Abstract
The electrochemical behavior of low-alloy steels rusted in the atmosphere have been examined using potentiostatic technique and by coulometry. Polarization diagrams obtained in 0.1M Na2SO4 are characterized by pronounced cathodic currents much greater than the limiting current of oxygen depolarization, which is attributed to the reduction of overlying ferric rust. The high rate of Fe2+ dissolution observed when rusted steels are immersed in the solution is consistent with the polarization behavior.
Assuming that the rate of anodic dissolution or the corrosion rate of rusted steel in deaerated neutral solution is equivalent to the reduction rate of ferric rust, the corrosion rate is determined as a function of immersion time, based on the coulometric measurement of un-reduced amount of ferric oxide on specimens which have been immersed previously in the solution for various time. The corrosion rate is high immediately after immersion and decreased rapidly with time, becoming almost constant in several hours.
The rate of corrosion at a given time of immersion combined with the corresponding corrosion potential gives a plot which precisely followed the analysed or true polarization curve of the similar specimen. This demonstrated the validity of the suggested corrosion mechanism of rusted steel associated with the reduction of rust.
In the course of corrosion process ferrous ion comes out into the solution, the origin of which is traced to the dissolution of the base metal under the rust layer. The rust reduced to lower oxide, on the other hand, does not dissolve in any significant amount.
