Potential-pH Diagrams for Ni-Cr-Fe Alloys and Pb Adsorption on Ni-Cr-Fe Alloys Surfaces in Water at 25°C and High Temperature (250-350°C)

E-pH (Pourbaix) diagrams are drawn at 25 and 300 degrees C for Ni-Cr-Fe alloys, taking into account the bimetallic spinel oxides, with activities of metals corresponding to Alloy 600 (Ni75Cr15Fe10) and Alloy 690 (Ni60Cr30Fe10). Gibbs free energies of formation for adsorbed Pb and standard potentials for equilibrium Pb electroadsorption on these Ni alloys surfaces in aqueous solution are derived from DFT calculations of Pb adsorption on model Ni-Cr-Fe alloys surfaces. E-pH diagrams showing the conditions of Pb underpotential deposition (UPD) on Alloys 600 and 690 are constructed at 25 degrees C, 250 degrees C, 300 degrees C and 350 degrees C. Overlap of the domain of stability of adsorbed Pb with the domains of Ni, Cr, Fe ions, monometallic oxides and FeCr2O4 is observed. Hence, from the presented diagrams one can predict the E-pH conditions where effects of adsorbed Pb on the dissolution and passivation of Ni-Cr-Fe alloys are expected. These predictions could help finding the mechanism for Pb-induced SCC of Ni-based alloys used as tubing materials in pressurized water nuclear reactors. (C) 2018 The Electrochemical Society.