8.Discussion
From the viaual observation it is clear that the tube was failed due the pitting (localized) corrosion and the leakages are found from bend and twisted region (Fig.3a and b),i.e. higher stress concentration area. Highly stressed acts as anodic with respect to its surroundings. The anodic region acts as a potential site to start localized corrosion [1]. As EDS analysis report indicates the presence of sulfur compond on the corroded areas, so it is clear that sulfur-di-oxide in the gas mixture accelerate the corrosion rate with the presence of moisture inside the tube. High temperatures (Fig. 7)[2] and lower pH (Table 3) also increase the tendency of pitting.
It is therefore presumed that following chemical reactions took place to promote corrosion:
During chemical reaction inside the tube a layer of FeSO4 has been formed. The characteristics of FeSO4 layer is soft and poorly adherent and therefore, layer of FeSO4 does not act as a protective layer and allows further corrosion by acid [6] (Fig.8).165 HV near surface where the surface was severely corroded and 206 HV near core where surface remain unaffected (Fig.4). This hardness profile is indicating comparatively softer region near the surface than the core.