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The electrochemical and surface properties of mild steel in stagnant aerated solutions containing 5 mM citric acid at pH 5 and pH 9 were investigated by open circuit potential, potentiodynamic polarization and electrochemical impedance measurements combined with X-ray photoelectron spectroscopy (XPS) characterization. The results show that the active dissolution behavior of steel in the citric acid solution at pH 5 is well described using a kinetic model consisting of a modified consecutive mechanism, in parallel with a catalytic path involving the divalent intermediate Fe[(OH) 2 )]ads. The surface film is composed of FeO, Fe(OH) 2 , FeOOH and Fe-citrate. Also, the analysis of in-depth distribution of composition indicates a high coverage of the organic layer. In the pH 9 citrate solution, a thin bilayer passive film was formed (with Fe(II) and Fe(III) located mainly in the inner and outer regions, respectively). Its overall composition is Fe 3 O 4 /FeOOH. Electrochemical measurements revealed the low protective ability of this film and this was confirmed by a modest XPS intensity ratio of Fe(III)/Fe(II). Moreover, no clear presence of Fe-citrate was detected in the passive film, and so the poor passivation behavior was correlated to insufficient adsorption of the tri-ionized citrate anion. The importance of solution flow to stimulate the adsorption of citrate anion is emphasized.

Corrosion behavior and surface properties of mild steel in aqueous citrate solutions