Intergranular Corrosion Susceptibility of Alloy 800 Weldments.

Alloy 800 was subjected to an autogenous welding treatment using two varying techniques, laser beam (LB) welding and tungsten inert gas (TIG) welding. The weldment regions such as weld fusion zone (WFZ), heat-affected zone (HAZ) and base metal were compared using reactivation technique, i.e., double loop electrochemical potentiodynamic reactivation (DL-EPR) test and ASTM G 28 test for intergranular corrosion (IGC) susceptibility. Alloy 800 showed no significant DOS values in WFZ of laser weldment as cooling rates were faster resulting in negligible carbide precipitation and chromium depletion which was ascertained by G28 test. While considerable reactivation was observed in HAZ of TIG and LB weldments as they are exposed to carbide precipitation temperatures during welding procedures as a result of heat input and cooling rates. Magnetic susceptibility measurements on weldments indicated extent of chromium depletion. The DOS values measured for Alloy 800 LB weldments were one order of magnitude less for all regions as compared to TIG weldment and corroborated with depth of attack obtained from IGC tests. The highest DOS values were observed for HAZ of TIG weldment. DOS values are an indirect measurement of chromium depletion in HAZ that will weaken the passive film formation and result in increased corrosion in ASTM G28 test. The increased grain boundary attack and grain dropping suggest decreased IGC resistance of Alloy 800 weldments by TIG welding compared to LB welding. Hence, laser welding technique is a better technique for welding/repair welding of SG tubes to avoid sensitization and reduce its susceptibility to intergranular corrosion/intergranular stress corrosion cracking (IGC/IGSCC) during operations. [ABSTRACT FROM AUTHOR]