High resolution characterization of Pb-SCC in Alloy 800 exposed to 330 °C mildly caustic environments

Alloy 800 was exposed to a 330 °C Pb-containing, mildly caustic (pH330°C 9.5) environment producing SCC. High resolution analytical TEM characterization of crack tips provided evidence to elucidate the Pb-caustic SCC mechanism and highlight a continuum of classical caustic SCC, facilitated at lower pH by the presence of Pb. Successive regions of Ni-enrichment and Cr-rich oxides were observed along the crack with selective Fe dissolution ahead of crack tips, de-alloying. Also, Pb is observed at oxide-metal interfaces. A hybrid film-rupture/de-alloying SCC mechanism is proposed, with Pb at oxide-metal interfaces acting to impair the passivity of the metastable Cr-rich oxide.