Cathodic Polarization Behavior of Rusted Carbon Steels Exposed to Atmospheric Environment for Different Duration
Authors: Aga Ichiro , Deguchi Hiroshi , Tsuchiya Hiroaki , Hanaki Koushu , Yamashita Masato , Fujimoto Shinji 
Published: December 2020

Carbon steel is most commonly employed as a structural material for infrastructures due to its superior mechanical strength as well as low cost. However, as carbon steel corrodes in atmospheric environment, improvement in corrosion resistance is required for steel infrastructures. It is well-known that during wet/dry cycle in atmospheric environment, the reduction reactions of rust on carbon steel as well as the oxygen reduction on rusted steel occur as cathodic reaction, affecting an anodic dissolution of underlying steel. Although the chemical composition and structure of rust layers on carbon steel have been extensively studied, the reduction reactions have not been focused. In this study, we have investigated reduction reactions on rusted carbon steels.

Specimens were rusted carbon steels that had been exposed to a coastal region in Japan for different periods of 3 months, 1 year and 3 years. In the present work, polarization test was carried out to investigate reduction reactions on the specimens. The polarization was performed in a deaerated aqueous solution containing chlorides and bicarbonate for 500 s~30000 s at -0.8 V where the reduction of rusts on the specimens mainly occurs under the deaerated condition. After the defined periods of the rust reduction (td
), air was introduced into the aqueous solution.

SEM observation of rusts on the rusted carbon steels revealed that the thickness of the rusts increased with prolonged periods of the exposure from 50 μm after 3 months to 130 μm after 3 years. In the polarization test, smaller reduction current density was observed on rusted steel with thicker rust layer during the potentiostatic polarization at -0.8 V under the deaerated condition. The subsequent introduction of air into the solution increased the reduction current density. The increase of the current density (ΔIc
) was attributed to oxygen reduction reaction on the specimens. Therefore, ΔIc
was defined as oxygen reduction current density. The ΔIc
varied depending on the periods of rust reduction (td
), that is, the ΔIc
increased with increasing td
and then decreased after maximum with td
. The maximum of ΔIc
was larger for specimen with thicker rust layer. Structures of rust layers were examined by XRD using synchrotron radiation before and after the polarization test. Obtained XRD patterns revealed that the main components of the rusts grown on the carbon steel were Fe3O4 and FeOOH. After the cathodic polarization, the fraction of Fe3O4 in the rust layers increased whereas that of FeOOH decreased with increasing td
.

According to the above results, the increased ΔIc
with td
in the initial stage of the polarization test can be attributed to the increased amount of electroactive Fe3O4, which facilitates electron transfer in the rust. However, the further increase of Fe3O4 may reduce the active surface area of Fe3O4, resulting in the decreased ΔIc
in the later stage of the test. Furthermore, the larger maximum ΔIc
observed on the specimen with thicker layer is also attributed to the amount of Fe3O4, that is, larger amount of Fe3O4 is formed in thicker rust layer. These results indicate that the reduction reactions on rusted steel are strongly related to the structure of rust layer, which will affect corrosion behavior of steel in atmospheric corrosion.