In situ X-ray absorption spectroscopy for identification of lead species adsorbed on a nickel surface in acidic perchlorate solution

In situ X-ray absorption spectroscopy (XAS) using a synchrotron radiation was applied to identify the Pb species adsorbed on Ni surface in acidic perchlorate solution containing 10−4 M Pb2+ with relation to the Pb-induced stress corrosion cracking (Pb-SCC) of Ni-base alloys used as tubing materials of steam generators in nuclear power plants. The periodical emersion method under potentiostatic polarization, using the Ni plate with large specific surface area (surface roughness, Sr = 78.3) as a working electrode, was developed to detect sensitively the sub-monolayer coverage of Pb on Ni. The Pb LIII absorption spectra in a scanning XAS mode were measured by monitoring the Pb Lα fluorescence line. The Pb LIII absorption near-edge structure (XANES) at the potential more positive than −0.245 V (SHE) corresponding to the equilibrium potential of Pb-electrodeposition in 10−4 M Pb2+ solution has revealed that the Pb species adsorbed on the Ni surface are metallic, providing the clear evidence of underpotential deposition (UPD) of Pb on the Ni surface. Moreover, the extended X-ray absorption fine structure (EXAFS) analysis was performed with a two-shell fit involving Pb–Ni and Pb–Pb interactions assuming that the Ni (1 1 1) plane is mainly exposed to the solution. The EXAFS results were convincingly explained in terms of the Pb-UPD model which represents the coexistence of the surface alloy phase and the adlayer with the same p(2 × 2) structure in the narrow potential range of −0.185 V (SHE) to −0.245 V (SHE).