Corrosion Initiation and Propagation of Two Duplex Stainless Steels Embedded in Concrete

Two duplex stainless steels rebars: UNS(1) S32304 and UNS S32101 were selected to investigate the corrosion initiation and propagation in reinforced concrete specimens. No chlorides were added to the concrete mix. Three of six embedded rebars had one inch concrete cover. A methodology was implemented to accelerate the chloride transport. The solution used was 20% NaCl by weight. First, cyclic ponding in a moderate elevated temperature (37°C) curing room took place, followed by a migration cell approach at laboratory room temperature. This methodology allowed corrosion to initiate in a relative short period of time (months rather than years). The electric field produced ionic current that polarized the rebars to potential range of values at which removal of Cr2O occurred, this likely lowered the amount of chlorides needed for corrosion to initiate. After corrosion had initiated and propagated for some time; selected specimens were terminated for visual examination. On specimens selected for autopsy, the rebars in the top row showed corrosion to various degrees. Corrosion had propagated to such extent on the terminated specimens that cracks had appeared at the concrete surface above one of the rebars. Stray current likely caused accelerated corrosion on rebars where corrosion had initiated first. Based on chloride concentrations measured at the rebar trace, corrosion initiated: on UNS S32101 rebars range 0.95 to 2.95 wt% cement, and on UNS S32304 rebars the chloride range 1.06 to 2.6 wt% cement. The findings also suggest that UNS S32304 rebars corroded at at a significantly slower rate than UNS S32101.