Understanding Pb-Alkaline Stress Corrosion Cracking (SCC) in Ni- and Fe-Based Alloys

Stress corrosion cracking (SCC) of Ni- and Fe-based alloys in Pb-alkaline conditions is relevant to the degradation of steam generator tubing materials exposed to the extreme ends of plausible conditions of secondary side heat transfer crevices. Self-loaded U-bend samples of Alloys 600, 690, and 800 were exposed to 10 wt.% NaOH solution and 2 wt.% NaOH with the addition of 100 ppm Pb at 280 °C for 120 h to study the effect of Pb on the initiation of SCC in a mildly caustic environment. Post-exposure characterization was performed to evaluate dealloying susceptibility and investigate the role of Pb in impairing protective oxide films. Alloy 600 (Ni-16.10Cr-9.69Fe), Alloy 690 (Ni-29.80Cr-10.08Fe), and Alloy 800 (Fe-30.14Ni-20.25Cr) behaved differently due to chemical composition differences, notably Ni and Cr content. Alloys 600 and 800 were susceptible to Pb-induced SCC while all alloys studied were resistant to SCC in 10 wt.% NaOH without Pb.