Over 35 Years of Providing Metallurgical Analysis

Corrosion Fatigue

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.

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

External Appearance:
Pinhole leak, thick edge longitudinally orientated crack, or cracking down both sides of the tube. Typically the cracks occur on the cold side of the tube; however it has been documented on the hot side. Some cracks can be circumferential but this is in the minority.

The damage usually first appears on the water side of buckstay attachments. The cracking pattern may be circular cracks surrounding the weld between the buckstay attachment and the water wall tube.  Microstructure analysis shows the cracks tend to be bulbous with numerous lobes. The crack tips themselves may be somewhat blunted but are oxide filled and transgranular.

Attachment type:
Construction attachments, buckstay attachments, membrane welds, lugs, fins etc.

Common Locations:
          Wind box casing attachments
          Buckstay connections
          Sidewall to slope connections
          Division wall at slope penetration
          Burner elevations
          Boiler water seals (weir box)
          Boiler ash hopper seal plates
          Gas recirculation duct attachments
          End of membrane connections
          Economizer fin welds
          Fin welded tubes
          Scalloped tie bars
          Water wall gusset plates
          Penthouse floor attachments
          Rear wall arch


Corrective Action: 
Start the unit up as slowly as possible to minimize the differential expansion strains.  Always start up with water chemistry under proper control. Inspect tubes via non-destructive examination, such as Phased Array or Radiography, for corrosion fatigue cracking, and replace tubes damaged by corrosion fatigue. Perform inspections in areas susceptible to corrosion fatigue. Monitor pH and dissolved oxygen content of the feedwater, specifically during start-up conditions and load swings, to prevent pitting corrosion and subsequent corrosion fatigue.