The Effect of Lead on Dealloying and Stress Corrosion Cracking (SCC) of Ni-Cr-Fe Alloys in Boiling Caustic Environments
Material Science , Corrosion , Stress Corrosion Cracking (SCC) , Dealloying
304L stainless steel (SS) samples were exposed to a highly caustic 50 wt.% NaOH environment at or near the boiling temperature (135-140 °C) in reducing conditions to study dealloying. Flat samples were used for electrochemical measurements, and SS 316L was used in some tests to evaluate the effect of Mo addition and subtle changes in Ni content. Twisted and U-bend samples of SS 304L were used to evaluate the effect of stress on dealloying and SCC. All exposed surfaces were characterized post-exposure using optical microscopy, scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS) chemical analysis with additional characterization done on some samples at the nanoscale using transmission electron microscopy (TEM). Following testing in pure caustic, SCC tests and electrochemical measurements were then repeated in lead-contaminated (5, 50, and 500 ppm), caustic environments with lead added as PbO. The effect of dissolved Pb concentration and its possible effect on dealloying susceptibility was evaluated. The effect of Pb addition on SS 304L was found to shift the open circuit potential towards a more positive value. It was also noted that the addition of Pb had a delaying effect on dealloying that consequently slowed SCC in comparison to the results obtained in pure caustic conditions. This effect was attributed to the deposition of metallic Pb on the Ni-rich surface formed after dealloying of a few atomic layers. Pb deposition blocked dissolution pathways which slowed dealloying kinetics and subsequently hindered SCC initiation in comparison to pure caustic conditions.