Investigating Pb-induced stress corrosion cracking of alloy 800 via in-situ XPS

Laboratory investigations, aimed at testing the extremes of secondary side crevice chemistry have demonstrated that Alloy 800NG materials can be susceptible to Pb-induced stress corrosion cracking (PbSCC) in caustic environments, provided sufficient stress is present. More recently, a mechanism for PbSCC of Alloy 800NG has been developed. Integral to this mechanism is the assumption that microscopic quantities of Pb are electrodeposited in metallic form (i.e., Pb⁰) on the metal surface, at the oxide-metal interface. The fact that only a few atomic layers of Pb are typically associated with this degradation process has posed considerable challenges for investigations of the chemical state of Pb on the material surface. To this end, building on the previous mechanistic work, an experimental approach was devised wherein PbSCC cracks were produced in Alloy 800NG notched pin specimens subjected to a 3-point bend loading and, subsequently, fractured in-situ within an ultra-high vacuum (UHV) pocket chamber attached to an X-ray Photoelectron Spectrometer (XPS). The combination of the high surface specificity, chemical state selectivity and UHV of XPS, allowed for unfettered analysis of the fracture face for the presence, and oxidation state determination, of Pb. This paper includes a discussion of the results of this examination in the context of the mechanism of Pb-assisted SCC of Alloy 800NG.