Simultaneous in situ Kelvin probe and Raman spectroscopy analysis of electrode potentials and molecular structures at polymer covered salt layers on steel

A probe head for Raman spectroscopy was installed in the sample chamber of a height-regulated Kelvin probe (KP) in order to focus a laser beam beneath the KP needle and to detect light scattered at the sample surface. This allowed a simultaneous spectroelectrochemical analysis of buried polyvinylbutyral/sodium sulfate/steel interfaces during exposure to humid and dry air. Alterations of sulfate spectra, Volta potential shifts and variations of the polymer surface topography were monitored and found to be interrelated. Bulging of the polymer layer during humidification lowered the Volta potential and thereby indicated a reduction of the interface stability. Partial solvation of sulfate particles manifested in the evolution of the symmetric SO stretching vibration peak at ∼980cm−1, but was retarded and did not induce a volume increase at the polymer/sulfate interface that was detectable at the sample surface. Drying of water saturated PVB/Na2SO4/steel interfaces instantaneously induced a depression of the polymer surface and increased Volta potentials. Pronounced potential shifts in the range of several hundred millivolts, however, were not observed before de-solvation of sodium sulfate initiated. These observations confirmed that the combined KP-Raman spectroscopy approach provides valuable complementary information that is not available from each technique separately.