Previous work has focused on the corrosion initiation behavior of rebar embedded in concrete. However, a complete assessment of the potential benefit afforded by new candidate rebar alloys from a corrosion resistance standpoint must address the corrosion propagation behavior and other factors that might affect the risk of corrosion-induced concrete cracking. In this study various electrochemical techniques were employed to characterize the radial (depth) and lateral (length) corrosion propagation behavior of 316LN stainless steel (S31653), 2101 duplex stainless steel (21% Cr, 1.6% Ni, 0.29% Mo, 4.8% Mn), and MMFX-2 (9.3% Cr, 0.089% Ni, 0.023% Mo, 0.46% Mn) compared to carbon steel in saturated Ca(OH)2 with NaCl additions. Radial corrosion was investigated by monitoring the anodic dissolution rate following propagation of local corrosion in a confined anode area. A pitting factor was also determined for each alloy, which describes the degree of corrosion localization. Lateral propagation was characterized using closed packed microelectrode arrays simulating a continuous electrode in order to monitor the spreading of active corrosion from initiated pit sites to adjacent surfaces. Radial pit growth was ohmically controlled but repassivated more readily at high potentials in the case of S31653 and 2101 stainless steels. Conversely, pit growth on carbon steel propagated at all applied anodic potentials and did not repassivate until deactivation by cathodic polarization. Stainless steel showed the highest resistance to lateral corrosion propagation from an active site during microelectrode array testing. In contrast, carbon steel was found to easily undergo widespread depassivation along the surface compared to stainless steel. 2101 and MMFX-2 duplex steels showed similar radial propagation behavior and corrosion morphology, which was intermediate between carbon steel and stainless steel.
Chloride induced corrosion of reinforcing bar leads to premature deterioration of concrete structures. Chlorides are introduced from the external environment typically from marine exposure or wintertime application of de-icing salts. For analysis of the life-cycle cost of any concrete bridge member exposed to chlorides, the corrosion-limited service life is considered to be the sum of the corrosion initiation and propagation phases. Currently, new corrosion resistant candidate rebar materials are being considered to extend the lifetime of reinforced concrete structures. In comparison to carbon steel, stainless steel rebar (containing 18% Cr) is passive through a much broader range of pH?s, due to thermodynamic passivity and dynamic passivity when Cr is added to Fe-based alloys. Moreover, Cr, Mo, and Ni containing stainless steels have a much greater resistance to chloride induced local corrosion compared to carbon steels and have a higher chloride threshold and, therefore, a much longer initiation stage prior to depassivation. However, once the chloride threshold is exceeded and local corrosion begins, it is not known how the corrosion propagation behavior of new rebar materials will affect the remaining service life of reinforced concrete.
Some of the observations made during a previous study of the corrosion initiation behavior in saturated Ca(OH)2 have hinted that different composition rebar materials may also have different corrosion propagation characteristics. A bar which exhibits the highest chloride threshold and longest initiation time may not necessarily also yield the best resistance to corrosion propagation nor optimal conversion of parent-metal into solid corrosion products that ultimately can damage concrete. It is suspected that, once the chloride threshold concentration is
