Using Fourier Transform Infrared Analysis to Detect Corrosion Products on the Surface of Metals Exposed to Atmospheric Conditions

Fourier transform Infrared (FTIR) analysis was applied to characterize the surface of atmospherically exposed metal plates. Early stages of atmospheric corrosion result in formation of corrosion films. Corrosion films are made up of layers of corrosion products initially caused from chemical reactions between the metal substrate and adsorbed gases and then by reactions between corrosion products and adsorbed gases. A protocol was developed for analysis of corrosion films in situ on zinc-galvanized and 55% Al-Zn surfaces. Using a variable angle reflectance accessory, modeling, and cautious interpretation, functional groups were deduced from IR absorbance spectra. The fingerprint region of the mid-IR was interpreted to postulate the effect of orientation and structure of key functional groups that made up part of the corrosion product. Efforts were made to assign bands in the region of < 600 cm^–1. Corrosion products based on a layer structure composed of metal hydroxides with metal chlorides or metal carbonates could be differentiated by observing hydroxide stretching and vibrational movements in the 3,000 cm^–1 to 4,000 cm^–1 and 800 cm^–1 to 1,100 cm^–1 regions, respectively. Corrosion layers that incorporated water coordinated or in a lattice could be identified. Effects of anions such as chloride, carbonate, and sulfate on the spectra of metal hydroxide-metal aqua complexes were discussed. The technique was corroborated using field emission gun scanning electron microscopy (FESEM) and energy-dispersive spectroscopy (EDS) to verify elemental composition and corrosion product homogeneity. Different types of corrosion products were identified and discussed.