Advanced microstructural characterization of transgranular Pb-caustic SCC in Alloy 690TT

Alloy 690TT was tested at 340 °C in mildly caustic environments containing lead (Pb) to investigate the transgranular cracking mechanism of Pb-induced stress corrosion cracking (PbSCC). Advanced electron microscopy characterization revealed that the fracture surfaces exhibited evidence of cleavage and regions with significant enrichment of metallic Ni, which maintained the same orientation as the original grain matrix. Moreover, there are some indications that crystallographic cracking along {100} and {110} planes is possible at high pH in Pb-containing environment. These experimental observations are consistent with a form of film-induced cleavage mechanism, assisted by de-alloying and exacerbated by planar slip in the matrix.