Mechanism-related modelling of pit evaluation in the CrNiMoV steel in simulated environment of low pressure nuclear steam turbine

Highlights: • Pit evolution in a welded joint was addressed in 3.5 wt.% NaCl solution at 180 °C. • Great pit density in weld metal approaching the fusion line was related to the galvanic effect and microstructure difference. • Different mechanisms of pit nucleation were observed in base and weld zones of the welded joint. • A model of pit depth distribution involving different mechanisms was developed. Pit initiation and growth in a 25Cr2Ni2MoV welded joint in the simulated environment of a low pressure nuclear steam turbine was addressed by using U-bend samples. Different pit densities in various zones of the welded joint were observed with the order: weld near heat affected zone > base metal near heat affected zone > heat affected zone. This was partly ascribed to the galvanic effect and material difference. Pits in the base metal emanated from local deep grooves of surface machining, while they nucleated from surface inclusions in weld metal. A model for pit distribution of the welded joint was developed and verified accordingly.