In Situ X-Ray Absorption Spectroscopy Study of Sn Underpotential Deposition on Ni from Perchloric Acid

Abstract

In situ X-ray absorption spectroscopy (XAS) was applied to investigate the Sn underpotential deposition (UPD) on Ni surface from 0.2 M HClO 4 solution containing 10 −3 M Sn 2+ with relation to the inhibition effect of Sn on aqueous corrosion of Ni. The periodical emersion method under potentiostatic polarization, using the surface-roughened Ni plate (surface roughness S r = 78.3) as a working electrode was employed to detect sensitively the sub-monolayer coverage of Sn on Ni. The Sn K-edge absorption spectra in a scanning XAS mode were measured by monitoring the Sn K α1 fluorescence line. The Sn K-edge absorption near-edge structure (XANES) in the Sn-UPD potential region has revealed that the Sn-UPD layer on Ni is oxygenated. The extended X-ray absorption fine structure (EXAFS) analysis was performed with a two shell fit of the nearest neighbor Sn-Ni and Sn-O interactions, assuming that the uppermost Ni surface exposed to the solution is mainly oriented to the (111) plane. The results have indicated that Sn atoms are substituted like a surface alloy at face-center-cubic sites in the first Ni layer and further bonded with oxygen atoms. The strong inhibition effect of Sn on aqueous corrosion of Ni is ascribed to the bond between Sn and O atoms in addition to the bond between Sn and Ni atoms. The formation of metal monolayer on a foreign metal electrode at potentials more noble than the equilibrium potential of bulk electrodeposition is termed underpotential deposition (UPD). The UPD of Sn on Pt 1-4 has been investigated with relation to electrocatalysts for electro-oxidation of CO, 5 HCOOH 6 or methanol 7-10 while Sn-plating has been widely used in steel industry due to the corrosion prevention of iron and steels by Sn.11-14 Our recent study 15 has revealed that 10 −3 M Sn 2+ added in 0.2 M HClO 4 inhibits completely the anodic dissolution of pure Ni, suggesting the participation of Sn-UPD in corrosion inhibition of Ni. We reported previously that the addition of Pb 2+ in acidic perchlorate solution suppresses the anodic dissolution of Ni and the suppression of Ni corrosion is ascribed to the Pb-UPD. 16 Moreover, it was confirmed by in situ surface X-ray absorption spectroscopy (XAS) that the Pb species deposited on Ni are in metallic state. 17The following differences, however, are distinct between inhibition effects of Pb 2+ and Sn 2+ on anodic dissolution of Ni. In the case of Pb 2+ , although the Pb atoms deposited on Ni in the UPD potential region suppress the anodic dissolution of Ni, the anodic dissolution of Ni is re-activated due to anodic stripping of the deposited Pb atoms as the potential shifts to the noble direction and Ni is subsequently passivated. 16 In contrast in the case of Sn 2+ , no anodic dissolution of Ni is observed in the wide potential region exceeding the Sn-UPD region.15 The X-ray photoelectron spectroscopy (XPS) analysis has showed that in the case of Pb 2+ , any Pb species are not detected on the passive surface of Ni, 16 while in the case of Sn 2+ , Sn species are detected on the pa...

Inhibition effect of underpotential deposition of metallic cations on aqueous corrosion of metals

“…The in situ XAS study combined with the periodical emersion method using an sr-Ni plate with a surface roughness of 78.3 has been performed by Seo et al (2014a) in order to investigate the chemical state and local structure of electro-deposited Sn on Ni under potential control in deaerated 0.2 m HClO 4 solution with 10 −3 m Sn 2+ . Figure 14 shows the normalized XANES spectra of Sn K-edge for the sr-Ni plates polarized at −0.15 V and 0.05 V in deaerated 0.2 m HClO 4 solution with 10 −3 m Sn 2+ .…”

“…The RSFs of the surface atomic structure models proposed for the Sn-UPD layer on Ni are simulated by using the program FEFF8.2 (Ankudinov et al, 1998(Ankudinov et al, , 2002 to compare wth the real RSFs in Figure 15 by curve fitting. As a result, a Sn-incorporation model is the most suitable in which Sn atoms are substituted like in a surface alloy at face-centered cubic sites in the first Ni layer and further coordinated with oxygen atoms (Seo et al, 2014a).…”

“…obtained with curve fitting for two shells of Sn-O and Sn-Ni based on the Sn-incorporation model are listed in Table 3 (Seo et al, 2014a). The normalized Sn K-edge XANES spectrum of a metallic β-Sn foil is also shown for comparison.…”

“…Reproduced with permission from The Electrochemical Society. (Seo et al, 2014a). The peaks, P A and P B , are assigned to the Sn-O and Sn-Ni interactions, respectively.…”

“…Most of studies of corrosion inhibition due to UPD suggested only the possibility of UPD from the electro-chemical measurements such as cyclic voltammetry and could not show any clear evidence of UPD. Nevertheless, the direct evidence of UPD on a corroding metal surface (Seo et al, 2008(Seo et al, , 2014aSeo, 2012) has been recently obtained from the detection and identification of UPD species by using surface-sensitive analytical methods such as X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (XAS), which could lead to a more quantitative insight of participation of UPD in aqueous corrosion.…”