Page 60 - Handbook of Materials Failure Analysis

54 CHAPTER 3 Boiler tube failures: Some case studies

FIGURE 3.5
Hydrogen damage, “window opening” and “thick-edge spilt” type failure [3].

With the destruction of protective magnetic oxide layer, concentrated NaOH reacts
with the tube material and forms atomic hydrogen as per the reaction
Fe + 2NaOH ¼ Na 2 FeO 2 +2H
The atomic hydrogen so produced reacts with Fe 3 C of pearlite constituent and forms
CH 4 which ultimately causes hydrogen damage, discussed earlier. The tube surface
deposits accumulate at locations where flow is disrupted such as, welds with backing
rings, at bends, in horizontal tube weld, and at high-heat input locations. Figure 3.2
shows the locations of the boiler where caustic corrosion can occur and Figure 3.6
shows caustic corrosion gouging of a boiler tube.

2 CASE STUDIES

All possible types of boiler tube failure mechanisms have been discussed in
detail earlier. Investigation into the probable causes of failure require gathering of
physical evidence or data through a systematic problem analysis. The important