Page 400 - Handbook of Materials Failure Analysis

398 CHAPTER 15 Welding-associated failures in power boilers

Table 15.5 Chemical Analysis of Elements of the Scale on the Tube (E2623)
C% V% Mn% Cr% Ni% Fe% Mg%
1.4 28.2 0.54 6.7 2.4 20.64 0.06

Pitting corrosion in steels is usually promoted by surface deposits that set up local
concentration cells and dissolved halides that produce local anodes by rupture of the
protective oxide film. Anodic corrosion inhibitors, such as chromates, can cause
rapid pitting if present in concentrations below a minimum value that depends on
the metal environment combination, temperature, and other factors. Pitting also
occurs at mechanical ruptures in protective organic coatings if the external environ-
ment is aggressive or if a galvanic cell is active. Pitting normally occurs in stagnant
environments. With corrosion-resistant alloys, such as SSs, the most common cause
of pitting corrosion is highly localized destruction of passivity by contact with mois-
ture that contains halide ions, particularly chlorides. Chloride-induced pitting
of SSs usually results in undercutting, producing enlarged subsurface cavities, or
caverns [36].
Since decreasing the chloride content below the current levels is impractical for
such applications, it has been recommended to consider replacing the austenitic SS
grade 304L by a SS of a higher pitting resistance number (e.g., 316L or 904L or even
duplex steel grades).

3.2 FAILURE OF A SUPER-AUSTENITIC STAINLESS STEEL
FIREFIGHTING WELDED PIPE LINE RESULTING FROM WRONG
WELDING PARAMETERS
A failure investigation on super-austenitic stainless steel (SASS: UNS S31254) fire-
fighting pipe line was reported recently [37]. SASSs are a special class of austenitic
SSs with higher Mo and Cr content for high chloride-containing environments. The
firefighting pipe was welded by GTAW process using a nickel base filler metal (ER
NiCrMo3). The firefighting network was established in the oil and gas field to pro-
vide the fire protection through piping seawater as firefighting water. Leakage of the
firefighting was observed at the weld line and/or adjacent to the weld zone (i.e., at
HAZ) after the firefighting network was pressurized and put in-service as shown in
Figures 15.4 and 15.5. The failure mechanism was identified to be pitting corrosion
along the weld joint. Microstructural examination of the failed area showed that the
failure started at the fusion zone between the weld metal and the HAZ as shown in
Figure 15.6. This area is called “unmixed zone” (UMZ) and has a microstructure of
dendrites of austenite similar to that of an autogenous weld [38].
EDX analysis showed that Mo and Cr levels varied across the joint. Mo has a
partitioning coefficient less than 1; hence, it tends to segregate during solidification
to the grain boundaries. Cr, also, segregates during solidification in a manner similar
to that of Mo but with lesser degree. Thus, the core of the dendrites within the UMZ