Abstract
Tube damage due to stress-assisted corrosion (SAC, or Corrosion Fatigue) is relatively common in recovery boilers. Another damage mechanism, known as "phosphate hideout," has been attributed to under-deposit corrosion in high-pressure boilers. Phosphate hideout is characterized by the retention of phosphate in the boiler during conditions of high pressure and the subsequent release of phosphate when the pressure is reduced. The condition is most often reported in high-pressure units, e.g., 2,500 psi (17.24 MPa). Paper mill recovery boilers typically operate at pressures below 1,500 psi (10.34 MPa), but may be frequently taken off-load, or swing, due to shut down of a paper machine or to conduct a "chill and blow," an operation procedure used to release boiler fireside deposits in the upper part of the boiler. This paper presents an unusual case example of recovery boiler waterside tube deterioration by a combination of SAC and a corrosion mechanism related to phosphate hideout. The phosphate corrosion damage appeared secondary, and cracking was present under deep SAC deposits. The secondary cracking was characteristic of intergranular stress corrosion cracking (SCC) mechanism. The combined SAC and SCC damage zones exhibited deposits mainly of sodium-iron-phosphate, or maricite, a chemical that is the reaction product between mono- and di-sodium phosphate and the boiler tube protective oxide layer magnetite.