This case study follows the investigation of a leak in a lateral water wall tube from a coal-fired boiler. The analysis determined the failure mechanism that caused the leak and identified possible operating conditions that would have caused this type of failure. The tube material was plain carbon steel.
A fissure in the tube was tight with no significant plastic deformation. A metallographically-prepared cross section of the tube revealed numerous tight cracks in the tube wall near the fissure, which was on the hot side of the tube. These cracks were discontinuous in the section plane and followed the material grain boundaries. Pearlite islands near the cracks were observed to be partially decarburized.
The inside surface of the tube was covered in stratified layers of copper and iron oxide deposits. These types of deposits are commonly an indication of upstream corrosion in other areas of the boiler. Improper water treatment, condenser in-leakage, or other water contamination can create high or low pH conditions that will facilitate corrosion.
A byproduct of the copper deposition reaction is atomic hydrogen. Some of the nascent hydrogen will diffuse into the tube material, where it reacts with the iron carbide in the steel, and generates methane gas. This gas collects at the material grain boundaries creating the small cracks observed in the cross section. Over time these cracks grew and coalesced, reducing the effective tube wall thickness. Eventually the tube wall was no longer able to withstand the internal operating pressure and ruptured.
In summary, the failure mechanism for the tube was hydrogen damage. The hydrogen damage embrittled the tube metal such that initiation and coalescence of intergranular cracks resulted in the eventual tube failure. Maintaining recommended water chemistry - and thereby preventing excessive scale and/or deposit formation - is one of the most effective ways to prevent hydrogen damage. Periodic tube and water sampling were recommended to monitor these conditions.
The work scope for this analysis consisted of light microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and metallography (microstructure analysis). For specific details on these methods, please see the Handbook of Analytical Methods of Materials on our website or contact us for more information.