High resolution characterization of Pb-SCC in Alloy 800 exposed to 330 °C mildly caustic environments. (1st August 2018) - Persaud, S.Y.
Highlights: Alloy 800 was susceptible to through-wall, mixed-mode SCC in Pb-caustic environments (pH330 °C = 9.5). Nano-scale characterization of transgranular and intergranular crack tips revealed similar local chemistries and SCC mechanisms. Crack chemistry: successive Ni-rich regions, segregated Fe- and Cr-rich oxides, and Pb at metal-oxide interfaces. Pb deposition at metal-oxide interfaces impairs Cr-rich oxide passivity, hindering repassivation kinetics and promoting oxide-rupture. Impaired oxide passivity led to de-alloying (Fe dissolution/Ni-enrichment) and ahybrid film-rupture/de-alloying Pb-caustic SCC mechanism. Abstract: 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.