SAC in Superheater Tube Failures: When Stress, Corrosion and Microstructural Changes Collaborate to Make a Complex Failure

Stress Assisted Corrosion (SAC) in superheater tubes is a puzzling challenge for boiler operators. Interestingly, this degradation mechanism shouldn't theoretically occur in superheater tubes, as they're designed to contain only superheated steam, not liquid water. The presence of SAC in these components often signals underlying operational issues, such as improper layup procedures during shutdowns.

SAC typically initiates with pitting on the internal surface of the tubes and is intensified by stresses induced by frequent start-stop cycles.

The failure of superheater (SH) tubes is sometimes multi-factorial, with several key mechanisms contributing to SAC progression:

1. Intermittent operations lead to thermal cycling, inducing residual stresses and potentially trapping moisture during shutdowns, which creates a corrosive environment.
2. If (for any reason), the tube temperatures exceed design limits, microstructural changes occur: carbon depletion near the surface of the tube, pearlite spheroidization and grain growth. Grain growth reduces strength and creep resistance by decreasing grain boundary area, while pearlite spheroidization diminishes the steel's overall strength and hardness.
3. Rapid cooling, especially during unplanned shutdowns, can cause twinning, introducing planes of weakness in the microstructure.

These factors synergistically degrade the tube material:

Untreated water in contact with the tubes, Thermal cycling creates and causes surface defects, microstructural changes from overheating weaken the material's resistance to deformation and crack propagation, and twinning (in some cases) provides potential crack initiation sites. Together, these mechanisms create a complex state of material vulnerability that, when combined with potential corrosive conditions, significantly increases the risk of SAC and ultimate failure in superheater tubes.

As industry professionals, it's crucial to remain vigilant, not only in implementing robust inspection regimes but also in ensuring proper shutdown and startup procedures. Particular attention should be paid to complete drainage and drying of superheater tubes during outages.

#BoilerMaintenance #MaterialsEngineering #CorrosionPrevention #PowerPlantOperations #Failure_Analysis