the TP347H FG alloy, whereas the scale was multi-layered with serious inner sulfide layer appearance and spallation. Similarly to the TP347H FG, the multi-layer oxides structures also formed on the Super304H, which consisted of Fe
₂
O
₃
(outer layer), Cr
₂
O
₃
(inner layer) and high amounts of sulfide were also detected. The corrosion resistance of the TP347H FG alloy was slightly higher than the Super304H, due to higher levels of Cr and fine grain sizes. The mechanism of ash corrosion was also discussed.