How does the ring/halo effect accelerates corrosion outside of localized repairs?

When a concrete patch is installed, fresh high pH concrete is placed in the repair area. The chemical makeup of the new concrete differs from that of the original concrete, therefore the steel in the surrounding areas of the patch is still relatively more active than the steel within the patch. The combination of having the same piece of reinforcement within the patch being passive, while the reinforcement outside the patch is being active, forms a corrosion cell that is fueled by its electrochemical imbalances.

Typically, the patching procedure leaves the reinforcing steel embedded in concrete with abruptly different corrosion potentials. The large difference in corrosion potential (voltage) combined with the short distance between anode and cathode, leads to accelerated corrosion in areas near the patch, resulting in spalling.