Local reducing atmosphere-acause of superheater corrosion inkraft recovery units

Fireside corrosion limits superheater steam temperature in kraft recovery units to a maximum of approximately 480oC. Particularly aggressive corrosion has frequently been attributed to molten deposits at the tube metal surface. The deposits are formed by two distinctly different mechanisms: carryover of black liquor particles and condensation of material vaporized in the lower furnace. In the lower superheater region, particularly on the windward side of the tubes, carryover is dominant. Freshly formed carryover deposits contain unburned black liquor residue, which can create a local reducing atmosphere at the metal surface. A field study on superheater corrosion was recently performed using temperature-controlled probes over a wide range of metal temperatures. Probes were made of ferritic steel (T-22) and of austenitic steel (Incoloy 800H). The results showed that the metal wastage was much higher on the windward side than on the leeward side of the probes. The greater metal wastage on the windward side was caused by a reducing environment that facilitates sulfidation corrosion. The effect of the reducing atmosphere on the corrosion was found to be more significant than the effect of chloride and potassium salts