Shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) studies of steel surface corrosion

Abstract

Electrochemical shell isolated nanoparticle enhanced Raman spectroscopy (SHINERS) in alkaline environments using Au@SnO2 core-shell particles is reported. Au nanoparticles covered by an approximately 4 nm thick SnO2 shell are shown to be pinhole free and provide strong surface enhanced Raman scattering of a pyridine derivative adsorbed on smooth gold surfaces. The Au@SnO2 core-shell particles are used to demonstrate the first electrochemical SHINERS spectra of corrosion products on 304 stainless steel and carbon steel. Unlike Au@SiO2 particles, the SnO2 shells are not degraded by the local high pH created by electrolysis reactions. The SHINERS spectra for 304 stainless steel are indicative of amorphous or microcrystalline Fe(OH)2 with a small contribution from Cr(VI) oxide at high overpotentials. In the presence of KCl, a band attributed to γ-FeOOH is found in the spectra. For carbon steel the SHINERS spectra did not show any evidence of Fe oxides, hydroxides or oxyhydroxides but rather the formation of the Fe-water complex, [Fe(H2O)6]n+ (n = 2,3) at the electrode surface. SHINERS data for carbon steel in the presence of an organic inhibitor, benzotriazole (BTA), indicate that BTA inhibits the formation of Fe-water complexes at the surface of the electrode that form in its absence.