ELECTROCHEMICAL AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDIES OF PASSIVE FILM ON TIN IN CITRATE BUFFER SOLUTION

Passive film formation as a function of applied potential on tin in ci trate buffer solution was studied by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. In order to avoid air oxidation and to m inimise contamination of the samples, an electrochemical preparation a nd transfer system attached to the ultrahigh vacuum system was used. Q uantitative evaluation of the electrochemical and XPS data showed the characteristic changes of the spectra in the prepassive and in the pas sive potential range. The prepassivation correlated with the clear pre sence of Sn-0, Sn2+ and Sn4+ species, while true passivation correlate d with the presence of Sn4+ species only, as expected on thermodynamic grounds. Three different oxygen-containing species were found in both potential ranges: oxide, hydroxide and water. The hydroxyl oxygen pea ks (oxygen atoms from tin-hydroxide species) were dominant in the O 1s spectra due to electrochemical conditions used (first scan) to produc e samples for XPS examination.