Time-dependent variation of the electrochemical impedance for thermo-mechanically treated versus plain low alloy steel rebars in contact with simulated concrete pore solution

Electrochemical studies (AC impedance and DC polarization) have been used to study the protective properties of passive films that form on reinforced steel embedded in concrete as a function of time and space. Experiments have been performed on passive/oxide films grown on the surface of steel reinforcement bars exposed to chloride-contaminated simulated concrete pore solution (SPS) prepared by dissolving a representative amount of sodium hydroxide and potassium hydroxide in distilled water and saturated with lime. A common type of locally available concrete reinforcement TMT (thermo-mechanically treated) mild steel with TM (tempered martensitic) and PF (pearlite ferrite) rings (source “A”) was investigated in comparison to a type of hot rolled low-alloy steel without thermo-mechanical treatment (source “B”). The results of the elemental analysis and metallographic microstructure coupled with electrochemical studies indicate that the coupled surface and sub-surface composition of TMT rebar has a time-dependent reversing effect on the protective properties of the passive oxide films that provide durability and protection to the steel rebars against corrosion in concrete. Steel with TM-PF rings has a better initial corrosion resistance in comparison to low alloyed steel. However, with the passage of time, its corrosion resistance decreases, and alloyed steel finally dominates over the TMT rebar.