Ellipsometric Study of the Formation of Films on Iron in Orthophosphate Solutions

Ellipsometry was used to study optical properties and kinetics of film growth on iron in 0.1IV sodium orthophosphate solutions differing in pH. Properties and thicknesses of films grown at room temperature (22°C) were found to vary within wide limits depending upon pH, potential, and presence or absence of oxygen. In aerated NasPO solution, γ-Fe2O3 or Fe3O4 film is formed with a refractive index n = 2.6 and absorption coefficient k = 0.20. At anodic polarization potentials and at pH 7 or higher, iron oxide films are also formed. On the other hand, at open-circuit potentials in aerated solutions of pH 7.0 and pH 9.1, a multiple phosphate film is produced. It is composed of several layers with n values oscillating between 1.4 and 1.5, and k values from 0 to 0.14. Such a film decreases the corrosion rate of iron shifting its open-circuit potential to more noble values. As a potential is reached at which iron oxide forms, a film composed of both iron oxide and iron phosphate is produced. The growth rate of iron oxide films, and that of phosphate films at higher pH values than 7.2, follow a logarithmic law. Below pH 7.2, both in aerated and deaerated solutions, an unprotective phosphate film is formed. It is composed of several layers with n values oscillating between 1.34 and 1.36, and k values from 0.013 to 0.45. Its thickness is a linear function of tune.