Mechanism:

Corrosion fatigue is the term commonly applied to water side cracking in regions of high stress.  When the local stress breaks the protective magnetite scale and exposes the steel to the corrosive environment.  The damage occurs during start up, when thermal strains may be highest due to differential expansion and the water chemistry is out of the control range.  Over numerous startups and many cycles, the damage progresses as water side cracking.  The cracks usually occur at attachments or locations on the tubing where stress and strain occur.  Higher stresses lead to more rapid corrosion when the water side chemistry is conducive to corrosion.  The common term for this is "corrosion fatigue", but a better description may be "stress-assisted corrosion".  Carbon steel is the most commonly affected material, as that is the most common material for water wall construction. Note that the stress must be high enough to crack the magnetite scale; and the boiler water needs to be corrosive; that is, either oxygen concentration must be too high or the pH must be out of the control range (strongly acidic or strongly basic). The damage is more common in cycling boilers that have had several hundred cold starts over their operating life.

Indications primarily initiate on the inside surface of the tube and appear as longitudinal cracks. Multiple cracks always exist next to the primary failure.