Synchrotron far-infrared spectroscopy of corroded steel surfaces using a variable angle of incidence

Far-infrared spectroscopy, using a synchrotron source, has been used to study carbon steel corroded in CO₂-saturated brine in the presence and absence of the corrosion inhibitor 2-mercaptopyrimidine (MPY), which allowed the steel surface roughness to be modified. The effect of the angle of incidence (θ_i, 30–80°) on the band intensity and observed bands of the spectra from these surfaces has been determined. For the MPY-treated steel (low surface roughness) the highest band intensity is observed at high θ_i (80°) and different bands were observed at different θ_i. In contrast, for the MPY-free steel (high surface roughness) the highest band intensity is observed at low θ_i (30°) and spectral content changes were not observed. The results are explained in terms of the roughness of the MPY-treated and MPY-free steels, and their effect on the level of diffusely reflected light of the incident infrared beam.