Influence of native oxide film age on the passivation of carbon steel in neutral aqueous solutions with a dicarboxylic acid.

• Increasing exposure in air produced more hydroxylated native oxide films on steel. • The passive film became increasingly more protective with age of native oxide film. • The passive films consist of a inner Fe(II)-rich layer and an outer FeOOH layer. • A kinetic model was reasonably optimized on the experimental impedance spectra. Electrochemical impedance spectroscopy (EIS), open circuit potential (E oc) measurements and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) were used to investigate the effect of exposure in air on the passivation of carbon steel electrodes in a neutral aerated glutarate solution. The electrochemical measurements showed that for native oxide films aged between 4 h and 120 h the passive films became increasingly more protective. Analysis of the XPS revealed a bilayer structure of the passive film (inner Fe(II)-rich, outer Fe oxyhydroxide), with increasing values of the Fe(III)/Fe(II) intensity ratio, and gave evidence that passivation was favored by progressively hydroxylated native oxide films. [ABSTRACT FROM AUTHOR]