Iufrared Absorption Spectra of the Chemical Conversion Coating Films of Aluminum

Abstract

Infrared spectra of chemical conversion coating films of aluminum produced by the Modified Bauer-Vogel (MBV), the Alrok and the Alodin processes were investigated by means of the transmission, the high sensitivity reflection and the conventional specular reflection methods. The spectra of these inorganic films obtained by the reflection methods were generally in agreement with that of the transmission method, although moderate band shifts and the change in relative intensity occurred in the spectra of phosphate films obtained by the high sensitivity reflection method. The measurement of spectrum with the reflection methods can thus be employed, with some caution, for studying the composition and the structure of inorganic films formed on metal surfaces.
The absorption spectra of the films obtained by the MBV and the Alrok processes showed strong absorption bands at 1500 and 1385 cm⁻¹, which can be assigned to the vibration of CO₃²⁻ ion in basic carbonate. A continuous absorption appearing in the region of 1100∼500 cm⁻¹ in the spectra of the same films is considered to be due to Al₂(CrO₄)₃. These facts clearly indicate that the films do not consist of oxides, compounds supposed by many investigators, but consist mainly of basic carbonate, probably of aluminum, and aluminum chromate.
The surface film obtained by the Alodin process is composed substantially of phosphates of Cr and Al. This film also contains a small amount of sodium chromate or chromate ion physically adsorbed in the film.